Photophysical and photochemical properties of the ...

Chemosphere 83 (2011) 1513–1523

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Photophysical and photochemical properties of the pharmaceutical compound salbutamol in aqueous solutions Leah G. Dodson 1, R. Aaron Vogt, Joyann Marks 2, Christian Reichardt, Carlos E. Crespo-Hernández ⇑ Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, United States

a r t i c l e

i n f o

Article history: Received 27 October 2010 Received in revised form 18 January 2011 Accepted 19 January 2011 Available online 12 February 2011 Keywords: Environmental photochemistry Pharmaceutical compounds Transient absorption spectroscopy Computational chemistry Aquatic systems Persistent organic pollutants

a b s t r a c t Salbutamol is a potent b2-adrenergic receptor agonist widely used in the treatment of bronchial asthma and chronic obstructive pulmonary disease. An increasing number of studies have detected salbutamol in natural water systems worldwide. Studies have shown that sunlight degrades salbutamol resulting in the formation of products; some showing higher toxicity to bacteria Vibrio fischeri than the parent compound. In this contribution, steady-state absorption and emission techniques, high-performance liquid chromatography, and transient absorption spectroscopy are used to investigate the photochemistry of salbutamol in aqueous buffer solutions at controlled pH values. Ground- and excited-state calculations that include solvent effects are performed to guide the interpretation of the experimental results. Salbutamol is sensitive to UVB light absorption in the pH range from 3 to 12, forming products that absorb light at longer wavelengths than the parent compound. Quantum yields of degradation reveal that the deprotonated species is 10-fold more photo-active than the protonated species. In line with this result, the fluorescence quantum yield of the protonated species is more than an order of magnitude higher than that of the deprotonated species. Transient absorption spectroscopy shows that population of the triplet state occurs with a rate constant of 7.1 108 s1 in the protonated species, while a rate constant of 1.7 1010 s1 is measured for the deprotonated species. While degradation of the deprotonated species is not affected by the presence of molecular oxygen, a twofold increase in the photodegradation yield of the protonated species in air-saturated conditions is observed. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Pharmaceutical and personal care products (PPCPs) are a class of aquatic pollutants that have been increasingly detected in field samples worldwide (Daughton and Ternes, 1999a; Kolpin et al., 2002a; Boreen et al., 2003; De˛bska et al., 2004; Richardson, 2004; Jones et al., 2005). There are several lines of evidence suggesting that photochemical degradation could play an important role in the fate of these compounds in the environment (Boreen et al., 2003, 2004, 2005; Packer et al., 2003b; Cosa, 2004; De˛bska et al., 2004; Eichhorn and Aga, 2004; Latch et al., 2005; Löffler et al., 2005; Sanderson et al., 2005; Werner et al., 2005; Wolters and Steffens, 2005). Many PPCPs contain aromatic rings and ⇑ Corresponding author. Tel.: +1 216 368 1911; fax: +1 216 368 3006. E-mail address: [email protected] (C.E. Crespo-Hernández). Fellow of the Support of Undergraduate Research and Creative Endeavors (SOURCE, CWRU). Present address: Department of Chemistry, California Institute of Technology, Pasadena, CA. 2 Department of Chemistry, Fisk University, Nashville, TN. Fellow of the Academic Careers in Engineering & Science (ACES, NSF) Undergraduate Summer Internship Program. Present address: Department of Chemistry, Macromolecular Interfaces Institute, Virginia Polytechnic and State University, Blacksburg, VA. 1

0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.01.048

functional groups that can absorb solar radiation or react with photo-generated reactive species found in natural water (Buser et al., 1998; Boreen et al., 2003; Latch et al., 2003; Packer et al., 2003a; Tixier et al., 2003). PPCPs often contain chemical structures similar to those found in pesticides, which are known to be lightsensitive and have detrimental effects in aqueous environments (Amor and Jori, 2000; Burrows et al., 2002a,b; Pelletier et al., 2006). A growing number of studies have found that PPCPs can be photo-reactive when studied in the context of phototoxicity (van Henegouwen, 1997; Vargas et al., 1999; Cosa, 2004). Because PPCPs found in surface water have already escaped the rigorous degradation environment of wastewater treatment, photochemical degradation in sunlit water could be expected to play an even more important role than biodegradation (Andreozzi et al., 2003; Boreen et al., 2003; Daughton and Ruhoy, 2009). It has yet to be determined whether levels found in drinking water pose a human health risk (Fent et al., 2006), but finding these compounds in drinking and surface water sources is a concern due to their possible low levels of action and widespread occurrence (Daughton and Ternes, 1999a; Kolpin et al., 2002b; Boxall, 2004). Salbutamol is a pharmaceutical compound commonly used to relieve the bronchospasms associated with asthma (Fig. 1)

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L.G. Dodson et al. / Chemosphere 83 (2011) 1513–1523

0 0 Fig. 1. Proposed micro-ionization scheme of salbutamol. Micro-ionization constants pK a1 ¼ 9:22, pK a1 ¼ 9:60, pK a2 ¼ 10:22, and pK a2 ¼ 9:84 (Ijzerman et al., 1984).

(Moxham and Costello, 1997). It is excreted in urine as a mixture of the un-metabolized drug and its conjugated metabolite (Boulton and Fawcett, 1995; Damasceno et al., 2000). Several studies have detected salbutamol in natural water sources worldwide (Ternes, 1998; Daughton and Ternes, 1999b; Calamari et al., 2003; MacLeod et al., 2007), including concentrations as high as 471 ng L1 in the Thames (Bound and Voulvoulis, 2006). The inability of waste treatment plants to remove this pharmaceutical completely from wastewater has also been documented (Jones et al., 2006, 2007; MacLeod et al., 2007; Daughton and Ruhoy, 2009). Recently, it has been found that exposure of aqueous salbutamol solutions to sunlight radiation results in the formation of products (Kirshnakumar et al., 2007; Akumar et al., 2008), some showing toxic properties to bacteria Vibrio fischeri (Sakkas et al., 2007). However, there is a need for background information regarding the steady-state and time-resolved photochemistry of salbutamol under controlled experimental conditions. Quantitative information on the photodegradation quantum yield and on the excited states and reactive species that are involved in its light-induced transformation in aqueous solutions are lacking. Such studies could assist in developing more efficient methods to degrade salbutamol and related PPCPs in water treatment plants by using advanced oxidation processes or photocatalytic methods. In this work we present results from laboratory studies of the photodegradation of salbutamol in aqueous buffered solutions. Quantum yields of fluorescence and of degradation as well as transient absorption experiments are performed at different pH conditions to investigate the photoreactivity of the different species of salbutamol. Ground- and excited-state calculations are also performed to guide the interpretation of the experimental results. Salbutamol is found to be susceptible to UVB light absorption (280–320 nm) in the pH range from 3 to 12 while showing increased degradation activity in basic conditions. In particular, exposure of the protonated and deprotonated species to UVB radiation results in the formation of several products, some of which absorb at longer wavelengths than the parent compound. Even though the concentration of these products in aquatic systems might be expected to be low, their formation has the potential of increasing the amount of toxic substances in natural water systems.

2. Materials and methods 2.1. Chemicals All chemicals were used as received. Salbutamol (P96% pure) and DL-tryptophan (P99% pure) were acquired from Sigma Aldrich (St. Louis, MO). Ultrapure water was used to prepare acid, neutral and basic buffer solutions. The pH 3 buffer solution was prepared using (C2H5)3N, NaH2PO4, and phosphoric acid, buffer solutions with values between 5 6 pH 6 8 were prepared using NaH2PO4 and Na2HPO4 salts, while pH 10 and 12 buffer solutions were prepared using Na2B4O710H2O and NaOH or Na2HPO4 and NaOH, respectively. 2.2. Steady-state absorption and emission spectrometers Absorption spectra were recorded on a Cary 100 Bio UV–Vis Spectrophotometer (Varian, Inc.) with a 1 cm optical path quartz cell (Starna Cells, Inc.). Fluorescence spectra were measured using a Cary Eclipse Fluorescence Spectrophotometer (Varian, Inc.). Fluorescence spectra were taken at medium PMT voltage (600 V) and the bandwidth used for emission and excitation monochromators was 2.5 or 5 nm with an averaging time of 0.1 s. The absorption of the solutions for the emission experiments was 0.2 in a 1 cm2 cell at the excitation wavelength. 2.3. Irradiation of salbutamol Buffered solutions at pH values of 3.0 ± 0.2, 10 ± 0.2 or 12.0 ± 0.2 were prepared to maximize the fraction of the protonated, zwitterionic/neutral or deprotonated species of salbutamol (Fig. 1), respectively. The solutions were irradiated with a 150 W ozonefree Xe lamp (Newport-Oriel, Apex Source Arc, source model 66453, lamp model 6255) using a band-pass filter (FGUV11S, Schott) that transmits 90% or more in the spectral range from 280 to 380 nm. Filtering the IR emanating from the lamp had no effect on the photodegradation of salbutamol showing that thermal decomposition reactions have an insignificant contribution to the reported results. The temperature of the salbutamol solutions while under irradiation was 22 ± 2 °C. The lamp is


1515

FracAk ¼ 1 10Ak

ð3Þ

equipped with a rear reflector and a 1.3 in. diameter output beam. The output beam was focused down to 0.95 cm (3/8 of an inch) in front of the cell for the irradiation experiments using a lens with a focal length of 450 mm that was placed 41 cm (16 in.) from the sample holder and 2 in. from the lamp source. Samples were placed in a spinette cell stirrer holder (Starna Cells, Inc.) and irradiated in a 1 cm2 quartz cell. The solutions were stirred continuously to ensure that a new solution volume was irradiated at all times. Continuous stirring also guarantees that the irradiation was performed on homogeneous solutions. The lamp source, optical components and cell holder were mounted on an optical table to ensure reproducibility of the results. The light-induced degradation of salbutamol at different pH values was monitored using UV–Vis and emission spectroscopy as well as high-performance liquid chromatography (see electronic Supporting Material, Fig. 1S). Separation of the product mixtures was performed using a Shimadzu HPLC (LC-20AD), which is integrated with a quaternary pump system and with a UV–Vis photodiode array detector (SPD-M20A). The photodiode array detector permits the detection of the absorption spectra of the eluted compounds. A Luna PFP(2) chromatographic column from Phenomenex (5 lm, 25 cm 4.6 mm) was used. The gradient method used a two-pump solvent mixture composed of pH 3 phosphate buffer and acetonitrile, with a flow rate of 0.75 mL min1. A linear increase in acetonitrile from 4% to 9% over 10 min was performed, which was then kept constant for 40 min (i.e., 91% pH 3 buffer: 9% acetonitrile mobile phase composition). 2.4. Determination of quantum yield of photodegradation Degradation quantum yields were determined in air- and N2saturated conditions. Calibration curves were constructed for salbutamol solutions at pH 3 and 12 using the HPLC. The area under the chromatographic fraction corresponding to the salbutamol species was then measured to determine the change in concentration of parent compound as a function of irradiation time. The incident photon flux was determined using the potassium ferrioxalate as a chemical actinometer following the method described by Calvert and Pitts (Calvert and Pitts, 1966). Potassium ferrioxalate has been recommend by IUPAC as a well established polychromatic actinometer (Kuhn et al., 2004). Since polychromatic light was used, the average quantum yield (molecules photon1) of the potassium ferrioxalate was obtained using:

R kf

Uav g ¼

ki

Ik Uk dk R kf ki Ik dk

ð1Þ

NFe2þ t ac Uav g

R kf FracAav g ¼

ð2Þ

where N Fe2þ is the number of Fe2+ ions formed and tac is the time interval for which the actinometer was irradiated. The number of Fe2+ ions was measured using the method described by Calvert and Pitts (Calvert and Pitts, 1966). The fraction of photons absorbed by the sample at a given wavelength FracAk is given by:

ki

Ik FracAk dk R kf ki Ik dk

ð4Þ

Since FracAavg changes with irradiation time, the average rate Ia;av g ðt i Þ (photons s1) at which photons are absorbed at any given time ti by the sample is given as:

Ia;av g ðti Þ ¼ FracAav g Io;av g

ð5Þ

The total number of photons absorbed up to a given time tn, ITa;av g ðtn Þ, by the sample is then given as

ITa;av g ðtn Þ ¼

n X ðIa;av g ðti ÞÞ ðt i t i1 Þ

ð6Þ

i¼0

where (ti – ti1) is the irradiation time interval during which Ia,avg(ti) is the rate of photon absorption. The average quantum yield of photodegradation of the sample Us,avg is then calculated by using:

Us;av g ¼

DNðtÞ ITa;av g ðtÞ

ð7Þ

where DN(t) is the number of molecules of the salbutamol species that have been degraded at a given irradiation time t. Eq. (7) was applied by graphing DN(t) as a function of the number of photons absorbed at a given irradiation time ITa;av g ðtÞ. The slope of a least squares linear regression gives the average degradation quantum yield (in molecules photon1) of the sample in the spectral range of the electromagnetic spectrum used for irradiation. 2.5. Determination of fluorescence quantum yield Determination of the fluorescence quantum yields was performed as described in detail elsewhere (Chen, 1967; Lakowicz, 1999) using tryptophan as a fluorescence standard. The fluorescence spectra were corrected for the wavelength-dependent sensitivity of the detector. The solutions of tryptophan (pH 6) and salbutamol at pH 3 or pH 12 were excited at 280 nm under identical instrument conditions. The absorbance of the solutions at the excitation wavelength was 0.39 ± 0.01. The fluorescence quantum yield at each pH was measured by using Eq. (8) (Lakowicz, 1999):

Uun f ¼

where Ik and Uk are the relative incident photon flux of the lamp source and quantum yield of the actinometer at a given wavelength, respectively, in the spectral range from 280 to 380 nm used in this work. The relative incident photon flux from the lamp source was measured after the FGUV11S band-pass filter using a spectrometer (Avantes, AvaSpec-2048) prior to quantum yield measurements. The quantum yields of potassium ferrioxalate used in Eq. (1) were interpolated from those reported by Murov et al. (1993). The wavelength dependence of this quantum yield is less than 6% in the spectral range from 280 to 380 nm (Murov et al., 1993). The average incident photon flux I0,avg (photons s1) was calculated using:

I0;av g ¼

where Ak is the absorbance at wavelength k. The average fraction of photons absorbed by the sample over the complete spectral range of irradiation wavelengths FracAavg was determined using:

n2un Iun ODs s U n2s Is ODun f

ð8Þ

where OD (the optical density) is the steady-state absorption of the standard (s) or unknown (un) at the excitation wavelength, I is the integrated area under the emission spectra of the standard or unknown, n is the refractive index of the respective solvent, and Uf is the fluorescence quantum yield. Note that we assume that the values for the refractive index of the solutions need not to be known in Eq. (8). This is because the solute will not change the refractive index of the diluted solutions significantly and similar buffer solutions and excitation wavelength were used for the unknown and the standard in this work. 2.6. Pump–probe femtosecond transient absorption system The femtosecond laser system and broadband transient absorption spectrometer used in this work have been described in detail elsewhere (Reichardt et al., 2009). Briefly, a Quantronix Integra-i/e 3.5 Laser generating 100 fs pulses at 800 nm with a repetition rate of 1 kHz was used to pump an optical parametric

1516


amplifier (TOPAS, Light Conversion). In this work, the optical parametric amplifier output was tuned to the excitation wavelengths of 290 or 305 nm for experiments in acidic and basic conditions, respectively. Potential contributions from other excitation wavelengths or polarizations were removed by using a Glan-Taylor prism and a reflective k-filter. Contribution from rotational relaxation effects in the kinetics were removed by using a depolarizing plate that randomizes the polarization of the excitation pulses. A continuously moving 2 mm CaF2 crystal was used for white light continuum generation and the probe pulses were corrected for group velocity dispersion (Nakayama et al., 1997) using a homemade LabView program (National Instruments, Inc.) (Reichardt et al., 2009). A broadband spectrometer was used for data collection (Helios, Ultrafast Systems, LLC). Data analysis was performed using Igor Pro 6.12 software (Wavemetrics, Inc.), as described elsewhere (Reichardt et al., 2009). The instrument response function varies at 210 ± 80 fs in the spectral range between 350 and 650 nm depending on the probe wavelength used, as determined from the coherent signal seen in solvent-only scans. Approximately 20 representative kinetic traces were used in the global fit analysis for each dataset covering the full range of probe wavelengths. The kinetic traces were globally fitted to a sum of two exponential terms and a constant offset convoluted with the appropriate instrument response function. The intensity of the excitation pulses was attenuated using a neutral density optical filter to minimize multiphoton absorption and cross-phase modulation effects resulting from the solute, solvent or cell windows (Lorenc et al., 2002). Still, a small contribution from multiphoton ionization signal of the solvent was observed even after minimizing the intensity of the excitation pulses. If needed, the transient absorption signals can be corrected from the contribution of multiphoton solvent signals as described previously (Crespo-Hernández and Kohler, 2004). Hence, the first lifetime obtained from the global fit analysis (0.2 ± 0.1 ps at pH 3) and (0.5 ± 0.2 ps at pH 12) more likely corresponds to the formation of a small amount of hydrated electrons upon ionization of water (Crespo-Hernández and Kohler, 2004) and this lifetime is thus not discussed further in this work. All reported uncertainties are at least twice the standard deviation (2r). The salbutamol concentration used for the laser experiments was (3.0 ± 0.4) 103 M at both pH 3 and pH 12 buffer conditions. The sample in the excited volume was continuously refreshed using a magnetic stirrer to avoid re-excitation of the pumped volume by successive laser pulses. Degradation of salbutamol was monitored by UV absorption spectroscopy and solutions were replaced by fresh ones if the steady-state absorbance at the excitation wavelength decreased by more than 10% during the course of the experiments. No changes in the transient absorption spectra or in the decay signals of salbutamol were observed during the course of the transient absorption experiments for solutions showing a decrease in steady-state absorbance of less than 10% at the excitation wavelength. 2.7. Ground-state calculations All calculations were performed using the Gaussian 03 suite of programs (Frisch et al., 2004) as described in detail in previous works (Reichardt et al., 2009; Reichardt and Crespo-Hernández, 2010). In short, protonated and deprotonated salbutamol structures were optimized using the unrestricted implementation of the B3LYP functional (Lee et al., 1988; Becke, 1993) and the 631G(d,p) standard basis set. Geometry optimizations were performed without any geometrical restriction except those imposed by symmetry. Solvent effects were taken into account in the ground-state optimizations by performing self-consistent reaction field (SCRF) calculations using the polarizable continuum model

(PCM) (Barone et al., 1997) with the integral equation formalism (SCRF = IEFPCM) (Cancès et al., 1997). The default water solvent parameters were used, as given in Gaussian 03. 2.8. Excited-state calculations Vertical excitation energies on the optimized ground-state structures that include solvent effects were performed using the unrestricted implementation of the PBE0 functional (Adamo and Barone, 1999; Adamo et al., 1999) and the 6-311++G(d,p) basis set (i.e., at the TD-PBE0/IEFPCM/6-311++G(d,p) level of theory), as described elsewhere (Reichardt et al., 2009; Reichardt and Crespo-Hernández, 2010). The PBE0 functional has been shown to provide reasonable excited-state energies (Improta et al., 2007a; Jacquemin et al., 2009) and correct excited-state ordering (Jacquemin et al., 2006; Improta et al., 2007a) when solvent effects are included in the calculations (Mennucci et al., 2001; Improta et al., 2007a,b; Crespo-Hernández et al., 2008; Reichardt and Crespo-Hernández, 2010; Vogt et al., 2010). The character of the excited states was estimated from a molecular orbital analysis of the principal configuration interaction transitions. 3. Results 3.1. Steady-state absorption and emission spectra Salbutamol exists as a racemic mixture of the S- and R-enantiomers in aqueous solutions (Brittain et al., 1973; Esquisabel et al., 1997; Schmekel et al., 1999). Until 1999, salbutamol was marketed as the racemate form but more recently as the pure R-enantiomer (also known as levosalbutamol), which is the biologically active enantiomer (Boulton and Fawcett, 2001). Being an amphoteric compound, each enantiomer can be present as four different species in the pH range between 6.0 and 8.5 found in aquatic systems (Fig. 1) (Morgan et al., 2007). Ijzerman et al. calculated the microscopic ionization constants associated with each salbutamol species (Ijzerman et al., 1984). These constants can be used to estimate the fraction of each species present at any pH (see Fig. 2). The protonated form of salbutamol is expected to be the primary species found in natural water systems with common pH values. The deprotonated species dominates at pH values higher than 11 while the zwitterionic (46.9%) and neutral (20.2%) species have the highest contribution at a pH near 10. Hence, irradiation studies at controlled pH conditions can provide evidence of which salbutamol species is more reactive upon sunlight absorption and of the role that photochemical degradation plays in aqueous environments. The normalized absorption and emission spectra of salbutamol at pHs 3, 10 and 12 are shown in Fig. 2. A bathochromic shift in the absorption and emission spectra is observed with an increase in the pH of the solution (see also Table 1). Zero–zero singlet state energies of 34 557 cm1 (300 nm) and 31 731 cm1 (316 nm) were estimated from the crossing point of the normalized absorption and emission spectra at pH 3 and 12 for the protonated and deprotonated species, respectively. Table 1 also reports the fluorescence quantum yield of the protonated and deprotonated species. The fluorescence yield of the protonated species is more than an order of magnitude greater than that of the deprotonated species. 3.2. Irradiation experiments and determination of degradation quantum yields Fig. 3 (left panel) shows the changes in the absorption spectra of salbutamol as a function of irradiation time for aqueous buffer solutions at pH 3, 10 and 12. Changes in the absorption spectra of salbutamol with irradiation time at pH 7 are qualitatively similar to those

1517


at pH 3 (see Fig. 2S), as expected from the higher contribution of the protonated species at neutral pH (Fig. 2). The absorption intensity of the salbutamol solutions increases with irradiation time and new absorption bands are formed at longer wavelengths; up to 450 nm depending on the pH of the solution. This is indicative of the formation of products that absorb at visible wavelengths. The photodegradation of salbutamol was also followed using emission spectroscopy. The changes in the emission spectra of salbutamol as a function of irradiation time at pH 3, 10 and 12 are shown in Fig. 3 (right panel). It can be observed that the integral of the main emission band of salbutamol decreases with irradiation time, while a new emission band is observed at longer wavelengths. The observation of an isoemissive point is evidence that at least one product is being formed, which emits at longer wavelengths than salbutamol. It is possible that a common product is responsible for the emission at longer wavelengths at different pH values. HPLC experiments suggest that common products are formed in the photodegradation of salbutamol at pH 3 and 12 (see ESM, Fig. 1S). The characterization of the products resulting from the photodegradation of salbutamol at different pHs and the evaluation of their toxicity is beyond the scope of the present work. We note, however, that the major products resulting from the direct and catalytic photodegradation of salbutamol have been characterized and their structures have been reported (Kirshnakumar et al., 2007; Sakkas et al., 2007; Akumar et al., 2008). Fig. 3 shows that products are formed that absorb at the same wavelengths as salbutamol. This precludes the determination of photodegradation yield using UV–Vis spectroscopy (CrespoHernández and Arce, 2000; Crespo-Hernández et al., 2000). Hence, the areas under the absorption peak of the chromatogram corresponding to salbutamol as a function of irradiation time were used to determine the quantum yield of degradation of the protonated and deprotonated species at pH 3 and 12, respectively. Quantum yields were determined in air- and N2-saturated conditions (Table 1). The quantum yield of the protonated species is an order of magnitude smaller than that of the deprotonated species. Interestingly, while the presence of molecular oxygen causes a 50% increase in the quantum yield of degradation of the protonated species, its presence does not appreciably affect the degradation of the deprotonated species.

Fraction of Species

1.0

0.8

0.6

Protonated Deprotonated Neutral Zwitterionic

0.4

0.2

0.0 6

7

8

9

10

11

12

13

pH 4.5 pH 3 pH 10 pH 12

Normalized Absorbance

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 225

250

275

300

325

350

Wavelength (nm) pH 3 pH 10 pH 12

Normalized Intensity

1.0

0.8

0.6

3.3. Transient absorption measurements 0.4

0.2

0.0 300

350

400

450

500

Wavelength (nm) Fig. 2. Estimated fraction of each salbutamol species present at a given pH (top). Relative fractions were calculated using the ionization constants reported by Imboden and Imanidas (1999). Normalized absorption (middle) and emission (bottom) spectra of salbutamol in aqueous buffer solutions at pH 3, 10, and 12.

Fig. 4 depicts the transient absorption spectra and contour plots of salbutamol at pH 3 and 12. Broad absorption bands are observed for salbutamol at pH 3 and 12 in the spectral window investigated in this work. At initial time delays, the transient absorption spectrum of the protonated species shows absorption maxima around 475 and below 350 nm, while that of the deprotonated species shows an absorption maximum near 525 nm. The initial absorption bands decay with the simultaneous formation of an absorption band with maximum at 375 nm at pH 3 and around 400 nm and above 650 nm at pH 12. An additional negative-amplitude band with maximum around 350 nm is observed at pH 12. This negative-amplitude band resembles the steady-state fluorescence

Table 1 Photochemical properties of the protonated and deprotonated species of salbutamol in aqueous buffer solutions. pH

A (kmax) (nm)

e (kmax) (mM1 cm1)

F (kmax) (nm)

E0,0 (cm1)

Uf

s1 (ps)a

Udeg, in air

Udeg, in N2

3 12

276 ± 1 295 ± 1

1.7 ± 0.2 2.8 ± 0.1

305 ± 2 352 ± 2

34 557 31 731

0.06 ± 0.01 0.003 ± 0.001

1400 ± 500b 55 ± 5

0.04 ± 0.01 0.37 ± 0.06

0.02 ± 0.01 0.36 ± 0.02

a s1 is the depopulation lifetime of the singlet state, which does not decay within the time window of our spectrometer and the amplitude of which is represented by a constant offset in the global fit analysis (see Section 2 for details). b The accuracy of this value is limited by the 3 ns time window used in this work.

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600

3.0 0.0 min 30.0 min 60.0 min 90.0 min 120 min 150 min 180 min 210 min 240 min

2.0 1.5 1.0 0.5 0.0 200

0.0 min 5.0 min 10.0 min 15.0 min 20.0 min

500

Intensity (a.u.)

Absorbance

2.5

400 300 200 100 0

250

300

350

400

450

300

500

350

Wavelength (nm) 2.0

450

500

800 0.0 min 1.0 min 2.0 min 3.0 min 4.0 min 5.0 min 6.0 min

0.0 min 1.0 min 2.0 min 3.0 min 4.0 min 5.0 min 6.0 min

700 600

Intensity (a.u.)

1.5

Absorbance

400

Wavelength (nm)

1.0

500 400 300 200

0.5

100 0

0.0 200

250

300

350

400

300

450

350

400

450

500

Wavelength (nm)

Wavelength (nm) 1.2

0.8 0.6 0.4 0.2 0.0 200

0.0 min 0.5 min 1.0 min 1.5 min 2.0 min 2.5 min 3.0 min 3.5 min 4.0 min

150

Intensity (a.u.)

1.0

Absorbance

180

0.0 min 0.5 min 1.0 min 1.5 min 2.0 min 2.5 min 3.0 min 3.5 min 4.0 min

120 90 60 30 0

250

300

350

400

450

500

Wavelength (nm)

320

360

400

440

480

520

560

Wavelength (nm)

Fig. 3. Left panel: Changes in the absorption spectra as a function of irradiation time of salbutamol in aqueous buffer solutions at pH 3 (top), 10 (middle) and 12 (bottom). Right panel: Changes in the fluorescence spectra as a function of irradiation time of salbutamol in aqueous buffer solutions at pH 3 (top), 10 (middle) and 12 (bottom). Excitation wavelengths are 260, 280 and 295 nm, respectively. The dashed line in the bottom panel shows a representative water Raman emission band when exciting at 295 nm.

emission band that has a maximum around 352 nm (Fig. 3). Table 1 reports the lifetimes obtained from a global fit analysis to the transient absorption data shown in Fig. 4. Representative decay signals at selected probe wavelengths are shown in Fig. 5 together with best global fit curves. 3.4. Ground-state optimizations Salbutamol is present as a mixture of the S- and R-enantiomers in aqueous solutions. Hence, we have calculated the thermodynamic stability of the protonated and the deprotonated salbutamol enantiomers in water solvent at the B3LYP/IEFPCM/6-31G(d,p)

level of theory (Table 1S). Two different conformers (rotamers) of each enantiomer were optimized to judge if more than one conformer is present under the experimental conditions used in our experiments. The S- and R-conformers for the protonated and deprotonated salbutamol species are shown in Figs. 3S and 4S, respectively. The optimized ground-state geometries for the neutral and zwitterionic forms of salbutamol are shown in Fig. 5S. 3.5. Vertical excitation energies Vertical excitation energies in aqueous solutions for all the ground-state optimized structures investigated in this work are


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Fig. 4. Transient absorption (top, left panel) spectra and contour plot (bottom, left panel) for salbutamol in aqueous buffer solutions at pH 3. Transient absorption spectra (top, right panel) and contour plot (bottom, right panel) for salbutamol in aqueous buffer solution at pH 12. Spectral evolution at delay times below 0.5 ps is not shown due to contributions from solvent signal (see Section 2 for details).

4. Discussion 410 nm 580 nm

2.0

ΔA / 10

3

1.6 1.2 0.8 0.4 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time / ns 6.0

370 nm 400 nm 500 nm

Δ A / 10

3

5.0 4.0 3.0 2.0 1.0 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time / ns Fig. 5. Decay signals at selected probe wavelengths for salbutamol in aqueous buffer solutions at pH 3 (top) and 12 (bottom).

shown in Table 2S. Also shown are the characters of the excited states and singlet–triplet energy gaps for the protonated and deprotonated species. For completeness, the vertical excitation energies for the neutral and zwitterionic forms of salbutamol are also given in the ESM.

The presence of salbutamol in aquatic systems worldwide has stimulated investigations aimed at understanding the fate of this pharmaceutical compound in the environment (Ternes, 1998; Daughton and Ternes, 1999b; Calamari et al., 2003; Bound and Voulvoulis, 2006; MacLeod et al., 2007). It has been demonstrated that exposure of salbutamol to sunlight can result in the formation of products in aqueous solutions (Kirshnakumar et al., 2007; Akumar et al., 2008), some of which have the potential to be toxic (Sakkas et al., 2007), thus contributing new harmful compounds to the already polluted natural water sources. However, quantitative information regarding the photodegradation yield, excited-states and reactive intermediate species involved in the sunlight-induced degradation of salbutamol in aqueous media is lacking. Such information can assist scientists aiming at optimizing sewage treatment plant protocols that use advanced oxidation processes and/or photochemical methods for the degradation of salbutamol and related PPCPs in aqueous media. Hence, we performed a comprehensive and systematic array of computational, steady-state and time-resolved experiments, which allowed us to gather essential insights about the primary photochemical relaxation pathways of salbutamol in aqueous solutions at controlled pH values. It is informative to discuss initially the results of calculations. Ground-state optimizations predict that a racemic mixture of the S(1) and R(1) enantiomers will be present in an aqueous solution of salbutamol at pH 3 (Table 1S, assuming that 0.6 kcal mol1 of thermal energy, kBT, is available at 298 K). The calculations for the protonated species are in good agreement with the experimental observations (Brittain et al., 1973; Esquisabel et al., 1997; Schmekel et al., 1999). Interestingly, Table 1S also shows that the S-conformers of deprotonated salbutamol are predicted to be more stable by approximately 1.6 kcal mol1 than the corresponding Rconformers at the B3LYP/IEFPCM/6-31G(d,p) level of theory. Thus, the calculations suggest that the S-enantiomers will be exclusively

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present in pH 12 buffer aqueous solutions at room temperature. In general, the calculations predict that the larger the dipole moment of a given conformer the more stable it is in water solution. The agreement of our calculations with the experimentally available data suggests that the level of theory used is adequate to provide reliable predictions of the salbutamol species present in the experiments. Table 2S shows the calculated vertical excitation energies of the protonated and deprotonated species in aqueous solutions. Results are presented for the first excited singlet state and the three lowest triplet states. This is because these are the excited states expected to be involved in the photochemistry of the protonated and deprotonated species under low-intensity, sunlight environmental conditions. Importantly, the excitation energies and the order of the excited states do not differ significantly in the R- and S-enantiomers of the protonated or deprotonated species. Hence, our calculations predict that the photochemistry of the protonated or deprotonated species should not depend significantly on the actual enantiomer present in solution. For this reason, the average vertical excitation energy of the first excited singlet state and that of the singlet–triplet energy gap are also reported in Table 2S. The average singlet state vertical excitation energy of the protonated species is predicted at 4.84 eV (256 nm), while that of the deprotonated species is 4.28 eV (291 nm) in aqueous solutions. These values are in reasonable agreement with the experimental results (Fig. 3 and Table 1), taking into consideration the expected accuracy of the calculations and that explicit solvent–solute interactions are not modeled (Reichardt et al., 2009; Reichardt and Crespo-Hernández, 2010). The character of the excited states was estimated from an inspection of the molecular orbital participating in the configuration interaction transitions describing each excited state (Reichardt et al., 2009). The calculations suggest that the first excited singlet state and the three lowest triplet states of protonated salbutamol have primarily pp⁄ character (Table 2S). In the case of the deprotonated salbutamol species, the first excited singlet state and the three lowest triplet states have substantial contributions from pp⁄ and charge transfer (CT) transitions. In addition, the average energy gap between the singlet state and the receiver triplet state (T3) of the protonated species is estimated to be 0.47 eV, while that for the deprotonated species is 0.03 eV. Hence, the excited-state calculations predict that the singlet to triplet intersystem crossing rate constant in the deprotonated species should be significantly larger than that in the protonated species in aqueous solutions. According to El-Sayed’s classical propensity rules (Turro et al., 2009), singlet and triplet states having the same character are expected to have smaller rate constants of intersystem crossing than those with different character. These predictions are in good agreement with the time-resolved and steady-state results discussed next. Fig. 4 shows the transient absorption spectra of the protonated and deprotonated species. The initial absorption bands in the visible are assigned to excited-state absorption by the first excited singlet state (i.e., Sn S1 transition) of the protonated and deprotonated species, respectively. The S1 states of the protonated and deprotonated species decay with a simultaneous formation of bands that have absorption maxima in the UV. The UV absorption bands are assigned to the lowest-energy triplet state (i.e., Tn T1 transition) of the protonated and deprotonated species. A broad absorption band with maximum above 650 nm is also observed in the transient absorption spectra of deprotonated species at long times, which is assigned to the hydrated electron absorption band originating from a multiphoton ionization of the solvent. The absorption band of the hydrated electron has absorption maximum around 720 nm in aqueous solutions (Jou and Freeman, 1979a,b). The hydrated electron band is also observed in pH 3 solutions,

albeit in smaller yield because it is more efficiently scavenged in the first few nanoseconds by the high concentration of hydronium ions (H3O+) (see Fig. 4, left panel). An additional negative-amplitude band with maximum around 350 nm is observed at pH 12 that resembles the steady-state fluorescence emission band of the deprotonated species (Fig. 3, right panel). This band is thus assigned to stimulated emission from the excited singlet state of deprotonated salbutamol. Fig. 5 shows that the excited singlet states of the protonated and deprotonated species decay in 1.4 ns and 55 ps, respectively, to populate the corresponding triplet excited states. Neither the triplet state of the protonated nor that of the deprotonated species decays within the time window of our experimental setup, explaining the need of a constant offset in the global fit analysis. The rate constant for the population of the triplet state in the protonated species is 7.1 108 s1, while that in the deprotonated species is approximately 25-fold larger at 1.8 1010 s1. These results are in line with the predictions of the calculations presented above. They also agree with the relatively small steady-state fluorescence quantum yields measured for both species, which suggest that other nonradiative decay pathways are competing with fluorescence emission. Fig. 6 summarizes a kinetic model that satisfactorily explains the theoretical and time-resolved results presented in this work. This kinetic model is used next to understand the steady-state photochemistry of salbutamol in acidic and basic aqueous solutions. Irradiation experiments in the pH range from 3 to 12 show that salbutamol is photoactive to UVB radiation; more so at basic pH values (Table 1). These results suggest that sunlight wavelengths reaching the earth’s surface (P280 nm) have enough energy to populate the S1 state of salbutamol regardless of the pH conditions of the aquatic systems. Hence, our laboratory’s experiments support previous works suggesting that exposure of salbutamol aqueous solutions to direct sunlight radiation results in the formation of products (Kirshnakumar et al., 2007; Akumar et al., 2008). Importantly, the systematic photochemical analysis performed in this work shows that the protonated form is expected to play the leading role in the photochemical transformation of salbutamol in natural water systems. The relatively small photodegradation yield reported in Table 1 for the protonated species suggests that sunlight-induced degradation of salbutamol in the environment should be a relatively slow process, not taking into consideration the possible presence of other reactive species in natural water systems that might accelerate its degradation. Table 1 also shows that molecular oxygen plays an important role in the degradation of protonated salbutamol, whereas its presence is insignificant in the degradation of the deprotonated species. As molecular oxygen is an efficient triplet quencher (Turro et al., 2009), the twofold increase in the degradation yield of

Fig. 6. Plausible kinetic model explaining the time-resolved photochemistry of salbutamol in aqueous solutions in acidic and basic conditions. Dashed lines represent nonradiative decay pathways. The model assumes that ultrafast internal conversion takes place in the triplet manifold. Quantitative values for the major photochemical decay pathways are reported in Table 1.


protonated salbutamol in air- versus N2-saturated conditions can be explained by the formation of reactive oxygen species through a type I or a type II mechanism, probably contributing to the formation of oxidation products (Foote, 1991). The increased susceptibility of protonated salbutamol to reactive oxygen species suggests that other indirect photosensitization routes of oxidation are likely to be present in the degradation of salbutamol in natural water systems. It is possible that the lack of participation of reactive oxygen species in the degradation of the deprotonated salbutamol is due to an inefficient sensitization of molecular oxygen by the triplet state of deprotonated salbutamol. A pH-dependent formation of reactive oxygen species has been documented in the photochemistry of other molecules including pharmaceutical compounds (Bionneau et al., 1975; de la Peña et al., 1997; Ogilby, 2010). However, the reasons behind this phenomenon are often poorly understood and depend sensitively on the actual molecule under study. Regardless of the specific mechanistic details explaining the O2-independent degradation, the strong CT-character of the singlet and triplet excited states (Table 2S) suggests that both states can play an important role in the photodegradation of deprotonated salbutamol.

5. Conclusions The results presented in this work show that salbutamol undergoes photochemical transformation in the spectral range of the sun’s radiation reaching the earth’s surface regardless of the pH of the aquatic system. According to Fig. 2, protonated salbutamol is expected to be the predominant species in natural water systems; assuming typical pH values in the range of 6.0–8.5 found in natural water systems (Morgan et al., 2007). This suggests that salbutamol will degrade at relatively slow rates in the environment, even though this will ultimately depend on the relative concentration of the different salbutamol species (i.e., pH of the water), the presence of other reactive species including reactive oxygen species, and the sun’s photon flux reaching the natural water surfaces. We consider that it is necessary to characterize the products resulting from the light-induced degradation of PPCP compounds in aqueous solutions and to investigate their potential toxicity in in vitro and in vivo studies. It has been documented that oxidation products formed from light-induced degradation of pharmaceutical compounds have the potential to be more toxic than the parent compound due to their increased ability to act as energy or electron transfer species (van Henegouwen, 1997; Vargas et al., 1999; Cosa, 2004). The constant intake of minute amounts of compounds formed by the direct or indirect photodegradation of PPCPs by living organisms might lead to toxicological effects and/or formation of mutations in cells in living organisms as has been documented for other pharmaceuticals and related compounds (Porter, 1982; Epstein, 1989; Miranda et al., 1998; Cosa, 2004). Our results for protonated salbutamol suggest that oxidation products are formed. This is because a doubling of its photodegradation yield was observed when air- versus N2-saturated conditions were used. In addition, the observation of new absorption bands at longer wavelengths in the absorption spectra of irradiated salbutamol in the pH range from 3 to 12 (Fig. 3) hints at the formation of oxidation and/or dimerization products. This work draws attention to the need for a comprehensive assessment of the potential toxicological impact of not only the PPCPs but also of their light-induced products in the aquatic systems. The synergistic effects that these pollutants might have to living organisms are unknown and merit attention.

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Acknowledgements The authors thank the Department of Chemistry, CWRU for initial support of this research. Acknowledgment is also made to the donors of the American Chemical Society Petroleum Research Fund and the NSF ADVANCE program at CWRU for partial support of this research. LGD acknowledges the support from the SOURCE program (Support of Undergraduate Research and Creative Endeavors) and thanks Dr. Amy Sage for initial assistance and training. JM thanks the support from the Academic Careers in Engineering & Science Program (ACES, NSF). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere.2011.01.048. References Adamo, C., Barone, V., 1999. Toward reliable density functional methods without adjustable parameters: the PBE0 model. J. Chem. Phys. 110, 6158–6170. Adamo, C., Scuseria, G.E., Barone, V., 1999. Accurate excitation energies from timedependent density functional theory: assessing the PBE0 model. J. Chem. Phys. 111, 2889–2899. Akumar, G.K., Kutty, K.K., Raju, K., Mohanan, S., Varghese, H.T., Panicker, C.Y., 2008. A novel thermo-acoustic analysis to detect photochemical reaction of salbutamol. Int. J. Chem. Sci. 6, 1081–1092. Amor, T.B., Jori, G., 2000. Sinlight-activated insecticides: historical background and mechanisms of phototoxic activity. Insect Biochem. Mol. Biol. 30, 915–925. Andreozzi, R., Raffaele, M., Nicklas, P., 2003. Pharmaceuticals in STP effluents and their solar photodegradation in aquatic environment. Chemosphere 50, 1319– 1330. Barone, V., Cossi, M., Tomasi, J., 1997. A new definition of cavities for the computation of solvation free energies by polarizable continuum model. J. Chem. Phys. 107, 3210–3221. Becke, A.D., 1993. A new mixing of Hartree–Fock and local density-functional theories. J. Chem. Phys. 98, 1372–1377. Bionneau, R., Pottier, R., Bagno, O., Joussotdubien, J., 1975. PH-dependence of singlet oxygen production in aqueous solutions using thiazine dyes as photosensitizers. Photochem. Photobiol. 21, 159–163. Boreen, A.L., Arnold, W.A., McNeill, K., 2003. Photodegradation of pharmaceuticals in the aquatic environment: a review. Aquat. Sci. 65, 320–341. Boreen, A.L., Arnold, W.A., McNeill, K., 2004. Photochemical fate of sulfa drugs in the aquatic environment: sulfa drugs containing five-membered heterocyclic groups. Environ. Sci. Technol. 38, 3933–3940. Boreen, A.L., Arnold, W.A., McNeill, K., 2005. Triplet-sensitized photodegradation of sulfa drugs containing six-membered heterocyclic groups: identification of an SO2 extrusion photoproduct. Environ. Sci. Technol. 39, 3630–3638. Boulton, D.W., Fawcett, J.P., 1995. Determination of salbutamol enantiomers in human plasma and urine by chiral high-performance liquid chromatography. J. Chromatogr. B: Biomed. Appl. 672, 103–109. Boulton, D.W., Fawcett, J.P., 2001. The pharmacokinetics of levosalbutamol: what are the clinical implications? Clin. Pharmacokinet. 40, 23–40. Bound, J.P., Voulvoulis, N., 2006. Predicted and measured concentrations for selected pharmaceuticals in UK rivers: implications for risk assessment. Water Res. 40, 2885–2892. Boxall, A.B.A., 2004. The environmental side effects of medication. EMBO Rep. 5, 1110–1116. Brittain, R.T., Farmer, J.B., Marshall, R.J., 1973. Some observations on the badrenoceptor agonist properties of the isomers of salbutamol. Br. J. Pharmacol. 48, 144–147. Burrows, H.D., Canle, L.M., Santaballa, J.A., Steenken, S., 2002a. Reaction pathways and mechanisms of photodegradation of pesticides. J. Photochem. Photobiol. B 67, 71–108. Burrows, H.D., Canle, L.M., Santaballa, J.A., Steenken, S., 2002b. Reaction pathways and mechanisms of photodegradation of pesticides. J. Photochem. Photobiol. B 67, 71–108. Buser, H.-R., Poiger, T., Muller, M.D., 1998. Occurrence and fate of the pharmaceutical drug diclofenac in surface waters: rapid photodegradation in a lake. Environ. Sci. Technol. 32. Calamari, D., Zuccato, E., Castiglioni, S., Bagnati, R., Fanelli, R., 2003. Strategic survey of therapeutic drugs in the rivers Po and Lambro in northern Italy. Environ. Sci. Technol. 37, 1241–1248. Calvert, J.G., Pitts, J.N., 1966. Photochemistry, 783–786. Cancès, E., Mennucci, B., Tomasi, J., 1997. A new integral equation formalism for the polarizable continuum model: theoretical background and applications to isotropic and anisotropic dielectrics. J. Chem. Phys. 107, 3032–3041. Chen, R.F., 1967. Anal. Lett. 1, 35–42. Cosa, G., 2004. Photodegradation and photosensitization in pharmaceutical products: assessing drug phototoxicity. Pure Appl. Chem. 76, 263–275.

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Photophysical and Photochemical Properties of the Pharmaceutical Compound Salbutamol in Aqueous Solutions Leah G. Dodson, R. Aaron Vogt, Joyann Marks, Christian Reichardt, Carlos E. CrespoHernández,* Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio, 44106. * Corresponding author, email: [email protected].

Electronic Supporting Material

Figure 1S. Representative HPCL chromatograms before and after UVB light irradiation of salbutamol in pH 12 (left panel, top) and 3 (left panel, bottom) aqueous buffer solutions. Right panels show the corresponding absorption spectra of several of the products formed (retention time of the products are shown in the legend), which were recorded on the solvent fluent conditions of the liquid phase. See methods section for the gradient conditions used.

Chemosphere 2011, 1S

2.0

Absorbance

1.5

1.0

0.5

0.0 200

250

300

350

400

450

500

550

600

Wavelength (nm)

Figure 2S. Changes in the absorption spectrum of salbutamol aqueous buffer solution at pH 7 as a function of irradiation time.


S(1)

S(2)

R(1)

R(2)

Figure 3S. Optimized structures for two different rotamers of the S- and R-enantiomers of protonated salbutamol in water at the B3LYP/IEFPMC/6-31G(d,p) level of theory.


S(1)

S(2)

R(1)

R(2)

Figure 4S. Optimized structures for two different rotamers of the S- and R-enantiomers of deprotonated salbutamol in water at the B3LYP/IEFPMC/6-31G(d,p) level of theory.


Figure 5S. Ground-state optimized structures for the neutral (top) and zwitterionic (bottom) forms of salbutamol in water at the BELYP/IEFPCM/6-31G(d,p) level of theory.


Table 1S. Optimization of protonated and deprotonated salbutamol species in water at B3LYP/IEFPCM/6-31G(d,p) level of theory Enantiomer Protonated S(1) S(2) R(1) R(2) Deprotonated S(1) S(2) R(1) R(2)

-1

Relative Energy / kcal mol

Dipole moment / D

0.00 0.95 0.25 3.70

12.01 5.32 11.19 7.17

0.00 0.14 1.61 1.69

19.23 18.08 17.70 18.83


Table 2S. Vertical excitation energies (eV) and character of the states [% ππ*, % CT] for protonated and deprotonated salbutamol species in water at TD-PBE0/IEFPCM/6311++G(d,p)//B3LYP/IEFPCM/6-31G(d,p) level of theorya State

S(1)

S(2)

R(1)

R(2)

3.49 [100, 0] 4.06 [100, 0] 4.42 [100, 0] 4.94 (0.0439) [100, 0]

3.45 [93, 7] 3.91 [100, 0] 4.39 [91, 9] 4.83 (0.0571) [100, 0]

3.48 [72, 28] 4.02 [100, 0] 4.43 [85, 15] 4.93 (0.0494) [89, 11]

3.45 [92, 8] 3.86 [100, 0] 4.37 [100, 0] 4.79 (0.0609) [100, 0]

∆E(S1 – T3)

0.52

0.44

0.50

0.42

∆E(S1 – T1)

1.45

1.38

1.45

1.34

3.34 [50, 50] 3.41 [65, 35] 4.26 [66, 34] 4.28 (0.0527) [31, 59]

3.31 [66, 34] 3.43 [62, 38] 4.26 [86, 14] 4.30 (0.0504) [50, 50]

3.32 [68, 32] 3.37 [63, 37] 4.24 [65, 35] 4.27 (0.0504) [88, 12]

3.33 [56, 44] 3.36 [48, 52] 4.26 [46, 54] 4.25 (0.0504) [41, 59]

∆E(S1 – T3)

0.02

0.04

0.03

-0.01

∆E(S1 – T1)

0.94

0.99

0.95

0.92

b

Protonated T1 T2 T3 S1

Deprotonated T1 T2 T3 S1

a

c

b

Oscillator strengths of the S1 states are given in parenthesis; Protonated species: Average S1 = 4.84 eV or 256.2 c nm; average ∆E(S1 – T3) = 0.47 eV; Deprotonated species: Average S1 = 4.28 eV or 290.5 nm; average ∆E(S1 – T3) = 0.025 eV.


I. Vertical excitation energies for the neutral form of salbutamol in water at the PBE0/IEFPCM/6-311++G(d,p) level of theory. Oscillator strength is given in parenthesis. Excited State 1: Triplet-A 63 -> 66 -0.50267 64 -> 67 0.50626 65 -> 67 -0.35844

3.49 eV; 355 nm

Excited State 2: Triplet-A 63 -> 67 -0.10097 64 -> 66 0.56978 64 -> 67 -0.10174 65 -> 66 -0.43796

4.09 eV; 303 nm

Excited State 3: Triplet-A 63 -> 66 0.58719 64 -> 67 0.34293 65 -> 67 -0.25628

4.47 eV; 277 nm

Excited State 4: Singlet-A 63 -> 67 -0.28987 64 -> 66 -0.37683 65 -> 66 0.50839

4.96 eV; 250 nm; (0.0399)

II. Vertical excitation energies for the zwitterionic form of salbutamol in water at the PBE0/IEFPCM/6-311++G(d,p) level of theory. Oscillator strength is given in parenthesis. Excited State 1: Triplet-A 65 -> 66 0.64886 65 -> 67 -0.11313 65 -> 68 -0.37637

3.14 eV; 394 nm

Excited State 2: Triplet-A 63 -> 66 0.18037 65 -> 66 0.37392 65 -> 68 0.62806 65 -> 71 0.12606

3.21 eV; 386 nm

Excited State 3: Singlet-A 63 -> 68 -0.14926 65 -> 66 0.65729

4. 20 eV; 295 nm; (0.0802)
