UNIVERSAL INTERACTIONS IN BINARY SOLUTIONS, STUDIED BY

UNIVERSAL INTERACTIONS IN BINARY SOLUTIONS, STUDIED BY SPECTRAL MEANSË LIVIA-VINCENŢA GHEORGHIEŞ1, MIHAELA DIMITRIU2, ELENA FILIP3, DANA-ORTANSA DOROHOI2 1

2

“Dunărea de Jos” University, Faculty of Sciences, Galaţi, Romania Al.I. Cuza University, Faculty of Physics, 11 Carol I Bdv., Iaşi, RO-700056 3 “AL. I. Cuza” University, Faculty of Chemistry, Iaşi Received December 15, 2008

Universal interactions in binary solutions of some polar molecules are studied by means of the electronic absorption spectra. The spectral shifts measured in solvents with different macroscopic parameters are correlated with the dipole moments of the studied molecules on the basis of the simple liquid theory. Two carbanion monosubstituted pyridazinium ylides are considered as spectrally active molecules in this study. The dipole moments in the ground state are estimated on the basis of the solvent influence on the intramolecular charge transfer band of the studied pyridazinium ylides. Key words: carbanion monosubstituted pyridazinium ylides, intramolecular charge transfer band, electric dipole moments.

1. INTRODUCTION

Intermolecular forces from liquids can modify the shapes of the electronic bands (bandwidth, intensity, symmetry, polarization), or can shift them in the wavelength scale [1, 2]. Occasionally, new bands can appear in the electronic spectra if the molecular symmetry is affected, or some complexes are formed between the molecules [3]. It is difficult to explain the electronic spectra modifications, due to the various types of interactions which could act in liquids and also to the circumstance that the total potential energy between the component molecules of the liquid is of the same magnitude as the thermal energy. For this last reason the local order established by the internal forces in liquids is continuously perturbed by the thermal motion. Only an averaged situation is reflected in the molecular electronic spectra [4,5]. In liquids can act universal and specific interactions (hydrogen bonds or different complex formed by charge transfer), depending on the molecules Paper presented at the 4th National Conference on Applied Physics, 25–26 September, 2008, Galati, Romania. Ë

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chemical structure and their physical properties. The first type of interactions acts at long distances, while the specific interactions act locally, at two or a few number of molecules able to be in quasi-chemical interactions. Universal or long range interactions are classified into orientation, induction, polarization and dispersive types. Consequently, the potential energy of universal interactions is usually represented as a sum of terms corresponding to these types of interactions.

U = U orient . + U ind . + U pol . + U disp.

(1)

The terms from relations (1) were expressed as functions on the microscopic parameters (ionization potential, I , electric dipole moment, µ , polarizability, α , Onsager radius, a ) of the molecules and macroscopic parameters (refractive index, n, electric permittivity, ε ) of the liquid solvent. In the theories about simple liquids (non-polar and non-polarizable), the specific interactions are neglected. For this reason the theory about the solvent influence on the electronic spectra of molecules is named theory of universal interactions. 2. THEORETICAL BACKGROUND

Based on Onsager [2] model for a continuous dielectric, Bakhshiev [3,4] expresses the spectral shifts measured in the electronic (absorption and fluorescence) spectra as function on the nature and strength of the intermolecular interactions in solutions considered as being homogeneous and continuous dielectrics. By integrating the pair potential due to the dispersive forces, Bakhshiev express the wavenumber ν disp. of the electronic absorption band in a non-polar

n2 − 1 , where n is refractive index, and n2 + 2 ionization potential I, as equation (2) shows.

solvent by the dispersive function f ( n ) =

3 II ' n 2 − 1 (2) 2a 3 I + I ' n 2 + 2 Equation (2) was obtained in the approximation that the solvent and solute molecules are non-polar ones and the solute molecule is polarizable. For the polarization interactions Bakhshiev uses a similar term with that obtained in Bayliss theory [6] to express the spectral shift due to this type of interactions. hc 2 f n 2 − 1 (3) hc∆ν pol . = hc(ν pol . −ν 0 ) = − 8π mν 0 a 3 n 2 + 2 hcν disp. = hcν 0 + (α g − α e )

The orientation-inductive forces in liquid produce a spectral shift which in Bakhshiev theory is expressed by relation (4).

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Universal interactions in binary solutions

hc∆ν or .ind . = hc(ν or .ind . −ν 0 ) = −

2   2n 2 + 1  2µ g ( µ g − µe cos ϕ )  ε − 1 n 2 − 1  ( µ g − µe ) n 2 − 1  − +     2 n2 + 2  a3 a3 n2 + 2  ε + 2 n + 2   

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(4)

The total spectral shift in electronic absorption spectra is a sum of the terms above mentioned: hc∆ν = hc ( ∆ν disp. + ∆ν pol . + ∆ν or .ind . ) (5) When equations (2), (3) and (4) are introduced in (5), after some calculations one obtains: 2    hc 2 f 3 II ' 2n2 + 1  2µ g ( µ g − µe cos ϕ ) ( µ g − µe )   n2 − 1  hc∆ν = (α g − α e ) 3 − + 2 −  3 3 3  n2 + 2 (6) 2a I + I ' 8π mν 0 a n +2  a a    

−

2n2 + 1 2µ g ( µ g − µe cos ϕ ) ε − 1 ε +2 n2 + 2 a3

Equation (6) can be written as it follows: hc∆ν = C1

n2 − 1 ε −1 + C2 2 ε +2 n +2

(7)

In equation (7) the following notations were made: C1 = (α g − α e )

2 2 3 II ' hc 2 f 2n 2 + 1  ( µ g − µe )    − + 2a 3 I + I ' 8π mν 0 a 3 n 2 + 2  a3  

C2 = −

2n 2 + 1 2 µ g ( µ g − µe cos ϕ ) n2 + 2 a3

(8)

(9)

In order to obtain information about the microscopic parameters of the spectrally active molecule one can use programs for regression computation when a great number of experimental sets of the type ν solv. , n, ε are available [7,8]. When Bakhshiev theory is applicable to the electronic spectra of the analyzed molecule, the computed and experimental values of the wave numbers are linearly dependent and the numerical values of the regression parameters can be used to estimate microscopic parameters of the spectrally active molecule. As equation (8) shows, it is very hard to separate the supply of the dispersive, polarization and inductive forces, because all terms which express their contribution to the spectral shifts in the electronic absorption spectra are multiplied by the dispersive function (1). Pyridazinium ylides are compounds able to participate to hydrogen bonds with the protic molecules, such as acids or alcohols. The specific interactions are

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neglected in Bakhshiev theory, so, to the right term or equation (7) a supplemental term must be added to express the spectral shift caused by this type of interactions. 3. EXPERIMENTAL

The pyridazinium ylides (Pi, i=1,2) with chemical structures from Fig. 1 and substitutes from Table 1 were prepared by known methods [9]. The study of solutions of ylides is important because these compounds are prepared in situ and are used in solutions.

Fig. 1 – Structural features and intramolecular charge transfer mechanism for the studied PY.

Pi, i=1,2 have an electronic absorption visible band due to an electron intramolecular charge transfer (ICT) from carbanion to the heterocycle [9,10]. The ICT mechanism is suggested in Fig. 1. The visible band has a low intensity; it disappears in acid medium and it shifts to blue when the ylide passes from an aprotic into a protic solvent, or from a non-polar into a polar one [8,9]. Table 1 Substituents of the studied pyridazinium ylides P p-Chloro-p-phenyl-pyridaziniump-nitro- benzoyl phenacylide (P1) p-Bromo-p-phenyl-pyridaziniump-nitro- benzoyl phenacylide (P2)

R −C6 H 4 − (Cl ) p

R1 −H

R2 −CO − C6 H 4 − ( NO2 ) p

−C6 H 4 − ( Br ) p

−H

−CO − C6 H 4 − ( NO2 ) p

The used solvents were treated for water elimination by known methods [11, 12]. Determination of ε at 20ºC was made using a Waine Kerr Autobalance Universal Bridge B-641 (300MHz) coupled with a Telmes TR-970 dielectric cell for liquids, thermostated with a U-10 Ultrathermostat (±0.2ºC precision). The concentration of the spectrally active molecules in solutions was of 10–4mol/L. The substances were weighted with a Mettler MDB-5 balance (±10–5grams). The electronic absorption spectra of Pi, i=1,2, in the range [50000– 20000]cm–1 are given in Fig. 2. Pyridazinium ylides have π → π * absorption

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bands in the UV range and a n → π * visible band with electronic charge transfer from the carbanion to heterocycle. When propionic acid is added in binary solution of Pi, i=1,2 in ethanol, the visible band caused by intramolecular charge transfer (ICT) disappears, because in the ylide protonation process, the lone electron pair of the carbanion is blocked. The values of the Pi, i=1,2 wavenumbers of ICT absorption band in different solvents are given in Table 2.

Fig. 2 – Visible electronic absorption spectra of P1 and P2; a) — in ethanol; b) -------- in ethanol with propionic acid. Table 2 Wavenumbers in the maximum of the ICT visible band of the studied monosubstituted pyridazinium ylides Nr. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Solvent Dioxane Benzene Toluene Anisol Chloroform n-Butyl acetate Dichloromethane Chlorobenzene Iso-Amyl acetate Ethyl acetate Methyl acetate n-Octyl alcohol Pyridyne n-Hexyl alcohol N-benzyl alcohol Cyclohexanol n-Amyl alcohol

ν ( cm −1 )

P1

P2

20120 20080 19900 19910 20150 20890 20190 19940 20850 20160 20190 20851 19910 20780 20710 20820 20860

20010 20010 20015 20010 20290 20710 20186 19840 20710 20140 20145 20680 19870 20711 20500 20470 20680

Nr.

Solvent

ν ( cm −1 )

P1 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

n-Butyl alcohol Methylethyl ketone Cyclohexanone Iso-Butyl alcohol Iso-Propyl alcohol n-Propyl alcohol Acetone Diaceton alcohol Acetyl acetone Ethyl alcohol Methanol Nitrobenzene Acetonitrile Dimethylformamide Ethylene glycol Dimethyl sulphoxide Formamide

20250 20130 20886 20610 20820 20400 20300 20710 21200 21590 20470 19870 20150 21235 21235 20220 20641

P2 20230 20190 20816 20571 20780 20360 20270 20641 21020 21550 20430 19910 20120 21170 21170 20200 20680

4. RESULTS SND DISCUSSIONS

From Table 2 it results an increase of the ICT wavenumber when the spectrally active substance passes from non-polar to polar or from aprotic to protic

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solvents. This fact demonstrates that the pyridazinium ylides are more polar in the ground state than in their excited state. The solvation energy of the carbanion monosubstituted pyridazinium ylides in the ground state is higher than that in the excited state. This phenomenon can be explained by the mechanism of the intramolecular charge transfer from the carbanion to the heterocycle, which decreases the molecular dipole moment. In the protic solvents the carbanion of the monosubstituted pyridazinium ylides can participate to a proton change between the hydroxyl group and the ylid carbanion. Consequently, relation (7) deriving from Bakhshiev theory can not be applied to the spectra recorded in protic solvents. A supplementary term must be added to right part of relation (7) counting the contribution of the specific interactions to the total spectral shift of the electronic band. The spectral shifts can be estimated by using relation (10): K

ν calc. = ν 0 + C1

n2 − 1 ε −1 + C2 + C3 ⋅ δ OH ( ppm ) 2 n +2 ε +2

(11)

In relation (10) ν 0 , C1 , C2 and C3 are regression coefficients and ν ( cm −1 ) , K

n , ε and δ OH ( ppm ) are parameters experimentally determined. The parameter

δ OH ( ppm ) represents the chemical shift measured in NMR spectra of the hydroxyl compounds. For the pyridazinium ylides under the study, the regression coefficients are listed in Table 3. Table 3 Regression coefficients for equation (10) K

K

Ylid

ν 0 ± ∆ν 0

C1 ± ∆C1

C2 ± ∆C2

C3 ± ∆C3

R

P1

19600 ± 350

0

1050 ± 15

60 ± 10

0.96

P2

19450 ± 280

0

1003 ± 34

80 ± 6

0.97

The regression parameter ν ( cm −1 ) has the significance of the wavenumber in the maximum of ICT band for gaseous phase of the spectrally active molecule. In the case of the monosubstituted pyridazinium ylides one can approximate the dipole moment in the excited state of the ICT transition as being null, as compared with its value in the ground state of the molecule [13]. By using this approximation in relation (9) one obtains:

µg =

a 3 ⋅ C2 n 2 + 2 2 2n 2 + 1

(11)

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Relation (11) was used to determine the dipole moment in the ground state of pyridazinium ylides. The obtained values are in Table 4. These values are in a good agreement with those obtained by HyperChem [14]. Table 4 Dipole moments of the pyridazinium ylides Ylid

C2 ( cm −1 )

a ( A)

µg ( D)

P1

1050

8

7.3

P2

1003

9

9.9

5. CONCLUSIONS

Orientation interactions are predominant in the solutions of carbanion monosubstituted pyridazinium ylides. The solvation energy in the ground state of the studied compounds is higher than that corresponding to the excited state. The solvent study of ICT visible band permits to estimate with a good precision the dipole moments in the ground state of ylides. REFERENCES 1. Molecular Liquids. Dynamics and Interactions, Eds. Barnes A.J., Orville Thomas W.J., Yarwood J., Reidel D., Publishing Company, Dordrecht, Boston, Lancester, Published in cooperation with NATO Scientific Aftaires Division, 1984. 2. Onsager, L., J. Am. Chem. Soc., 58, 1485, 1936. 3. Bakhshiev, H.G., Opt. i. Spectrosc. 10, 717, 1961; 16, 821, 1964. 4. Bakhshiev, H.G., Spectroscopia Mejmoleculiarn’x vzaimodeistvii, Izd. Mir, Leningrad, 1972. 5. Abe, T., Bull. Chem. Soc. Japan, 38, 1314, 1965; 39, 937, 1966. 6. Bayliss, N., J. Chem. Phys., 18, 292, 1950 7. Dorohoi, D.O., J. Mol. Struct., 704, 1–3, 31, 2004; 792–794, 86, 2006. 8. Gheorghieş C., Gheorghieş L.V., Dorohoi D.O., J. Mol. Struct. 887, 122, 2008. 9. Zugrăvescu, I., Petrovanu, M., N-Ylid Chemistry, Acad. Press, London, New York, 1976. 10. Dima, S., Creţu, R., Dima, M.M., Rev. Chim., Bucureşti., 58, 11, 2007, p.1016. 11. Vogel, A.I., A Text Book of Practical Organic Chemistry, Longmans, New York, 1978. 12. Reichart, C., Solvents and Solvent Effects in Organic Chemistry, 3, Wiley-VCH, Verlag GmbA & Co. KGaA, Weinheim, 2003, p. 329 13. Dorohoi, D.O., Holban, V., J. Mol. Struct., 292 (1–3), 133, 1993. 14. Dimitriu M., Metode spectrale şi de modelare moleculară pentru estimarea unor parametri electro-optici şi de structură ai moleculelor organice, Ed. Pim, Iasi, 2009.

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