Identification of valence electronic states of aqueous acetic acid in acid-base equilibrium using site-selective x-ray emission spectroscopy. Yuka Horikawaa,b ...
Identification of valence electronic states of aqueous acetic acid in acid-base equilibrium using site-selective x-ray emission spectroscopy
Yuka Horikawaa,b, Takashi Tokushimaa, Yoshihisa Haradaa,c,d, Osamu Takahashie, Ashish Chainania, Yasunori Senbaf, Haruhiko Ohashif, Atsunari Hirayab and Shik Shina,g a b
RIKEN/SPring-8, Kouto, Sayo, Hyogo 679-5148, Japan
Department of Physical Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan c
Department of Applied Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan d
e
Department of Chemistry, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan f
g
JST, CREST, Kawaguchi, Saitama 332-0012, Japan
JASRI/SPring-8, Kouto, Sayo, Hyogo 679-5198, Japan
Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Abstract Occupied valence electronic structures in the neutral and the anionic forms of aqueous acetic acid and their pH dependence under ambient condition are identified for the first time exploiting the site-selectivity of resonant x-ray emission spectroscopy (XES) in combination with calculation based on density functional theory (DFT).
Acetic acid aq.
=
Intensity (arb. unit)
O 1s XES A : pH 11.44 B : pH 6.30 C : pH 5.71 D : pH 5.20 E : pH 4.73 F : pH 4.30 G : pH 3.73 H : pH 3.28 I : pH 0.29
+
CH3COOH
CH3COO
515 520 525 Emission Energy / eV
515
520 525 Emission Energy / eV
515
520
molar fraction
1.0
525
Emission Energy / eV
Ratio analysis based on XES spectra
1
0.5
0.0 2
4
pH
6
8
The electronic structure of a material is important for many fields of science, because valence electrons play important roles in properties of many systems including molecules in liquids and solutions. Due to its importance in chemistry and biology, recent studies have applied direct methods to observe electronic state of aqueous solutions. Unoccupied valence states were observed for glycine by x-ray absorption spectroscopy (XAS), and the dissociation of amino and carboxyl groups could be
detected1. Core level observations were reported using photoemission
spectroscopy with a liquid jet technique, and the nitrogen 1s chemical shift of lysine along with the dissociation of amino groups were observed 2. A more recent study using the same technique showed that rapid proton exchange between two Nitrogen atoms of neutral imidazole leads to distinct chemical shifts3. For occupied valence states, we have reported electronic structure changes of iron atoms in myoglobin using element-specific XES observation4. All these studies have utilized the element-specificity of these methods to separate out signals from the solvent and solute molecules. However, since many molecules subjected to study in chemistry and biology consists of more than one atom of the same light element such as carbon, oxygen, and nitrogen, it is necessary to use a method which can distinguish the same element in different chemical environments. The method we applied for the study of aqueous acetic acid solutions is x-ray emission spectroscopy. In recent studies, XES was successfully used as a powerful element-selective tool to elucidate the electronic states of liquid water5-8. We have recently reported on the site-selective observation of two oxygens in different chemical environments, C=O and OH, using resonant XES at the oxygen 1s edge9 for liquid acetic acid (CH3COOH) under ambient conditions. Using the site-selectivity of resonant x-ray emission spectroscopy, we report here chemical state selective observations of the valence electronic states for acetic acid molecules in an aqueous solution. Furthermore, we report a systematic pH dependence of the occupied valence electronic structure of aqueous acetic acid solutions under ambient conditions. Acetic acid is a simple and common acid containing a single carboxyl group (-COOH) and has been well studied in terms of acid-base equilibrium. The acid dissociation constant, pKa, of acetic acid in aqueous solution is accurately known to be 4.756 at room temperature 10. It represents a model case to discuss deprotonation causing the neutral to anionic transition. Deprotonation plays an important role in many biological and chemical reactions. The pH dependence of chemical reactivity and property is known to be directly associated with geometric/steric and electronic changes accompanying deprotonation. Well-known examples include sulphuric acid, phenol, the helix structure of poly L-glutamine, pH dependence of micellization, pH sensitive fluorescence of proteins, etc11-14. It is expected that the valence electrons contributing to chemical bonding of hydrogen undergo changes upon deprotonation. Hence, it is extremely important to study the occupied valence electronic structure of solutions in acidbase equilibrium. XAS and XES spectra were recorded at synchrotron radiation facility SPring-8 using circularly polarized soft x-rays at the beamline BL17SU 15. Details of the emission spectrometer and the liquid flow cell used for these measurements are described elsewhere 8, 16. The energy resolution of the XAS and XES spectra are 0.1 eV and 0.42 eV, respectively. 2M aqueous acetic acid solutions at various pH conditions were prepared by mixing acetic acid (with a purity of better than 99.7%, Wako Pure Chemical Industries Ltd.), ultra pure water (Millipore Simpli Lab UV purification system), and a pH adjuster. 6M HCl or 5M NaOH solution (Reagent grade, Wako Pure Chemical Industries Ltd.) were used as pH adjusters. pH of the sample solutions were confirmed before and after the measurements to exhibit a pH within +/−0.1 of the reported values using a pH meter (Horiba F-54).
2
Fig. 1 shows the O 1s XAS spectra of 2M aqueous acetic acid samples for different pH values: strongly basic (pH 13.05), pH value close to the pKa of the acetic acid (pH 4.73) and strongly acidic (pH 0.81). XAS spectra of water and liquid acetic acid are also shown in Fig. 1. In the spectra of aqueous acetic acid, an isolated peak is observed at ~ 532 eV which corresponds well to the peak of liquid acetic acid but is absent in the XAS spectrum of water. This peak corresponds to the resonant excitation OC=O 1s → π* of the acetic acid molecule, as reported before9. The resonance peak exhibits a small (0.4 eV) energy shift with the increase of the solution pH. In the strongly acidic solution, acetic acid molecules exist in the neutral form. In contrast, for the strongly basic solution, the anionic form of acetic acid is dominant species. Therefore, this energy shift is attributed to the deprotonation of the carboxyl group as a function of solution pH. A similar peak shift of the resonance peak in O 1s XAS was reported for pH dependence of aqueous glycine and was also assigned to the deprotonation of the carboxyl group 1. For energies above 533 eV, the spectra of aqueous acetic acid are dominated by the O 1s absorption of solvent water, as only 1/24 of molecules belong to acetic acid for 2M concentration. In order to perform a selective excitation XES of acetic acid molecules in solvent water, we have tuned the incident energy to the OC=O 1s → π* resonance excitation, which is absent in water. Fig. 2 shows the XES spectra obtained for the samples at pH 0.29 (Fig. 2a) and pH 11.44 (Fig. 2b). The origin of the XES spectral feature at pH 11.44 and that at pH 0.29 are readily explained by chemical equilibrium considerations. According to well-known Henderson– Hasselbalch equation, more than 99.9% of acetic acid at pH 11.44 exists in the anionic form while the neutral form dominates (more than 99.9%) at pH 0.29. In Fig. 2 a, previously reported XES spectrum of liquid acetic acid is shown for comparison9. The similarity in the XES peak structure of aqueous acetic acid at pH 0.29 and pure liquid acetic acid9, though with a slight difference seen at around 520 eV, matches well with the fact that the neutral form is also dominant in pure liquid acetic acid. While acetic acid molecules in an aqueous solution are surrounded by water molecules, molecules of liquids acetic acid are surrounded by acetic acid. Thus differences of spectral features between liquid and aqueous acetic acid can be considered to be associated with solute-solvent interactions, i.e. solvation effect. Further confirmation of the neutral and anionic forms is provided by performing DFT calculations of XES spectra for both forms of the molecule. The spectral calculations were performed using the ground state wave functions by StoBedeMon code
17, 18
. In this code, relative intensity of calculated XES spectra are evaluated by dipole transition
probabilities between a core and valence orbitals
19
. In order to determine the energy position of the excited states,
normal ∆Kohn-Sham calculations were performed to compute the ionization energies of the highest occupied molecular orbital (HOMO) and a core orbital. The core-excited oxygen atom was described using the IGLO-III basis of Kutzelnigg et al.20, while (5211/411/1) and (311/1) basis sets were applied for carbon and hydrogen atoms, respectively. The non-core-excited oxygen atom was described by effective core potentials
21
. However, it is hard to determine a
reliable model of the molecular geometry in solutions as their complexity. In addition, for the molecule in liquid phase, the electronic structure remains essentially unchanged compared to isolated molecule as we demonstrated in the previous report9. Hence, we have performed calculations for an isolated molecule as an approximate approach. The calculated energy levels for the XES spectra of neutral and anionic acetic acid are shown along with the measured spectra in Fig. 2 a and 2 b, respectively. The both calculated spectra were shifted by −1.2eV to fit the experimental peak feature with highest intensity. All peak structures in the experiment are well reproduced in the calculation with small differences in energy positions (see supplementary information for detail). From the XES spectra obtained at OC=O 1s site-selective resonant excitation and comparison with density functional (DFT) calculation of XES spectra, it
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is confirmed that we could successfully extract the occupied valence electronic structure of the acetic acid molecules in aqueous solution. So far, valence electronic structures of anionic and neutral acetic acid at extreme pH conditions, strongly acidic and strongly basic, are determined. It is interesting to examine the electronic structure at intermediate pH conditions especially at around a pH value of the acid dissociation constant where molar fraction of anionic and neutral acetic acid varies drastically and therefore suspected to cause spectral change by interaction between them. In Fig. 3a, we plot the area-normalized O 1s XES spectra of 2M acetic acid solutions obtained using an excitation energy of 532.1 eV. The spectra show systematic changes as a function of pH values. Importantly, five isoemissive points are clearly observed (arrows in Fig. 3a) indicating the spectra consist of the overlapping signals of neutral and anionic forms. In fact, the obtained XES spectra of intermediate pH values can be nicely reproduced by summing spectra of the two forms. Fig. 3b show the experimental XES spectrum and the reproduced XES spectrum at pH 4.73 (E) obtained by summing with appropriate ratio the spectra of anionic form at pH 11.44 (A) and that of neutral form at pH 0.29 (I). Ratios between two spectra are optimized to have the minimum difference between the reconstructed spectra and measured spectra. The reconstructed spectra show a residual error for area under the curve of less than 6 % for all the intermediate pH value (experimental and reconstructed spectra are shown in the supplementary information). As we have shown in the differences of XES spectra between liquid and aqueous acetic acid, valence electronic structure observed through XES is sensitive to surrounding environment. If the interaction between solute molecules is strong, the molecular electronic states can be perturbed by the interaction, and isoemissive points cannot be observed due to interaction between the two species. Thus, we conclude that the anionic and neutral form of acetic acid in aqueous solutions do not interact with each other to form new states, and only population of the two components changes depending on solution pH. In order to carry out a quantitative analysis of the ratio between the neutral and anionic forms, we used the component ratio analysis obtained from the fitting procedure. The intensity of the reconstructed spectra is described as following equation;
I=
P ⋅ x ⋅ I Anionic + (1 − x) ⋅ I Neutral P ⋅ x + (1 − x)
(1),
where IAnionic and INeutral indicate the normalized intensity of anionic form (spectrum A in Fig. 3 b) and neutral form (spectrum I in Fig. 3 b) respectively. x is a molar fraction of anionic form. P ⋅ x + (1 − x ) in the denominator is an term to normalize the reconstructed spectra. P is a relative emission efficiency (efficiencies of whole emission process including absorption process) for the anionic and neutral forms and is found to be 1.0±0.05 from intensity analysis of XES spectra in pH dependence (see supplementary information). As shown in Fig. 3 c, ratio analysis using Eq. 1 leads to a successful matching of molar fraction derived from XES spectra and the Henderson-Hasselbalch equation. The result shows validity of quantitative analysis using XES and the non-interactive nature in terms of electronic structure between anionic form and neutral form of acetic acid molecule in an aqueous solution at low concentration. The occupied valence electronic structure of aqueous acetic acid in acid-base equilibrium was observed for the first time exploiting the site-selectivity of resonant XES. From the XES spectra obtained at C=O site-selective O 1s resonant excitation, electronic structures in neutral and anionic forms of acetic acid are identified in combination with DFT calculations. A quantitative molar fraction analysis based on identified XES spectra of neutral and anionic forms demonstrates that the molar fraction obtained by XES study is consistent with the standard Henderson-Hasselbalch
4
equation. These results show the potential of element- and site-selective XES as a quantitative tool to observe valence electronic states of solute molecules in aqueous solutions of biological and chemical systems.
Acknowledgements This work is partly supported by the Japanese Ministry of Education, Science, Sports and Culture through a Grant-inAid for Scientific Research (Nos. 16104004 and 17340097). This experiment was carried out at SPring-8 BL17SU with the approval of the RIKEN SPring-8 Center (Proposal Nos. 20070135 and 20080064). We thank M. Oura and T. Takeuchi for valuable support, Y. Hitaka for technical assistance.
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5
O1s XAS
Liquid acetic acid
Intensity (arb.unit)
Aqueous Acetic acid pH 13.05 pH 4.73 pH 0.81
Water
530
535
540 545 Photon Energy / eV
550
Figure 1 O 1s XAS spectra of 2M aqueous acetic acid for strongly basic (pH 13.05), pH value close to the pKa (pH 4.73) and strongly acidic (pH 0.81). O 1s XAS spectra of pure liquid acetic acid and pure water are also shown. Arrow at 532.14eV indicates the excitation energy of XES measurement.
Intensity (arb.unit)
a
CH3COOH
b
CH3COO OR
Exp. Liquid acetic acid Exp. Aqueous acetic acid pH 0.29
Exp. Aqueous acetic acid pH 11.44
Calculation
Calculation
520
525
Emission energy / eV
520
525
Emission energy / eV
Figure 2 Calculation (bar graph) and measured (line) O 1s XES spectra of neutral acetic acid (a) and anionic acetic acid (b). XES spectrum of pure liquid acetic acid is also shown above the spectrum of neutral acetic acid for comparison.
6
a Intensity (arb. unit)
O1s XES Aqueous acetic acid
515
A: B: C: D: E: F: G: H: I :
pH 11.44 pH 6.30 pH 5.71 pH 5.20 pH 4.73 pH 4.30 pH 3.73 pH 3.28 pH 0.29
520
525 Emission Energy / eV
c Aqueous acetic acid
1.0
pH 4.73
Experimental Reconstructed
molar fraction
Intensity (arb. units)
b 0.8
Anionic form
Neutral form
0.6 0.4 0.2 0.0
515
2
520 525 Emission energy / eV
4
6
8
pH
Figure 3 (a) O 1s XES spectra of acetic acid in various pH solutions. Observed isoemissive points in pH dependence are indicated by arrows in the spectra. (b) Comparison between measured (green line) and reconstructed (black line) XES spectra for sample E obtained by summing spectra A and I. Blue and red lines below the experimental spectrum indicate two components of the reconstructed spectra, i.e. spectra of A and I respectively. (c) Molar fractions of the anionic and neutral forms evaluated from the measured spectra using Eq.1 (open circles) and molar fraction curves calculated from the Henderson–Hasselbalch equation (solid curves).
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