We calculated ionization potentials (IP) and C 1s-π* bound-state excitation energies for unhydrated and hydrated species of glyoxal and methylglyoxal in the gas phase. It is extremely difficult for quantum chemical calculations to be accurate for complex systems (e.g., liquids) due to computational limitations. Gas-phase calculations can be used instead to describe the electronic excitations. Typically, the core-to-valence excitation energies of molecules in the gas phase show only small shifts compared to the excitation energies of these molecules in liquid phase or as a part of a cluster. However, the IP of the molecule is higher in the gas phase compared to condensed phases. The Rydberg series converging to the ionization threshold are shifted to higher energies for the lower Rydberg and shifted to lower energies or quenched for the higher Rydberg by the surroundings . The results of the quantum chemical calculations are presented in Table . In the case of glyoxal, we observe that the excitation energies of C=O bonds of the unhydrated and monohydrated form differ by approximately 0.5 eV. The IP of the C=O moieties (295.4 for the unhydrated form and 295.15 eV for the monohydrated form) are higher than those of the C–OH moieties by 0.54 and 0.29 eV, respectively.

For methylglyoxal, the C 1s-π* excitation energies were calculated for the unhydrated, monohydrated and dihydrated forms. We also calculated energies of enol forms of methylglyoxal and hydrated methylglyoxal because they have been identified as possible structures in aqueous methylglyoxal solutions by . They applied density functional theory (DFT) calculations to study the hydration of 1 M of methylglyoxal at 298 K and showed that the formation of enol structures and aldol condensation products, which have in their chemical structure a C=C bond, are the thermodynamically most stable species in the methylglyoxal system. This is a very favorable chemical path when methylglyoxal is hydrated and thus taken into account in our calculations as well. Figure  shows the transition from the monohydrated methylglyoxal to the enol formation which is the intermediate step for the aldol condensation products.

The excitation energies of C=O moieties at 286.23–287.05 eV were lower than those of C–OH(CH2) moieties, formed by keto–enol tautomerism of the monohydrated methylglyoxal, but higher than the excitation energies of the CH2 moieties, at 284.85–285.91 eV. The IP of the C=O moieties (294.73–295.07 eV) are higher by approximately 0.7 eV than the C–OH and C–OH(CH3) moieties (294.25–294.53 eV) but much higher than the C–OH(CH2) moieties of the enol structure at 293.13 and 292.87 eV. This indicates that the hydration and keto-enol tautomerism lower the C 1s ionization threshold.

In Table  we also present the calculated oscillator strengths (f).

Figure  presents the recorded spectra for the C 1s absorption edge of binary glyoxal–water and ternary solutions formed by the addition of either NaCl or Na2SO4 together with the excitation energies of our quantum chemical calculations. The results of the quantum calculations are presented with colored lines that correspond to hydrated and unhydrated molecules in the gas phase, and the length of the lines is scaled in respect with the calculated oscillator strengths. Apart from the absorption probability of a molecule, the absorption intensity of the presented spectra is also related to the relative amount of the species in the solution and the interactions that take place in the liquid phase, and thus we cannot directly use oscillator strengths as expected intensity ratios in the experimental spectra.

The spectra with and without the addition of inorganic salts only have a single feature at around 289.6 eV. The location of the absorption edge of aqueous solutions of pure glyoxal was at 289.6±0.1 eV (0.5 M) and 289.5±0.1 eV (1 and 2 M), and upon the addition of inorganic salts, the absorption edge energy systematically increased by 0.1 eV in all cases. All values are, however, close to the experimental accuracy. All absorption edge energies can be found in the Supplement (Table S1). Based on our calculations (Table ), the absorption edge energies are close to the C–OH excitation energies of the monohydrated and dihydrated form, calculated at 289.97 and 289.99 eV, respectively. Photon energies above 291 eV exceed the C 1s binding energy and ionization starts. We do not observe any peaks at the calculated C=O energies of the unhydrated and monohydrated glyoxal at 286.18 and 286.66 eV, respectively. This indicates that the dihydrated form of glyoxal overwhelmingly dominates in our liquid solution samples. According to the calculations, a change in hydration state upon the addition of salts towards the mono- or dihydrated form of glyoxal would have shown up as new spectral features around 286–287 eV, which are not observed.

The dominance of hydrated species in aqueous glyoxal has been reported before. Measurements of near-UV molar absorptivities of aqueous solutions at 25 ∘C have shown that 98 % of glyoxal is in dihydrated form and the rest is in monohydrated form . applied DFT calculations to find the favorable hydration paths of glyoxal and of the subsequent products of hydration and used the Poisson–Boltzmann (PB) continuum approximation to describe the interaction with water. Their results show that the hydrated species are the most thermodynamically favorable, and they can subsequently be involved in reactions to form oligomers. studied aqueous solutions with 1 M glyoxal with nuclear magnetic resonance (NMR) spectroscopy. In agreement with our study, they did not detect unhydrated glyoxal and concluded that the glyoxal in the solution is mainly in dihydrated form and contains oligomers. and identified potential oligomers containing acetal groups in aqueous glyoxal. Here, we did not calculate absorption energies of oligomers, but based on the C 1s absorption energies of similar O–C–OH functional groups found in saccharides , we estimate that in oligomers acetal moieties would have higher absorption energies compared to C–OH moieties. Their contribution would therefore be embedded in the broad continuum part of the XAS spectrum. The observed increase in absorption edge energy with decreasing concentration of glyoxal and the addition of salts could be related to changes in the degree of oligomerization. This is in line with the study of , who observed deoligomerization with the addition of Na2SO4 and NaCl.

In Fig.  we present the spectra of C 1s of methylglyoxal in aqueous mixtures. We observe three distinct absorption features at 285.8, 287.9 and approximately 290 eV. Figure  shows five peaks, denoted as A, B, C1, C2 and C3, that were identified during the fitting process of the spectrum of methylglyoxal in the aqueous phase. We correlate these peaks with the absorption energies of functional groups presented in our solutions (Table ). We also present the excitation energies and ionization potentials of our quantum chemical calculations.

The first peak, designated as A (Fig. ), is found at 285.8 eV. We assign this peak to the CH2 moiety of the enol form of the monohydrated methylglyoxal, in good agreement with the predicted C=C excitation energy of 285.91 eV. Usually, the C=C peaks are found at 285 eV, but an increase in the energy of C=C transitions has been observed before in organic compounds with similar structure. Phenols and quinones in solid phase were studied by , using scanning transmission X-ray microscopy (STXM) under a He atmosphere. These compounds have the same characteristic moiety, and excitations between 286.05 and 286.35 eV were attributed to C 1s → πC=C∗. C=C bonds can also result from radiation damage of the sample. However, consecutive scans of the solutions did not show any significant changes in the spectra, and thus radiation damage is considered to be small in our experiments.

The next strong peak in Fig.  at 287.9 eV (peak B in Fig. ) can be assigned to the C–OH(CH2) transitions in the enol form of monohydrated methylglyoxal. The calculated energy of this transition is at 287.53 eV (lower by 0.4 eV).

The asymmetrical shape of our spectra suggests underlying peaks. We fit three peaks, according to the observed shoulders on the two main peaks, at 286.5 eV (peak C1), 287.1 eV (peak C2) and 288.6 eV (peak C3). The predicted C=O absorption energies are at 286.51 and 287.05 eV for the unhydrated and monohydrated methylglyoxal in the gas phase, respectively. studied solid organic compounds in ultrahigh vacuum conditions, applying XAS in the near-edge region, and has also assigned the C=O transitions at 286.5 eV. applied soft X-ray spectromicroscopy at atmospheric pressure, and used STXM with XAS at 0.5 atm to characterize functional groups of dry particles, and they both correlated the C=O transitions with absorption energies at around 286.7 eV. The calculated and reported values for C=O transitions are lower than the respective peak at 288.3 eV in the gas-phase spectrum (presented in Fig. S1 of the Supplement). The calculated C 1s-π* excitation energies of the C–H moieties are at 286.6 and 287.64 eV for the unhydrated and monohydrated form, respectively. C–H Rydberg states have not been calculated but could be mixed with the calculated states and could interact with surrounding water molecules in aqueous solutions. C–H transitions have been assigned by at 287.3 eV and at the energy range from 287 to 288.4 eV by . In conclusion, in this energy range we expect peaks related to C=O excitations associated with the unhydrated methylglyoxal, C=O(CH3) excitations of the monohydrated methylglyoxal and also C–H excitations of the hydrated species. Based on previous studies, we expect to have mainly monohydrated and dihydrated forms of the compound and a negligible fraction of unhydrated methylglyoxal . C=O and C–H excitations from hydrated species that contain enol structures and originate from monohydrated methylglyoxal should also be considered as they can co-exist in the solutions. Due to the variety of possible species containing C=O and C–H bonds in the aqueous solutions, we suggest an energy range from 286.5 to 288.6 eV for the transitions of C=O and C–H groups.

The wide feature at around 290 eV in Figs.  and  is related to C–OH transitions and ionization of C 1s electrons. The calculated C–OH absorption energy of the monohydrated form at 290.27 eV matches well with the energy from where the broad structure starts. The IP are expected to be shifted by some eVs compared to the calculated, depending on the surroundings where the nearest neighbor is dominant.

In the case of methylglyoxal, the addition of salts did not introduce observable shifts to absorption energies. Due to the presence of overlapping peaks in the spectra of the salt-free systems, we cannot estimate the absolute concentrations of unhydrated and hydrated methylglyoxal and, thus, we cannot observe how the addition of salts affects the concentration of the hydrates. Changes in the relative ratios of unhydrated and hydrated forms would be expected due to changes in the molecular interactions (water–methylglyoxal, water–water) when ions co-exist in the solution. Such phenomena have been reported in previous experimental  and computational  studies, which have shown a decrease in the co-solubility of methylglyoxal in aqueous solutions with the addition of NaCl and Na2SO4 (salting-out effect). Here we observe a small change in the relative intensities between the peaks A and B in pure methylglyoxal solutions, compared to solutions containing Na2SO4 and NaCl. In salt-containing solutions, the relative intensity for peak A (CH2(enol)) becomes slightly smaller than for peak B (C–OH(CH2)enol) or the absorption edge (∼291 eV). Considering that other hydrated forms of methylglyoxal contribute to the spectra in the energy range of peak B, this can indicate smaller abundance of the enol form compared to other hydrated forms in salt solutions. However, the addition of salts changes the shape of the background due to close-lying ionization continua of the Cl and S L shells , and thus these changes remain inconclusive.