Computational Vibrational Sum Frequency Spectra of Formaldehyde

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Computational Vibrational Sum Frequency Spectra of Formaldehyde and Hydroxymethanesulfonate at Aqueous Interfaces Nicholas A. Valley† and Geraldine L. Richmond*,‡ †

California Northstate University College of Health Sciences, 2910 Prospect Park Drive, Rancho Cordova, California 95670, United States ‡ Department of Chemistry, University of Oregon, Eugene, Oregon 97403, United States S Supporting Information *

ABSTRACT: The identity and arrangement of aqueous species at the interface of atmospheric aerosol impacts aerosol properties including albedo and propensity to uptake additional gas phase species. Formaldehyde and sulfur dioxide are two common atmospheric species that alter (individually and in concert) aqueous atmospheric aerosol interfaces. Vibrational sum frequency (VSF) spectroscopy studies of planar aqueous formaldehyde solution surfaces have shown alteration during exposure to sulfur dioxide gas. Additional changes were observed once exposure was ceased. The results suggested the formation of a new organic species, hydroxymethanesulfonate (HMS), which acts as a thermodynamic sink for aqueous sulfur dioxide and lead to acidification of the aerosol particles. The coherent nature of the VSF response and its strong dependence on surface species present and their orientations, however, made definite vibrational assignments for exact species notoriously difficult. The focus of this paper is on elucidating the species and orientations that give rise to specific experimentally derived spectral features through VSF spectra calculated using a combination of classical molecular dynamics (MD) and density functional theory (DFT). The results demonstrate that the most prevalent surface species for a formaldehyde containing aqueous solution is hydrated formaldehyde in the form of methylene glycol. Calculated VSF spectral frequencies for the proposed product HMS are in agreement with experiment. Furthermore, changes in the experimental spectra both during and after the flow of sulfur dioxide are consistent with HMS in different interfacial orientations.

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INTRODUCTION The chemistry and properties of atmospheric aerosol continues to be a major source of uncertainty in assessing global atmospheric phenomena. Aerosol impact on climate, air quality, and health are large unknowns. Uncertainty is due in part to the complex mixture of organic and inorganic components comprising aerosol particles and the complex interplay of chemistry occurring both in and at the surface of the aerosol. Reactions between species in different phases can occur at and uptake of gaseous species into the particle is governed by the particle/gas-phase interface.1−6 Answers to many outstanding questions in the atmospheric community rely upon understanding the dynamics of aerosols. This includes understanding migration of species to and from aerosol surfaces, reactions occurring at the surface or in the bulk, and facilitation of the uptake of gaseous species. An important model system for addressing many basic questions of interactions of organic and inorganic material at aerosol surfaces is the reaction of formaldehyde with sulfur dioxide (SO2). Formaldehyde, a pollutant common in urban environments, is readily taken up by aqueous particles due to its rapid hydration to methylene glycol.7 Sulfur dioxide, a common © 2016 American Chemical Society

atmospheric pollutant with a strong affinity for water surfaces, is known to facilitate aerosol acidification and lead to changes in aerosol composition.8−10 Formaldehyde and SO2 are known to react to form hydroxymethanesulfonate (HMS).11,12 The produced HMS resides at the surface and can impact further SO2 uptake. This system demonstrates the complex interactions between organic and inorganic species as well as the surface and bulk chemistry that can occur in aerosol. In many atmospheric aerosol studies, the overarching questions are similar. Where does the chemistry occur: in the gas-phase, at the air-particle interface, or in the bulk of the particle? What is the distribution of reactants and products within the aerosol particle? How is continued uptake of gas-phase species into the aerosol impacted by the changing surface conditions as the reaction proceeds? Examining the complicated chemistry between formaldehyde and SO2 will elucidate changes in surface composition, gas uptake, and surface to bulk partitioning in this atmospherically realistic system. Previous work in the Received: April 1, 2016 Revised: June 9, 2016 Published: June 13, 2016 14122

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molecular dipole moment and polarizability with respect to the relative degrees of freedom of each atom in the molecule. These requirements demand the use of electronic structure methods. Generally, a choice is made to either sacrifice sampling size for accuracy of the response properties (ab initio MD, QM/MM), to sacrifice a portion of accuracy and generalizability for larger systems and longer sampling by parametrizing molecular response properties (classical MD), or to sacrifice predictive power by disregarding dynamics (tensor rotations of static ab initio calculations). The approach we employ is a version of the energy representation method27−29 of calculating VSF spectra, using classical molecular dynamics (MD), density functional theory (DFT), and an in-house code. The classical molecular dynamics are used to determine the conformations and orientations present at an aqueous interface. The change in the dipole moment and polarizability with respect to the vibrational normal modes is calculated using DFT for a representative set of static structures. Finally, the in-house code generates VSF spectra by combining the requisite information from the classical MD and DFT calculations. Reasonably accurate VSF spectra for similar small organic molecules at aqueous interfaces have been obtained using this approach.30−34 This computational methodology is applied in an effort to elucidate specific information relating to the formaldehyde and hydroxymethanesulfonate aqueous interfaces previously studied experimentally.

Richmond laboratory has used vibrational sum frequency spectroscopy to examine the behavior of aqueous formaldehyde interfaces in the presence of flowing SO2 gas,5 laying the groundwork for the work presented here. The previous formaldehyde-SO2 studies in the Richmond laboratory performed by Ota et al.5 had three major conclusions. First, in accord with bulk thermodynamics,13−15 at the aqueous interface, formaldehyde exists in its hydrated form, methylene glycol. Second, it was concluded that HMS is formed when aqueous formaldehyde surfaces are exposed to sulfur dioxide gas, a conclusion based in part on VSF spectroscopy studies of aqueous solutions containing HMS. Finally, for aqueous HMS solutions in the presence of flowing sulfur dioxide gas, there is a clear alteration in the spectral signal that does not persist once SO2 flow is terminated. The alteration of the spectrum was attributed to formation of an HMS-SO2 complex at the surface when SO2 is present above the reacted solution. Further molecular details of such a complex, however, could not be obtained. The prior experimental work has demonstrated the complex chemistry that occurs at the surface during formaldhyde’s exposure to SO2 gas; however, the overlapping spectral signatures of formaldehyde, methylene glycol, hydroxymethanesulfonate, and other possible products made definitive assignments difficult. This study seeks to obtain a more detailed picture of the exact species and interfacial orientations corresponding to the major spectral features observed in the experimental studies through theoretical computations of vibrational sum-frequency (VSF) spectra for direct comparison and analysis. Interpretation of experimental VSF spectra is difficult due to the nature of the VSF response. VSF spectroscopy involves overlapping polarized visible and infrared laser beams in space and time at an interface that results in a molecular response, generating signal at the sum of the visible and IR frequencies. Stronger signal occurs when the IR frequency is resonant with vibrational transitions in the molecule thus generating a vibrational spectrum. The selection rules of the VSF response are such that local centrosymmetry results in destructive interference. Due to this, VSF spectroscopy is transparent to the bulk and is surface selective due to the anisotropy present there. Additionally, different polarization combinations of the incident and signal beams can probe vibrations perpendicular (SSP polarization scheme) or parallel (SPS polarization scheme) to the interface. The same selection rules that make VSF spectroscopy a surface selective technique also give rise to phase interferences that occur between neighboring vibrational modes. The phase interferences can alter apparent peak positions, apparent peak intensities, and line shapes. Spectral assignments can be made using semiarbitrary fitting of experimental spectra, but this leaves considerable room for alternative interpretations. Calculated VSF spectra can serve to confirm and expand on interpretations made from a specific analysis of experimental spectra. There are many approaches to calculating VSF spectra16−26 ranging from parametrized classical molecular dynamics24,25 to ab initio molecular dynamics.26 Calculation of accurate VSF spectra is a tremendous challenge due to the nature of the problem spanning multiple time and length scales. It requires knowledge of the equilibrium orientations and conformations of the molecules of interest at a realistic water interface. This necessitates large numbers of water molecules and long time dynamics. It also requires knowledge of the response of the

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COMPUTATIONAL METHODOLOGY Optimized geometries, harmonic vibrational frequencies, as well as dipole moments and static polarizabilities at displaced geometries were calculated at the B3LYP/6-311++G(2d,2p) level of theory. Anharmonic frequencies were afforded by use of second-order vibrational perturbation theory. All DFT calculations were performed using the Gaussian 09 program package.35 Structures and vibrational frequencies were visualized using Avogadro36,37 and Jmol,38 respectively. Second-order susceptibility tensors (χ) were obtained from the product of dipole (μ) and polarizability (α) derivatives with respect to the displacements (Q) of each normal mode (q) calculated using three-point differentiation according to the following equation: χijk(2), q ∝

∑ Cabc a,b,c

∂αab , q ∂μc , q ∂Q q ∂Q q

(1)

Classical MD calculations were performed using the Amber 12 suite of programs.39 Initial configurations for eight methylene glycol, formaldehyde, or hydroxymethanesulfonate molecules in a 30 Å cube with 900 water molecules were created using the PACKMOL program.40 One dimension of the box was extended, and periodic boundary conditions are applied to generate a slab geometry with two water interfaces. Polarizable force fields (pol3 for water, ff02pol for the organics) were used with a 1 fs time step and the SHAKE algorithm for treating bonds to hydrogen. Atomic charges were determined using iterative RESP fitting and for hydroxymethanesulfonate were scaled by 0.8 similar to what is appropriate in modeling ionic liquids41 to prevent overestimating aggregation. The use of such a scaling factor is likely required because the force fields do not allow breaking and formation of bonds that would better describe the acid−base equilibria of water and HMS. VSF 14123

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The lowest energy structures on the gas phase landscape correspond to the fMG1 and HMS1 structures. In the solvation environment present in the classical MD calculations, the fMG3 and HMS1 conformers are most populous. The change in population suggests that the conformers of MG are highly sensitive to solvation, as would be anticipated for a molecule containing a geminal diol. In the interfacial region where water density decreases, contributions from the fMG1 and fMG2 conformers increase. While effects of solvation greatly alter the relative energies of the conformers for fMG and to a lesser extent HMS, the CH stretching frequencies are minimally impacted. DFT calculations of the gas-phase MG and HMS structures microsolvated with 3−6 waters shifted CH stretching frequencies by at most ∼10−15 cm−1. This is on the order of the error in the experimental spectra. Similar calculations for FRM resulted in shifts of ∼60−100 cm−1. Formaldehyde VSF Spectra. In the bulk, formaldehyde hydrates nearly completely (∼99%).13−15 However, the depleted water density at the aqueous interface can lead to an alteration of the hydration equilibrium when compared to the bulk. It is unclear if the bulk thermodynamic ratio of formaldehyde’s level of hydration extends to the surface. Molecules similar to formaldehyde, specifically methylglyoxal, have shown changes in the ratio of hydrated:doubly hydrated forms at the surface relative to the bulk.30 The experimental VSF studies of formaldehyde surfaces could not determine the interfacial hydrated:unhydrated ratio explicitly.5 Comparison of experimental and calculated spectra are used to verify that the hydrated form of formaldehyde is still the dominant species at the interface. Calculated VSF spectra in both SSP and SPS polarization schemes for formaldehyde and methylene glycol are shown in Figure 2 with experimental spectra of aqueous formaldehyde shown for comparison. The calculated spectra are generated using all MG conformations shown in Figure 1. Surface conformer populations and orientations were obtained from analysis of the MD calculations. The response properties of the corresponding DFT conformers in each orientation are used to generate a VSF spectrum as outlined in the methodology and previous works. The experimental SSP VSF spectra from ref 5 shows three main features: a dominant peak at 2910 cm−1 with a sloping shoulder at 2870 cm−1, a smaller peak around 2785 cm−1, and another small peak near 2995 cm−1. In the experimental studies, the dominant peak was assigned to the methylene glycol symmetric stretch mode, while the shoulder and two smaller peaks could be attributed to modes from formaldehyde, methylene glycol, or longer-chain oligomers. This uncertainty in the spectral assignments makes interpretation regarding formaldehyde hydration difficult. However, comparison of the experimental spectra (black dots), with our calculated spectra of formaldehyde (blue) and methylene glycol (red) in Figure 2, shows the majority of the experimental spectral features correspond best with the calculated methylene glycol spectrum. The calculated spectrum of methylene glycol exhibits a main peak at 2910 cm−1 composed of contributions from the symmetric methylene stretches of fMG3 and fMG1. This feature in the calculated fMG spectrum corresponds remarkably well to the main experimental feature at 2910 cm−1. Furthermore, the sloping intensity to the red of the main peak in the experimental spectrum is captured by intensity arising from the symmetric methylene stretches of fMG2 and fMG4 in the calculated spectrum. Finally, contributors to the experimental intensity near 2995 cm−1 also arise from fMG,

spectra and other results are calculated using 20 ns evolution trajectories. An in-house code is used to calculate VSF spectra using the outputs of the DFT and MD calculations. Stick spectra obtained by inspecting the susceptibility tensor are broadened using a Voigt-like profile, which convolutes Lorentzian and Gaussian functions to better describe contributions from homogeneous and inhomogeneous broadening. All peaks in the calculated spectra have been broadened using a Lorentzian line width of 4 cm−1 and a Gaussian line width of either 8 or 40 cm−1 to best show the spectral contributions to the spectrum. It should be noted that the calculated spectra reveal the normal modes of the organic species only and thus we focus our analysis on the CH stretching region where the water contributions are generally small. Contributions from the water background are not included because accurate water spectra in the region of the CH stretching region are difficult to obtain within the energy representation. Additionally, due to their low concentration, it is unfeasible to consider contributions from hydronium ions though they would presumably have a greater impact on the CH stretching region than contributions from water. Nonresonant contributions are also not included in the current implementation. Fits of the experimental spectra suggest ignoring the water and nonresonant contributions is a modestly reasonable assumption for this system.

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RESULTS AND DISCUSSION DFT Structures. The three major species in the aqueous formaldehyde−SO2 system are presumed to be formaldehyde (FRM), its hydration product methylene glycol (fMG), and formaldehyde and SO2’s reaction product hydroxymethanesulfonate (HMS). In water, formaldehyde reacts to form methylene glycol according to CH2O + H2O ⇌ HOCH2OH. In the presence of water and sulfur dioxide, formaldehyde reacts to form HMS according to CH2O + HSO3− ⇌ HOCH2SO3−. Optimized gas-phase geometries for formaldehyde, methylene glycol, and hydroxymethanesulfonate are shown in Figure 1. The unhydrated formaldehyde, having no

Figure 1. Gas phase structures and relative gas-phase energies in kcal/ mol calculated at the B3LYP/6-311++G(2d,2p) level of theory. The population percentages in aqueous solution from analysis of dihedrals in classical MD calculations are given in bold.

rotatable bonds, is described by a single conformer: FRM1. A total of four conformers are necessary to represent the hydrated formaldehyde, fMG, with rotations about both C−O bonds. Two unique conformers, HMS1 and HMS2, describe hydroxymethanesulfonate considering rotation about the C− O bond. 14124

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Comparison of calculated SPS VSF spectra with experimental SPS VSF spectra provides a corresponding picture. The experimental SPS VSF spectrum shows a broad feature from ∼29802800 cm−1. In the experimental studies, this broad feature was assigned to CH stretching modes; however, the poor signal-to-noise in SPS made spectral assignments difficult. The calculated SPS spectrum, however, captures the features of the experimental spectrum, enabling clarification in mode assignments. For the calculated fMG spectrum, contributions due to fMG1 and fMG3 are spaced ∼20 cm−1 apart and centered around 2950 cm−1. Given the uncertainty inherent in the DFT calculations and the comparison of calculated gasphase frequencies to experimental condensed phase frequencies they both may be closer to 2950 cm−1. In the limit of strong hydrogen bonding, it would be expected that the four MG conformers should become indistinguishable. Indeed, these modes for the two conformers are closer together when 4−6 water molecules are added to the DFT calculations. These features near 2950 cm−1 correspond to the asymmetric methylene stretches of fMG1 and fMG3. Considering intensity contributions from both peaks brings the relative intensity closer in line with what is seen experimentally. The feature near 2910 cm−1 is due to the symmetric methylene stretch of fMG3. Again, the feature at lower wavenumbers could be assigned to either the symmetric C−H stretch of the unhydrated formaldehyde or the symmetric methylene stretch of fMG4. Even though the broadness of the experimental spectra is not accurately captured by either fMG or FRM spectra, the density of spectral modes of fMG more accurately capture the intensity observed experimentally throughout the CH region. The comparison of the experimental spectra and the calculated methylene glycol spectra in both SSP and SPS schemes indicates methylene glycol is the dominant interfacial species in aqueous formaldehyde solutions. Methylene Glycol Interfacial Orientation. The comparison of experimental and theoretical spectra supports the conclusion that methylene glycol is the major species at the interface in aqueous formaldehyde solutions. While there is methylene glycol present at the interface in the molecular dynamics trajectories, calculation and analysis of the density profile does not show increased surface concentration relative to the bulk population. The methylene glycol molecules in the MD simulations that are present at the interface adopt the orientations displayed in Figure 3. The most common tilt angle of the H−C−H bisector relative to the surface normal is approximately 70°. This corresponds to the H−C−H bisector pointing slightly into the gas phase while the O−C−O bisector points slightly into the bulk. The twist angle of 0 or 180° corresponds to orientations where the oxygen−oxygen interatomic axis is parallel to the surface normal. This orientation makes it possible for both hydroxyl groups to point slightly into the bulk presumably allowing for greater solvation of these moieties at the interface. VSF Spectra of HMS. The final question remaining from the experimental formaldehyde−SO2 studies regards understanding the effect of hydroxymethanesulfonate surfaces on further SO2 uptake. Experiments from ref 5 have demonstrated that, after exposure to SO2 gas, HMS is formed in solution and is present at the aqueous interface. The experimental spectra further suggest that SO2 interacts with HMS at the surface during continued SO2 flow, perhaps forming an HMS−SO2 complex. Still, the existence and exact nature of this complex were uncertain and warrant further study.

Figure 2. Calculated VSF spectra (using combined MD and DFT data) of formaldehyde (blue) and methylene glycol (red) in the SSP (top) and SPS (bottom) polarization schemes. Experimental spectra from ref 5 are included for comparison (black dots).

likely the asymmetric methylene stretches of the fMG1, fMG2, and fMG3 conformers, which are calculated to fall between 2942 and 2985 cm−1. The minor feature near 2785 cm−1 in the experimental SSP spectrum is more difficult to characterize. This feature may correspond to the symmetric methylene stretch mode of fMG4 near 2800 cm−1 but may also arise from the symmetric C−H stretch of the unhydrated formaldehyde around 2730 cm−1. While both resonances appear near the peak in question, it is more likely due to the later as the effects of hydrogen bonding between water and the unhydrated formaldehyde should increase the C−H bond strength, generating a shift to higher frequencies and broadening of the peak as is observed experimentally. Even if the feature at 2785 cm−1 arises from unhydrated formaldehyde molecules at the interface, the small relative intensity of this mode in the experimental spectrum indicates unhydrated formaldehyde comprises only a small percentage of the total surface population of formaldehydederived species. The calculated SSP spectral intensity per molecule of formaldehyde is considerably stronger than for methylene glycol. To obtain a relative intensity that would match the experimental spectrum well, the unhydrated formaldehyde would only have a population on the order of, at most, a few percent. This evidence of low formaldehyde surface population is consistent with bulk hydration ratios of aqueous formaldehyde solutions and confirms the interpretations put forward in ref 5. 14125

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has two main features. There is a dominant peak around 2920 cm−1 and a smaller peak near 2850 cm−1. During the flow of SO2 gas, the lower frequency peak becomes much stronger and both peaks shift slightly to higher frequencies. The previous experimental work attributed the higher frequency peak to a HMS methylene stretch. The lower frequency peak that grows in was attributed to the formation of an HMS−SO2 complex. Focusing first on peak position, the calculated SSP HMS spectrum shows a large feature at 2855 cm−1 and a smaller feature at 2920 cm−1, which agrees fairly well with the peak positions in both during and after SO2 flow experimental spectra. There is, however, a large difference in relative intensities of the two peaks in the calculated spectra relative to the experimental spectra. This discrepancy suggests that the orientations in the theoretical calculations do not accurately reflect the orientations present at equilibrium in the experimental time scale. This could result from a poor description by the molecular dynamics force fields. It is also possible that reaching the equilibrium state that occurs in the experimental spectrum after flow of SO2 (possibly ∼1 s) is beyond the time scale investigated in the dynamics trajectories (∼20 ns). The experimental SPS spectral intensities after flow is stopped are very weak (sitting barely above the noise), so the spectrum is not reproduced here. The spectrum does have peaks near 2910 and 2950 cm−1. Similarly to the SSP spectra, the SPS spectrum during flow of SO2 shows minor frequency shifting of the major peaks and the peak at lower frequency increases in relative intensity. The SPS spectrum calculated using the combination of MD and DFT also shows two main peaks. The first appears around 2950 cm−1 in agreement with experiment. The lower frequency peak, however, is around 2850 cm−1 in the calculated spectrum, quite far from the experimental peak. Again, these discrepancies between the experimental and calculated spectra suggest the species’ orientations in the calculated system are not adequately capturing the realities of the experimental system. The lack of water in the DFT could be one cause; however, microsolvation in the DFT calculations predicts that the associated mode is not greatly affected by solvation. Again, similarly to the SSP results, there is no clear association between the experimental spectra and the calculated spectra using the combination of MD orientations and DFT response. It is posited that inaccurate representations of the HMS orientations at the interface may be the cause of the poor predictive capacity of the calculated spectra. This is not necessarily a fault of the methodology that can be remedied. The relative intensities in the calculated SSP spectrum are closer to those seen in the experimental spectrum with SO2 flowing. Under flowing conditions it is likely that the dynamics at the interface are disrupted by the increased and continual uptake of SO2. It is likely that the time-scale of the dynamics that govern the orientation of HMS probed by the experimental spectrum after flow of SO2 has stopped is much longer than the time-scale of the dynamics that govern the orientation of HMS either in the MD calculations or probed by the experimental spectrum during flow of SO2. If this were the case, the dynamics and spectra observed in the MD calculations would be more representative of the experimental interface perturbed by SO2 than the interface with only HMS present (after SO2 flow stops). If the disagreement in the relative intensities of the spectra is simply a matter of nonmatching interfacial orientations, as was

Figure 3. Orientation of methylene glycol in the vicinity of the air− water interface obtained from analysis of the MD calculations. Bisector of the H−C−H bond angle tilt relative to the surface normal and twist of the H−H segment relative to parallel to the surface normal. Colors correspond to different levels of populations from low to high in order purple, blue, light blue, green, red, orange, and yellow.

Calculated theoretical SSP spectra of HMS are displayed in Figure 4 and compared to experimental spectra from ref 5 taken during and after the flow of SO2 gas has ceased. The experimental SSP spectrum after the flow of SO2 has stopped

Figure 4. Calculated VSF spectra (using combined MD and DFT data) of hydroxymethanesulfonate (green) in the SSP (top) and SPS (bottom) polarization schemes. Experimental spectra from ref 5 during (black circles) and after (gray triangles) flow of sulfur dioxide included for comparison. 14126

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Figure 5. Dependence of calculated SSP (top) and SPS (bottom) VSF spectra of HMS1 (left) and HMS2 (right) on the tilt angle relative to the surface normal. Calculated using rotated gas phase DFT tensors.

posited above, it should be possible to find an orientation of the gas-phase DFT structures that gives a calculated spectrum that matches experiment. The dependence of the calculated spectrum on orientation is determined by rotating the DFT calculated β tensor and using the appropriate tensor element(s) to generate spectra. Figure 5 and the associated discussion detail how the VSF spectrum calculated using only static DFT structures depends on the orientation of each conformer of HMS. Figure 6 shows composite VSF spectra assuming a 1:1 ratio of the conformers and a specific orientation of HMS. Spectra generated by the rotation of DFT calculated β tensors are shown in Figure 5 for each HMS conformer in both the SSP and SPS polarization schemes. The series of graphs correspond, at the bottom, to the sulfur in the C−S bond pointing toward the gas phase (0°) to, at the top, pointing directly into the bulk (180°) with representative graphs every 10° in between. The shown graphs are for a single twist angle of the molecule relative to an initial structure, and while not shown, similar data are also generated for every 10° of twist angle possibilities. The twist angle for the data shown is such that, as the molecule is tilted at 60°, the H−C−O bond angle bisector points almost directly into the bulk. The specific twist angle was chosen for the displayed theoretical spectra in Figure 5 as it is one of a hand full of twist angles which lead to spectra that best match the after SO2 flow experimental spectra. Asterisks mark the clearest example of this type of spectrum. A representation of the corresponding molecular orientation, relative to the surface normal, is also

Figure 6. Calculated VSF spectra (from rotation of DFT structures assuming 50% HMS1 and 50% HMS2) of hydroxymethanesulfonate (blue) in the SSP polarization scheme. Experimental SSP spectrum from ref 5 after (gray triangles) flow of sulfur dioxide included for comparison.

shown at right in Figure 5. At the asterisks-marked molecular orientation (blue), contributions in the calculated SSP spectrum from HMS1 at 2850 and 2950 cm−1 will be small while the contribution from HMS2 near 2910 cm−1 would be large. This agrees with what is seen experimentally after sulfur dioxide flow has been stopped. In the calculated SPS spectrum, 14127

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majority, if not all, of the VSF spectral intensity of aqueous formaldehyde solutions is due to methylene glycol. This is in agreement with observations made in the experimental work. At most, a few percent of the interfacial molecules are in the unhydrated (formaldehyde) form, showing no significant difference from the hydrated:unhydrated ratio observed in the bulk. The most prominent MG surface species is fMG3 with an average orientation where the methylene H−C−H bisector points slightly toward the gas phase and the plane containing the H−C−H angle is perpendicular to the interfacial plane. This orientation allows the hydroxyl functional groups to be well solvated at the interface and puts the methylene moiety in a position that may make reactions with other interfacial species more facile. Calculated spectral features of HMS reproduce the experimental peak positions well, and differences in relative intensities are attributed to differences in time scales. The majority of peaks that are experimentally observed both during and after flow of sulfur dioxide can be assigned to HMS, with the observed vibrational modes corresponding to the two HMS conformers. The calculations also demonstrate the orientational dependence of VSF spectra, with such orientation calculations providing a much-improved agreement of relative intensities when compared to experimentally measured ones. Such analysis goes further to demonstrate that the unperturbed surface of aqueous HMS most likely adopts an arrangement with the methylene pointing out of the bulk, the hydroxyl group pointing into the bulk, and the C−S bond pointing slightly out of the bulk. While still reasonable that HMS is forming a complex with SO2 under flow conditions, the theoretical spectra show the changes in the experimental VSF spectra are consistent with minor alterations in the orientation of HMS at the interface.

moderate contributions from HMS1 at 2850, 2915, and at 2950 cm−1 arise. Continued analysis of the dependence of the calculated VSF spectra on the orientation of HMS shows that even relatively minor changes in the tilt and/or twist angles can result in a return to calculated spectra that match the experimental spectra under flow of SO2. The yellow calculated spectra in Figure 5 that appear below the spectra marked with asterisks are an example of such a situation. These spectra correspond to an orientation where the angle between the C−S bond and the surface normal is about 30°. Spectra calculated based off this orientation and a few others are consistent with the experimental spectra with SO2 flowing. A composite SSP spectrum generated using the data from rotated DFT structures seen in Figure 5 is presented in Figure 6. As the interfacial orientations and relative conformer populations are not exactly known, a 1:1 ratio of the conformers has been assumed for this analysis. It has also been assumed that both conformers adopt the same orientation. The calculated SSP spectrum is plotted with the experimental SSP spectrum after SO2 flow has stopped for comparison. This should be a direct comparison. The calculated SPS spectrum is not shown as the experimental spectrum after SO2 flow is too noisy to make reasonable comparisons. Still, the agreement between the experimental and calculated SPS spectra seems to improve considerably. Use of the alternative method of calculating VSF SSP spectra provides much improved agreement with experiment. This does, however, require assumptions about the orientation and conformer ratio that the combined MD/DFT methodology does not require. The agreement for the SSP spectra is much improved with both peak positions and relative intensities in very close agreement between the calculated and experimental spectra as can be seen in Figure 6. Tentative assignments of the experimentally observed peaks are made based on the proximity of the calculated and experimentally observed vibrational frequencies, and assuming that the differences in relative intensities arise from dynamics beyond the time scale of the classical molecular dynamics performed. Under these assumptions, the features in the experimental SSP spectrum near 2850 and 2910 cm−1 are assigned to the symmetric stretches of HMS1 and HMS2 respectively. In the experimental SPS spectrum with sulfur dioxide flowing, the feature near 2950 cm−1 corresponds to the asymmetric stretch of HMS1. While the agreement is less clear, the feature near 2900 cm−1 in the experimental SPS spectrum falls between the symmetric stretches of HMS1 and HMS2. There still remain many possible explanations for the differences in the experimental spectra during and after SO2 flow, including reversible complexes formed from intermolecular or covalent interactions of HMS with SO2. Nevertheless, the calculated VSF spectra obtained by rotating the gas-phase species suggest that there is sufficient variability in the calculated VSF spectrum of HMS alone as a function of orientation to generate the changes in the experimental spectrum that are observed during SO2 flow.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03311. Optimized geometries and harmonic and anharmonic vibrational frequencies for DFT structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (541)-346-4635. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Laura McWilliams and Claire Valley for assistance in preparing the manuscript. The authors are grateful for financial support for this work from National Science Foundation Grant CHE-1505891.

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CONCLUSIONS Comparison of calculated VSF spectra of formaldehyde, methylene glycol, and hydroxymethanesulfonate to previously published work have confirmed and illuminated many of the molecular details of these systems. Calculated spectra of formaldehyde and methylene glycol have shown that the

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REFERENCES

(1) Donaldson, D. J.; Vaida, V. The Influence of Organic Films at the Air-Aqueous Boundary on Atmospheric Processes. Chem. Rev. 2006, 106, 1445−1461.

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DOI: 10.1021/acs.jpcc.6b03311 J. Phys. Chem. C 2016, 120, 14122−14129

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DOI: 10.1021/acs.jpcc.6b03311 J. Phys. Chem. C 2016, 120, 14122−14129