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On the Assignment of the Vibrational Spectrum of the Water Bend at the Air/Water Interface Chayan Dutta, and Alexander V Benderskii J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02678 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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The Journal of Physical Chemistry Letters

Dutta, Benderskii, TOC ACS Paragon Plus Environment


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Dutta, Benderskii, Figure 1 ACS Paragon Plus Environment

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On the Assignment of the Vibrational Spectrum of the Water Bend at the Air/Water Interface Chayan Dutta and Alexander V Benderskii* Department of Chemistry, University of Southern California, Los Angeles, CA90089

ABSTRACT. We have previously reported the spectrum of the water bend vibrational mode (ν2) at the air/water interface measured using Sum-Frequency Generation (SFG). Here, we present experimental evidence to aid the assignment of the ν2 spectral features to H-bonded classes of interfacial water, which is in general agreement with two recent independently published theoretical studies. The dispersive line shape shows an apparent frequency shift between SSP vs. PPP polarization combinations (SFG-visible-IR). This is naturally explained as an interference effect between the negative (1630 cm-1) and positive (1662 cm-1) peaks corresponding, respectively, to “free-OH” and “H-bonded” species, which have different orientations and thus different amplitudes in SSP vs. PPP spectra. Surfactant monolayer of sodium dodecyl sulfate (SDS) was used to suppress the free OH species at the surface, and the corresponding SFG spectral changes indicate that these water molecules with one of the hydrogens pointing up into the air phase, contribute to the negative peak at 1630 cm-1.

KEYWORDS. Air/water interface, Water/vapor interface, Water surface, Vibrational spectroscopy, water bend, SFG, nonlinear spectroscopy.

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*Corresponding Author E-mail: [email protected]

TOC Graphic


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The dynamic hydrogen bonding network of liquid water is the microscopic underpinning of most, if not all, of its physical, chemical, and biological properties, including interfacial phenomena such as surface tension and hydrophobic/hydrophilic interactions.

Vibrational

spectroscopy, predominantly in the OH-stretching region of the spectrum, has been applied extensively to study the H-bond structures, distribution, and ultrafast dynamics in bulk1-3 and interfacial water.4-12 Recently, several groups started exploring the water bend vibrational mode (ν2) for the studies of the aqueous H-bonding at interfaces.13-16 Water bend spectroscopy may provide molecular information complementary to that available from the OH-stretch spectra.15 For example, while the OH-stretch frequency is predominantly affected by a single donor Hbond, the water bend mode, which involves both hydrogens, is sensitive to at least two donor Hbonds (in addition to possibly being affected by the acceptor H-bonds through the two lone pairs).

Intramolecular coupling between the two local OH-stretch modes on each water

molecule, as well as intermolecular coupling between neighboring molecules significantly complicate the spectral and orientational analysis of the vibrational SFG measurements in the OH-stretch region because, in general, the direction of the transition dipole does not represent the orientation of any single OH-bond.17 In contrast, the water bend SFG spectroscopy should be free of many of these complications since there is only one mode per molecule, and the intermolecular coupling is weaker due to the smaller transition dipole.15 The bend mode is also a doorway state for vibrational relaxation in water, situated right in the middle of the ~3000 cm-1 band gap between the OH-stretch modes and the librational, rotational, and translational modes. Of considerable interest is therefore the mechanism of water bend mode coupling to the librational overtones, which provide a broad background on which the ν2 spectral peak is observed in liquid water18 and at the air/water interface.13


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In our preceding publication, we reported the first surface-selective vibrational sum-frequency generation (VSFG) spectrum of the water bend at the air/water interface.13 We offered an “Occam’s razor” interpretation of the spectrum in terms of the minimum number of Lorentzians sufficient to fit the observed line shape within the available signal-to-noise. Briefly, although the observed line shape (Fig. 1) appears to have both positive and negative peaks, it rides on top of a strong background signal which presumably is due to the librational overtones in this spectral region. Thus, the band shape can be satisfactorily approximated by an interference of a single main Lorentzian peak with a spectrally-flat background. Shortly thereafter, Nagata and Bonn14 suggested an assignment of the ν2 line shape based on molecular dynamics (MD) simulations. Analysis of the MD trajectories revealed that the VSFG spectrum has indeed two distinct contributions of opposite sign: the negative peak (less blue-shifted from the gas-phase ν2 frequency) due to water molecules with 0 or 1 donor H-bond – predominantly, the “free-OH” species with one of the hydrogens not participating in H-bonding, and the positive peak (stronger blue-shifted) due to species with an average of 2 donor H-bonds. Recently, Ni and Skinner15 presented a mixed quantum-classical calculation by first computing the spectroscopic maps of the water bend that correlate the transition frequency, dipole, and couplings with the local electric field, and then applying them to the MD trajectories to obtain the ν2 spectrum of both bulk liquid and interfacial water . As with the previously published OH-stretch calculations,19-21 analysis of the MD trajectories allowed them to separate out spectral signatures of different Hbonding classes. While the contributions from each H-bonding class are broad and overlap one another, a clear general trend indicates negative contributions from the “free-OH” species (1N, 2S, and 3S H-bonded classes in their notation) on the red side of the spectrum and a positive blue-


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shifted contributions from H-bonded structures (3D, 4D H-bonded classes). Here, we present experimental evidence that support the recent theoretical assignments. The theoretical calculations focused on the SSP spectrum (obtained using P-polarized infrared light resonant with the (ν2) mode, S-polarized nonresonant visible light, and detecting Spolarized component of the SFG signal), which contains a single tensor element of (2) the nonlinear susceptibility xxz .

In

contrast, PPP polarized SFG spectra contain contributions from multiple tensor elements ( zzz , yyz , yzy , zyy ) and therefore (2)

(2)

(2)

(2)

provide different orientational averaging of the chromophores.

Thus comparing

SSP vs. PPP spectra can be useful in situations where distinct species with different orientations are present, e.g. different H-bonded classes proposed by theoretical

simulations14-15

described

above. Figure Figure 1. Top panel: Vibrational SFG spectra of the water bend mode at the air/water interface for PPP (red) and SSP (blue) polarization combinations of SFG, visible, and IR. Black solid lines show fit described in the text. Bottom panel: Two principal resonant Lorentzians (positive and negative) used in the fit (left axis). The background signal fit is shown on top (right axis).

1

shows

experimentally

measured SFG spectra of the water bend at the air/water interface obtained using SSP and PPP polarization combinations.


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The main spectral feature appears to be red-shifted by ~15 cm-1 in the PPP spectrum relative to the SSP spectrum. In light of the proposed assignment of this dispersive spectral feature to the free-OH species contributing a negative peak at lower frequency and H-bonded species contributing a positive peak at higher frequency, the apparent PPP-SSP spectral shift has a natural explanation.

The two interfering peaks of opposite sign have different relative

amplitudes in the PPP vs. SSP spectra, and thus the overall spectrum appears shifted even though the component peaks have the same center frequencies. This is illustrated in Figure 1 using two Lorentzians of opposite sign, negative centered at 1630.5 cm-1 and positive at 1662.5 cm-1, to fit the main spectral feature. The different amplitudes of the negative and positive components are indeed expected since molecules belonging to different H-bonding classes have different orientation relative to the surface normal. The overall fit (solid black lines in Figure 1, upper panel) also includes two other Lorentzians and a constant background term with a phase factor which collectively describe the ‘pedestal’ underlying the resonant water bend peaks.

The

intensity spectrum of this background ‘pedestal’ is also shown in the bottom panel of Figure 1. We note that this is not a “nonresonant” background usually used in SFG spectral fits, since there are known librational overtone modes in this spectral region.13, 18 The frequencies of the two main Lorentzians (1 and 2) were constrained to be the same for SSP vs. PPP spectra, within the 1 cm-1 experimental uncertainty, while their amplitudes were varied.

The line width

parameters 1 and 2 were also constrained within 45 cm-1 to 60 cm-1 range. The fitting parameters describing the background ‘pedestal’ were allowed to vary. The fitting parameters are summarized in Table 1 in Supporting Information. We emphasize that the two-Lorentzian fit of the main spectral feature is an oversimplification used here only to illustrate our argument. Several H-bonded classes likely contribute to each


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peak,15 each having its unique spectral shape and average molecular orientation. We therefore do not attempt fitting of both SSP and PPP spectra using the deconstructed H-bonding class contributions presented by Ni and Skinner,15 since they were calculated only for the SSP spectrum. It is well known that the free-OH species observed at the air/water interfaces and oil/water interfaces is greatly suppressed when water is in contact with a hydrophilic substance. In particular, spreading a monolayer of an anionic amphiphilic surfactant on the water surface effectively eliminates Figure 2.The OH-stretch SFG spectrum of the air/water interface in absence (red) and presence (blue) of a monolayer of SDS (1 mM SDS solution)., for both SSP and PPP polarizations.

the

free-OH

species,

presumably by H-bonding the freeOH

groups

to

the

surfactant

headgroups.22-25 This is demonstrated in Figure 2 which shows the SFG spectrum of the free OH-stretch mode at the air/water interface of pure water in comparison with the 1mM solution of sodium dodecyl sulfate (SDS), which forms a monolayer of SDS at the water surface. The free OH peak at ~3700 cm-1 is almost entirely suppressed in the presence of SDS, in both SSP and PPP spectra. We therefore expect the corresponding changes in the water bend SFG spectra to point to the contribution of the free-OH species. Figure 3 shows the comparative SFG spectra of the water bend at the air/water interface of pure water and 1 mM SDS solution. It is evident that the negative peak at 1630 cm-1 is


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Figure 3.Comparison of the water bend SFG spectra of the air/water interface in absence (top left) and presence (top right) of an SDS monolayer, for SSP (blue) and PPP (red) polarizations. Solid black lines represent fit to the model described in text. Bottom panels show the two main Lorentzians used in the fitting.

significantly diminished in the presence of the SDS monolayer for both SSP and PPP polarizations. Also, the frequency shift between SSP and PPP spectra is no longer apparent, since the interference effect is now diminished due to lower amplitude of the negative peak. This solidifies the assignment of the negative 1630 cm-1 feature in the water bend SFG spectrum to the free OH species (1N, 2S, 3S H-bonded classes). We note that the SDS molecule itself does not give any SFG signal in this frequency range. This was verified experimentally by recording


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spectra of D2O/SDS mixtures under the identical experimental conditions (the spectra are presented in the Supporting Information). Figure 3 also shows the curve fitting to the same model as in Figure 1, with the main spectral feature described using two Lorentzians of opposite signs. The fitting parameters can be found in Table 1 in Supporting Information. The complete decomposition of the spectra are presented in the Supporting Information section. Interestingly, the fitting indicates that the negative peak at 1630 cm-1, while diminished, does not entirely disappear in the presence of 1 mM SDS. This is in contrast with the free OH stretch peak that is completely suppressed (Figure 2). Clearly, there are contributions to the negative spectral amplitude at 1630 cm-1 other than the free OH species. Indeed, Ni and Skinner calculations suggest that the fully tetrahedrally-coordinated species (4D H-bonded class) provide a negative amplitude signal at the lower-frequency end of the spectrum as well as positive signal on the blue-side of the spectrum. Another possible source of the residual negative amplitude at 1630 cm-1 could be the librational overtone resonances, which are not included in the theoretical treatments and only crudely approximated by our fitting. Indeed, the constant term in our fitting equation has a phase relative to the resonant Lorentzian terms which is not zero, and further, is different for different polarizations (SSP vs. PPP) and also different in the absence vs. presence of SDS (Supporting Information, Table 1). This indicates that this term does not simply describe a nonresonant background, but rather approximates a spectrally broad collection of resonant signals. The assignment of the broad blue-shifted peak observed at ~1750 cm-1 in the PPP spectrum of the pure air/water interface remains an open question. Although it is clearly present in the PPP spectrum, the SSP spectrum shows a much weaker blue-shifted feature at ~1700 cm-1(due to interference effects and the significant line width, frequency assignment of these peaks is


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somewhat ill-determined). MD simulations of the SSP spectrum have not found appreciable spectral density in this frequency range,14-15 but no attempt has been made so far to calculate the PPP spectrum. It is possible that this feature is due to the librational overtones of water which were not explicitly included in the calculations, or H-bonded structures with extreme blue-shift (2) of the bend frequency which contribute to susceptibility tensor elements other than xxz , or

perhaps a combination band of the water bend ν2 and low-frequency collective modes. We also cannot completely rule out the possibility of an impurity contributing to this spectral feature. In summary, the experimental evidence presented here strongly supports the recent theoretical assignment of the water bend SFG spectrum of the air/water interface in terms of the hydrogenbonded classes. The main resonant spectral feature of the water bend at the surface has a dispersive line shape which arises due to interference between a lower-frequency negative peak and higher frequency positive peak. The negative red-shifted peak at 1630 cm-1 is mostly associated with the ‘free OH’ species with zero or one donor H-bond, although there is also a discernible contribution from the fully hydrogen-bonded species. The positive blue-side peak at 1662 cm-1 is mostly due to the structures where both hydrogens are tied up in donor H-bonds. The nearly quantitative agreement between the experimentally measured and theoretically calculated SSP spectra of the pure air/water interface is both an encouraging validation of the spectroscopic maps and MD simulations and a valuable input for future spectroscopic studies of aqueous interfaces.

In particular, since different hydrogen-bonded species are spectrally

separated, the calculated spectroscopic parameters can be used to independently characterize the molecular orientation of water molecules at the surface involved in different H-bonding motifs.

ACKNOWLEDGMENT


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This research is supported by AFOSR grant No. FA9550-15-1-0184 and NSF grant CHE1153059. ABBREVIATIONS

SFG Sum Frequency Generation; VSFG Vibrational Sum Frequency Generation.


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