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, California 90089, United States * S Supporting Information ABSTRACT: We previously reported the spectrum of the water bend vibrational mode (ν 2 ) at the air/water interface measured using sum-frequency generation (SFG). Here, we present experimental evidence to aid the assignment of the ν 2 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 and PPP polarization combinations (SFG-visible-infrared). This is naturally explained as an interference effect between the negative (1630 cm -1 ) and positive (1662 cm -1 ) peaks corresponding to “free-OH” and “H-bonded” species, respectively, which have different orientations and thus different amplitudes in SSP and PPP spectra. A 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 . T he 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 and hydrophilic interactions. Vibrational spectroscopy, pre- dominantly in the OH-stretching region of the spectrum, has been applied extensively to study the H-bond structures, distribution, and ultrafast dynamics in bulk 1-3 and interfacial water. 4-12 Recently, several groups started exploring the water bend vibrational mode (ν 2 ) 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 H-bond, the water bend mode, which involves both hydrogens, is sensitive to at least two donor H-bonds (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 sum-frequency generation (SFG) measure- ments 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 because there is only one mode per molecule, and the intermolecular coupling is weaker because of 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 ν 2 spectral peak is observed in liquid water 18 and at the air/water interface. 13 In our preceding publication, we reported the first surface- selective vibrational sum-frequency generation (VSFG) spec- trum 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 (Figure 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 Bonn 14 suggested an assignment of the ν 2 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 ν 2 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 Skinner 15 presented a mixed quantum-classical calculation by first computing the spectro- scopic 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 ν 2 Received: November 16, 2016 Accepted: January 9, 2017 Published: January 9, 2017 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 801 DOI: 10.1021/acs.jpclett.6b02678 J. Phys. Chem. Lett. 2017, 8, 801-804