Hydrogen-bonded OH Stretch Research Articles

Unified picture of vibrational relaxation of OH stretch at the air/water interface

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.

Vibrational Sum Frequency Generation Spectra of Water-Vapor Interfaces Covered by Alcohols: Effects of Surface Coverage and Coupling between Oscillators.

The present study deals with the effects of varying coverage of water surface by alcohols on the vibrational sum frequency generation (VSFG) spectrum of interfacial water. We have considered two different alcohols: Tertiary butyl alcohol (TBA) whose alkyl part is fully branched and stearyl alcohol (STA) which has a long linear alkyl chain with larger hydrophobic surface area than that of TBA. With increase of the alcohol concentration, the hydrogen bonded OH stretch region of the VSFG spectrum is found to change following a regular trend for the STA-water system, whereas non-monotonic variation of the VSFG spectrum is observed for the TBA-water system which can be correlated with the presence of very different interactions of TBA molecules at different concentrations. On increasing the concentration of TBA, the hydrophobic groups get more tilted towards the water phase and significant hydrophobic interactions are introduced at higher concentrations. Whereas, for STA, there is a gradual increase in the hydrophilic interaction. Because of stacking interactions between the long chain alkyl groups, the hydrophobic parts stay outward from the water phase at higher concentrations and a regular change in the VSFG spectrum is observed. We have also presented a computationally efficient scheme to calculate the VSFG spectrum of interfacial systems for coupled oscillators which is expected to be beneficial for the treatment of coupling where the interfacial system size is inherently large.

Molecular Dynamics Driven by the Many-Body Expansion (MBE-MD).

We present a protocol for classical and nuclear quantum dynamics, in which the energies and forces are generated by the many-body expansion (MBE), and apply it to water clusters using the TTM2.1-F and MB-Pol interaction potentials at various temperatures. We carry out MBE-molecular dynamics (MD) classical and nuclear quantum dynamical simulations, in which the energies and forces of the full system are approximated by the two-, three-, and four-body terms of the MBE, and compare the average potential and the vibrational density of states with the full simulation, i.e., the one for which no MBE is used. Our results indicate that the thermally averaged potential energy from the MBE up to the four-body term converges with near-identical behavior to the one from the full simulation. The three-body makes a substantial contribution (∼20%) to the energy, whereas the four-body is necessary for obtaining quantitatively accurate energetics and forces, albeit making a small contribution to each (∼2%). We further show that the harmonic frequencies are reproduced to within a few wavenumbers (cm-1) at the four-body level and that the slowest modes to converge with the MBE rank are those involving the strongest hydrogen bonds. Anharmonicity exacerbates this effect, so that a four-body description of the energies and forces is needed to achieve accurate anharmonic vibrational frequencies in the hydrogen-bonded OH-stretching region. We also discuss the asymptotic scaling of the MBE-MD protocol with respect to the cost of the underlying potential energy evaluation, suggesting that electronic structure methods that scale at least as N4, N being the size of the system, are needed to result in savings over the traditional full MD simulation. We anticipate that the MBE-MD protocol can evolve into a powerful and practical method, which will allow for highly accurate ab initio MD simulations on a much broader range of molecular systems than can be currently handled.

Guided Diffusion Monte Carlo: A Method for Studying Molecules and Ions That Display Large Amplitude Vibrational Motions.

Diffusion Monte Carlo provides an effective and efficient approach for calculating ground state properties of molecular systems based on potential energy surfaces. The approach has been shown to require increasingly large ensembles when intra- and intermolecular vibrations are weakly coupled. We recently proposed a guided variant of diffusion Monte Carlo to address these challenges for water clusters [Lee, V. G. M.; McCoy, A. B. J. Phys. Chem. A 2019, 123, 8063-8070]. In the present study, we extend this approach and apply it to more strongly bound molecular ions, specifically CH5+ and H+(H2O)n=1-4. For the protonated water systems, we show that the guided DMC approach that was developed for studies of (H2O)n can be used to describe the OH stretches and HOH bends in the solvating water molecules, as well as the free OH stretches in the hydronium core. For the hydrogen bonded OH stretches in the H3O+ core of H+(H2O)n and the CH stretches in CH5+, we develop adaptive guiding functions based on the instantaneous structure of the ion of interest. Using these guiding functions, we demonstrate that we are able to obtain converged zero-point energies and ground state wave functions using ensemble sizes that are as small as 10% the size that is needed to obtain similar accuracy from unguided calculations.

Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide-Water Binary Complex.

The gas-phase vibrational spectrum of the isolated iodide-water cluster ion (I-·H2O), first reported in 1996, presents one of the most difficult, long-standing spectroscopic puzzles involving ion microhydration. Although the spectra of the smaller halides are well described in the context of an asymmetrical ground-state structure in which only one OH group is hydrogen-bonded to the ion, the I-·H2O spectrum displays multiplet structures with partially resolved rotational patterns that are additionally influenced by quantum nuclear spin statistics. In this study, this complex behavior is unraveled with a combination of experimental methods, including ion preparation in a temperature-controlled ion trap and spectral simplification through applications of tag-free, two-color IR-IR double-resonance spectroscopy. Analysis of the double-resonance spectra reveals a vibrational ground-state tunneling splitting of about 20 cm-1, which is on the same order as the spacing between the peaks that comprise the multiplet structure. These findings are further supported by the results obtained from a fully coupled, six-dimensional calculation of the vibrational spectrum. The underlying level structure can then be understood as a consequence of experimentally measurable, vibrational mode-dependent tunneling splittings (which, in the case of the ground vibrational state, is comparable to the rotational energy spacing between levels with Ka = 0 and 1), as well as Fermi resonance interactions. The latter include the hydrogen-bonded OH stretches and combination bands that involve the HOH bend overtones and soft-mode excitations of frustrated translation and rotation displacements of the water molecule relative to the ion. These anharmonic couplings yield closely spaced bands that are activated in the IR by borrowing intensity from the OH stretch fundamentals.

Structures, energetics, and infrared spectra of the cationic monomethylamine-water clusters

The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, [(CH3NH2)(H2O)n]+ (n=1–5), have been studied using quantum chemical calculations at the MP2/6-311+G(2d,p) level. The results reveal that the formation of proton-transferred CH2NH3+ ion core structure is preferred via the intramolecular proton transfer from the methyl group to the nitrogen atom and the water molecules act as the acceptor for the O⋯HN hydrogen bonds with the positively charged NH3+ moiety of CH2NH3+, whose motif is retained in the larger clusters. The CH3NH2+ ion core structure is predicted to be less energetically favorable. Vibrational frequencies of CH stretches, hydrogen-bonded and free NH stretches, and hydrogen-bonded OH stretches in the calculated IR spectra of the CH2NH3+ and CH3NH2+ type structures are different from each other, which would afford the sensitive probes for fundamental understanding of hydrogen bonding networks generated from the radiation-induced chemical processes in the [(CH3NH2)(H2O)n]+ complexes.

Temperature Dependence of Intramolecular Vibrational Bands in Small Water Clusters.

Cyclic water clusters are pivotal for understanding atmospheric reactions as well as liquid water, yet the temperature (T) dependence of their dynamics and spectroscopy is poorly studied. The development of highly accurate water potentials, such as MB-pol, partly rectifies this. It remains to account for the quantum nuclear effects (NQE), because quantum nuclear dynamics become increasingly inaccurate at low temperatures. From a practical point of view, we find that NQE can be accounted for simply by subtracting a constant from the frequencies obtained from the velocity autocorrelation functions (VACF) of classical nuclear dynamics, resulting in unprecedented agreement with experiment, mostly within 5 cm-1. We have performed classical simulations of (H2O)n clusters (n = 2-5) from 20 K and up to their melting temperature, calculating both all-atom and partial VACF, thus generating the temperature dependence of the vibrational frequencies (IR and Raman bands). Focusing on the hydrogen-bonded (HBed) OH stretch and HOH bend, we find opposing T dependencies. The HBed OH modes blue shift linearly with T, attributed to ring expansion rather than any specific conformational change. The lowest-frequency Raman concerted mode is predicted to show the largest such shift. In contrast, the HOH bend undergoes a red-shift, with the highest frequency concerted band undergoing the largest red-shift. These results can be explained by a coupled-oscillator model for n hydrogen atoms on a ring, constrained to move either tangentially (stretch) or perpendicularly (bend) to the ring. With increasing temperature and weakening of HBs, the intrinsic force constant increases (stretch) or remains constant (bend), while the nearest-neighbor coupling constant decreases, and this results in the interesting behavior revealed herein. T-dependent Raman studies are required for testing some of these predictions.

Interpreting Quasi-Thermal Effects in Ultrafast Spectroscopy of Hydrogen-Bonded Systems.

Vibrational excitation of molecules in the condensed phase relaxes through vibrational modes of decreasing energy to ultimately generate an equilibrium state in which the energy is distributed among low-frequency modes. In ultrafast vibrational spectroscopy, changes in the vibrational features of hydrogen-bonded NH and OH stretch modes are typically observed to persist long after these high-frequency vibrations have relaxed. Due to the resemblance to the spectral changes caused by heating the sample, these features are typically described as arising from a hot ground state. However, these spectral features appear on ultrafast time scales that are much too fast to result from a true thermal state, and significant differences between the thermal difference spectrum and the induced quasi-thermal changes in ultrafast spectroscopy are often observed. Here, we examine and directly compare the thermal and quasi-thermal responses of the hydrogen-bonded homodimer of 7-azaindole with temperature-dependent FTIR spectroscopy and ultrafast mid-IR continuum spectroscopy. We find that the thermal difference spectra contain contributions from both dissociation of the hydrogen bonds and from frequency shifts due to changes in the thermal population of low-frequency modes. The transient spectra in ultrafast vibrational spectroscopy are also found to contain two contributions: initial frequency shifts over 2.3 ± 0.11 ps associated with equilibration of the initial excitation, and frequency shifts associated with the excitation of several fingerprint modes, which decay over 21.8 ± 0.11 ps, giving rise to a quasi-thermal response caused by a distribution of fingerprint modes being excited within the sample ensemble. This resembles the thermal frequency shifts due to population changes of low-frequency modes, but not the overall thermal spectrum, which is dominated by features caused by dimer dissociation. These findings provide insight into the changes in the vibrational spectrum from different origins and are important for assigning, analyzing, and comparing features in thermal and ultrafast vibrational spectroscopy of hydrogen-bonded complexes.

Kinetic Energy Density as a Predictor of Hydrogen-Bonded OH-Stretching Frequencies.

This work considers the nature of the intermolecular hydrogen bond in a series of 15 different complexes with OH donor groups and N, O, P, or S acceptor atoms. To complement the existing literature, room-temperature gas-phase vibrational spectra of the methanol-pyridine, ethanol-pyridine, and 2,2,2-trifluoroethanol-pyridine complexes were recorded. These complexes were chosen, as they exhibit hydrogen bonds of intermediate strength as compared to previous investigations that involved strong or weak hydrogen bonds. Non Covalent Interactions (NCI) theory was used to calculate various properties of the intermolecular hydrogen bonds, which were compared to the experimental OH-stretching vibrational red shifts. We find that the experimental OH-stretching red shifts correlate strongly with the kinetic energy density integrated within the reduced density gradient volume that describes a hydrogen bond [G(s0.5)]. Given that vibrational red shifts are commonly used as a metric of the strength of a hydrogen bond, this suggests that G(s0.5) could be used as a predictor of hydrogen bonding strength.

Infrared absorption of methanol-water clusters (CH3OH)n(H2O), n = 1-4, recorded with the VUV-ionization/IR-depletion technique.

We recorded infrared (IR) spectra in the CH- and OH-stretching regions of size-selected clusters of methanol (M) with one water molecule (W), represented as MnW, n = 1-4, in a pulsed supersonic jet using the photoionization/IR-depletion technique. Vacuum ultraviolet emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer to detect clusters MnW as protonated forms Mn-1WH+. The variations in intensities of Mn-1WH+ were monitored as the wavelength of the IR laser light was tuned across the range 2700-3800 cm-1. IR spectra of size-selected clusters were obtained on processing of the observed action spectra of the related cluster-ions according to a mechanism that takes into account the production and loss of each cluster due to IR photodissociation. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increases, whereas those in the CH region are similar for all clusters. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted with the M06-2X/aug-cc-pVTZ method for the methanol-water clusters are consistent with our experimental results. For dimers, absorption bands of a structure WM with H2O as a hydrogen-bond donor were observed at 3570, 3682, and 3722 cm-1, whereas weak bands of MW with methanol as a hydrogen-bond donor were observed at 3611 and 3753 cm-1. For M2W, the free OH band of H2O was observed at 3721 cm-1, whereas a broad feature was deconvoluted to three bands near 3425, 3472, and 3536 cm-1, corresponding to the three hydrogen-bonded OH-stretching modes in a cyclic structure. For M3W, the free OH shifted to 3715 cm-1, and the hydrogen-bonded OH-stretching bands became much broader, with a weak feature near 3179 cm-1 corresponding to the symmetric OH-stretching mode of a cyclic structure. For M4W, the observed spectrum agrees unsatisfactorily with predictions for the most stable cyclic structure, indicating significant contributions from branched isomers, which is distinctly different from M5 of which the cyclic form dominates.

Dominant conformer of tetrahydropyran-2-methanol and its clusters in the gas phase explored by the use of VUV photoionization and vibrational spectroscopy.

Tetrahydropyran-2-methanol (THPM) is a typical alcohol containing a six-member cyclic ether, which can be considered as the model molecule of cyclic sugar. Herein, vacuum ultraviolet (VUV) photodissociation spectroscopy is employed to study fragmentation pathways and infrared (IR) plus VUV photoionization spectroscopy to investigate the structures of neutral THPM and its clusters with the size up to the trimer. Qualitative structural assignments are confirmed for the neutral species and ions based on MP2/aug-cc-pVTZ and ωB97X-D/cc-pVTZ calculations. The fragment cations at m/z = 84, 85, and 98 arise by the losing of CH2OH, CH3OH, and H2O from the monomer, respectively, as a result of C-C bond and C-O bond dissociation under the VUV (118 nm) radiation. It is found that the loss of CH3OH and H2O involves hydrogen transfer from the CH2 group to the dissociating CH2 and OH groups. Comparing the observed and calculated spectra of the monomer THPM, it suggests that the conformer containing a chair tetrahydropyran ring and an intramolecular hydrogen bond would be dominantly survived in a supersonic beam. Moreover, the IR spectra of larger clusters n > 1 (n = 2, 3) show only the broad hydrogen bonded OH stretch mode, and thus these larger clusters would form a closed-cyclic structure, where all OH groups are participating in hydrogen bonding. Partially the CH stretch positions of THPM clusters do not change significantly with the increasing of cluster size, thus the CH and CH2 groups are not involved in H-bonding interactions.

Effect of hydrogen bonding on the infrared absorption intensity of OH stretch vibrations

We consider how the infrared intensity of a hydrogen-bonded OH stretch varies from weak to strong H-bonds using a theoretical model. We obtain trends for the fundamental and overtone transition intensities as a function of the donor–acceptor distance, a common measure of H-bond strength. Building upon our earlier work using a two-diabatic state model, we introduce a Mecke function-based dipole moment for the H-bond and calculate transition moments using one-dimensional vibrational eigenstates along the H-atom transfer coordinate. The fundamental intensity is found to be over 20-fold enhanced for strong H-bonds, where non-Condon effects are significant. We analyse isotope effects, including the secondary geometric isotope effect. The first overtone intensity varies non-monotonically with H-bond strength; suppression occurs for weak bonds but strong enhancements are possible for strong H-bonds. We also study how these trends are affected by Mecke parameter variations. For a few specific dimers, we compare our results with earlier works.

Vibrational Coupling at the Topmost Surface of Water Revealed by Heterodyne-Detected Sum Frequency Generation Spectroscopy.

Unraveling vibrational coupling is the key to consistently interpret vibrational spectra of complex molecular systems. The vibrational spectrum of the water surface heavily suffers from vibrational coupling, which hinders complete understanding of the molecular structure and dynamics of the water surface. Here we apply heterodyne-detected sum frequency generation spectroscopy to the water surface and accomplish the assignment of a weak vibrational band located at the lower energy side of the free OH stretch. We find that this band is due to a combination mode of the hydrogen-bonded OH stretch and a low-frequency intermolecular vibration, and this combination band appears in the surface vibrational spectrum through anharmonic vibrational coupling that takes place exclusively at the topmost surface.

Capturing the Ultrafast Vibrational Decoherence of Hydrogen Bonding in Interfacial Water.

Vibrational sum-frequency generation (vSFG) measurements in the frequency and time domains reveal that the interfacial hydrogen bonded OH stretch at the water/calcium fluoride interface is composed of two populations oriented oppositely. The time-resolved vSFG free-induction decay suggested that, whereas the strongly hydrogen bonded OH vibrational stretches, centered near 3140 ± 11 cm-1, are oriented toward bulk water and lose their collective coherence within ∼70 ± 7 fs, the weakly hydrogen bonded OH species, centered near 3410 ± 12 cm-1, are pointed toward the interface and dephase within ∼50 ± 6 fs.

Efficient Spectral Diffusion at the Air/Water Interface Revealed by Femtosecond Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy.

Femtosecond vibrational dynamics at the air/water interface is investigated by time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy and molecular dynamics (MD) simulation. The low- and high-frequency sides of the hydrogen-bonded (HB) OH stretch band at the interface are selectively excited with special attention to the bandwidth and energy of the pump pulses. Narrow bleach is observed immediately after excitation of the high-frequency side of the HB OH band at ∼3500 cm(-1), compared to the broad bleach observed with excitation of the low-frequency side at ∼3300 cm(-1). However, the time-resolved spectra observed with the two different excitations become very similar at 0.5 ps and almost indistinguishable by 1.0 ps. This reveals that efficient spectral diffusion occurs regardless of the difference of the pump frequency. The experimental observations are well-reproduced by complementary MD simulation. There is no experimental and theoretical evidence that supports extraordinary slow dynamics in the high-frequency side of the HB OH band, which was reported before.

Calculations of the IR spectra of bend fundamentals of (H2O)n=3,4,5 using the WHBB_2 potential and dipole moment surfaces.

Stimulated by new experiments from the Havenith group, we report IR spectra of the bend fundamentals of (H2O)n=3,4,5, using anharmonic, coupled-mode VSCF/VCI calculations, done in a subspace of modes consisting of all the monomer bends plus the hydrogen-bonded OH stretches. Double-harmonic spectra are also reported. All calculations employ a faster version of the ab initio WHBB potential and also a more accurate representation of the dipole moment surface, reported previously. Comparisons at the harmonic level are made with previous high-level ab initio calculations, notably those of Howard and Tschumper and also with harmonic frequencies from the semi-empirical TTM3-F potential, which have been reported previously by Howard and Tschumper. The calculations provide energies and intensities of the hydrogen-bonded OH stretches and these are also presented and briefly discussed.

Probing Water and CO₂ Interactions at the Surface of Collapsed Titania Nanotubes Using IR Spectroscopy.

Collapsed titania nanotubes (cTiNT) were synthesized by the calcination of titania nanotubes (TiNT) at 650 °C, which leads to a collapse of their tubular morphology, a substantial reduction in surface area, and a partial transformation of anatase to the rutile phase. There are no significant changes in the position of the XPS responses for Ti and O on oxidation or reduction of the cTiNTs, but the responses are more symmetric than those observed for TiNTs, indicating fewer surface defects and no change in the oxidation state of titanium on oxidative and/or reductive pretreatment. The interaction of H2O and CO2 with the cTiNT surface was studied. The region corresponding to OH stretching absorptions extends below 3000 cm−1, and thus is broader than is typically observed for absorptions of the OH stretches of water. The exchange of protons for deuterons on exposure to D2O leads to a depletion of this extended absorption and the appearance of new absorptions, which are compatible with deuterium exchange. We discuss the source of this extended low frequency OH stretching region and conclude that it is likely due to the hydrogen-bonded OH stretches. Interaction of the reduced cTiNTs with CO2 leads to a similar but smaller set of adsorbed carbonates and bicarbonates as reported for reduced TiNTs before collapse. Implications of these observations and the presence of proton sources leading to hydrogen bonding are discussed relative to potential chemical and photochemical activity of the TiNTs. These results point to the critical influence of defect structure on CO2 photoconversion.

2D heterodyne-detected sum frequency generation study on the ultrafast vibrational dynamics of H2O and HOD water at charged interfaces.

Two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy is applied to study the ultrafast vibrational dynamics of water at positively charged aqueous interfaces, and 2D HD-VSFG spectra of cetyltrimethylammonium bromide (CTAB)/water interfaces in the whole hydrogen-bonded OH stretch region (3000 cm(-1) ≤ ωpump ≤ 3600 cm(-1)) are measured. 2D HD-VSFG spectrum of the CTAB/isotopically diluted water (HOD-D2O) interface exhibits a diagonally elongated bleaching lobe immediately after excitation, which becomes round with a time constant of ∼0.3 ps due to spectral diffusion. In contrast, 2D HD-VSFG spectrum of the CTAB/H2O interface at 0.0 ps clearly shows two diagonal peaks and their cross peaks in the bleaching region, corresponding to the double peaks observed at 3230 cm(-1) and 3420 cm(-1) in the steady-state HD-VSFG spectrum. Horizontal slices of the 2D spectrum show that the relative intensity of the two peaks of the bleaching at the CTAB/H2O interface gradually change with the change of the pump frequency. We simulate the pump-frequency dependence of the bleaching feature using a model that takes account of the Fermi resonance and inhomogeneity of the OH stretch vibration, and the simulated spectra reproduce the essential features of the 2D HD-VSFG spectra of the CTAB/H2O interface. The present study demonstrates that heterodyne detection of the time-resolved VSFG is critically important for studying the ultrafast dynamics of water interfaces and for unveiling the underlying mechanism.

Communication: Ultrafast vibrational dynamics of hydrogen bond network terminated at the air/water interface: A two-dimensional heterodyne-detected vibrational sum frequency generation study

Ultrafast vibrational dynamics of hydrogen bond network at the air∕water interface is revealed by two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy. Three diagonal peaks are clearly observed in the 2D HD-VSFG spectrum, which correspond to the negative and positive hydrogen-bonded OH stretch bands and the positive "free" OH stretch band in the steady-state HD-VSFG spectrum. A diagonally elongated bleaching lobe of the hydrogen-bonded OH at 0 ps indicates that it is partly inhomogeneously broadened. This diagonal elongation vanishes in a few hundred femtoseconds, implying the ultrafast spectral diffusion in the hydrogen-bonded OH band. Off-diagonal cross peaks between each OH oscillator are clearly observed instantaneously within the time resolution of 0.2 ps, suggesting that they are vibrationally coupled through anharmonicity and∕or energy transfer.

Experimental and Theoretical Investigations of Energy Transfer and Hydrogen-Bond Breaking in the Water Dimer

The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D(0)) of (D(2)O)(2). The value obtained, 1244 ± 10 cm(-1) (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm(-1) (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H(2)O)(2). In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H(2)O)(2) and (D(2)O)(2) were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment.