Vibrational predissociation of the phenol–water dimer: a view from the water (original) (raw)
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The Journal of Physical …, 2010
The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl-H 2 O dimer were studied following excitation of the HCl stretch of the dimer. Velocity-map imaging and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, HCl fragments were detected by 2 + 1 REMPI via the f 3 ∆ 2 r X 1 Σ + and V 1 Σ + r X 1 Σ + transitions. REMPI spectra clearly show HCl from dissociation produced in the ground vibrational state with J′′ up to 11. The fragments' center-of-mass translational energy distributions were determined from images of selected rotational states of HCl and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well when using a dimer dissociation energy of D 0 ) 1334 ( 10 cm -1 . The rotational distributions in the water cofragment pair-correlated with specific rotational states of HCl appear nonstatistical when compared to predictions of the statistical phase space theory. A detailed analysis of pair-correlated state distributions was complicated by the large number of water rotational states available, but the data show that the water rotational populations increase with decreasing translational energy. † Part of the "Reinhard Schinke Festschrift".
The Journal of chemical …, 2011
The bond dissociation energy (D 0 ) of the water dimer is determined by using state-to-state vibrational predissociation measurements following excitation of the bound OH stretch fundamental of the donor unit of the dimer. Velocity map imaging and resonance-enhanced multiphoton ionization (REMPI) are used to determine pair-correlated product velocity and translational energy distributions. H 2 O fragments are detected in the ground vibrational (000) and the first excited bending (010) states by 2 + 1 REMPI via theC 1 B 1 (000) ←X 1 A 1 (000 and 010) transitions. The fragments' velocity and center-of-mass translational energy distributions are determined from images of selected rovibrational levels of H 2 O. An accurate value for D 0 is obtained by fitting both the structure in the images and the maximum velocity of the fragments. This value, D 0 = 1105 ± 10 cm −1 (13.2 ± 0.12 kJ/mol), is in excellent agreement with the recent theoretical value of D 0 = 1103 ± 4 cm −1 (13.2 ± 0.05 kJ/mol) suggested as a benchmark by Shank et al.
Imaging H2O Photofragments in the Predissociation of the HCl− H2O Hydrogen-Bonded Dimer
Journal of Physical …, 2011
The state-to-state vibrational predissociation (VP) dynamics of the hydrogenbonded HCl-H 2 O dimer was studied following excitation of the dimer's HCl stretch by detecting the H 2 O fragment. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, H 2 O fragments were detected by 2 þ 1 REMPI via the C 1 B 1 (000) r X 1 A 1 (000) transition. REMPI spectra clearly show H 2 O from dissociation produced in the ground vibrational state. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational states of H 2 O and were converted to rotational state distributions of the HCl cofragment. The distributions were consistent with the previously measured dissociation energy of D 0 = 1334 ( 10 cm -1 and show a clear preference for rotational levels in the HCl fragment that minimize translational energy release. The usefulness of 2 þ 1 REMPI detection of water fragments is discussed.
Imaging the State-Specific Vibrational Predissociation of the Ammonia-Water Hydrogen-Bonded Dimer
The journal of physical …, 2009
The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-water dimer were studied following excitation of the bound OH stretch. Velocity-map imaging (VMI) and resonanceenhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the bound OH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B 1 E′′ r X 1 A 1 ′ transition. The REMPI spectra show that NH 3 is produced with one and two quanta of the symmetric bend (ν 2 umbrella mode) excitation, as well as in the ground vibrational state. Each band is quite congested, indicating population in a large number of rotational states. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with zero, one, or two quanta in ν 2 and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well by using a dimer dissociation energy of D 0 ) 1538 ( 10 cm -1 . The rotational state distributions in the water cofragment pair-correlated with specific rovibrational states of ammonia are broad and include all the J KaKc states allowed by energy conservation. The rotational populations increase with decreasing c.m. translational energy. There is no evidence for ammonia products with significant excitation of the asymmetric bend (ν 4 ) or water products with bend (ν 2 ) excitation. The results show that only restricted pathways lead to predissociation, and these do not always give rise to the smallest possible translational energy release, as favored by momentum gap models. Figure 1. Equilibrium geometry of the ammonia-water dimer (ref. 32). R(N-O) ) 2.989 Å; θ N ) 23.1°; θ O ) 49.
Imaging bond breaking and vibrational energy transfer in small water containing clusters
CPL Frontiers, 2013
This letter presents a brief overview of our recent experimental studies of state-to-state vibrational predissociation (VP) dynamics of small hydrogen bonded (H-bonded) clusters following vibrational excitation. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) are used to determine accurate bond dissociation energies (D 0 ) of (H 2 O) 2 , (H 2 O) 3, HCl-H 2 O and NH 3 -H 2 O. Pair-correlated product energy distributions from the VP of these complexes are also presented and compared to theoretical models. Further insights into mechanisms are obtained from the recent quasi-classical trajectory (QCT) calculations of Bowman and coworkers. The D 0 values for (H 2 O) 2 and (H 2 O) 3 are in very good agreement with recent calculated values, and the results are used to estimate the contributions of cooperative interactions to the H-bonding network.
State-resolved spectroscopy of high vibrational levels of water up to the dissociative continuum
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012
We summarize here our experimental studies of the high rovibrational energy levels of water. The use of double-resonance vibrational overtone excitation followed by energy-selective photofragmentation and laser-induced fluorescence detection of OH fragments allowed us to measure previously inaccessible rovibrational energies above the seventh OH-stretch overtone. Extension of the experimental approach to triple-resonance excitation provides access to rovibrational levels via transitions with significant transition dipole moments (mainly OH-stretch overtones) up to the dissociation threshold of the O–H bond. A collisionally assisted excitation scheme enables us to probe vibrations that are not readily accessible via pure laser excitation. Observation of the continuous absorption onset yields a precise value for the O–H bond dissociation threshold, 41 145.94 ± 0.15 cm −1 . Finally, we detect long-lived resonances as sharp peaks in spectra above the dissociation threshold.
J. Am. Chem. Soc. , 2012
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.
Hydrogen-Bond Disruption by Vibrational Excitations in Water
Journal of Physical Chemistry A, 2007
An excitation of the OH-stretch ν OH of water has unique disruptive effects on the local hydrogen bonding. The disruption is not an immediate vibrational predissociation, which is frequently the case with hydrogenbonded clusters, but instead is a delayed disruption caused by a burst of energy from a vibrationally excited water molecule. The disruptive effects are the result of a fragile hydrogen-bonding network subjected to a large amount of vibrational energy released in a short time by the relaxation of ν OH stretching and δ H 2 O bending excitations. The energy of a single ν OH vibration distributed over one, two, or three (classical) water molecules would be enough to raise the local temperature to 1100, 700, or 570 K, respectively. Our understanding of the properties of the metastable water state having this excess energy in nearby hydrogen bonds, termed H 2 O*, has emerged as a result of experiments where a femtosecond IR pulse is used to pump ν OH , which is probed by either Raman or IR spectroscopy. These experiments show that the H 2 O* spectrum is blue-shifted and narrowed, and the spectrum looks very much like supercritical water at ∼600 K, which is consistent with the temperature estimates above. The H 2 O* is created within ∼400 fs after ν OH excitation, and it relaxes with an 0.8 ps lifetime by re-formation of the disrupted hydrogen-bond network. Vibrationally excited H 2 O* with one quantum of excitation in the stretching mode has the same 0.8 ps lifetime, suggesting it also relaxes by hydrogen-bond re-formation.
J. Phys. Chem. A, 2013
We report a joint experimental-theoretical study of the predissociation dynamics of the water trimer following excitation of the hydrogen bonded OH-stretch fundamental. The bond dissociation energy (D 0 ) for the (H 2 O) 3 → H 2 O + (H 2 O) 2 dissociation channel is determined from fitting the speed distributions of selected rovibrational states of the water monomer fragment using velocity map imaging. The experimental value, D 0 = 2650 ± 150 cm −1 , is in good agreement with the previously determined theoretical value, 2726 ± 30 cm −1 , obtained using an ab initio full-dimensional potential energy surface (PES) together with Diffusion Monte Carlo calculations [Wang; Bowman. J. Chem. Phys. 2011, 135, 131101]. Comparing this value to D 0 of the dimer places the contribution of nonpairwise additivity to the hydrogen bonding at 450−500 cm −1 . Quasiclassical trajectory (QCT) calculations using this PES help elucidate the reaction mechanism. The trajectories show that most often one hydrogen bond breaks first, followed by breaking and re-forming of hydrogen bonds (often with different hydrogen bonds breaking) until, after many picoseconds, a water monomer is finally released. The translational energy distributions calculated by QCT for selected rotational levels of the monomer fragment agree with the experimental observations. The product translational and rotational energy distributions calculated by QCT also agree with statistical predictions. The availability of low-lying intermolecular vibrational levels in the dimer fragment is likely to facilitate energy transfer before dissociation occurs, leading to statistical-like product state distributions.
Dynamics and Infrared Spectroscopy of the Protonated Water Dimer
Angewandte Chemie International Edition, 2007
Accurate infrared spectroscopy of protonated water clusters that are prepared in the gas phase has become possible in recent years, thus opening the door to a deeper understanding of the properties of aqueous systems and the hydrated proton, which are of high interest in central areas of chemistry and biology. Several computational studies have appeared in parallel, providing a necessary theoretical basis for the assignment and understanding of the different spectral features. It has been recently demonstrated that the H 5 O 2 + motif, also referred to as the Zundel cation, plays an important role in protonated water clusters of six or more water molecules and, together with the Eigen cation (H 9 O 4 + ), as a limiting structure of the hydrated proton in bulk water. 11] The importance of the hydrated proton and the amount of work devoted to the problem contrast with the fact that the smallest system in which a proton is shared between water molecules, H 5 O 2 + , is not yet completely understood, and an explanation of the most important spectral signatures and the associated dynamics of the cluster is lacking.