Observation of the Low‐Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces (original) (raw)
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Spectroscopic determination of the water dimer intermolecular potential-energy surface
The Journal of Chemical Physics, 2002
Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP ͑anisotropic site potential͒ potential form to microwave, terahertz, and midinfrared cavity ringdown (D 2 O) 2 spectra via a rigorous calculation of the water dimer eigenstates with the PSSH ͑pseudo-spectral split Hamiltonian͒ method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H 2 O and D 2 O. The calculated dimer structure and dipole moment are close to those determined from microwave spectroscopy and high level ab initio calculations, except that the O-O distance ͑2.952 Å͒ is significantly shorter than the currently accepted experimental value. The dimer binding energy ͑4.85 kcal/mol͒ is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor-acceptor interchange tunneling barriers and the cyclic water trimer and tetramer structures and binding energies.
Terahertz laser spectroscopy of the water dimer intermolecular vibrations. I. (D2O)2
The Journal of Chemical Physics, 2000
Terahertz laser VRT spectra of the water dimer consisting of 731 transitions measured with an average precision of 2 MHz and involving four ͑D 2 O͒ 2 intermolecular vibrations ͑one previously published͒ have been measured between 65 and 104 cm Ϫ1. The precisely determined energy level patterns differ both qualitatively and quantitatively from the predictions of several dimer potentials tested, and reveal an ordering of the intermolecular vibrations which differs dramatically from that predicted by standard normal mode analysis. Strong coupling is indicated between the low barrier tunneling motions and the intermolecular vibrations as well as among different vibrations. Particularly, the 83 cm Ϫ1 ͑acceptor wag͒ and 90 cm Ϫ1 ͑D 2 O͒ 2 ͑acceptor twist͒ vibrations interact through a Coriolis perturbation. These spectra provide the basis for our recent determination of the water pair potential. The corresponding data set for ͑H 2 O͒ 2 is presented in an accompanying paper.
Spectroscopic Determination of the Water Pair Potential
Science, 1999
A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D 2 O) 2 spectra through a rigorous calculation of the water dimer eigenstates. It accurately reproduces most ground state vibration-rotation-tunneling spectra and yields excellent second viral coefficients. The calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations. The dimer binding energy and acceptor switching and donor-acceptor interchange tunneling barriers are in excellent agreement with recent ab initio theory, as are cyclic water trimer and tetramer structures and binding energies.
Determination of a flexible (12D) water dimer potential via direct inversion of spectroscopic data
The Journal of Chemical Physics, 2002
We report the determination of two dimer water potential energy surfaces via direct inversion of spectroscopic data. The first surface, rigid, employs the MCY functional form originally fitted by Clementi and co-workers from ab initio calculations, modified by adjunction of a fifth, uncharged, site to improve the dispersion component. The vibration-rotation-tunneling energy levels were computed by means of the pseudospectral split Hamiltonian method that we developed previously. The fitted surface shows considerable improvement as compared to the original one: transitions among the ground-state manifold are in error by at most 0.2 cm Ϫ1 , and excited state band origins ͑up to 150 cm Ϫ1) are reproduced to within 0.5 to 3 cm Ϫ1. For the second surface, flexible, we used the same modified MCY functional form, considered now to depend on the 12 internal degrees of freedom, and augmented by the monomer potential energy terms. The water dimer is described in its full dimensionality by collision-type coordinates in order to access the whole configuration sampled by this floppy system. Internal motions of the monomers ͑stretches and bends͒ are explicitly considered by invoking an adiabatic separation between the slow ͑intermonomeric͒ and fast ͑intramonomeric͒ modes. This (6ϩ6)d adiabatic formulation allows us to recast the calculations into an equivalent six-dimensional dynamics problem (ϳpseudorigid monomers͒ on an effective potential energy surface. The resulting, fitted, fully flexible dimer potential leads to a much better agreement with experiment than does the rigid version, as examplified by the standard deviation on all observed frequencies being reduced by a factor of 3. It is shown that monomer flexibility is essential in order to reproduce the experimental transitions.
Molecular Physics, 2003
We report the observation of extensive a-and c-type rotation-tunnelling (RT) spectra of (H 2 O) 2 for K a ¼ 0-3, and (D 2 O) 2 for K a ¼ 0-4. These data allow a detailed characterization of the vibrational ground state to energies comparable to those of the low-lying (70-80 cm À1 ) intermolecular vibrations. We present a comparison of the experimentally determined molecular constants and tunnelling splittings with those calculated on the VRT(ASP-W)III, SAPT-5st, and VRT(MCY-5f) intermolecular potential energy surfaces. The SAPT-5st potential reproduces the vibrational ground state properties of the water dimer very well. The VRT(MCY-5f) and especially the VRT(ASP-W)III potentials show larger disagreements, in particular for the bifurcation tunnelling splitting.
IR spectroscopy of pyridine–water structures in helium nanodroplets
Physical Chemistry Chemical Physics, 2014
We present the results of an IR spectroscopic study of pyridine-water heterodimer formation in helium nanodroplets. The experiments were carried out in the frequency range of the pyridine C-H stretch region (3055-3100 cm À1) and upon water deuteration in the DO stretch region (2740-2800 cm À1). In order to come to an unambiguous assignment we have determined the angle between the permanent dipole and the vibrational transition moment of the aggregates. The experiments have been accompanied by theoretical simulations which yielded two minimum structures with a 16.28 kJ mol À1 energy difference. The experimentally observed bands were assigned to two structures with different H-bonds: an NÁ Á ÁH bond and a bifurcated OÁ Á ÁH-C bond.
Local vibrational modes of the water dimer – Comparison of theory and experiment
Chemical Physics Letters, 2012
Local and normal vibrational modes of the water dimer are calculated at the CCSD(T)/CBS level of theory. The local H-bond stretching frequency is 528 cm À1 compared to a normal mode stretching frequency of just 143 cm À1. The adiabatic connection scheme between local and normal vibrational modes reveals that the lowering is due to mass coupling, a change in the anharmonicity, and coupling with the local HOH bending modes. The local mode stretching force constant is related to the strength of the H-bond whereas the normal mode stretching force constant and frequency lead to an erroneous underestimation of the Hbond strength.
A new ab initio ground-state dipole moment surface for the water molecule
The Journal of Chemical Physics, 2008
A valence-only ͑V͒ dipole moment surface ͑DMS͒ has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation ͑C͒ and relativistic ͑R͒ effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H 2 16 O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539Ϯ 0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546͑6͒ D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge ͓J. Chem. Phys. 113, 16 ͑2000͔͒. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces ͑PESs͒ has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions.
Chemical Physics, 1983
The dipole-moment derivatives and infrared-absorption intensities of the water dimer including scveml det~terxed 3pscis.s have been calculated using sb initio SCF techniques. The resuhs xs compsrrd \\ith ths :malogous qtuntirirrs for monomeric w;lter. In addition to the highly enhanced intensity of the immmolsculnr OH zmxch mew inrem~olaxdar modes that oc'cur in the 90400 cm-' region are also found to be very intense. An electrostatic model for the w:tter dimsr ha been explored with I view to devrtoping a possible scheme for the cltlculntion of infrared intensities of Ixger clusters. AS ;L rsa~lt of the significrtnt exchange and charge-transfer effects such a model is found to hs unrsliablr in dacrihing thi: drpok-mamat derivatives that directly involve the hydrogen bond.
The Journal of chemical physics, 2015
We report the first high-resolution spectra for the out-of-plane librational vibration in the water dimer. Three vibrational subbands comprising a total of 188 transitions have been measured by diode laser spectroscopy near 500 cm(-1) and assigned to (H2O)2 libration-rotation-tunneling eigenstates. The band origin for the Ka = 1 subband is ˜524 cm(-1). Librational excitation increases the interchange and bifurcation hydrogen bond rearrangement tunneling splittings by factors of 3-5 and 4-40, respectively. Analysis of the rotational constants obtained from a nonlinear least squares fit indicates that additional external perturbations to the energy levels are likely.