The infrared absorption spectrum of the protonated water dimer (H 5 O

The dynamics and infrared absorption spectrum of the protonated water dimer are reported by full quantum simulation. Strong couplings between the spectroscopically active proton-transfer motion and low-frequency, large-amplitude torsional modes are clearly identified and their role in the cluster dynamics is explained. These couplings are responsible for the characteristic doublet-peak around 1000 cm-1, which was not understood and subject of debate. This spectral feature is reproduced, assigned and explained.Comment: 7 pages, 5 figures, accepted version in Angewandte Chemie. The content is the same as in older versions, the style was adapted to match the requirements of Ange. Che

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Quantum-dynamical full-dimensional (15D) calculations are reported for the protonated water dimer (H5O2+) using the multiconfiguration time-dependent Hartree (MCTDH) method. The dynamics is described by curvilinear coordinates. The expression of the kinetic energy operator in this set of coordinates is given and its derivation, following the polyspherical method, is discussed. The potential-energy surface (PES) employed is that of Huang et al. [J. Chem. Phys. 122, 044308 (2005)]. A scheme for the representation of the PES is discussed which is based on a high-dimensional model representation scheme, but modified to take advantage of the mode-combination representation of the vibrational wave function used in MCTDH. The convergence of the PES expansion used is quantified and evidence is provided that it correctly reproduces the reference PES at least for the range of energies of interest. The reported zero point energy of the system is converged with respect to the MCTDH expansion and in excellent agreement (16.7 cm(-1) below) with the diffusion Monte Carlo result on the PES of Huang et al. The highly fluxional nature of the cation is accounted for through use of curvilinear coordinates. The system is found to interconvert between equivalent minima through wagging and internal rotation motions already when in the ground vibrational state, i.e., T=0. It is shown that a converged quantum-dynamical description of such a flexible, multiminima system is possible.

Ab initio calculations on the six-dimensional cis--trans double minimum potential energy surface of the electronic ground state of the HONO molecule were performed using a coupled cluster approach. An analytic fit to the data points was established. The interconversion barrier was calculated to be 4105 cm(-1). The nuclear motion problem was solved variationally using a full six-dimensional Hamiltonian in internal coordinates. The eigenstates up to about 3650 cm(-1) were tentatively assigned by harmonic quantum numbers. The assignment was based on the mean values of the internal coordinates of the six-dimensional eigenfunctions and on a comparison of the eigenenergies with those calculated by second-order perturbation theory from a full quartic force field in dimensionless normal coordinates. In cold matrices the trans- and the cis-OH nu(1) stretching modes and the first trans- and cis-NO 2nu(2) stretching overtones lead to isomerization. In the isolated molecule these modes (J=0) were found to be entirely localized. However, several overtones of the nu(4) ONO bending and nu(5) N-O stretching, which are close in energy to the OH stretch and combined with the torsional mode, were found to be strongly cis-trans delocalized.

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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.