Potential energy surfaces, quasiadiabatic channels, rovibrational spectra, and intramolecular dynamics of (HF)2 and its isotopomers from quantum Monte Carlo calculations

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Abstract
We report analytical representations of the six‐dimensional potential energy hypersurface for (HF)2, the parameters of which are closely adjusted to low energy experimental properties such as hydrogen bond dissociation energy (D0=1062 cm−1 ) and vibrational–rotational spectra in the far and mid infrared. We present a detailed analysis of properties of the hypersurface in terms of its stationary points, harmonic normal mode amplitudes, and frequencies for the Cs minimum and C2h saddle point and effective Morse parameters and anharmonic overtone vibrational structure for the hydrogen bond and the HF stretching vibrations. The comparison between experimental data and the potential energy surface is carried out by means of accurate solutions of the rotational–vibrational Schrödinger equation with quantum Monte Carlo techniques, which include anharmonic interactions between all modes for the highly flexible dimer. Two extensions of the quantum Monte Carlo technique are presented, which are based on the clamped coordinate quasiadiabatic channel method and allow for the approximate calculation of excited rotational and vibrational levels. Predictions include dissociation energies D0 for isotopomers (XF)2 with X=μ, D, T (D0=477; 1169; 1217 cm−1 ). Unusual anharmonic isotope effects predicted for the out‐of‐plane bending fundamental ν6 [378; 276; 295; and 358 cm−1 for (HF)2, (DF)2, (HFDF), (DFHF)] can be understood in simple terms. Centrifugal effects both for the high frequency a‐axis rotation and low frequency c‐axis rotation are accurately calculated for the vibrational ground state and some excited states, with a best equilibrium center of mass distance Req.ab=5.14a0 between the HF monomers. A very large anharmonic interaction constant x46≊−16 cm−1 is predicted for the hydrogen bond vibration ν4 and for out‐of‐plane bending ν6. This leads to assignment of our earlier experimental observation of a band at 383 cm−1 as ν6+ν4−ν4(K=1←0) at almost exactly the predicted position. The fundamental ν4 is predicted at 130±10 cm−1. A new, indirect assignment of our experimental data gives ν4≊125 cm−1. Monte Carlo calculations are presented for quasiadiabatic channels and transition states for hydrogen bond dissociation. We present a discussion of symmetry correlations for these channels and symmetry effects in predissociation by rotation, nuclear spin symmetry, and parity violation. Large effects from zero point energy on the three‐dimensional quantum centrifugal barriers for rotational predissociation are found. On the basis of the new data, a much improved statistical mechanical estimate for the equilibrium 2HF=(HF)2 is obtained.