An a b i n i t i o calculation of the stretching energies for the HF dimer

Abstract

For the HF dimer we calculate the fundamental HF stretching vibration frequencies, and the fundamental and overtone frequencies of the intermolecular (HF–HF) stretching vibration, using an ab initio potential energy surface and the previously developed semirigid bender Hamiltonian. The ab initio surface used involves the addition of 459 nuclear geometry points to the 1061 reported in our earlier work. These extra points have been chosen to give more information on those parts of the surface that involve distortions of the HF bond lengths. We have fitted these 1520 points to an analytic expression, slightly modified from our previous work, that involves 39 adjustable parameters and one constrained parameter; the weighted standard deviation of the fit is 29.3 cm−1. To calculate the vibrational frequencies, and the tunneling splittings in these vibrationally excited states, we use the semirigid bender Hamiltonian to average over the trans-tunneling path. We also calculate from the ab initio surface the effect of the adiabatic corrections, for the HF stretching states, that arise from the separation of the tunneling mode. In the adiabatically corrected calculation we obtain ν1=3926 cm−1, and ν2=3875 cm−1, which are in good agreement with the experimental results (3930.9 and 3868.1 cm−1, respectively). We also predict ν4=146 cm−1, 2ν4=280 cm−1, and 3ν4=405 cm−1. The value obtained for ν4 enables us to explain the observed perturbation of the lower tunneling component of the K=4 ground state level as being due to interaction with the lower tunneling component of the ν4=1, K=3 level.

Partially coupled electrical model of vibrational frequency shifts in weak atom-diatomic and diatomic-diatomic complexes

“…With an effective potential to include anharmonic effects, they obtained a difference in the monomer frequencies of 52.0 cm-l (from w1 = 3926.0 and 0 2 = 3874.0). Among the results in the extensive study by Quack and SuhmSo were H F monomer stretching frequencies obtained from one-dimensional cuts through the potential surface of Bunker et al 51 In this particular set of results, the difference in the two monomer stretches was 27 cm-1 when the slices followed the H-F stretching coordinates but 113 cm-l when they followed normal coordinates. These calculations of vibrational frequencies from full ab initio potential surfaces do not correspond to the treatment of our model; however, they suggest that the tendency of our model to make the monomers slightly more equivalent than they are is a consequence of the neglect of coupling, both harmonic and anharmonic, between the monomer stretches.…”

Double Tunneling: An Overlooked Quantum Effect in Anionic Molecular Clusters

“…It is very diffuse and is located at the positive end of the cluster dipole, i.e., close to the dangling hydrogen(s). Because of a weak excess electron binding, the structure of the neutral dimer changes only moderately upon electron attachment [17].Tunneling in neutral hydrogen-bonded dimers is a well-known phenomenon confirmed by both terahertz spectroscopy and quantum calculations, e.g., for water and hydrogen fluoride dimers [7,[18][19][20][21][22][23][24][25]. What happens, however, with the tunneling process if an excess electron is attached to these dimers?…”

“…As a matter of fact, much more attention has been paid to its neutral precursor-the hydrogen fluoride dimer. Numerous experimental [19][20][21] and theoretical [22][23][24][25][26][27][28] studies have been devoted to the elucidation of the structure and the energetics of this system. High quality potential energy surfaces exist for the neutral dimer [27,28] and vibrational calculations in full dimensionality have been performed providing spectroscopic data, in very good agreement with experiment [22].…”

“…Therefore, we have used this method and basis set for an extensive scan of the ͑HF͒ 2 2 potential energy surface [29]. Potentials and structures along the tunneling coordinate [24], which is a minimum energy path connecting the two equivalent minima of ͑HF͒ 2 2 via a corresponding saddle point, are shown in Fig. 1.…”

Spectrum and vibrational predissociation of the HF dimer. I. Bound and quasibound states

“…[21][22][23][24] On the theoretical side, the dimer has also been studied in a variety of ways, using quantum Monte Carlo methods, [25][26][27] four-dimensional rigid rotor 28 -31 and full six-dimensional ͑6D͒ bound state calculations, [32][33][34][35][36][37] as well as vibrational predissociation calculations. 33,[37][38][39] In the theoretical work, several potential energy surfaces ͑PESs͒ have been used, 25,[40][41][42] 43 which is based on explicitly correlated second-order Møller-Plesset calculations, and which is adjusted to reproduce the experimental dissociation energy and monomer stretch frequencies. This potential has been used to describe the dimer interactions in He n (HF) 2 clusters 44 and the HF trimer, 45 but so far no rigorous test of this potential for the dimer proper has been published.…”