Rotational Structure of Small ⁴He Clusters Seeded with HF, HCl, and HBr Molecules

Abstract

Diffusion Monte Carlo calculations are performed for ground and excited rotational states of HX(4He)N, complexes with N<or=20 and X=F, Cl, Br. The calculations are done using ab initio He-HX intermolecular potentials whose computation is described. Intermolecular energies and He radial and angular probability density distributions are computed as a function of the number of solvent atoms. Excited states are calculated using fixed-node diffusion Monte Carlo methods, and molecule-solvent angular momentum coupling is studied as a function of cluster size and potential anisotropy. Nodal surfaces of the many-body wave function are computed approximately by making an adiabatic Born-Oppenheimer-like separation of radial and angular degrees of freedom of the cluster. This procedure is extended to include radial dependencies in the adiabatic nodal function. We predict that the observed decrease in the gas-phase rotational constants for HCl and HBr in a 4He nanodroplet will be smaller than that observed for HF, despite HF's having the largest (by far) gas-phase rotational constant of the three molecules. This suggests that the specifics of the solvation dynamics of a molecule in a 4He cluster are the result of a delicate interplay between the magnitude of the gas-phase rotational constant of the molecule and the anisotropic contributions to the atom-molecule potential energy.

Chaos in the classical mechanics of bound and quasi-bound HX–4He complexes with X = F, Cl, Br, CN

“…They concluded that it is the strength and anisotropy of the He-molecule interaction, rather than the gas phase rotational constant itself, that is mainly responsible for the reduction of the rotational constant in the nanodroplet regime. A more recent study 7 of HX-He complexes with X = F, Cl, and Br obtained results in agreement with the conclusions of ref. 16.…”

“…All calculations described in this paper assume pairwise molecule-He interaction potentials. The PESs for the HF-He, HCl-He and HBr-He dimers have been described elsewhere 7 and consist of the following expansion in Legendre polynomials…”

“…The study of the quantum and classical dynamics of very floppy van der Waals complexes such as HCl-Ar continues to be, despite its long history, an area of active research. [1][2][3][4][5][6][7][8] In particular, these systems can be thought of as laboratories for the study of quantum chaos. 9 However, the classical and semiclassical behavior of the weakest possible van der Waals molecules, i.e., those involving He atoms, e.g., HF- 4 He, have been studied much less.…”

“…A reasonable working definition of quantum solvation is the following: as molecules dissolved in 4 He droplets become surrounded by an increasing number of solvent atoms, angular momentum coupling with the solvent atoms eventually saturates. 6,7 Normally, a molecule dissolved in a liquid is not expected to rotate freely as it might in the gas-phase. However, in an ultra-cold (T E 0.37 K) droplet of 4 He atoms, the molecule appears to rotate freely but with a different spectroscopic constant-typically the gas phase rotational constant, B 0 , seems to decrease.…”

Complexes of helium with neutral molecules: Progress toward a quantitative scale of bonding character

“…We first compared our predicted geometries and interaction energies with the accurate values available from the literature for the majority of the presently investigated He(M) systems (the only previously unreported are the HeClR, R = CN, CCH, OH, SH, CH 3 , SiH 3 ). With respect to these benchmark data (listed in Table S3), typically obtained at the CCDS(T) or SAPT levels of theory with very large basis sets, the bond distances quoted in Figure , and the listed in Table feature a mean unsigned deviation (MUD), respectively, of 0.0908 a 0 , and 1.20 cm −1 .…”