Electronic-Rotational Coupling in Cl–para-H2 Van der Waals Dimers

We examine the interaction between an open-shell chlorine atom and a para-H2 molecule in the region of configuration space that corresponds to a weakly bound Cl–para-H2 van der Waals dimer. By constructing and diagonalizing the Hamiltonian matrix that represents the coupled Cl atom electronic and H2 rotational degrees of freedom, we obtain one-dimensional energy curves for the Cl–para-H2 system in this region of configuration space. We find that the dimer exhibits fairly strong electronic-rotational coupling when the Cl–H2 distance R is close to ; however, this coupling does not modify substantially the positions and depths of the van der Waals wells in the dimer’s curves. An approximation in which the para-H2 fragment is treated in the strict limit thus appears to yield an accurate representation of those states of the weakly bound Cl–para-H2 dimer that correlate with H2 in the limit. 1. Introduction Experimental studies [1] of the infrared absorption spectra of solid para-H2 matrices that contain chlorine atoms as substitutional impurities indicate that Cl-H2 interactions raise the transition energy associated with the spin-orbit (SO) transitions of the Cl impurities. In these systems, the H2 molecules in the Cl atom’s first “solvation shell” reside in the van der Waals region of the Cl-H2 potential energy surface [2]. A detailed analysis of the matrix-induced blue shift of the SO transition for Cl atoms embedded in solid para-H2 would thus provide insight into the shape of the Cl-H2 potential energy surface in this region of configuration space. This in turn could help us better understand the dynamics of the Cl + H2 → HCl + H reaction, which has long been considered a benchmark system in chemical reaction dynamics [3]. For example, theoretical studies [4] of the HCl/DCl product branching ratio of the Cl + HD reaction suggest that the van der Waals region of the potential energy surface plays a key role in controlling the reaction’s dynamics at low collision energies. In the para-H2 matrix, the Cl atom’s SO transition is blue shifted by about 60？cm？1 by Cl-H2 interactions [1], which amounts to a shift of about 5？cm？1 for each of the twelve H2 molecules in the Cl atom’s first solvation shell. The matrix-induced blue shift of the Cl SO transition can be qualitatively understood as arising from subtle differences in the van der Waals interactions of the ground and excited SO states of the Cl atom with nearby H2 molecules. For a simulation to reproduce quantitatively the observed blue shift, we might therefore anticipate that the potential energy