Hindered internal rotation and o r t h o-H2 enrichment in t r a n s-stilbene–H2/D2 complexes

Abstract

A supersonic free jet expansion has been used to prepare trans-stilbene–H2 and D2 complexes. The cooling in the jet collapses most of the ortho and para H2 and D2 rotational population to the lowest rotational levels of a given nuclear spin symmetry: j=0 and j=1. The laser-induced fluorescence excitation spectrum of stilbene–D2 shows a well-resolved doublet at the origin due to stilbene–D2( j=0) and stilbene–D2( j=1) complexes. The 4.9 cm−1 splitting of these transitions indicates that the D2 molecule is undergoing hindered internal rotation in the complex and that the barrier to internal rotation changes upon electronic excitation. The relative intensities of the stilbene–D2( j=0) and stilbene–D2( j=1) origins depend on the D2 concentration in the jet. At low D2 flows the transitions arising from stilbene–D2( j=1) are favored while at high D2 flows the ( j=0)/(j=1) transition intensities approach the 2:1 intensity ratio given by their nuclear spin statistical weights. By contrast, in stilbene–H2 we observe only a single transition at the origin which we assign to stilbene–H2( j=1). We are able to place an upper bound on the stilbene–H2( j=0) transition intensity of 5% of the stilbene–H2( j=1) intensity. Dispersed fluorescence spectra are used to bracket the binding energies of the stilbene–H2/D2 complexes in both ground and excited states. In the ground state of stilbene–H2( j=1) and stilbene–D2( j=0,1) complexes, 169≤D′0 ≤249 cm−1, so that the stilbene–H2( j=0) transitions cannot be missing by virture of the complex being unbound. We propose a simple kinetic scheme which supports the possibility that selective exchange reactions, in which more strongly bound H2( j=1) preferentially displaces H2( j=0) from stilbene, contribute to the enrichment of stilbene–H2( j=1) complexes in the jet. We also propose that H2( j=2) molecules may contribute to the suppression of stilbene–H2( j=0) complexes by virture of the fact that the H2( j=2) level is above the dissociation threshold for the stilbene–H2 complex while the D2( j=2) level is likely not.