VIBRATIONAL SHIFTS IN p-DICHLOROBENZENE - RARE GAS VAN DER WAALS COMPLEXES

Author Institution: Department of Chemistry, University of Pittsburgh; Department of Chemistry, Virginia Commonwealth UniversityFluorescence excitation spectra have been obtained for jet-cooled $p-C_{6}H_{4}Cl_{2}$ and $p-C_{6}D_{4}Cl_{2}$ and their van der Waals complexes with Ar and Kr. The van der Waals shifts for the orgins of the Ar and Kr complexes are the same for both $p-C_{6}H_{4}Cl_{2}$ and $p-C_{6}D_{4}Cl_{2}$: however, the van der Waals shifts for vibronic transitions in the complexes are isotopically dependent. The vibrationally dependent van der Waals shifts are largest for $16a^{2}_{0}$: this appears to be a general property of complexes of rare gases with aromatic systems. The effect of vibrational excitation of $\nu_{16a}$ of p-dichlorobenzene on the van der Waals potential is modeled by using the results of ab initio normal mode calculations and atom-atom pair potentials. The model calculations are not in quantitiative agreement with experiment, but they correctly predict that for $16a^{2}_{0}$ the vibrationally dependent shift is larger for Kr complexes than for Ar complexes, and, for a given rare gas atom, the vibrationally dependent shift is larger for $p-C_{6}H_{4}C1_{2}$ complexes than for $p-C_{6}D_{4}Cl_{2}$ complexes. In addition, the model calculations lend support to out tentative assignment of $16a^{2}_{0}$ in $p-C_{6}D_{4}Cl_{1}$ - He