THE SPECTROSCOPY AND DYNAMICS OF PI-HYDROGEN-BONDED COMPLEXES - BENZENE-HCL/DCL AND TOLUENE-HCL/DCL

The benzene-HCl/DCl and toluene-HCl/DCl complexes have been studied using both fluorescence and multiphoton ionization detection. These complexes are prototypical of π hydrogen-bonded complexes involved in the chemically important process of electrophilic aromatic attack. Laser-induced fluorescence (LIF) etalon scans of the 601 rotational band contour are used to determine the S1 state geometry of the benzene-HCl as one in which HCl is on the sixfold axis with a center-of-mass separation of 3.64±0.03 Å. The lack of significant van der Waals' intensity points to the complex having a hydrogen-bonded geometry similar to that found in the ground state. Dispersed fluorescence scans are used to put crude bounds on the S0 and S1 binding energies of the benzene-HCl complex of 1.8≤D″0 ≤ 3.8 kcal/mol and 1. 5≤D′0≤3.5 kcal/mol. The fluorescence lifetimes of bound levels of the complexes are factors of 7-12 times shorter than the corresponding levels of the free molecules. In contrast, the C6H 6-CH3Cl complex, which has a similar geometry and binding energy, has a fluorescence lifetime nearly as long as C6H 6. We argue that the differences observed are consequences of the hydrogen bonding interactions present in benzerte-HCl and toluene-HCl. Two-color multiphoton ionization experiments in which the delay between the S 0-S1 laser and the ionization lasers is continuously scanned give evidence that the hydrogen bonding interactions lead to enhanced intersystem crossing to the triplet state. One-color RE2PI scans show that fragmentation of the [benzene-HCl]+ and [toluene-HCl]+ ionic complexes proceeds nearly quantitatively (≥98%) from the hydrogen-bonded S1 state. This fragmentation occurs by virtue of the repulsive geometry formed for the ionic complex in Franck-Condon excitation from the S1 state. © 1990 American Institute of Physics.