|(a) Dynamics of vibrational predissociation in aromatic-noble gas complexes:
i. p-aminophenol-argon van der Waal’s complex: Resonantly enhanced multiphoton ionization and dispersed fluorescence studies, as well as ab initio calculations on the van der Waal’s complex of p-aminophenol with argon were carried out. From the absence of features in the REMPI spectrum beyond 6a, the binding energy of the complex was truncated between 440 and 482 cm-1 for the excited state and between 380-450 cm-1 for the ground state. For the ab initio part, it was necessary to include an extended basis set on the heavy atoms since the calculations at all levels (viz. HF, MP2, DFT) using 6-31+G* basis set resulted in an unbound potential. The binding energy of the optimized complex including BSSE and ΔZPE was calculated as 439 cm-1, which compared well with the experimentally determined range. The dispersed fluorescence spectra of the complex at various excitations were used to obtain information about the dynamics of vibrational predissociation (VP). The character of the dispersed fluorescence spectra and the VP rate constant obtained using serial model were intermediate between the aniline–argon and aniline–CH4 cases.
ii. Hydroquinone-argon van der Waal’s complex: The REMPI spectrum of the Hydroquinone-argon complex displayed features due to the complex up to ~1250 cm-1, which is unusually high for such complexes. The complex, like the monomer, was found to exist in both cis- and transconformational forms. Ab initio calculations carried out at the MP2/6-311G* level for the S0 state of the complex yielded pi-bonded as well as H-bonded structures. The experimentally observed conformers are assigned pi-bonded structures. The appearance of the complex in the REMPI spectrum at relatively high energies was attributed to slow vibrational predissociation rate constants (on the time scale of the transit time through the extraction grids; typically 1 Ás) due to the existence of multiple minima in the potential energy surface. Calculations on the S0 state of the H-bonded complexes (Ar atom bound to the phenolic OH group) of hydroquinone as well as aminophenol indicated that the OH stretching frequency in these complexes was blue shifted by 12-13 cm-1 contrary to conventional H-bonding characteristic.