THE SPECTROSCOPY AND PHOTOPHYSICS OF $\pi$ HYDROGEN-BONDED COMPLEXES: BENZENE-$CHCl_{3}$
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Date
1993
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Publisher
Ohio State University
Abstract
A vibronic level study of the spectroscopy and photophysics of the $C6H6-CHCl3$ complex has been carried out using a combination of laser-induced fluorescence and resonant two-photon ionization (R2Pl). In $C_{6}H_{6}-CHCl_{3}$, the $S1-S0$ origin remains forbidden while the $16^{1}_{0}$ transition is weakly induced. Neither $6^{1}_{0}$ nor $16^{1}_{0}$ are split by the presence of the $CHCl_{3}$ molecule. On this basis, a $C_{3v}$ structure is deduced for the complex, placing $CHCl_{3}$ on the six-fold axis of benzene. The large blue-shift of the complex's absorption relative to benzene $(+178 cm^{-1})$ and the efficient fragmentation of the complex following one-color R2Pl reflect a hydrogen-bonded orientation for $CHCl3$ relative to benzene's $\pi$ cloud. Dispersed fluorescence scans place a firm upper bound on the ground state binding energy of the complex of $2024cm^{-1}$. Both the $6^{1}$ and $6^{1} l^{1}$ leves do not dissociate on the time-scale of the $S_{1}$ fluorescence and show evidence of extensive state mixing with van der Waals' levels primarily built on the $0^{0}$ level of benzene. The $C_{6}H_{6} (CHCl_{3})_{2}$ cluster shows extensive intermolecular structure beginning at $+84 cm^{-1}$, a strong origin transition, and splitting of $6^{1}$. A structure which places both $CHCl_{3}$ molecules on the same side of the benzene ring is suggested on this basis. The vibronic level scheme used to deduce the structure of $C_{6}H_{6}-CHCl_{3}$ is tested against previous data on other $C_{6}H_{6}-X$ complexes. The scheme is found to be capable, in favorable cases, of deducing the structures of $C_{6}H_{6}-X$ complexes based purely on vibronic level data. Finally, the results on $C_{6}H_{6}-CHCl_{3}$ are compared with those on $C_{6}H_{6}-HCl$ and $C_{6}H_{6}-H_{2}O$ to evaluate the charaeteristics of the hydrogen bond.
Description
Author Institution: Department of Chemistry, Purdue University