The Spectroscopy and Photophysics of π Hydrogen‐BondedComplexes: Benzene–CHCl₃

Abstract

A vibronic level study of the spectroscopy and photophysics of the C 6 H 6 –CHCl 3 complex has been carried out using a combination of laser‐induced fluorescence and resonant two‐photon ionization (R2PI). In C 6 H 6 ‐CHCl 3 , the S 1 –S 0 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 R2PI reflect a hydrogen‐bonded orientation for CHCl 3 relative to benzene’ π cloud. Dispersed fluorescence scans place a firm upper bound on the ground state binding energy of the complex of 2,024 cm –1 . Both the 6 1 and 6 1 1 1 levels 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 characteristics of the n hydrogen bond.