THE SPECTROSCOPY AND PHOTOPHYSICS OF PI-HYDROGEN-BONDED COMPLEXES - BENZENE-CHCL3

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 (R2PI). In C6H6-CHCl3, the S1-S0 origin remains forbidden while the 1601 transition is weakly induced. Neither 601 nor 1601 are split by the presence of the CHCl3 molecule. On this basis, a C3v structure is deduced for the complex, placing CHCl3 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 CHCl3 relative to benzene's 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 61 and 6111 levels do not dissociate on the time-scale of the S1 fluorescence and show evidence of extensive state mixing with van der Waals' levels primarily built on the 00 level of benzene. The C6H6-(CHCl3)2 cluster shows extensive intermolecular structure beginning at +84 cm-1, a strong origin transition, and splitting of 61. A structure which places both CHCl3 molecules on the same side of the benzene ring is suggested on this basis. The vibronic level scheme used to deduce the structure of C6H6-CHCl3 is tested against previous data on other C6H6-X complexes. The scheme is found to be capable, in favorable cases, of deducing the structures of C6H6-X complexes based purely on vibronic level data. Finally, the results on C6H6-CHCl3 are compared with those on C6H6-HCl and C6H6-H2O to evaluate the characteristics of the π hydrogen bond.