Knowledge Bank

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 $C6H6-CHCl3$, 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 $(+178cm-1)$ 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 $61$ and $61l1$ leves 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 $+84cm-1$, 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 charaeteristics of the hydrogen bond.