Knowledge Bank

Extending an earlier study of the benzene-$Br2$ complex in the solid $phase.1$ we are studying the infrared spectrum of benzene isolated in $SO2$ and $I2$ matrices at --- $180\circ C$. In the first system, it has been possible to measure the intensities of all the gas-phase-active fundamentals. Compared with the gas phase values, we observe a large increase in the intensity of the out-of-plane bending vibration, $\nu 11$. while the total intensity in the region of the C-H stretching mode, $\nu 18$ is considerably decreased. The other modes do not change as much, and there are only very small frequency shifts. One remarkable feature of the benzene-$I2$ system is the appearance of \emph{all} the fundamental modes of vibration, including the inactive $a2g$ mode $\nu a$, at $\sim 1346cm-1$ and the $b2g$ modes, $\nu 4$ and $\nu 5$, at $\sim 700$ and $\sim 995cm-1$, respectively. We believe this system may be unique, in that the interaction is weak enough so that frequencies of the benzene in the matrix are not appreciably different from the free molecule, and yet strong enough so that every one of the fundamental modes may be observed directly.