Knowledge Bank

In isotopic mixed crystals of benzene, it has been possible to observe the effect of the $C1$ site crystal field (non-resonance interactions) on the vibrational transitions of a 1% isotopic impurity. These isotopic impurities are of two distinct kinds: a) those possessing at least a trigonal axis in the gas phase $(C6H6,C6D6$, and $C6H3D3)$; and b) those of lower than $D3h$ symmetry in the gas phase $(1,4C6H4D2,1,3C6H4D2$ and $C6H3D)$. The solvents for these systems are $C6H6$ and $C6D4$. In category a) only the well-known site group splitting is observed on gas phase degenerate vibrations, and no splitting is observed on any vibration known to be nondegenerate. The magnitude of these site group splittings is in general $4-7cm-1$. The splittings are found to be different in magnitude and different in relative intensity for the components of the doublets in the crystal-allowed $e2u$ and molecule-allowed $e1u$ vibrations. In isotopic mixed crystals containing solutes of category b), all observed bands, known group-theoretically to be molecule nondegenerate, are found to be doublets. These splittings, termed orientational effects,'' are $3-4 cm^{-1}$. It can be shown that, due to the reduced symmetry of these isotopic species, there are three distinct orientations of each in the $C_{1}$ sites of the crystal. For $p-C_{6}H_{4}D_{2}, m-C_{6}H_{4}D_{2}$ and $C_{6}H_{5}D$, two of these orientations are environmentally quite similar giving an effective site of $C_{2h}$. Only two peaks are observed in these cases with an intensity ratio of 2:1. Evidence for such effects is also found in the pure crystal spectra of these isotopic species. The interesting case of 1, 3, 4 $C_{6}H_{3}D_{3}$, with only a plane of symmetry, is shown to have 6 orientations in the $C_{1}$ site but 3 in the $C_{2h}$ physical environment.'' Calculations of both orientational effects and site group splittings are in progress and will be discussed.