Spectroscopic study on deuterated benzenes. I. Microwave spectra and molecular structure in the ground state.

We observed microwave absorption spectra of some deuterated benzenes and accurately determined the rotational constants of all H/D isotopomers in the ground vibrational state. Using synthetic analysis assuming that all bond angles are 120°, the mean bond lengths were obtained to be r₀(C-C) = 1.3971 Å and r₀(C-H) = r₀(C-D) = 1.0805 Å. It has been concluded that the effect of deuterium substitution on the molecular structure is negligibly small and that the mean bond lengths of C-H and C-D are identical unlike small aliphatic hydrocarbons, in which r₀(C-D) is about 5 mÅ shorter than r₀(C-H). It is considered that anharmonicity is very small in the C-H stretching vibration of aromatic hydrocarbons. [ABSTRACT FROM AUTHOR]