On the Vibrational Interlacing Rule in Deuterated Benzenes: I. Out-of-plane Vibrations*

If A and B t are two molecules that differ by a single isotopic substitution then in the harmonic approximation vibrational frequencies of molecule A are interlaced with vibrational frequencies of molecule B t . This interlacing rule is a generalization of the well-known order rule. In the case of planar molecules, the interlacing rule applies separately to in-plane and to out-of-plane vibrations. This rule is particularly simple in the case of out-of-plane vibrations. The validity of the interlacing rule is verified for benzene and deuterated benzene molecules. Violation of this rule indicates (very unlikely) strong anharmonicity effects, or (more likely) erroneous vibrational assignments and/or erroneous experimental data. Hence this rule can help to analyze vibrational spectra, in particular in the assignments of experimental frequencies to various vibrational types.