Knowledge Bank

“The experimental values of the centrifugal distortion constant $DJ$ for a series of cyclic molecules are analyzed in the light of existing theory. For symmetric-top molecules which have a planar-ring structure the constant $Dj$ can be represented by Kratzer’s formula for diatomic molecules, namely [ D_{J} = \frac{4B_{s}^{*}}{\omega^{s}} ] if the value of the totally-symmetric-ring breathing frequency is used for $\omega$. For slightly asymmetric tops which also possess a planar-ring configuration, the same relationship applies if, for the frequency $\omega$, one uses the value that corresponds most closely to the totally-symmetric-ring breathing vibration if the molecule were a genuine symmetric top. As it is $known1$ that in planar molecules only the in-plane modes contribute to the constant $DJ$, Kratzers’ relationship is expected to break down for cyclic molecules with a non-planar-ring configuration. The validity of Kratzer’s formulka has been noted previously for the methyl $halides2$. The present study indicates that this relationship may be a useful first approximation to the $DJ$-value in the analysis of the microwave spectra of cyclic molecules which have a planar configuration.”