l-Type Doubling in Polyatomic Molecules and Its Application to the Microwave Spectrum of Methyl Cyanide and Methyl Iso-Cyanide

The theory of the $l$-type doubling for polyatomic molecules is developed and applied to explain certain anomalies observed in the microwave spectrum of C${\mathrm{H}}_3$CN and C${\mathrm{H}}_3$NC. The theory predicts for the transition $J=1\mathrm{}\to J=2\mathrm{}$, $\Delta K=0\mathrm{}$, where the molecule is in an excited $e$ vibration state, a pattern consisting of two lines separated by about 100 mc/sec. with two lines near the center of this doublet which are about 2 or 3 mc/sec. apart. The predicted patterns can be made to agree quantitatively with the observed patterns by correcting the degenerate Coriolis term for centrifugal distortion.