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 CH₃CN and CH₃NC. The theory predicts for the transition J=1→J=2, ΔK=0, 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.