Microwave Spectrum and Internal Barrier of Methylphosphine

The microwave spectrum of methylphosphine has been investigated in the regions of 12–33 and 46–49 kMc. The rotational constants are found to be A = 71,869.5, B = 11,792.6, C = 11,677.7 Mc. Stark effect measurements yield a dipole moment of 1.100±0.010 D. The barrier to internal rotation of the methyl group, as determined from the A—E splittings of rotational lines in the ground torsional state and from the satellite frequency pattern of the 000—101 transition, is 685.2±5 cm—1. The structure of the molecule and the magnitude of the internal barrier in relation to other molecules are discussed.