Force Constants in Methylamine-A Determination by the Use of 15N Isotope Shifts

Infrared absorption spectrum of methylamine—15N has been examined in the gaseous state, and the amounts of 15N isotope shifts have been determined for 12 absorption bands of undeuterated methylamine. Force constants in the methylamine molecule were adjusted on the bases of the vibrational frequencies of CH3NH2, CH3ND2, CD3NH2, and CD3ND2 and of the 15N isotope shifts of CH3NH2 by the method of the least squares. The 15N isotope shifts have been found to be useful data for selecting the best set of force constants among several sets which are equally good as far as only the agreement of the observed and calculated frequencies are taken into account.