Inversion and internal rotation in methyl- d-amine: Microwave spectrum

Abstract

The microwave spectrum of CH 2DNH 2 has been observed in the 8–74 GHz region. The spectrum shows that this molecule takes essentially two distinguishable conformers, trans and gauche forms, although a small amount of coupling between them can be detected. For each line of the trans form a small inversion splitting has been found. It is 93.97 MHz at K = 0 and a periodic function of K. The mean frequencies of the inversion pairs of lines are well explained as the frequencies of a rigid rotor. The gauche spectrum is extremely complicated; each rotational line splits into four because of inversion and gauche-gauche tunneling interactions. The analysis was carried out based on the theory developed in the preceding paper. Tunneling energy parameters of internal-rotation, ϵ gg and ϵ tg , and inversion, δ gg and δ tg , were determined as ϵ gg = 3476.6 MHz, ϵ tg = 3233.1 MHz, δ gg = 2790.6 MHz and δ tg = 3052.7 MHz. Energy difference between trans and gauche conformers Δ E tg was estimated to be 7.060 cm −1 from these values of parameters and also on the basis of the observed anomaly in the Q branch series of trans form which is due to an accidental degeneracy between the K = 1 level of trans and K = 2 level of gauche. The effects on the internal-rotation of other internal motions have also been discussed.