The effects of proton tunneling, N quadrupole coupling, and methyl internal rotations in the microwave spectrum of ethyl methyl amine.

The spectra of N-ethyl methyl amine, CH(NH)CHCH, were measured using a molecular jet Fourier transform microwave spectrometer in the frequency range of 2 GHz-26.5 GHz. Splittings due to proton inversion tunneling, Coriolis coupling, N quadrupole coupling, and methyl internal rotation were fully resolved. The experimentally deduced rotational constants are A = 25 934.717(21) MHz, B = 3919.8212(23) MHz, and C = 3669.530(21) MHz. The proton tunneling causes (+) ↔ (-) splittings of about 1980.9 MHz for all c-type transitions between the lowest symmetric and the higher anti-symmetric energy levels. The splittings of the (+) ← (+) and (-) ← (-) levels, mainly influenced by Coriolis coupling, were also observed and assigned for b-type transitions, yielding the coupling constants F = 0.3409(71) MHz and F = 163.9(14) MHz. The N quadrupole coupling constants were determined to be χ = 2.788 65(55) MHz and χ - χ = 4.630 45(91) MHz. Fine splittings arising from two inequivalent methyl rotors are in the order of 150 kHz, and the torsional barriers are determined to be 1084.62(41) cm for the CHNH methyl group and 1163.43(80) cm for the CHCH methyl group. The experimental results are in good agreement with those of quantum chemical calculations.