The effects of proton tunneling, ¹⁴N quadrupole coupling, and methyl internal rotations in the microwave spectrum of ethyl methyl amine Available to Purchase

The spectra of N-ethyl methyl amine, CH₃(NH)CH₂CH₃, 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, ¹⁴N 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_bc = 0.3409(71) MHz and F_ac = 163.9(14) MHz. The ¹⁴N quadrupole coupling constants were determined to be χ_aa = 2.788 65(55) MHz and χ_bb − χ_cc = 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⁻¹ for the CH₃NH methyl group and 1163.43(80) cm⁻¹ for the CH₂CH₃ methyl group. The experimental results are in good agreement with those of quantum chemical calculations.