The microwave spectrum of gauche-ethylamine

The microwave spectrum of the ground state of the normal species of gauche-ethylamine CH₃CH₂NH₂ and that of -NHD, -NDH, as well as -ND₂ isotopic species were measured and assigned. The ground state splits into four substates due to two internal large amplitude motions: inversion (s and a) and internal rotation (o and e) about the CN axis. Intersystem transitions due to tunneling as well as vibrational-rotational perturbations affect not only the absorption frequencies but also the Stark effect and NQHFS. The rotational constants for the two symmetrical inversion states (s) were fitted for the normal species as (all values in MHz) A^se = 32 423.470 ± 0.184, B^se = 8 942.086 ± 0.039, and C^se = 7 825.520 ± 0.048, and A^so = 32 378.733 ± 0.182, B^so = 8 940.906 ± 0.052, and C^so = 7 825.551 ± 0.042 with the interaction constants Q_a^s = 151.12 ± 0.52 and Q_b^s = 44.4 ± 7.0. The antisymmetrical inversion states (a) were fitted as A^ae = 32 423.347 ± 0.142, B^ae = 8 942.027 ± 0.029, and C^ae = 7 825.525 ± 0.031, and A^ao = 32 378.720 ± 0.142, B^ao = 8 940.984 ± 0.029, and C^ao = 7 825.573 ± 0.031 with the interaction constants Q_a^a = 167.10 ± 0.31, Q_b^a = 48.1 ± 5.4. The energy splitting due to intersion was determined (in MHz) as Δν_inv = 1 391.39 ± 0.19 and that due to internal rotation as Δν_tors = 1 170.58 ± 0.18. The cis barrier separating the two equivalent torsional states was calculated as 690 cm^?1, and the inversion barrier between the inversion states was calculated as 1400 cm^?1, both using the Dennison-Uhlenbeck model. A simple model explaining the inversion splittings of the monodeuterated species is proposed. Comparing the relative intensities for several temperatures the gauche form was observed to be energetically higher than the trans form by 110 ± 50 cm^?1. The dipole moment could only be fitted by taking into account the internal motions yielding (in Debye) μ_a^eff = 0.11 ± 0.01, μ_b^eff = 0.65 ± 0.01, and μ_c^eff = 1.014 ± 0.015. The quadrupole coupling constants (in MHz) were found as χ_aa = ?χ⁺ = 2.268 ± 0.043 and χ_bb ? χ_cc = χ^? = 3.120 ± 0.035.