Microwave spectrum, structure and internal rotation of the methylamine–hydrogen chloride dimer

Abstract

The rotational spectra of three asymmetric-top isotopomers of the methylamine–hydrogen chloride dimer have been detected in the ground vibrational state by pulsed-nozzle, Fourier-transform microwave spectroscopy. Rotational constants, centrifugal distortion constants and Cl and N nuclear quadrupole coupling constants were determined. Analysis of the observed rotational constants leads to the conclusion that the dimer has a tetrahedral arrangement at nitrogen with the HCl lying in the methylamine symmetry plane. When H of HCl is assumed to lie between the N and Cl atoms the distance r(N⋯Cl)= 3.004(3)Å and angle ϕ= 78.4(4)° are determined by fitting observed moments of inertia. Comparison of the nuclear-quadrupole coupling constant χ_aa(³⁵Cl) with those of related dimers shows a small amount of proton transfer from Cl to N, though the dimer is best described as the hydrogen-bonded form, CH₃NH₂⋯HCl.

The microwave spectrum is split into closely spaced doublets as a result of internal rotation of the CH₃ group about the C—N bond. The splittings were analysed using an IAM treatment to obtain V₃= 474.3(4) cm^–1. The threefold barrier is considerably reduced from that of free methylamine (683.0 cm^–1).