MICROWAVE-SPECTRUM, STRUCTURE AND INTERNAL-ROTATION OF THE METHYLAMINE HYDROGEN-CHLORIDE DIMER

The rotational spectra of three asymmetric-top isotopomers of the meth ylamine-hydrogen chloride dimer have been detected in the ground vibra tional state by pulsed-nozzle, Fourier-transform microwave spectroscop y. Rotational constants, centrifugal distortion constants and Cl and N nuclear quadrupole coupling constants were determined. Analysis of th e observed rotational constants leads to the conclusion that the dimer has a tetrahedral arrangement at nitrogen with the HCl lying in the m ethylamine symmetry plane. When H of HCl is assumed to lie between the N and Cl atoms the distance r(N...Cl) = 3.004(3) angstrom and angle p hi = 78.4(4)degrees are determined by fitting observed moments of iner tia. Comparison of the nuclear-quadrupole coupling constant chi(aa)(Cl -35) with those of related dimers shows a small amount of proton trans fer from Cl to N, though the dimer is best described as the hydrogen-b onded form, CH3NH2...HCl. The microwave spectrum is split into closely spaced doublets as a result of internal rotation of the CH3 group abo ut the C-N bond. The splittings were analysed using an IAM treatment t o obtain V3 = 474.3(4) cm-1. The threefold barrier is considerably red uced from that of free methylamine (683.0 cm-1).