Methane as a proton acceptor: rotational spectrum and internal dynamics of a weakly bound dimer of methane and hydrogen cyanide

Abstract

The rotational spectra of the isotopomers CH4⋯HC14N, 13CH4⋯HC14N, CH4⋯DC14N, CH4⋯HC15N, CH4⋯DC15N, CH3D⋯HC14N and CH3D⋯HC15N of a weakly bound dimer formed between methane and hydrogen cyanide have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Each dimer species containing a Td methane isotopomer exhibits a symmetric-top type spectrum consisting of two K= 0 transitions and two K= 1 transitions. The spectrum in each of the four states has been analysed to give an operationally defined rotational constant B and centrifugal distortion constant Dj, as well as 14N and/or D nuclear quadrupole coupling constants, as appropriate. For the species CH3D⋯HC14N and CH3D⋯HC15N, spectra in three K= 0 states were observed and similarly analysed. These observations have been interpreted in terms of a C3v equilibrium geometry of the dimer in which HCN lies along a C3 axis of methane and forms a hydrogen bond to one of the four equivalent faces of the methane tetrahedron. The observed spectral patterns are discussed on the basis of the internal rotaion of the methane subunit which permutes the site of weak binding among the faces of the tetrahedron, especially with reference to the correlation diagram (due to Ohshima and Endo, J. Chem. Phys., 1990, 93, 6256) linking the free rotor states of methane in the low-barrier limit with the K states in the rigid C3v geometry limit. The distance r(C⋯C)= 3.837 (3)Å between the C atoms of CH4 and HCN is remarkably insensitive either to variation of the internal rotation state or the nature of the isotopomer. The same conclusion holds for the intermolecular stretching force constant which has the value kσ= 1.60 (2) N m–1 for isotopomers containing HCN.