Distortions of ethyne when complexed with a cuprous or argentous halide: the rotational spectrum of CH⋯CuF

A new molecule CH⋯CuF has been synthesized in the gas phase by means of the reaction of laser-ablated metallic copper with a pulse of gas consisting of a dilute mixture of ethyne and sulfur hexafluoride in argon. The ground-state rotational spectrum was detected by two types of Fourier-transform microwave spectroscopy, namely that conducted in a microwave Fabry-Perot cavity and the chirped-pulse broadband technique. The spectroscopic constants of the six isotopologues CH⋯CuF, CH⋯CuF, CH⋯CuF, CH⋯CuF, CD⋯CuF and CD⋯CuF were determined and interpreted to show that the molecule has a planar, T-shaped geometry belonging to the molecular point group C, with CuF forming the stem of the T. Quantitative interpretation reveals that the ethyne molecule is distorted when subsumed into the complex in such manner that the C≡C bond lengthens (by δr) and the two H atoms cease to be collinear with the C≡C internuclear line. The H atoms move symmetrically away from the approaching Cu atom of CuF, to increase each ∗≡C-H angle by δA = 14.65(2)°, from 180° to 194.65(2)°. Ab initio calculations at the explicitly-correlated level of theory CCSD(T)(F12∗)/aug-cc-pVTZ lead to good agreement with the experimental geometry. It is shown that similar distortions δr and δA, similarly determined, for four complexes CH⋯MX (M = Cu or Ag; X = F, Cl or CCH) are approximately linearly related to the energies D for the dissociation process CH⋯MX = CH + MX.