Distortions of ethyne when complexed with a cuprous or argentous halide: the rotational spectrum of C₂H₂⋯CuF

A new molecule C₂H₂⋯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 ¹²C₂H₂⋯⁶³Cu¹⁹F, ¹²C₂H₂⋯⁶⁵Cu¹⁹F, ¹³C₂H₂⋯⁶³Cu¹⁹F, ¹³C₂H₂⋯⁶⁵Cu¹⁹F, ¹²C₂D₂⋯⁶³Cu¹⁹F and ¹²C₂D₂⋯⁶⁵Cu¹⁹F were determined and interpreted to show that the molecule has a planar, T-shaped geometry belonging to the molecular point group C_2v, 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 C₂H₂⋯MX (M = Cu or Ag; X = F, Cl or CCH) are approximately linearly related to the energies D_e for the dissociation process C₂H₂⋯MX = C₂H₂ + MX.