Distortion of ethyne on formation of a π complex with silver chloride: C ₂H ₂Ag-Cl characterised by rotational spectroscopy and ab initio calculations

C ₂H ₂Ag-Cl was formed from ethyne and AgCl in the gas phase and its rotational spectrum observed by both the chirped-pulse and Fabry-Perot cavity versions of Fourier-transform microwave spectroscopy. Reaction of laser-ablated silver metal with CCl ₄ gave AgCl which then reacted with ethyne to give the complex. Ground-state rotational spectra of the six isotopologues ¹²C ₂H ₂ ¹⁰⁷Ag ³⁵Cl, ¹²C ₂H ₂ ¹⁰⁹Ag ³⁵Cl, ¹²C ₂H ₂ ¹⁰⁷Ag ³⁷Cl, ¹²C ₂H ₂ ¹⁰⁹Ag ³⁷Cl, ¹³C ₂H ₂ ¹⁰⁷Ag ³⁵Cl, and ¹³C ₂H ₂ ¹⁰⁹Ag ³⁵Cl were analysed to yield rotational constants A ₀, B ₀, and C ₀, centrifugal distortion constants Δ _J, Δ _JK, and δ _J, and Cl nuclear quadrupole coupling constants χ _aa(Cl) and χ _bb(Cl) -χ _cc(Cl). A less complete analysis was possible for ¹²C ₂D ₂ ¹⁰⁷Ag ³⁵Cl and ¹²C ₂D ₂ ¹⁰⁹Ag ³⁵Cl. Observed principal moments of inertia were interpreted in terms of a planar, T-shaped geometry of C _2v symmetry in which the AgCl molecule lies along a C ₂ axis of ethyne and the Ag atom forms a bond to the midpoint () of the ethyne π bond. r ₀ and rm(1) geometries and an almost complete r _s-geometry were established. The ethyne molecule distorts on complex formation by lengthening of the CC bond and movement of the two H atoms away from the CC internuclear line and the Ag atom. The rm(1) bond lengths and angles are as follows: r(Ag) 2.1800(3) Å, r(C-C) 1.2220(20) Å, r(Ag-Cl) 2.2658(3) Å and the angle H-C has the value 187.79(1)°. Ab initio calculations at the coupled-cluster singles and doubles level of theory with a perturbative treatment of triples (F12)cc-pVTZ yield a r _e geometry in excellent agreement with the experimental rm(1)version, including the ethyne angular distortion.