Distortion of ethyne on formation of a π complex with silver chloride: C2H2⋯Ag-Cl characterised by rotational spectroscopy and ab initio calculations.

C(2)H(2)⋯Ag-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 (12)C(2)H(2)⋯(107)Ag(35)Cl, (12)C(2)H(2)⋯(109)Ag(35)Cl, (12)C(2)H(2)⋯(107)Ag(37)Cl, (12)C(2)H(2)⋯(109)Ag(37)Cl, (13)C(2)H(2)⋯(107)Ag(35)Cl, and (13)C(2)H(2)⋯(109)Ag(35)Cl 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 (12)C(2)D(2)⋯(107)Ag(35)Cl and (12)C(2)D(2)⋯(109)Ag(35)Cl. 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 r(m)(1) geometries and an almost complete r(s)-geometry were established. The ethyne molecule distorts on complex formation by lengthening of the C≡C bond and movement of the two H atoms away from the C≡C internuclear line and the Ag atom. The r(m)(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 r(m)(1) version, including the ethyne angular distortion.