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

C2H2 center dot center dot center dot 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 CCl4 gave AgCl which then reacted with ethyne to give the complex. Ground-state rotational spectra of the six isotopologues (C2H2)-C-12 center dot center dot center dot(AgCl)-Ag-107-Cl-35, (C2H2)-C-12 center dot center dot center dot(AgCl)-Ag-109-Cl-35, (C2H2)-C-12 center dot center dot center dot(AgCl)-Ag-107-Cl-37, (C2H2)-C-12 center dot center dot center dot(AgCl)-Ag-109-Cl-37, (C2H2)-C-13 center dot center dot center dot 107Ag(35)Cl, and (C2H2)-C-13 center dot center dot center dot(AgCl)-Ag-109-Cl-35 were analysed to yield rotational constants A(0), B-0, and C-0, centrifugal distortion constants Delta J, Delta JK, and delta J, and Cl nuclear quadrupole coupling constants chi aa(Cl) and chi bb(Cl) -chi cc(Cl). A less complete analysis was possible for (C2D2)-C-12 center dot center dot center dot(AgCl)-Ag-107-Cl-35 and (C2D2)-C-12 center dot center dot center dot(AgCl)-Ag-109-Cl-35. 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 pi bond. r(0) and r(m)((1)) geometries and an almost complete rs-geometry were established. The ethyne molecule distorts on complex formation by lengthening of the C equivalent to C bond and movement of the two H atoms away from the C equivalent to C internuclear line and the Ag atom. The r(m)((1)) bond lengths and angles are as follows: r(*center dot Ag) = 2.1800(3) A degrees, r(C-C) = 1.2220(20) A degrees, r(Ag-Cl) = 2.2658(3) A degrees and the angle H-C equivalent to* has the value 187.79(1)degrees. Ab initio calculations at the coupled-cluster singles and doubles level of theory with a perturbative treatment of triples (F12*)/cc-pVTZ yield a re geometry in excellent agreement with the experimental r(m)((1))version, including the ethyne angular distortion. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4761895]