Distortion of ethyne on coordination to silver acetylide, C ₂H₂⋯AgCCH, characterised by broadband rotational spectroscopy and ab initio calculations

The rotational spectra of six isotopologues of a complex of ethyne and silver acetylide, C₂H₂⋯AgCCH, are measured by both chirped-pulse and Fabry-Perot cavity versions of Fourier-transform microwave spectroscopy. The complex is generated through laser ablation of a silver target in the presence of a gas sample containing 1% C₂H₂, 1% SF₆, and 98% Ar undergoing supersonic expansion. Rotational, A ₀, B₀, C₀, and centrifugal distortion Δ_J and Δ_JK constants are determined for all isotopologues of C₂H₂⋯AgCCH studied. The geometry is planar, C_2v and T-shaped in which the C₂H₂ sub-unit comprises the bar of the "T" and binds to the metal atom through its π electrons. In the r₀ geometry, the distance of the Ag atom from the centre of the triple bond in C₂H₂ is 2.2104(10) Å. The r(HC≡CH) parameter representing the bond distance separating the two carbon atoms and the angle, (CCH), each defined within the C₂H₂ sub-unit, are determined to be 1.2200(24) Å and 186.0(5)°, respectively. This distortion of the linear geometry of C ₂H₂ involves the hydrogen atoms moving away from the silver atom within the complex. The results thus reveal that the geometry of C₂H₂ changes measurably on coordination to AgCCH. A value of 59(4) N m^-1 is determined for the intermolecular force constant, k_Σ, confirming that the complex is significantly more strongly bound than hydrogen and halogen-bonded analogues. Ab initio calculations of the r_e geometry at the CCSD(T)(F12^*)/ACVTZ level of theory are consistent with the experimental results. The spectra of the ¹⁰⁷Ag¹³C¹³CH and ¹⁰⁹Ag ¹³C¹³CH isotopologues of free silver acetylide are also measured for the first time allowing the geometry of the AgCCH monomer to be examined in greater detail than previously.