Changes in the Geometries of C2H2 and C2H4 on Coordination to CuCl Revealed by Broadband Rotational Spectroscopy and ab-Initio Calculations

C2H2 The molecular geometries of isolated complexes in which a single molecule of C2H4 or C2H2 is bound to CuCl have been determined through pure rotational spectroscopy and ab-initio calculations. The C2H2 center dot center dot center dot CuCl and C2H4 center dot center dot center dot CuCl complexes are generated through laser vaporization of a copper rod in the presence of a gas sample undergoing supersonic expansion and containing C2H2 (or C2H4), CCl4, and Ar. Results are presented for five isotopologues of C2H2 center dot center dot center dot CuCl and six isotopologues of C2H4 CuCl. Both of these complexes adopt C-2v, T-shaped geometries in which the hydrocarbon binds to the copper atom through its pi electrons such that the metal is equidistant from all H atoms. The linear and planar geometries of free C2H2 and C2H4, respectively, are observed to distort significantly on attachment to the Cu Cl unit, and the various changes are quantified. The angle(*-C-H) parameter in C2H2 (where * indicates the midpoint of the C C bond) is measured to be 192.4(7)degrees in the r(0) geometry of the complex representing a significant change from the linear geometry of the free molecule. This distortion of the linear geometry of C2H2 involves the hydrogen atoms moving away from the copper atom within the complex. Ab-initio calculations at the CCSD(T)(F12*)/AVTZ level predict a dihedral angle(HCCCu) angle of 96.05 degrees in C2H4 center dot center dot center dot CuCl, and the experimental results are consistent with such a distortion from planarity. The bonds connecting the carbon atoms within each of C2H2 and C2H4, respectively, extend by 0.027 and 0.029 angstrom relative to the bond lengths in the isolated molecules. Force constants, k(sigma), and nuclear quadrupole coupling constants, chi(aa)(Cu), [chi(bb)(Cu) - chi(cc)(Cu)], chi(aa)(Cl), and [chi(bb)(Cl) - chi(cc)(Cl)], are independently determined for all isotopologues of C2H2 center dot center dot center dot CuCl studied and for four isotopologues of C2H4 center dot center dot center dot CuCl.