Interaction of a pseudo-π C-C bond with cuprous and argentous chlorides: Cyclopropane•••CuCl and cyclopropane•••AgCl investigated by rotational spectroscopy and ab initio calculations

Strongly bound complexes (CH2)(3)center dot center dot center dot MCl (M = Cu or Ag), formed by non-covalent interaction of cyclopropane and either cuprous chloride or argentous chloride, have been generated in the gas phase by means of the laser ablation of either copper or silver metal in the presence of supersonically expanded pulses of a gas mixture containing small amounts of cyclopropane and carbon tetrachloride in a large excess of argon. The rotational spectra of the complexes so formed were detected with a chirped-pulse, Fourier transform microwave spectrometer and analysed to give rotational constants and Cu and Cl nuclear quadrupole coupling constants for eight isotopologues of each of (CH2)(3)center dot center dot center dot CuCl and (CH2)(3)center dot center dot center dot AgCl. The geometry of each of these complexes was established unambiguously to have C-2v symmetry, with the three C atoms coplanar, and with the MCl molecule lying along a median of the cyclopropane C-3 triangle. This median coincides with the principal inertia axis a in each of the two complexes (CH2)(3)center dot center dot center dot MCl. The M atom interacts with the pseudo-p bond linking the pair of equivalent carbon atoms C-F (F = front) nearest to it, so that M forms a non-covalent bond to one C-C edge of the cyclopropane molecule. The (CH2)(3)center dot center dot center dot MCl complexes have similar angular geometries to those of the hydrogen-and halogen-bonded analogues (CH2)(3)center dot center dot center dot HCl and (CH2)(3)center dot center dot center dot ClF, respectively. Quantitative details of the geometries were determined by interpretation of the observed rotational constants and gave results in good agreement with those from ab initio calculations carried out at the CCSD(T)(F12*)/aug-cc-pVTZ-F12 level of theory. Interesting geometrical features are the lengthening of the C-F-C-F bond and the shrinkage of the two equivalent C-B-C-F (B = back) bonds relative to the C-C bond in cyclopropane itself. The expansions of the C-F-C-F bond are 0.1024(9) angstrom and 0.0727(17) angstrom in (CH2)(3)center dot center dot center dot CuCl and (CH2)(3)center dot AgCl, respectively, according to the determined r(0) geometries. The C-C bond lengthening is in each case about four times that observed by similar methods in the corresponding complexes of MCl with ethyne and ethene, even though the cyclopropane complexes are more weakly bound than their ethyne and ethene analogues. Reasons for the larger increase in r(CC) in the pseudo-pi complexes are discussed. (C) 2015 AIP Publishing LLC.