Abstract
The structure of the CH2ClF center dot center dot center dot HCCH dimer has been determined using both chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy. The complex has C-s symmetry and contains both a double C-H center dot center dot center dot pi interaction, in which one pi-bond acts as acceptor to two hydrogen atoms from the CH2ClF donor, and a weak C-H center dot center dot center dot Cl interaction, with acetylene as the donor. Analysis of the rotational spectra of four isotopologues ((CH2ClF)-Cl-35 center dot center dot center dot(HCCH)-C-12-C-12, (CH2ClF)-Cl-37 center dot center dot center dot(HCCH)-C-12-C-12, (CH2ClF)-Cl-35 center dot center dot center dot(HCCH)-C-13-C-13, and (CH2ClF)-Cl-37-(HCCH)-C-13-C-13) has led to a structure with C-H center dot center dot center dot pi distances of 3.236(6) angstrom and a C-H center dot center dot center dot Cl distance of 3.207(22) angstrom, in good agreement with ab initio calculations at the MP2/6-311++G(2d,2p) level. Both weak contacts are longer than those observed in similar complexes containing a single C-H center dot center dot center dot pi interaction that lies in the C-s plane; however, this appears to be the first double C-H center dot center dot center dot pi contact to be studied by microwave spectroscopy, so there is little data for direct comparison. The rotational and chlorine nuclear quadrupole coupling constants for the most abundant isotopologue are: A = 5262.899(14) MHz, B = 1546.8074(10) MHz, C = 1205.4349(7) MHz, chi(aa) = 28.497(5) MHz, chi(bb) = -65.618(13) MHz, and chi(cc) = 37.121(8) MHz.