Observation of a double C–H⋯π interaction in the CH2ClF⋯HCCH weakly bound complex

Abstract

The structure of the CH₂ClF⋯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⋯π interaction, in which one π-bond acts as acceptor to two hydrogen atoms from the CH₂ClF donor, and a weak C–H⋯Cl interaction, with acetylene as the donor. Analysis of the rotational spectra of four isotopologues (CH₂³⁵ClF⋯H¹²C¹²CH, CH₂³⁷ClF⋯H¹²C¹²CH, CH₂³⁵ClF⋯H¹³C¹³CH, and CH₂³⁷ClF–H¹³C¹³CH) has led to a structure with C–H⋯π distances of 3.236(6) Å and a C–H⋯Cl distance of 3.207(22) Å, 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⋯π interaction that lies in the C_s plane; however, this appears to be the first double C–H⋯π 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, χ_aa = 28.497(5) MHz, χ_bb = −65.618(13) MHz, and χ_cc = 37.121(8) MHz.