Alkynes as CH/π Acceptors: Microwave Spectra and Structures of the CH₂F₂···Propyne and CH₂ClF···Propyne Dimers

Rotational spectra of weakly bound complexes of chlorofluoromethane (CH₂ClF) and difluoromethane (CH₂F₂) with propyne (HCCCH₃) have been measured using chirped-pulse and resonant-cavity Fourier-transform microwave spectroscopy, adding to a relatively small body of high resolution spectroscopic data on propyne complexes. Both dimers contain CH/π contacts, as well as secondary contacts between one or both halogen atoms and the methyl group of propyne. A detailed structural determination for CH₂F₂···propyne has been made by study of the normal, one deuterated and four ¹³C substituted isotopologues, with the second lowest energy configuration predicted from ab initio calculations agreeing well with the observed structure. Experimental rotational constants for the most abundant isotopologue of CH₂F₂···propyne are A₀₀ = 5815.5858(15) MHz, B₀₀ = 1341.1191(5) MHz, C₀₀ = 1099.2040(4) MHz (uncorrected for internal rotation effects), and the dipole moment components, determined by Stark effect measurements, are μ_a = 1.568(2) D, μ_b = 0.587(2) D, and μ_tot = 1.674(3) D. For CH₂ClF···propyne, only ³⁵Cl and ³⁷Cl isotopologues have been assigned, providing rotational constants and chlorine atom coordinates consistent with the lowest energy structure from a series of ab initio predictions. Rotational constants for the ³⁵Cl isotopologue are A = 3423.639(7) MHz, B = 1253.7562(20) MHz, and C = 1200.4828(15) MHz and the diagonal and two off diagonal components of the quadrupole coupling tensor have also been determined.