Rotational spectrum, inversion, and geometry of 2,5-dihydrofuran⋯ethyne and a generalization about Z⋯H–C hydrogen bonds

The ground-state rotational spectra of nine isotopomers of a complex formed between 2,5-dihydrofuran and ethyne were recorded with a pulsed-jet, Fourier-transform microwave spectrometer. Rotational and centrifugal distortion constants were obtained for C4H6O⋯HCCH, C4H6O⋯DCCH, C4H6O⋯HCCD, C4H6O⋯DCCD, [3,4-D2]–C4H6O⋯HCCH, C4H6O⋯HC13CH, C4H6O⋯HCC13H, [2-C13]–C4H6O⋯HCCH, and [3-C13]–C4H6O⋯HCCH. The C13 substituted species were studied in their natural abundances. For the more abundant isotopomers, weak c-type transitions as well as strong a-type transitions were observed. The primary intermolecular binding was shown to consist of a hydrogen bond formed by the ethyne subunit acting as the proton donor and the O atom of 2,5-dihydrofuran as the proton acceptor. The complex has a plane of symmetry that includes the O atom and the ethyne subunit, with a pyramidal configuration at oxygen. A fit of the principal moments of inertia of all nine isotopomers under the assumption of unperturbed 2,5-dihydrofuran and ethyne geometries yielded the values r(O⋯H)=2.127(8)Å, ϕ=57.8(18)°, and θ=16.2(32)°, where ϕ is the angle made by the HCCH subunit at O and θ is the angular deviation of the O⋯H–C nuclei from collinearity. This geometry is compared with those obtained by ab initio calculations conducted with a range of basis sets and with electron correlation taken into account at the MP2 (Møller-Plesset second order) level of theory. A small inversion doubling (≈20–30kHz) of c-type transitions, well resolved only for the parent isotopomer and [3,4-D2]–C4H6O⋯HCCH, was attributed to a vibrational motion that inverts the configuration at oxygen. A one-dimensional model for this motion was used with a double minimum potential energy function of the type V(ϕ)=αϕ4+βϕ2 to estimate the observed separation ΔE01 of the lowest pair (v=0 and v=1) of associated energy levels. The predicted ΔE01 had the same magnitude as that deduced from the inversion doubling of the c-type transitions. The geometry of C4H6O⋯HCCH is compared with those other B⋯HCCH, where B is vinyl fluoride, oxirane, and thiirane. A rationalization of the angular geometries of various B⋯HX, where X=F, Cl, Br, or CCH, is presented.