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...H13CCH, C4H6O...HC13CH, , and [3(13C]-C4H6O...HCCH. The 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) A, phi=57.8(18) degrees , and theta=16.2(32) degrees, where phi is the angle made by the HCCH subunit at O and theta 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 (approximately equal to 20-30 kHz) of c-type transitions, well resolved only for the parent isotopomer and [3HCCH, 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(phi)=alphaphi(4)+betaphi(2) to estimate the observed separation DeltaE(01) of the lowest pair (v=0 and v=1) of associated energy levels. The predicted DeltaE(01) 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.