MICROWAVE-SPECTRA AND STRUCTURE OF HCN-BF3 - AN ALMOST WEAKLY-BOUND COMPLEX

Rotational spectra of the complex HCN-BF3 have been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectra have been observed for the B-11 and B-10 species with both (HCN)-N-14 and (HCN)-N-15, and the hyperfine structure has been analyzed. The complex has the expected C3v structure, with the nitrogen end of the HCN toward the boron, but the observed BN bond length of 2.473(29) angstrom is notably shorter than that in the related weakly bound systems N=N-BF3 and N=C-C=N-BF3. The out-of-plane distortion of the BF3 cannot be determined accurately but is probably less than 3-degrees. We compare the structure, force constants, quadrupole coupling constants, and binding energy of the complex with those of other species formed from BF3 and a variety of nitrogen donors. Despite the rather short B-N bond length, the force constant for the intermolecular bond, as well as the boron and nitrogen nuclear quadrupole coupling constants, is comparable to those of a truly weakly bound system.