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 ¹¹B and ¹⁰B species with both HC¹⁴N and HC¹⁵N, and the hyperfine structure has been analyzed. The complex has the expected C_3v structure, with the nitrogen end of the HCN toward the boron, but the observed BN bond length of 2.473(29) Å 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°. 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.