Microwave Spectrum, Structural Parameters, and Quadrupole Coupling for 1,2-Dihydro-1,2-azaborine

The first microwave spectrum for 1,2-dihydro-1,2-azaborine has been measured in the frequency range 7−18 GHz, providing accurate rotational constants and nitrogen and boron quadrupole coupling strengths for three isotopomers, H6C4 11B14N, H6C4 10B14N, and H5DC4 11B14N. The measured rotational constants were used to accurately determine coordinates for the substituted atoms and provide sufficient data to determine most of the important structural parameters for this molecule. The spectra were obtained using a pulsed beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of 14N, 11B, and 10B nuclear quadrupole hyperfine interactions. High-level ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The rotational constants for the parent compound are A = 5657.335(1), B = 5349.2807(5), and C = 2749.1281(4) MHz, yielding the inertial defect Δ0 = 0.02 amu·Å2 for the ground-state structure. The observed near-zero and positive inertial defect clearly indicates that the molecular structure of 1,2-dihydro-1,2-azaborine is planar. The least-squares fit analysis to determine the azaborine ring structure yielded the experimental bond lengths and 2σ errors R(B−N) = 1.45(3) Å, R(B−C) = 1.51(1) Å, and R(N−C) = 1.37(3) Å for the ground-state structure. Interbond angles for the ring were also determined. An extended Townes−Dailey population analysis of the boron and nitrogen quadrupole coupling constants provided the valence p-electron occupancy p c = 0.3e for boron and p c = 1.3e for nitrogen.