Microwave spectra, molecular structure, and aromatic character of 4a,8a-azaboranaphthalene

The microwave spectra for seven unique isotopologues of 4a,8a-azaboranaphthalene [hereafter referred to as BN-naphthalene] were measured using a pulsed-beam Fourier transform microwave spectrometer. Spectra were obtained for the normal isotopologues with B-10, B-11, and all unique single C-13 and the N-15 isotopologue (with 11B), in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the (BN)-B-11-N-14 isotopologue are A = 3042.712 75(43) MHz, B = 1202.706 57(35) MHz, C = 862.220 13(35) MHz, D-J = 0.06(1) kHz, 1.5 chi(aa) (N-14) = 2.5781(61) MHz, 0.25(chi(bb)-chi(cc)) (N-14) = -0.1185(17) MHz, 1.5 chi(aa) (11B) = -3.9221(75) MHz, and 0.25(chi(bb) -chi(cc)) (11B) = -0.9069(24) MHz. The experimental inertial defect is Delta = -0.159 amu angstrom(2), which is consistent with a planar structure for the molecule. The B-Nbond length from the experimentally determined structure is 1.47 angstrom, which indicates p-bonding character between the B and N. The measured quadrupole coupling strengths provide important and useful information about the bonding, orbital occupancy, and aromatic character for this aromatic molecule. Extended Townes-Dailey analyses were used to determine the B and N electron sp(2)-hybridized and p-orbital occupations. These results are compared with electron orbital occupations from the natural bond orbital option in theoretical calculations. From the analyses, it was determined that BN-naphthalene has aromatic character similar to that of other N-containing aromatics. The results are compared with similar results for B-N bonding in 1,2-dihydro-1,2-azaborine and BN-cyclohexene. Accurate and precise structural parameters were obtained from the microwave measurements on seven isotopologues and from high-level G09 calculations. (C) 2016 AIP Publishing LLC.