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

Aaron M. Pejlovas, Adam M. Daly, Arthur J. Ashe, Stephen G. Kukolich

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8 Scopus citations


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 10B, 11B, and all unique single 13C and the 15N isotopologue (with 11B), in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the 11B14N isotopologue are A = 3042.712 75(43) MHz, B = 1202.706 57(35) MHz, C = 862.220 13(35) MHz, DJ = 0.06(1) kHz, 1.5χaa (14N) = 2.5781(61) MHz, 0.25(χbb - χcc) (14N) = - 0.1185(17) MHz, 1.5χaa (11B) = - 3.9221(75) MHz, and 0.25(χbb - χcc) (11B) = - 0.9069(24) MHz. The experimental inertial defect is Δ = - 0.159 amu Å2, which is consistent with a planar structure for the molecule. The B - N bond length from the experimentally determined structure is 1.47 Å, which indicates π-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 sp2-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.

Original languageEnglish (US)
Article number114303
JournalJournal of Chemical Physics
Issue number11
StatePublished - Mar 21 2016

ASJC Scopus subject areas

  • General Physics and Astronomy
  • Physical and Theoretical Chemistry


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