Microwave Spectra, Structure, and the Aromatic Character of 1-Chloroborepin

High resolution microwave spectra for the somewhat unstable compound 1-chloroborepin were measured in the 5-10 GHz range using a pulsed beam Fourier transform microwave spectrometer. Transitions were assigned and measured for three isotopologues, which include the most abundant isotopologue, ¹¹B³⁵Cl, and the less abundant ¹⁰B³⁵Cl and ¹¹B³⁷Cl isotopologues. The molecular parameters (MHz) determined for the ¹¹B³⁵Cl isotopologue are A = 3490.905(35), B = 1159.38520(79), C = 870.59492(56), 1.5χ_aa (¹¹B) = -0.220(22), 0.25(χ_bb - χ_cc) (¹¹B) = -1.5300(99), 1.5χ_aa (³⁵Cl) = -54.572(33), and 0.25(χ_bb - χ_cc) (³⁵Cl) = 4.7740(79). The inertial defect is calculated to be Δ = -0.174 amu Ų from the experimental rotational constants, indicating a planar structure with some out of plane vibrational motion. An extended Townes-Dailey analysis was performed on the ¹¹B and ³⁵Cl nuclei to determine the electron occupations in the valence hybridized orbitals using the experimental quadrupole coupling strengths. From the analysis it was determined that Cl is sharing some electron density with the empty p-orbital on B. The B-Cl bond length determined from the data is 1.798(1) Å, and the B-C bond lengths are 1.533(10) Å. The structural parameters and electronic structure properties of 1-chloroborepin are consistent with an aromatic boron-containing molecule.