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, 11 B35 Cl, and the less abundant 10 B35 Cl and 11 B37 Cl isotopologues. The molecular parameters (MHz) determined for the 11 B35 Cl isotopologue are A = 3490.905(35), B = 1159.38520(79), C = 870.59492(56), 1.5χaa (11 B) = -0.220(22), 0.25(χbb - χcc ) (11 B) = -1.5300(99), 1.5χaa (35 Cl) = -54.572(33), and 0.25(χbb - χcc ) (35 Cl) = 4.7740(79). The inertial defect is calculated to be Δ = -0.174 amu Å2 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 11 B and 35 Cl 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.