Microwave measurements and ab initio calculations of structural and electronic properties of N-Et-1,2-azaborine

Rotational transitions for N-Et-1,2-azaborine were measured in the 5-13 GHz range using a Flygare-Balle type Fourier transform spectrometer system. Twelve distinct rotational transitions with over 130 resolved hyperfine components, which included a-dipole and b-dipole transitions, were measured and analyzed to obtain rotational constants and (11)B and (14)N nuclear quadrupole coupling constants in the principal rotational axis system. Rotational constants obtained are A=4477.987(4), B=1490.5083(7), and C=1230.6728(6) MHz. The quadrupole coupling constants for (11)B are eQq(aa)=-1.82(1), (eQq(bb)-eQq(cc))=-3.398(4) MHz, and for (14)N, eQq(aa)=1.25(1), (eQq(bb)-eQq(cc))=0.662(4) MHz. Quantum electronic structure calculations predict a ground-state structure with the ethyl group perpendicular to the azaborine plane and rotational constants in very good agreement with the measured structure and rotational constants. The theoretical conformational analysis of the ethyl group rotation around the N[Single Bond]C bond in relation to the heterocyclic ring yielded an asymmetric torsional potential energy surface with barrier heights of about 900 and 1350 cm(-1) for the N-Et-1,2-azaborine. Results of the measurements and calculations indicate that the basic molecular structure of N-Et-1,2-azaborine is similar to ethylbenzene. Electrostatic potential calculations qualitatively show that pi-electron density is somewhat delocalized around the 1,2-azaborine ring.