Microwave spectrum, torsional frequency, barrier to internal rotation and structure of the BCl2 group in phenylboron dichloride

Abstract

The microwave spectra of three isotopic species (C₆H₅¹¹B³⁵Cl₂, C₆H₅¹¹B³⁵Cl³⁷Cl and C₆H₅¹⁰B³⁵Cl₂) of phenylboron dichloride have been observed. The spectra of the first four excited torsional states of C₆H₅¹¹B³⁵Cl₂ and the first two excited torsional states of C₆H₅¹¹B³⁵Cl³⁷Cl have been assigned, the variation of the inertial defect with torsional quantum number gives a torsional frequency of 37 ± 2 cm^–1 and a barrier to internal rotation of V₂= 13.6 ± 0.8 kJ mol ^–1. The r_s structure of the BCl₂ group is r_B–Cl= 176.0 ± 0.8 pm and ClBCl = 121.7 ± 0.5°. With the assumption of regular benzene ring (r_C–C= 139.7 pm and r_C–H= 108.4 pm) a least squares fitting procedure to the observed rotational constants gives the following structural parameters r_B–Cl= 176.0 ± 0.8 pm, r_B–C= 155.6 ± 1.4 pm and ClCl = 121.8 ± 0.4°.