The molecular structure of diphenylchloroborane studied by gas-phase electron diffraction and ab initio MO calculations including computational studies of conformational preferences and quadratic force fields

Abstract

The molecular structure of diphenylchloroborane, (C 6H 5) 2BCl, has been determined by gas-phase electron diffraction (GED) at 415 K and by ab initio MO calculations at the HF/3-21G∗ level, both being consistent with C 2 molecular symmetry. Corresponding ab initio fundamental frequencies are reported and an approximate HF/3-21G∗//HF/3-21G∗ scaled quantum mechanical (SQM) force field was established, transferring scale factors from a concurrent study of dichlorophenylborane, C 6H 5BCl 2. Vibrational amplitude quantities needed in the GED structural analysis were computed from the SQM force field. Planarity of the phenyl groups and the ClBC 2 framework was assumed in addition to C 2 molecular symmetry, and the geometrical parameters determined in the GED analysis, with effects from correlation in the data and uncertainty in the s-scale (0.1%) included in the standard deviations are: r a(CH)(para) = 110.8(6) pm, r a(CC)(ortho) = 141.0(2) pm, r a(BC) = 155.5(7) pm, r a(BCl) = 178.1(8) pm, ∡ α C B Cl = 117.2(5)° , and ф α( B C) = 29.5(1.8)° . Other geometry-defining parameters such as tilt of the phenyl groups and their distortion from the benzene structure, for example ∡ α C C C(ipso) = 117.9° , were taken from the computational structure. For comparison purposes the HF/3-21G∗ computational structures are also reported for the three other molecules in the four-molecule series of (C 6H 5) 3− n BCl n , n = 0, 1, 2, 3. For the three phenylboranes ( n = 2, 1, 0) there is an elongation of the BC bonds as the number of phenyl groups increases (154.4, 155.8 and 157.2 pm, with ф-values of 0, 27.7 and 33.7°), whereas for the chloroboranes ( n = 1, 2, 3) the BCl bonds become shorter (179.8, 176.9 and 174.7 pm) with an increasing number of chlorines attached to boron.