Molecular structure and scaled quantum mechanical force fields of bis(dimethylamino)chloroborane, [(CH 3) 2N] 2BCl, determined by gas-phase electron diffraction and molecular orbital calculations; and supplementary computations for [(CH 3) 2N] 3B, (CH 3) 2NBCl 2, and BCl 3

Abstract

The molecular structure of bis(dimethylamino)chloroborane, [(CH 3) 2N] 2BCl, has been determined by gas-phase electron diffraction (GED) at 320 K and by molecular orbital (MO) calculations at the HF, MP2, B3LYP, and B3PW91/6-311++G** levels of theory. Vibrational analyses were carried out resulting in refined scaled quantum mechanical force fields based on literature data with some reassignments. B3PW91/6-311++G** results were used for interplay with the GED structural investigation. All results were consistent with C 2 molecular symmetry with slightly non-planar B-NC 2 groups. The final results were obtained with shrinkage corrections for the skeletal non-bond distances. The two types of carbon atoms are identified by E and Z subscripts, referring to C–N–B–N, and the results with effects from correlation in the data and uncertainty in the s-scale (0.1%) included in the standard deviations, are: r a(B–Cl)=180.9(3) pm, r a(B–N)=142.6(3) pm, r a(N–C E)= r a(N–C Z)+0.04 pm=145.6(2) pm, ∠N–B–N=124.7(3)°, ∠B–N–C E=123.0(5)°, ∠B–N–C Z=123.8(4)°, θ (N–B–N–C E)=−161.2(12)°, and Δ θZ=187.2(16)° giving θ (N–B–N–C Z)=Δ θZ+ θ (N–B–N–C E)=26.4(11)°. This GED structure has a difference Δ(NC–BN)=3.0(3) pm. The locations of the hydrogen atoms relied heavily on MO-constraints using only one distance and one valence angle variable to give 〈r a ( C– H)〉=111.1(2) pm, and 〈∠N–C–H〉=110.6(4)°, and two independent torsional angles that in the final refinement were fixed to the computational values of θ (B–N–C E–H)=8.0° and θ (B–N–C Z–H)=6.0°. The experimental and computational results are compared with literature GED results for the three additional compounds in the [(CH 3) 2N] 3− n BCl n ( n=0–3) series of molecules and own results from compatible computations for these molecules. The bond-length variations are discussed in relation to different possibilities for π back donation to the electron deficient trivalent boron atom and strengthening of the σ-bond as the number of electronegative ligands attached to the central atom increases.