TORSIONAL BARRIERS IN H2B=XCH(3) (X=NH, O, PH, AND S)

Ab initio MO calculations at the MP2/6-31G level are Performed on H2B -XCH(3) structures (X = NH, O, PH, and S) to study the conformational preferences of the methyl group. In all the molecules, the eclipsed co nformer (one of the C-H bonds of the methyl group eclipses B=X) is fou nd to be a minimum. The threefold methyl rotational barrier is found t o decrease as the electronegativity of X increases. The barrier is fur ther reduced on descending down the group. A qualitative argument base d on the interaction of the fragment orbitals is used to rationalize t he observed trends. In contrast to methylaminoborane, methylphosphinob orane possesses a pyramidal environment around phosphorus. The predict ed IR spectrum of this yet to be synthesized molecule is also reported .