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 conformer (one of the C-H bonds of the methyl group eclipses B=X) is found to be a minimum. The threefold methyl rotational barrier is found to decrease as the electronegativity of X increases. The barrier is further reduced on descending down the group. A qualitative argument based on the interaction of the fragment orbitals is used to rationalize the observed trends. In contrast to methylaminoborane, methylphosphinoborane possesses a pyramidal environment around phosphorus. The predicted IR spectrum of this yet to be synthesized molecule is also reported.