Structures of H2Al=XHCH3 and H2Al=YCH3 (X = N, P, and As; Y = O, S, and Se) systems were investigated using ab initio method at the G(2) level to study the conformational preferences of the methyl group. In all of the molecules, the eclipsed C-s symmetry arrangement (one of the C-H bonds of the methyl group eclipses Al=X(Y)) conformer is found to be more stable than the staggered C, symmetry arrangement (the C-H bond is trans to Al=X(Y))conformer. The G(2) energetic results show that the 3-fold methyl rotational barrier is found to decrease as the electronegativity of X(Y) increases. They also show that this 3-fold methyl rotational barrier decreases when descending in the corresponding periodic table column, from nitrogen (or oxygen) to arsenic (or selenium) atoms. A qualitative argument based on the interaction of the fragment orbitals is used to rationalize the observed trends. The thermodynamic values of the methyl transfer reactions are examined. The possible dissociation processes of H2Al=XH(Y)CH3 systems into HAlXH(Y) and CH4 or into HAlXCH3 and molecular H-2 are also examined and reported.