Rigorous interpretation of electronic density functions of axial and equatorial conformers of dimethylphosphinoylcyclohexane, 2-(dimethylphosphinoyl)-1,3,5-trithiane, and 2-(dimethylphosphinoyl)-1,3-diethiane-1,1,3,3-tetraoxide

Theoretical analysis within the frame of the Topological Theory of Atoms in Molecules confirms the repulsive steric interaction between an axial dimet hylphosphinoyl group and the syn-diaxial hydrogens in cyclohexane derivativ e 2-ax. In seemingly good agreement with experiment, equatorial isomer 2-eq was calculated to be 1.49 kcal/mol more stable than 2-ax. (Experimental en ergy difference in (diphenylphosphinoyl)cyclohexane, DeltaH(0) = 1.96 kcal/ mol.) In contrast, axial 2-(dimethylphosphinoyl)-1,3,5-trithiane, 3-ax, was calculated to be 6.38 kcal/mol more stable than 3-eq. (Experimentally, the axial conformer of 2-(diphenylphosphinoyl)-1,3,5-trithiane, was found to b e 1.43 kcal/mol more stable than the equatorial conformer, in solvent chlor oform.) Theoretical analysis, in particular the electron density at the bon d critical point within the C(4,6)-H . . .O=P bonding trajectory, implies s ignificant bonding in this segment of interacting atoms. By the same token, substantial positive charge is acquired by the C-H bonds adjacent to the s ulfonyl groups in disulfone 4. Hydrogen bending between the phosphoryl grou p and H(4,6) leads to stabilization of 4-ax, which is estimated to be 5.0 k cal/mol lower in energy than 4-eq. This conclusion is supported by examinat ion of P=O . . .H-C(4,6) bond trajectories, as well as from evaluation of t he critical point properties along those interacting moieties. By contrast, fluorinated derivative 5 is more stable in the equatorial conformation, in dicating a repulsive electrostatic interaction of the C-F . . .O-P entity i n 5-ax.