Cyclophosphamide and isophosphamide - DFT conformational studies in the gas phase and solution

Cyclophosphamide and isophosphamide have been subjected to comprehensive conformational studies in the vacuum and solution using the SMD solvation model. Vacuum calculations were conducted using the B3LYP, M05-2X, M06-2X and ωB97XD functionals. Natural bond orbital (NBO) analysis has been performed for selected geometries. A preference for a chair conformation with the axial P=O bond is shown (¹C₄). The ⁵S₀ conformation is 1.25-2.31 kcal/mol and 1.72-2.92 kcal/mol higher in energy than the global minimum conformations of cyclophosphamide and isophosphamide, respectively. In the gas phase, the chair conformation with the equatorial P=O bond (⁴C₁) is of comparable stability or less stable than the skew form, depending on the method used, while it is slightly more favored than the ⁵S₀ conformation in solution. The stereoelectronic effects do not differentiate the ring conformer stability. The steric strains between N(EtCl)_1-2 and the C4 and C6 carbon atoms mainly influence the stability of cyclophosphamide and isophosphamide conformers.