Energies of minima and TS as well as characteristic NMR chemical shifts are reported for the parent unsaturated
phospho-organic molecules with CP2 backbone. Within the 14 relevant isomers of CH4P2, the two most stable
structures are 1,2-diphospha-l-propene, 3, and diphosphirane (cyclo-CH2(PH)2, 1, E
r
el = 8 kJ/mol). The relative
energies in kJ/mol at MP2/6-31G(d,p) are 84 for 1,3-diphospha-2-propene, 54 for 1,2-diphospha-2-propene,
and for the phosphinidenes, they are 63 (P−PH−CH3) and 102 (P−CH2−PH2). Although the potential
intermediate products (PH−CH2−PH, PH−PH−CH2, and cyclo-CH2−PH2−P) of diphosphirane rearrangements have the relative energies 182, 157, and 158 kJ/mol, respectively, and other minima were found to
have E
r
el between 287 and 322 kJ/mol. Rupture of endocyclic bonds of 1a displays a preference for opening
the P−C bond toward the CH2−PH−PH structure, which is stabilized by allyl conjugation. The lowest energy
pathway for isomerization of 1 has a barrier of 213 kJ/mol and goes via the intermediate cyclo-(CH2)(PH2)(P) toward PH2−PCH2. The calculated energy barriers of CH3−PPH, 3, indicate that this isomer should
be kinetically stable as isolated molecule.