Structural stability and vibrational analysis of dihalophosphoric acids HO-POX₂ and aminophosponic dihalides NH₂ -POX₂ (X = F, CI)

The structural stability of dihalophosphoric acids X₂POOH and aminophosphonic dihalides (phosphoramidic dihalides) X₂PONH ₂ (X = F, Cl) was investigated by DFT-B3LYP and ab initio MP2 levels of theory using extended 6-311+G basis set. From the calculations difluorophosphoric acid was calculated to exist in planar structure (hydrogen eclipses P=O bond), while dichlorophosphoric acid to exist as a non-planar structure with OPOH torsional angle of about 14-30°. The P-O rotational barrier was calculated to be about 2-3 kcal/mol in difluorophosphoric acid. Aminophosphonic difluoride and dichloride were predicted to have nonplanar structure. The planar structure of the two amides was predicted to have an imaginary frequency by both levels of calculations. The vibrational frequencies were computed, normal coordinate analyses were carried out and potential energy distributions (PED) were calculated for the four molecules in their low energy structures. On the basis of PED values and comparison with experimental data of the acids reliable assignments were provided for the normal modes of the four molecules.