Structural and electronic aspects of hydrogen bonding in two polymorphs of butylene-N,N'-bis(O,O'-diarylphosphoramidate).

The bisphosphoramidate (C(6)H(5)O)(2)P(O)NH(CH(2))(4)NHP(O)(OC(6)H(5))(2) crystallizes in two polymorphs, one (ndl) with a needle habit from tetrahydrofuran (THF)/ethanol and another (prm) which forms prisms from H(2)O/ethanol. The molecules in the two forms differ from each other in some torsion angles and the orientation of the diaminobutane bridge, although the differences between the similar bond lengths are not significant for the two polymorphs. The geometry optimizations at the B3LYP/6-31+G* level for isolated molecules show that the two conformers which exist in the crystalline state also represent local gas-phase energy minima. The decrease in the N-H distance from the optimized to the crystal structures has been described in terms of the decrease in electron density (ρ) at the bond-critical point (b.c.p.) of the N-H bond path when the molecule participates in hydrogen bonding, comparing the results of atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses for fully optimized structures ndl and prm with their hydrogen-bonded model clusters.