Effects of substituents upon the P···N noncovalent interaction: the limits of its strength.

Previous work has documented the ability of the P atom to form a direct attractive noncovalent interaction with a N atom, based in large measure on the charge transfer from the N lone pair into the σ* antibonding orbital of the P-H that is turned away from the N atom. As the systems studied to date include only hydrides, the present work considers how substituents affect the interaction and examines whether P···N might compete with other attractive forces such as H-bonds. It is found that the addition of electron-withdrawing substituents greatly strengthens the P···N interaction to the point where it exceeds that of the majority of H-bonds. The highest interaction energy occurs in the FH(2)P···N(CH(3))(3) complex, amounting to 11 kcal/mol. A breakdown of the individual forces involved attributes the stability of the interaction to approximately equal parts electrostatic and induction energy, with a smaller contribution from dispersion.