Substituent effects on cooperativity of pnicogen bonds.

Substituent effects on cooperativity of P···N pnicogen bonds are studied in XH2P···NCH(2)P···NCY (X=F, Cl; Y=H, F, CN, OH, NH(2)) complexes using high-level ab initio calculations. An increased attraction or a positive cooperativity is observed on introduction of a third molecule to the XH(2)P···NCH(2)P and NCH(2)P···NCY dyads. The shortening of the each pnicogen bond distance in the triads is dependent on the strength of the P···N bond and is increased in the order Y = NH(2) > OH > H > F > CN. The energy decomposition analysis indicates that the polarization energy is the important element in the interaction energy of P···N bond and may be regarded as being responsible for the stabilization in these systems. Natural bond orbital theory is used to characterize the interactions and analyze their enhancement with varying orbital interactions.