Substituent Effects on the Properties of Pnicogen-Bonded Complexes H2XP:PYH2, for X, Y = F, Cl, OH, NC, CCH, CH3, CN, and H

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on the pnicogen-bonded homodimers (PH2X)(2) and the binary complexes H2XP:PYH2, for X, Y = F, Cl, OH, NC, CCH, CH3, CN, and H. The binding energies of these complexes are influenced by the nature of the X,Y pair, the intermolecular distance, the relative orientation of the interacting molecules, and the charge-transfer energies from the lone pair of one P to the s-hole of the other. Binary complexes with X,Y = F, Cl, OH, and NC, as well as the homodimers, have a trans arrangement of the P-A and P-A' bonds with respect to the P center dot center dot center dot P bond, with A and A', the atoms of X and Y, respectively, bonded to the P atoms. The trendlines for the homodimers in plots of the binding energy versus the P-P distance, and the binding energy versus the total charge-transfer energy, exhibit better correlations than the trendlines for the binary complexes. The trendlines for the homodimers mark the boundary of the region in which points for the binary complexes appear. Pnicogen-bond radii for P in PH2X molecules have been determined from the P-P distances in the homodimers. The sum of these radii provides an excellent approximation to the P-P distance in the corresponding binary complex. EOM-CCSD spin-spin coupling constants (1p)J(P-P) have also been computed for all complexes. Coupling constants for the dimers and binary complexes exhibit a similar linear increase as the P-P distance decreases.