Pnicogen-Bonded Cyclic Trimers (PH2X)3 with X = F, Cl, OH, NC, CN, CH3, H, and BH2

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the structures and binding energies of cyclic trimers (PH2X)(3) with X = F, Cl, OH, NC, CN, CH3, H, and BH2. Except for [PH2(CH3)](3), these complexes have C-3h symmetry and binding energies between -17 and -63 kJ mol(-1). Many-body interaction energy analyses indicate that the two-body terms are dominant, accounting for 97-103% of the total binding energy. Except for the trimer [PH2(OH)](3), the three-body terms are stabilizing. Charge transfer from the lone pair on one P atom to an antibonding sigma* orbital of the P atom adjacent to the lone pair plays a very significant role in stabilization. The charge-transfer energies correlate linearly with the trimer binding energies. NBO, AIM, and ELF analyses have been used to characterize bonds, lone pairs, and the degree of covalency of the P center dot center dot center dot P pnicogen bonds. The NMR properties of chemical shielding and P-31-P-31 coupling constants have also been evaluated. Although the P-31 chemical shieldings in the five most strongly bound trimers increase relative to the corresponding isolated monomers, there is no correlation between the chemical shieldings and the charges on the P atoms. EOM-CCSD P-31-P-31 spin spin coupling constants computed for four (PH2X)(3) trimers fit nicely onto a plot of (1p)J(P-P) versus the P-P distance for (PH2X)(2) dimers. A coupling constant versus distance plot for the four trimers has a second-order trendline which has been used to predict the values of (1p)J(PP) for the remaining trimers.