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

Abstract

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 C3h 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 σ* 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···P pnicogen bonds. The NMR properties of chemical shielding and (31)P-(31)P coupling constants have also been evaluated. Although the (31)P 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 (31)P-(31)P 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(P-P) for the remaining trimers.