Using (FH)2 and (FH)3 to Bridge the σ-Hole and the Lone Pair at P in Complexes with H2 XP, for X=CH3 , OH, H, CCH, F, Cl, NC, and CN.

Abstract

Ab initio MP2/aug'-cc-pVTZ calculations are used to investigate the binary complexes H2 XP:HF, the ternary complexes H2 XP:(FH)2 , and the quaternary complexes H2 XP:(FH)3 , for X=CH3 , OH, H, CCH, F, Cl, NC, and CN. Hydrogen-bonded (HB) binary complexes are formed between all H2 XP molecules and FH, but only H2 FP, H2 ClP, and H2 (NC)P form pnicogen-bonded (ZB) complexes with FH. Ternary complexes with (FH)2 are stabilized by F-H⋅⋅⋅P and F-H⋅⋅⋅F hydrogen bonds and F⋅⋅⋅P pnicogen bonds, except for H2 (CH3 )P:(FH)2 and H3 P:(FH)2 , which do not have pnicogen bonds. All quaternary complexes H2 XP:(FH)3 are stabilized by both F-H⋅⋅⋅P and F-H⋅⋅⋅F hydrogen bonds and P⋅⋅⋅F pnicogen bonds. Thus, (FH)2 with two exceptions, and (FH)3 can bridge the σ-hole and the lone pair at P in these complexes. The binding energies of H2 XP:(FH)3 complexes are significantly greater than the binding energies of H2 XP:(FH)2 complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone-pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge-transfer energies across the pnicogen bond correlate with the intermolecular P-F distance, while charge-transfer energies across F-H⋅⋅⋅P and F-H⋅⋅⋅F hydrogen bonds correlate with the distance between the lone-pair donor atom and the hydrogen-bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron-donor atom and the hydrogen-bonded F atom. EOM-CCSD spin-spin coupling constants (2h) J(F-P) across F-H⋅⋅⋅P hydrogen bonds, and (1p) J(P-F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F-P coupling data than pnicogen bonds, despite the longer F⋅⋅⋅P distances in F-H⋅⋅⋅P hydrogen bonds compared to P⋅⋅⋅F pnicogen bonds. There is a correlation between the two bond coupling constants (2h) J(F-F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes.