Reduced nucleophilicity: an intrinsic property of the Lewis base atom interacting with H in hydrogen-bonds with Lewis acids HX (X = F, Cl, Br, I, CN, CCH, CP)

Abstract

Equilibrium hydrogen-bond dissociation energies D_e for the process B⋯HX = B + HX are calculated at the CCSD(T)(F12c)/cc-pVDZ-F12 level for ∼190 complexes B⋯HX. As established earlier, D_e values for such complexes can be described by the equation D_e = cN_BE_A, in which N_B and E_HX are the nucleophilicity and electrophilicity of the Lewis base B and the Lewis acid HX, respectively, and the constant c = 1 kJ mol⁻¹. Graphs of D_e as the ordinate and E_HX as the abscissa are presented for 26 series of hydrogen-bonded complexes B⋯HX. The Lewis base is fixed and HX is HF, HCl, HBr, HI, HCN, HCCH and HCP in each series. Each plot yields a good straight line, the slope of which is the nucleophilicity N_B of B. The Lewis bases are chosen for their simplicity and all have at least one non-bonding electron pair carried by the atom directly involved in the B⋯HX hydrogen bond. The direction of the minimum value σ_min of the molecular electrostatic surface potential on the 0.001 e bohr⁻³ iso-surface in the chosen bases B usually coincides with the axis of a non-bonding electron pair. The gradient of a graph of D_e/σ_min plotted against E_HX defines a reduced nucleophilicity И_B = N_B/σ_min in the sense that И_B appears to be a property only of the atom of B that is directly involved in the B⋯HX hydrogen bond, independent of the remainder of B. For example, the values of the reduced nucleophilicity for the series of isocyanide complexes CH₃NC⋯HX, HNC⋯HX and FNC⋯HX are 0.0343(16), 0.0337(18) and 0.0332(18), respectively, while those for the corresponding series of cyanide complexes are 0.0337(23), 0.0329(24) and 0.0333(23).