| Sumario: | Equilibrium hydrogen-bond dissociation energies De for the process B HX = B + HX are calculated at the CCSD(T)(F12c)/cc-pVDZ-F12 level for B190 complexes B HX. As established earlier, De values for such complexes can be described by the equation De = cNBEA , in which NB and EHX 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 De as the ordinate and EHX 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 NB 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 smin 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 De/smin plotted against EHX defines a reduced nucleophilicity NB = NB /smin in the sense that NB 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).
|
|---|
