The Electrophilicities of XCF 3 and XCl (X=H, Cl, Br, I) and the Propensity of These Molecules To Form Hydrogen and Halogen Bonds with Lewis Bases: An Ab Initio Study

Equilibrium dissociation energies, D , of four series of halogen- and hydrogen-bonded complexes B⋅⋅⋅XCF (X=H, Cl, Br and I) are calculated ab initio at the CCSD(T)(F12c)/cc-pVDZ-F12 level. The Lewis bases B involved are N , CO, PH , C H , C H , H S, HCN, H O and NH . Plots of D versus N , where the N are the nucleophilicities assigned to the Lewis bases previously, are good straight lines through the origin, as are those for the corresponding set of complexes B⋅⋅⋅XCl. The gradients of the D versus N plots define the electrophilicities E and E of the various Lewis acids. The determined values are: E =2.58(22), 1.40(9), 2.15(2) and 3.04(9) for X=H, Cl, Br and I, respectively, and E =4.48(22), 2.31(9), 4.37(27) and 6.06(37) for the same order of X. Thus, it is found that, for a given X, the ratio E / E is 2 within the assessed errors, and therefore appears to be independent of the atom X and of the type of non-covalent interaction (hydrogen bond or different varieties of halogen bond) in which it is involved. Consideration of the molecular electrostatic surface potentials shows that D and the maximum positive electrostatic potential σ (the most electrophilic region of XCF and XCl, which lies on the symmetry axes of these molecules, near to the atom X) are strongly correlated.