The Electrophilicities of XCF₃ and XCl (X=H, Cl, Br, I) and the Propensity of These Molecules To Form Hydrogen and Halogen Bonds with Lewis Bases: An <i>Ab Initio</i> Study

Abstract Equilibrium dissociation energies, D e , of four series of halogen‐ and hydrogen‐bonded complexes B⋅⋅⋅XCF 3 (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 2 , CO, PH 3 , C 2 H 2 , C 2 H 4 , H 2 S, HCN, H 2 O and NH 3 . Plots of D e versus N B , where the N B 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 e versus N B plots define the electrophilicities E XCF3 and E XCl of the various Lewis acids. The determined values are: E XCF3 =2.58(22), 1.40(9), 2.15(2) and 3.04(9) for X=H, Cl, Br and I, respectively, and E XCl =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 XCl / E XCF3 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 e and the maximum positive electrostatic potential σ max (the most electrophilic region of XCF 3 and XCl, which lies on the symmetry axes of these molecules, near to the atom X) are strongly correlated.

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