Substituent effects on noncovalent bonds: complexes of ionized benzene derivatives with hydrogen cyanide.

Here, we report the first experimental and computational study of the noncovalent binding energies and structures of ionized benzenes containing electron-withdrawing substituents solvated by one to four HCN molecules. Measured by ion mobility mass spectrometric equilibrium studies, the bond dissociation enthalpies of the first HCN molecule to the fluorobenzene (C6H5F•+), 1,4-difluorobenzene (C6H4F2•+), and benzonitrile (C6H5CN•+) ions (11.2, 11.2, and 9.2 kcal/mol, respectively) are similar to those of HCN with the benzene (C6H6•+) and phenyacetylene (C6H5CCH•+) radical cations (9.2 and 10.5 kcal/mol, respectively). DFT calculations at the B3LYP/6-311++G(d,p) level show that HCN can form in-plane hydrogen bonds to ring hydrogens, or bind electrostatically to positively charged carbon centers in the ring. The electron-withdrawing substituents increase the bond energy by increasing the partial positive charge on the ring hydrogens that form CHδ+---NCH bonds, and by creating a π hole, as evidenced by positiv...