title: Experimental measurement and theory of substituent effects in π-hydrogen bonding: complexes of substituted phenols with benzene.
abstract: IR spectroscopic experiments and theoretical DFT computations reveal the effects of aromatic substituents on π-hydrogen bonding between monosubstituted phenol derivatives and benzene. Simultaneous formation of two π-hydrogen bonds (red-shifting O-H···π and blue-shifting ortho-C-H···π) contribute to the stability of these complexes. The interaction of the acidic phenol O-H proton-donating group with the benzene π-system dominates the complex formation. The experimental shifts of O-H stretching frequencies for the different phenol complexes vary in the range 45-74 cm(-1). Strong effects on hydrogen-bonding energies and frequency shifts of electron-withdrawing aromatic substituents and very weak influence of electron-donating groups have been established. Experimental quantities and theoretical parameters are employed in rationalizing the properties of these complexes. The acidities of the proton-donating phenols describe quantitatively the hydrogen-bonding process. The results obtained provide clear evidence that, when the structural variations are in the proton-donating species, the substituent effects on π-hydrogen bonding follow classic mechanisms, comprising both resonance and direct through-space influences. The performance of three alternative DFT functionals (B3LYP, B97-D, and PBE0 combined with the 6-311++G(2df,2p) basis set) in predicting the O-H frequency shifts upon complexation is examined. For comparison, O-H frequency shifts for several complexes were also determined at MP2/6-31++G(d,p).
authors.affiliation: Department of Chemistry, University of Sofia , 1 James Bourchier Avenue, Sofia 1164, Bulgaria.
authors.forename: Valia
authors.initials: V
authors.lastname: Nikolova
authors.forename: Sonia
authors.initials: S
authors.lastname: Ilieva
authors.forename: Boris
authors.initials: B
authors.lastname: Galabov
authors.forename: Henry F
authors.initials: HF
authors.lastname: Schaefer
mesh_terms: D001554:Benzene / Q000737:chemistry*; D004583:Electrons; D006860:Hydrogen Bonding; D010636:Phenols / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics
chemical_list: D010636:Phenols; D001554:Benzene