Portal de Periódicos da CAPES

Arene−Cation Interactions of Positive Quadrupole Moment Aromatics and Arene−Anion Interactions of Negative Quadrupole Moment Aromatics

2006; American Chemical Society; Volume: 110; Issue: 46 Linguagem: Inglês

10.1021/jp065175v

1520-5215

Aimee Clements, Michael Lewis,

Advanced Chemical Physics Studies

Intermolecular interactions involving aromatic π-electron density are widely believed to be governed by the aromatic molecular quadrupole moment, ϑzz. Arene−cation binding is believed to occur primarily with negative ϑzz aromatics, and arene−anion binding is believed to occur largely with positive ϑzz aromatics. We have performed quantum mechanical computations that show the cation binding of positive ϑzz aromatics and the anion binding of negative ϑzz aromatics is quite common in the gas phase. The π-electron density of hexafluorobenzene, the prototypical positive ϑzz aromatic (experimental ϑzz = +9.5 ± 0.5 DÅ), has a Li+ binding enthalpy of −4.37 kcal/mol at the MP2(full)/6-311G** level of theory. The RHF/6-311G** calculated ϑzz value of 1,4-dicyanobenzene is +11.81 DÅ, yet it has an MP2(full)/6-311G** Li+ binding enthalpy of −12.65 kcal/mol and a Na+ binding enthalpy of −3.72 kcal/mol. The π-electron density of benzene, the prototypical negative ϑzz aromatic (experimental ϑzz = −8.7 ± 0.5 DÅ), has a F- binding enthalpy of −5.51 kcal/mol. The RHF/6-311G** calculated ϑzz of C6H2I4 is −10.45 DÅ, yet it has an MP2(full)/6-311++G** calculated F- binding enthalpy of −20.13 kcal/mol. Our results show that as the aromatic ϑzz value increases the cation binding enthalpy decreases; a plot of cation binding enthalpies versus aromatic ϑzz gives a line of best of fit with R2 = 0.778. No such correlation exists between the aromatic ϑzz value and the anion binding enthalpy; the line of best fit has R2 = 0.297. These results are discussed in terms of electrostatic and polarizability contributions to the overall binding enthalpies.