Molecular Clusters of pi-Systems: Theoretical Studies of Structures, Spectra, and Origin of Interaction Energies

Olefinic vs. aromatic pi-H interaction: a theoretical investigation of the nature of interaction of first-row hydrides with ethene and benzene

The nature and origin of the pi-H interaction in both the ethene (olefinic) and benzene (aromatic) complexes of the first-row hydrides (BH(3), CH(4), NH(3), H(2)O, and HF) has been investigated by carrying out high level ab initio calculations. The results indicate that the strength of the pi-H interaction is enhanced as one progresses from CH(4) to HF. Unlike conventional H-bonds, this enhancement cannot be simply explained by the increase in electrostatic interactions or the electronegativity of the atom bound to the pi H-bonded proton. The contributions of each of the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy are important. Thus, the inductive energy is highly correlated to the olefinic pi-H interaction as we progress from CH(3) to HF. On the other hand, both electrostatic and inductive energies are important in the description of the aromatic pi-H interaction. In either case, the contribution of dispersion energies is vital to obtain an accurate estimate of the binding energy. We also elaborate on the correlation of various interaction energy components with changes in geometries and vibrational frequencies. The red-shift of the nu(Y-H) mode is highly correlated to the inductive interaction. The dramatic increase in the exchange repulsion energies of these pi complexes as we progress from CH(4) to HF can be correlated to the blue-shift of the highly IR active out-of-plane bending mode of the pi system.

Blue-Shift in the Frequencies of the CH Stretches of Chloro- and Fluoroform Induced by C–H...π Hydrogen Bonding with Benzene Derivatives: the Influence of Electron Donating and Withdrawing Substituents

Mixed clusters of benzene derivatives and trihalogenmethanes have been prepared in a supersonic beam and studied with infrared vibrational predissociation spectroscopy detected by resonant two photon ionization (IR/R2PI). We report the spectra of chloroform and fluoroform attached to the chromophores fluorobenzene, benzene, toluene,