Theoretical study on some inter- and intra-molecular interactions

Ab-initio MP2&CI and DF calculations were used to study some chemical topics that involve inter- and intra-molecular so-called weak interactions. These topics include: i) What is the physical origin of the single bond rotational barrier, e.g. of ethane? Our answer is that the kinetic Pauli repulsion between CH bond pairs is much more important than hyperconjugative attraction of CH bond pairs through virtual CH σ* orbitals. ii) What is the physical origin of the bond length expansion of electron-rich main-group molecules, e.g. F 2 etc.? It is here dominantly explained by inter-atomic lone pair repulsion, with possible contributions also from atomic hybridization effects of the bonding AOs. The importance of the tails of the lone pairs is stressed. iii) What is the physical origin of reduced nonbonded interatomic separations? We found that most so-called reduced distances in the literature are simply due to the contraction of positively charged atoms. If the ubiquitous charge dependence of effective atomic radii is accounted for, a few really reduced distances survive. They are caused by specific orbital interactions of heavy nonmetal atoms, by specific charge attractions or by clamping bridges. iiii) What is the origin of the different orientations of fluorescence of dye molecules in zeolite channels? Oxonine was studied. We can explain the results of single molecule fluorescence microscopy. Correct van der Waals radii, silica - dye molecule - attractions and rotation of the optical transition moment due to orbital interactions are more important than the electrostatic Stark effect.