Analysis of Intermolecular Interactions by Ab Initio Molecular Orbital Calculations: Importance for Studying Organic Crystals

Abstract

Ab initio molecular orbital calculation is becoming a powerful method for studying intermolecular interactions. The analysis of the nature of intermolecular interactions by ab initio calculation is important for studying the roles of intermolecular interactions in determining crystal structures, although the intermolecular interactions have been discussed mainly based on crystal structures. It is often dangerous to judge the nature of intermolecular interactions solely on crystal structures. For example, it is sometimes claimed that the CH/π interaction is weak hydrogen bond, and the CH/π interaction is important for determining crystal structures probably due to the similarity of the structures of CH/π interaction and π hydrogen bond. But ab initio molecular orbital calculations show that the nature of the CH/π interaction is completely different from that of π hydrogen bond. The hydrogen bond is strong and directional, and therefore, the hydrogen bond is important in determining orientation of molecules in crystals. On the other hand, the CH/π interaction is weak, and the directionality is negligible in general, which suggests that most of the CH/π interaction cannot play important roles in determining orientation of molecules in crystals.