Molecules in Crystals — What Makes Them Different?

Abstract

The goal of molecular design is to create molecular aggregates and materials with special, tailored properties. Such an endeavour will be more successful the more we know about molecular interactions and how the properties of a molecule change in different surroundings. As long as a molecule keeps its essential identity in the bound state, it is appealing to retain the concept of’ a molecule’ even within a crystal or a liquid, because it relates so closely to well known chemical concepts. However, the properties of many molecules, including the water molecule, are known to be sensitive to their surroundings and their states of aggregation. For instance, the average O…O distance in the gaseous water dimer is 2.98 Å, while in liquid water it is only ~2.85 Å, and in normal ice ~2.75 Å. This gradual shortening of intermolecular bond distances is a manifestation of the cooperative nature of the intermolecular bonding, where certain geometrical arrangements give rise to large changes in molecular properties. In this paper, I will use the water molecule as an example, because water is a good and important representative of molecules which exhibit long-ranged interactions. Such molecules affect their neighbours, but the interaction also leads to changes in the molecule itself - a feature highly relevant in the context of molecular design.