Hard and soft electrons and holes

The principle of hard and soft acids and bases (HSAB) has given chemists a broad understanding of the observed selectivity in a variety of reaction classes. As we become increasingly aware of the principle's serious limitations, this study provides an alternative approach. The distinction between hard and soft electrons and holes (HSEH) adds to our understanding of reactivity. Because radicals are typically better stabilized at soft sites and lone pairs are better stabilized at hard sites, we can easily distinguish them. Simple electron density differences (from three single-point density functional theory [DFT] calculations) can be used to visualize this effect and condense the differences into a numerical descriptor. The usefulness of the concept is demonstrated by reproducing the experimentally observed reactivity of a wide range of molecules, including larger examples relevant to the material and pharmaceutical sciences.