Observations and Descriptions versus Explanations-An Example: Does Nature, Does Theory Know About Steric Hindrance?

Quantum chemical investigations in the literature sometimes underpin traditional chemical concepts (such as those of different primary and secondary types of chemical bonding or of steric repulsions) but sometimes also seem to undermine them. Conceptual pitfalls in recent discussions in this and other leading journals, when combining empirically and quantum chemically based reasoning, can be avoided when taking four points into account: 1) The construction of theoretical concepts should be guided by both, physical rigor and the aim of recovering the notions that have been found chemically fruitful. 2) Partitioning of a chemical phenomenon into elementary physical steps should not only consider the stationary initial and final states but also include non‐stationary intermediate states that exhibit the physically characteristic internal relaxation mechanism. 3) Even if sufficiently accurate, the partitioning into large contributions of opposite sign is less valuable for a qualitative chemical understanding, as long as the trend of one big decisive term or sum of terms cannot be made obvious (for instance on the basis of the variation principle—or by chemical intuition). 4) Causal explanatory notions, even if more vague or fuzzy than some more rigorous and unique descriptive concepts, have to be advanced towards a more fruitful chemical theory. Physical description—chemical explanation: Traditional chemical concepts such as steric hindrance or the various types of chemical bonding should not be rejected; they can indeed be underpinned by quantum chemical theory. Toward this aim more is needed, however, than the measurement and calculation of the precursors and products. Also the analysis of non‐stationary intermediate states is required which disclose the internal relaxation mechanism of the geometric and electronic structures. Thereby, conceptual pitfalls are avoided.