This chapter explores how the directed nature of chemical bonding affects molecular collisions and chemical reactions. Whereas the concept of size or cross-section leads to numbers (scalar quantities), the concept of chemical shape leads to vectors (numbers tied to directions). Consequently, this topic is more mathematically challenging, but its study yields important insights into the nature of chemical transformations. As expected, just as the size of a molecule depends on the probe selected to measure this quantity, the chemical shape of a molecule also depends sensitively on the probe chosen for the measurement. It is important to stress that the physical shape of a molecule is most commonly determined from molecular spectroscopy; it is usually expressed in terms of bond angles and bond lengths. The chemical shape of a molecule refers to the apparent size and shape of a molecule as experienced by another atom or molecule that collides with it.

The steric factor and early history of stereodynamics

The notion of spatial requirements for a chemical reaction dates to the introduction of a steric factor p in simple versions of the collision theory of reactions, Section 3.2.5. This was forced upon the theory by the smaller than expected, or even much smaller, overall magnitude of the reaction cross-section. Unfortunately, the early theory gave no clue for determining the value of p, which has largely been treated as an adjustable parameter to make simple collision theory agree with observation.