A @[email protected] that is attributable to isotopic substitution of an atom to which bonds are neither made nor broken in the @[email protected] or in a @[email protected] of a specified reaction, and is therefore not a @[email protected] One speaks of α, β, (etc.) secondary isotope effects, where α, β, (etc.) denote the position of isotopic substitution relative to the reaction centre. The corresponding @[email protected] on the @[email protected] of such a reaction is called a 'secondary @[email protected]'. Secondary isotope effects have been discussed in terms of the conventional electronic effects of physical organic chemistry, e.g. induction, @[email protected], @[email protected], etc., since these properties are determined by the electron distribution, that depends on vibrationally averaged bond lengths and angles which vary slightly with isotopic substitution. While this usage is legitimate, the term 'electronic @[email protected]' should be avoided, because of the misleading implication that such an effect is electronic rather than vibrational in origin.