A simple theory for predicting the effects of substituent changes on transition-state geometry

Spin Equilibria in Monomeric Manganocenes: Solid-State Magnetic and EXAFS Studies

“…62 In general, as the stability of the carbocations on the reaction coordinate increases, the transition state is moved closer to the reactants and the magnitude of the solvent isotope effect decreases. 52,63,64 Since the highly anionic 1 is able to greatly stabilize cationic guest molecules, the observed shift in mechanism from A-1 to A-S E 2 upon encapsulation may be a direct effect of the preference of monocationic guests to be encapsulated in 1.…”

Isotope effects in nucleophilic substitution reactions. IV. The effect of changing a substituent at the α carbon on the structure of SN2 transition states

“…Once the actual transition state structures had been determined, the effect of changing a substituent at the a carbon could be compared with the theoretical predictions based on Thornton's reacting bond rule (50) and More O'Ferrall's energy surface (6,51). According to Thornton's rule, the overall effect of a S,2 reaction is comprised of the substituent effect on the perpendicular and the parallel vibrations of the transition state.…”

The transition-state acidity and carbonyl stretching frequency in ester hydrolysis. A substituent-effect approach

“…pK a = 47 -34 n CO (2) Calculation of pK a (TS) should allow the estimation of n ‡ CO , which in turn leads to the calculation of f ν ‡ , or the corresponding value for the stretching frequency of the carbonyl oscillator in the transition state.…”

“…2 Reasonable atom connectivities, geometry and force field are the goals to achieve for the description of these transient molecular entities. The first two features are based on chemical intuition or isotopic labelling experiments, whereas the third comes from the molecular analysis of kinetic measurements or from theoretical calculations.…”

“…Major electronic effects are expected to operate through the relatively weak and polarisable bonds of the TS complex, which are significantly being perturbed during the course of the reaction. 2 The strong bonds in reactant molecules and those "non-reacting" bonds in the TS complex are less likely to be perturbed by the action of substituents. As a way of comparison, this Hammett ρ parameter is certainly larger than ρ = 0.47, for the ionisation of ArCH 2 -CO 2 H in water solvent at 25 °C.…”