Resonance energy transition states

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 681), we invoke the Hanunond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes the carbocation by resonance ... 

R. Sadeghi, R.T. Skodje, High energy transition state resonances in the H+H2 reaction, J. Chem. Phys. 98 (1993) 9208 R. Sadeghi and R.T. Skodje, Spectral quantization of high energy transition state resonances in the H+H2 reaction, J. Chem. Phys. 99, 5126 (1993). 

This effect occurs when the frequency of the incident beam is near a resonant energy transition of the atom or molecule that is responsible for the nonlinear behavior. Near the resonance the electrons occupy a real excited state for a finite period of time. For low intensity light, the population redistribution results in a change of the index of reflection, since it is mostly determined by the molecules in the ground state. A typical value for population redistribution susceptibility is x 10 esu, with a time response 10 s. 

Scheme 6.37a. A representation of the course of the reaction between bromine (612) and 1,3-butadiene (CH2=CH-CH=CH2) showing the potential surface and the rate-determining lowest-energy transition state ( 1) leading to a resonance stabilized intermediate. The reaction of the intermediate with bromide anion (Br") over the lowest-energy transition state (feO leading from the intermediate leads to the kinetic product. The transition state leading to the thermodynamic product ( 2) is lower than the rate-determining transition state but higher than that leading to the kinetic product.

In an electron spin resonance spectrometer, transitions between the two states are brought about by the application of the quantum of energy hv which is equal to g H. The resonance condition is defined when hv = g H and this is achieved experimentally by varying H keeping the frequency (v) constant. Esr spectroscopy is used extensively in chemistry in the identification and elucidation of structures of radicals. 

At the time the experiments were perfomied (1984), this discrepancy between theory and experiment was attributed to quantum mechanical resonances drat led to enhanced reaction probability in the FlF(u = 3) chaimel for high impact parameter collisions. Flowever, since 1984, several new potential energy surfaces using a combination of ab initio calculations and empirical corrections were developed in which the bend potential near the barrier was found to be very flat or even non-collinear [49, M], in contrast to the Muckennan V surface. In 1988, Sato [ ] showed that classical trajectory calculations on a surface with a bent transition-state geometry produced angular distributions in which the FIF(u = 3) product was peaked at 0 = 0°, while the FIF(u = 2) product was predominantly scattered into the backward hemisphere (0 > 90°), thereby qualitatively reproducing the most important features in figure A3.7.5. 

The substituent effects in aromatic electrophilic substitution are dominated by resonance effects. In other systems, stereoelectronic effects or steric effects might be more important. Whatever the nature of the substituent effects, the Hammond postulate insists diat structural discussion of transition states in terms of reactants, intermediates, or products is valid only when their structures and energies are similar. 

In bicyclic azines, as in the monocyclic azines already discussed, the faster of two nucleophilic substitutions proceeds via the transition state which has the lower free energy (with respect to the reactants) due to the stabilizing effects of resonance, hydrogen bonding, or electrostatic attractions. Different nucleophiles and different leaving... 

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