Activation energy-lowering effect

Assuming that the Ih-lp excitations do not affect the hopping parameter calculate the energy lowering effect on U of the inclusion of the Ih-lp excitations combined with the electron replacement in the active space using the numerical data from Tables 5.1 and 5.2. 

The effect of temperature on the non-catalysed reaction was difficult to disentangle, for at lower temperatures the autocatalytic reaction intervened. However, from a limited range of results, the reaction appeared to have an experimental activation energy of c. +71 kj moh. ... 

The contribution of this polar structure to the bonding lowers the energy of the transition state. This may be viewed as a lower activation energy for the addition step and thus a factor which promotes this particular reaction. The effect is clearly larger the greater the difference in the donor-acceptor properties of X and Y. The transition state for the successive addition of the same monomer (whether X or Y substituted) is structure [V] ... 

The transition is fully classical and it proceeds over the barrier which is lower than the static one, Vo = ntoColQl- Below but above the second cross-over temperature T 2 = hcoi/2k, the tunneling transition along Q is modulated by the classical low-frequency q vibration. The apparent activation energy is smaller than V. The rate constant levels off to its low-temperature limit k only at 7 < Tc2, when tunneling starts out from the ground state of the initial parabolic term. The effective barrier in this case is neither V nor Vo,... 

There is, however, no direct evidence for the formation of Cl", and it is much more likely that the complex is the active electrophile. The substrate selectivity under catalyzed conditions ( t j = 160fcbenz) is lower than in uncatalyzed chlorinations, as would be expected for a more reactive electrophile. The effect of the Lewis acid is to weaken the Cl—Cl bond, which lowers the activation energy for o-complex formation. 

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