The Reaction Co-ordinate

Chemiluminescence must be the result of a non-adiabatic reaction in that the ground state potential energy surface describing the molecule undergoing reaction must intersect with a different, excited state surface. 

Potential energy surfaces for the reactions can be obtained with varying degrees of precision. Some are merely illustrative, [37, 38, 39] while others lend themselves to calculation [40, 41]. Among the factors which may influence the population of the excited state are the following. 

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Significantly, the pre-exponential factors decrease with increasing reactivity, and this suggests that the Wheland intermediate is more nearly formed in the transition state, the more reactive the compound. Or, considered another way, the position along the reaction co-ordinate at which a given amount of carbon-halogen bond formation occurs is nearer to the ground state the more reactive the compound. 

OC1 with the sulfinyl sulfone thus appears to represent a substitution at sulfinyl sulfur where it is necessary to postulate an intermediate of the type shown in [47] on the reaction co-ordinate in order best to explain the behavior observed. 

Such bimolecular substitutions at a sulfenyl sulfur, just as was true for analogous substitutions at sulfinyl sulfur, can in principle take place either by a mechanism in which bond making and bond breaking are concerted (173a), or alternatively, by one (173b) where bond making precedes bond breaking and an intermediate [60] is present on the reaction co-ordinate. 

The relationship between geometric and charge progression along the reaction co-ordinate 186... 

A plot of the energies of the two configurations, R X and R+ X, in the gas phase as a function of the reaction co-ordinate is illustrated by the bold lines in Fig. 5. Each curve shows a minimum, the R X configuration because of the out-of-phase resonance, R X <— R, X (the in-phase resonance describing one of the triplet forms of the R—X bond), while R + X " shows a minimum due to the electrostatic attraction between cation and anion. 

The reaction mechanism and hence the reaction co-ordinate will, themselves, be governed by the nature of the configurations from which the profile is built up. Thus, for example, for the set of nucleophilic substitution reactions a large contribution of the carbocation configuration will endow the entire reaction co-ordinate with SN1 character. 

From this brief description it is quite apparent that the qualitative elements of the Marcus treatment for an electron transfer process are identical to the CM model. In CM terms the reaction involves the avoided crossing of reactant (Fe2+ + Fe3+) with product (Fe3+ + Fe2+) configurations, with the reaction co-ordinate just being the distortion-relaxation motion of the solvation sphere. Thus in CM terms any electron transfer reaction involves the avoided crossing of the DA (donor-acceptor) and D+A" configurations, and for such reactions at least, based on the equivalence with Marcus theory, the CM model has a solid foundation. 

THE RELATIONSHIP BETWEEN GEOMETRIC AND CHARGE PROGRESSION ALONG THE REACTION CO-ORDINATE... 

The concept of a reaction co-ordinate in organic chemistry is one that is not simply defined, since for a polyatomic system of n atoms it may depend on as many as 3n — 6 parameters for complete characterization. The reaction co-ordinate used by organic chemists in their representation of a reaction profile has only a qualitative significance. For example, that for an SN2 process on a methyl derivative is generally equated with the change in either the... 

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