Page 1093 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1078 changes. The rate characteristics of the reactions following radiation can be determined
from these spectroscopic changes. Various other techniques have been developed to
CHAPTER 12 follow the exceedingly fast changes that occur immediately after excitation. Some of
Photochemistry them can detect changes that occur over 10–100 fs (10 −15 s).
A useful technique for indirectly determining the rates of certain reactions involves
measuring the quantum yield as a function of quencher concentration. A plot of the
inverse of the quantum yield versus quencher concentration is then made (Stern-Volmer
plot). As the quantum yield indicates the fraction of excited molecules that go on to
product, it is a function of the rates of the processes that result in other fates for the
excited molecule. These processes are described by the rate constants k (quenching)
q
and k (other nonproductive decay to ground state):
n
k r
= (12.2)
k +k Q
+k
r q n
A plot of 1/ versus [Q] then gives a line with the slope k /k . It is often possible to
q
r
assume that quenching is diffusion controlled, permitting assignment of a value to k .
q
The rate of photoreaction, k , for the excited intermediate can then be calculated.
r
In the sections that follow, the discussion centers on the reactions of excited
states, rather than on the other routes available for dissipation of excitation energy.
The chemical reactions of photoexcited molecules are of interest for several reasons:
1. Excited states have excess energy and can therefore undergo reactions that
would be highly endothermic if initiated from the ground state. For example,
from the relationship E = h we can calculate that excitation by 350-nm light
corresponds to 82 kcal/mol in energy transfer.
2. The population of one or more antibonding orbitals in the excited state allows
the occurrence of chemical transformations that are electronically forbidden to
ground state species.
3. Both singlet and triplet excited states have unpaired electrons, whereas
closed-shell species are involved in most thermal processes (free radical
reaction being an exception). This permits the formation of intermediates that
are unavailable under thermal conditions.

Another important distinction between ground state and excited state reactions
involves the relative rates of conformational interconversion. In thermal reactions, as
stated by the Curtin Hammett principle (p. 296) conformers are normally in equilibrium,
but the position of the equilibrium does not determine the reaction pathway. Many
steps in photochemical reactions occur sufficiently rapidly that various conformers
are not in equilibrium. This is the principle of nonequilibrium of excited rotamers
4
(NEER). Thus in analyses of photochemical reactions, it is often necessary to consider
conformational issues in order to interpret the reaction. Reaction dynamics refers to
the conformational and other geometrical aspects of the reactions.
As we describe photochemical reactions, we note repeatedly that photochemical
reactions involve unpairing and re-pairing of electrons. Frequently, atom and group
migrations occur prior to the final electron re-pairing. Although discerning these
unpairing/re-pairing schemes is a first step in understanding photochemical mecha-
nisms, we also want to consider the structure of excited states and reaction inter-
mediates. As is the case for transition structures in thermal reactions, computational
approaches have provided a new level of insight.
4
H. J. C. Jacobs and E. Havinga, Adv. Photochem., 11, 305 (1979).