Page 209 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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456 Organic Chemical Systems, Theory

low-energy configurations are constructed by considering same minimum from which the initial excitation occurred,
all suitable occupancies of the MOs of the reactants on the process is viewed as photophysical. If it is not, a net
the left-hand side and of the products on the right-hand chemical reaction has occurred and the process is labeled
side. The symmetry of each is again identified as antisym- photochemical.
metric or symmetric with respect to each of the above At times the excited S 1 surface may touch or nearly
symmetry elements, using the rule S × S = A × A = S touch the S 0 surface, in which case the return to S 0 is very
and A × S = A. Correlation lines are now drawn from fast. Such areas in S 1 are often referred to as funnels since
left to right by keeping the occupancy of each MO in they very effectively return molecules to the ground state.
each configuration constant. This often produces cross- In order to understand photochemical reaction paths it
ings of configurations of the same symmetry. Accord- is thus important to have an understanding of the location
ing to the noncrossing rule these must ultimately be of barriers as well as minima and funnels in the S 1 and
avoided. T 1 surfaces, plus a sufficient understanding of the S 0 sur-
This is accomplished by introducing configuration in- face to allow a prediction or rationalization of the fate of
teraction, which converts the configuration correlation di- a molecule that lands in a known region of this surface.
agram to the desired state correlation diagram, as indicated Correlation diagrams are often useful for this purpose. For
in Fig. 10. Clearly, the crossing of correlating MOs in the instance, the diagram for the face-to-face cycloaddition of
case of the ethylene + ethylene cycloaddition causes a two ethylenes shown in Fig. 10 shows the presence of a
similar crossing of lines in the configuration correlation minimum in the S 1 surface in the general area of geome-
diagram. Since the effects of configuration mixing, which tries at which the pericyclic transition state occurred in the
produces the final state diagram, are generally relatively ground state. While the latter was energetically unfavor-
small, a memory of the crossing at the geometry of the able in the S 0 state, making the reaction highly unlikely
transition state survives and results in a large barrier in the sincethemoleculeswillprobablyfindotherreactionpaths,
energy of the ground state in the middle of the correlation the minimum in the S 1 state provides an efficient driving
diagram. It is then concluded that the transition state is force for the photochemical cycloaddition to proceed ef-
unfavorable relative to the case of the ethylene + butadi- ficiently. Thus, reactions that fail to occur in the ground
ene process, in which no such barrier is imposed by the state are often smooth when performed photochemically
correlation. and vice versa.
In general, by virtue of molecules landing at otherwise
improbable and highly energetic areas on the S 0 surface,
B. Photochemical Reactions
photochemical processes are capable of producing very
In photochemical reactions, initial electronic excitation is highly energetic ground state products. Yet, frequently
introduced by the absorption of a photon or by an energy the same perturbations, such as substituent effects, that
transfer from another molecule. It is normally followed increase the stability of a molecule in the ground state
by a very rapid, radiationless conversion to the lowest ex- also facilitate its photochemical reactions by lowering
citedsingletorthelowesttripletenergysurface,depending the barriers encountered along the way. The interplay of
on the multiplicity of the initial excited state. Also, any these two aspects of the excited-state surfaces—minima
vibrational energy in excess of that dictated by the tem- and barriers—make the consideration of photochemical
perature of the surrounding medium, whether generated processes far more complex than the study of thermal
by the initial excitation or by the radiationless process, reactions.
is rapidly lost to the solvent, unless one works in a gas
phase at low pressure. Thus, in a matter of a few picosec- SEE ALSO THE FOLLOWING ARTICLES
onds or less the molecule ends up in one or another of
the local minima in the S 1 or T 1 surface. Further motion HYDROGEN BOND • INFRARED SPECTROSCOPY • KINET-
on the surface may follow, depending on the tempera- ICS (CHEMISTRY) • MOLECULAR ELECTRONICS • OR-
ture and the height of the barriers surrounding the local GANICCHEMISTRY,SYNTHESIS•PERIODICTABLE(CHEM-
minimum. Also, radiationless conversion from the S 1 to ISTRY) • PHOTOCHEMISTRY,MOLECULAR • POTENTIAL
the T 1 state, known as intersystem crossing, can occur. ENERGY SURFACES • QUANTUM MECHANICS • RAMAN
This often happens on a nanosecond time scale. Sooner or SPECTROSCOPY
later a radiationless return to the S 0 state ensues. The final
fate of the molecule is further loss of excess vibrational BIBLIOGRAPHY
energy and thermal equilibration at the bottom of one or
another catchment basin in the S 0 surface, depending on Albright, T. A., Burdett, J. K., and Whangbo, M.-H. (1985). “Orbital
where on the S 0 surface the molecule landed. If this is the Interactions in Chemistry,” Wiley, New York.