Page 1098 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

H 1083

131.25 SECTION 12.2
1.088
Photochemistry of
C 1.360 C Alkenes, Dienes, and
Polyenes
1.088
1.134 107.10
147.09
H
H H
H 118.75 S
1
1.085
H C C
H 1.455
121.58

H
T
1
H C 1.335 C H H
H
1.085
121.70
S
0
Fig. 12.5. Minimum energy structures (MP2/6-31G ) for S 0 , S 1 , and T 1 states of ethene.
∗
Reproduced from Int. J. Quantum Chem., 82, 242 (2001), by permission of Wiley-VCH.

pyramidalized CI in Figure 12.6. The CI with carbene character is also a common
feature of alkene chemistry and can be considered to form by hydride migration from
the zwitterionic excited state. Similar structures involving carbon migration are also
found for substituted alkenes.

H H : H : H
C C H C C C C H
+
H H H
H H H
T 1 S 1
zwitterionic carbenoid
conical intersection
There are two relatively low-lying singlet states for styrene. S is associated
1
primarily with the benzene ring, whereas S is a singlet quinoid structure. The S state
2 1

1.09Å
1.12Å (a) (b)
1.41Å
156.7° 113.3°
1.07Å 1.09Å 1.19Å
1.09Å 94.9° 1.75Å
1.46Å 1.09Å
1.09Å
Fig. 12.6. Structures of lowest-energy conical intersections for the singlet excited state of ethene:
(a) ethylidene-like structure; (b) twisted and pyramidalized structure. Reproduced from Chem. Phys.,
259, 237 (2000), by permission of Elsevier.