Page 781 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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764 d. The hydrocarbon 10-E is easily reduced to a dianion. The HNMR spectrum
of the dianion shows an average downfield shift relative to the hydro-
CHAPTER 8 13
carbon. The central carbon shows a large upfield shift in the C-NMR
Aromaticity spectrum.

10-E

8.11. The HMOs for acenaphthene are shown below. The atomic coefficients for the
orbital that is the LUMO in the neutral compound and the HOMO in the dianion
are given at the right.

–2.36
–1.92 +0.322 –0.322
–1.43 +0.230 1 2 –0.230
–1.31 8a 8b 2a +0.388
–1.00 –0.388 8 3
7 5a 4
– 0.28 –0.120 6 5 +0.120
+0.63 +0.422 –0.422
+0.83
+1.00
+1.69
+2.47

Comment on the aromaticity, antiaromaticity, or nonaromaticity of acenaphthene
and its dianion on the basis of the following physical measurements:
a. The bond lengths of acenaphthene are given below. Compare them with
the bond lengths for naphthalene given on p. 18. What conclusions can you
draw about the aromaticity of acenaphthene?

1.395
1.466
1.44 1.381
1.386 1.424
1.433 1.382

b. Both X-ray crystallography and NMR data indicate that the C(1)−C(2) bond
lengthens significantly in the dianion, as indicated below (X-ray data). There
is also a different pattern of bond length alternation. What conclusions can
you draw about the aromaticity of the acenaphthene dianion?

1.42
1.44
1.43 1.43
1.30
1.42 1.41