Computational Chemistry

calculated NMR shielding (Section 5.5.5) of a “ghost nucleus” with no charge
or electrons placed at or above the ring center. A very comprehensive review of the
NICS test [61a] has been published, and an updated, superior variant [61b] has
been presented. Calculation of the changes inwand in NICS along the reaction
coordinate for the known reaction, the isomerization of diademane to triquinacene
(Fig.9.9), showed that the transition state, but neither the reactant nor the product,
was aromatic [ 60 ]. Homoaromaticity in transition states and in a few neutral,
ground-state molecules has been claimed [ 62 ].
The absence of homoaromaticity in triquinacene is presumably due to the three
pairs of nonbonded carbons being too far apart, 2.533 A ̊, from X-ray diffraction;
in the transition state (Fig.9.9), in contrast, the nonbonded CC distance has
been reduced to 1.867 A ̊ according to a B3LYP/6–311+G** (Section 7.2.3.4c)
calculation. Significantly, the measured C¼C length, 1.319 A ̊, is close to the normal
C¼C length (calculated and measured parameters of triquinacene are cited in [ 60 ]).

9.2 To the Literature.........................................................

A feast of information on computational chemistry is available, a small selection of
which is given below.

9.2.1 Books.............................................................

Books on computational chemistry and some related topics, in alphabetical order of
the author’s (or first-listed author’s) name. The terse characterization of a particular
book is a personal impression, and does not necessarily imply that it does not share
the virtues ascribed to some other book. The list does not claim to by any means
include all the good books on computational chemistry.

9.2.1.1 B

Computational Organic Chemistry, S. M. Bachrach, Wiley-Interscience, Hoboken,
NJ, 2007.

diademane transition state triquinacene

Fig. 9.9The isomerization of diademane to triquinacene proceeds through an aromatic transition
state, as shown by the magnetic susceptibility and NICS values for the three species

572 9 Selected Literature Highlights, Books, Websites, Software and Hardware