Computational Chemistry

relative energies, the MP2 geometries were subjected to single-point calculations
(Section 5.5.2) using the QCI method (Section 5.4.3), with the results shown
(Fig.9.3). At the QCI/6-31G//MP(fc)/6-31G level pyramidane is predicted to
be a relative minimum with a barrier of 96 kJ mol"^1 for its lowest-energy isomeri-
zation path, to the tricyclic carbene, which lies 87 kJ mol"^1 above it. This presents
us with the astonishing possibility that the exotic hydrocarbon may be isolable at
room temperature, the threshold barrier for being isolable at room temperature
being about 100 kJ mol"^1 [ 22 ]. Other properties of pyramidane, including ionization
energy and electron affinity (Section 5.5.5), heat of formation (Section 5.5.2.2c), and
NMR spectra (Section 5.5.5) were calculated [21b].

9.1.1.4 Polynitrogens. More Than a Computational Playground?

There has in recent years been considerable interest in the possibility of making
allotropes of nitrogen with more than two atoms per molecule. Nitrogen polymers
are interesting because to any chemist with imagination the idea of a form of pure
nitrogen that is not a gas at room temperature is fascinating, and because any such
compound would be thermodynamically very unstable with respect to decomposi-
tion to dinitrogen. The challenge is to identify computationally a realistic candidate
for synthesis and to make it. A faint hope is that a compound (an allotrope) may be
found with enough kinetic stability to be handled at room temperature. Such a
substance is potentially a useful high-energy-density material.

pyramidane

C4v

0

C2v

148

Cs

87.1

Cs

87.1

Cs

118

Cs

55.3

28.5

D2d

kJ mol–1

Cs

96.1

Fig. 9.3 (Part of) the pyramidane potential energy surface. QCICD(T)/6-31G//MP2(fc)/6-31G
calculations

9.1 From the Literature 565