The structures and energies (see table 5:2) of all thirteen isomers of compounds of the type C6(NO2)6-n(N3)n have been calculated at semiempirical PM3 and ab initio at HF level of
theory. The molecular structures are displayed in figure 5.1 with the abbreviated and rational names given in table 5.1.
Qualitatively, the structures are very similar at PM3 and at HF level of theory. All compounds possess an essentially planar aromatic C6 backbone which is substituted with nitro (-NO2)
and/or azido (-N3) groups. In all compounds both the nitro and azido groups show structural
parameters which are in good accord with increased valence bond (VB) considerations. For example, in the symmetrically substituted TATN isomer (HF/6-31G level) the terminal N-N bond in the azide units corresponds with 1.10 Ǻ almost to a triple bond whereas the Nα-Nβ
bond corresponds with 1.27 Ǻ to a very strong single or better a weak double bond [79]. For comparison, typical values for N-N single, double and triple bonds are as follows: 1.45 Ǻ, 1.24 Ǻ and 1.098 Ǻ [80]. The N atoms of the nitro groups lie as it is the case for all NO2
substituted isomers which are discussed in this thesis within the C6 plane whereas the two
oxygen atoms are rotated above and below the plane, respectively. The N-O bond lengths are with 1.22 – 1.23 Ǻ relatively short and correspond nearly to N=O double bonds with typical N-O single and N=O double bond values being 1.44 Ǻ and 1.20 Ǻ, respectively [80]. It is noteworthy to stress that in this compound all azide groups – though significantly out of the C6 plane – point to the same direction (i.e. all “up” or all “down”).
Whereas - as already stated above - usually the six carbon atoms with the six N atoms directly bound to C form a plane and the two oxygen atoms of the NO2 groups and the central and
terminal N atoms of the azide (-N3) substituents are rotated below and above this plane some
of the molecules under investigation show some additional remarkable structural features. The symmetrically substituted TATN has essentially Cs symmetry with the mirror plane being
perpendicular to the C6 plane. The hexaazido derivative (HAB) is nearly a planar molecule
with a molecular symmetry very close to C6h. This result may be surprising but both the PM3
and the HF calculation were performed in C1 without any symmetry constraints and were
It can generally be stated that the azide groups tend to move more to positions within or close to the C6 plane whereas the nitro groups are always arranged with a torsion angle of between
45° and 90° (perpendicular) to the C6 plane. For example, in PNP, all five NO2 groups are
rotated significantly out of the aromatic plane whereas the one N3 group lies essentially within
the C6 plane. The same is true for DATN2 where again the O atoms of the NO2 groups are
below and above the C6 plane whereas in this case the two azide units are almost within the
C6 plane. On the other hand, in DATN1 the four NO2 groups are “out of plane” with one N3
moiety pointing above and the second one (in ortho-position) pointing below the C6 plane. In
the tetraazido dinitro derivative (TADN2) the four azide groups are arranged in an up-down- up-down fashion. Finally, the pentaazido nitro benzene compound has the two azide groups in
ortho position out of the C6 plane whereas the remaining three N3 groups lie still slightly but
significantly less out of the plane with a much smaller torsion angle.
The optimised structures of all thirteen isomers of compounds of the type C6(NO2)6-n(N3)n are
shown in figures 5.2 – 5.14.
Figure 5:2 Molecular structure of hexanitrobenzene fully optimized at HF/6-31G(d) level level of theory.
Figure 5.3 Molecular structure of pentanitrophenyl azide fully optimized at HF/6-31G(d) level level of theory.
Figure 5:4 Molecular structure of 1,2-diazido 3,4,5,6-tetranitrobenzene fully optimized at HF/6-31G(d) level level of theory
Figure 5.5 Molecular structure of 1,3-diazido 2,4,5,6-tetranitrobenzene fully optimized at HF/6-31G(d) level level of theory.
Figure 5.6 Molecular structure of 1,4-diazido 2,3,5,6-tetranitrobenzene fully
Figure 5:7 Molecular structure of 1,3,5-triazido 2,4,6-trinitrobenzene fully optimized at HF/6-31G(d) level level of theory
Figure 5.8 Molecular structure of 1,2,3-triazido 4,5,6-trinitrobenzene fully optimized at HF/6-31G(d) level level of theory.
Figure 5.9 Molecular structure of 1,2,4-triazido 3,5,6-trinitrobenzene fully optimized at HF/6-31G(d) level level of theory.
Figure 5.10 Molecular structure of 2,3,5,6-tetra-azido 1,4-dinitrobenzene fully optimized at HF/6-31G(d) level level of theory.
Figure 5.11 Molecular structure of 1,2,3,4-tetra-azido 5,6-dinitrobenzene fully optimized at HF/6-31G(d) level levelof theory.
Figure 5.12 Molecular structure of 1,3,5,6-tetra-azido 2,4-dinitrobenzene fully optimized at HF/6-31G(d) level level of theory.
Figure 5.13 Molecular structure of penta-azido nitrobenzene fully optimized at
HF/6-31G(d) level level of theory.
Figure 5.14 Molecular structure of hexa-azido benzene fully optimized at HF/6-31G(d) level level of theory.
As previously described in the introduction of this thesis the thermally unstable product obtained by Huisgen and Ugi in 1957 by the reaction of benzenediazonium chloride with lithium azide in methanol at – 40 °C was formulated as phenylpentazole on the basis of 15N labelling studies (see reaction scheme 6.1 below) [47]. In the main reaction (65%) of the benzenediazo azide decomposes at low temperature (-50°C in methanol) to yield phenyl azide and primary nitrogen, simultaneously the remaining (35%) of benzenediazo azide undergoes ring closure to form phenylpentazole. At 0°C the phenylpentazole decomposes to yield phenyl azide and secondary nitrogen.Using 15N labelled benzenediazonium chloride allowed the determination of the decomposition of phenylpentazole which yielded equal amounts of labelled and unlabelled nitrogen whereas the decomposition of benzenediazo azide only yielded unlabelled nitrogen
Scheme 6.1 Reaction scheme for the reaction of 15N labelled benzenediazonium