Pentazole, calculations

Calculated molecular dimensions for pentazole (4) are summarized in Table 1. These may be compared with x-ray crystal structure dimensions of the ring in l-(p-dimethylaminophenyl)pentazole (16) (see Section 4.25.3). 

Figure 1 Symmetric and asymmetric decomposition of pentazole from SCF and (MBPT2) calculations...

Table 2 Theoretical calculations on pentazole protonation energies...

The availability of ever faster computers is responsible for the continuously improving quality in the calculations of the stability and the properties of pentazole compounds. Azidopentazole has been identified as the most stable of the known Ns polynitrogen compounds and considerable work has gone into the question whether a pentazole salt of the Ns+ ion would be stable. 

In almost all experimentally known pentazoles, the pentazole ring is attached to a substituted phenyl ring. Calculations using these well-known pentazoles were used to determine the mechanism of the formation and decomposition of the pentazoles (see Section 6.18.2.1). Here the theoretical data can be compared to experimental data for verification. As the calculated data for the known pentazole derivatives model their characteristics quite well, calculations can also be used to extrapolate the stability of yet unknown pentazole derivatives. The high sensitivity of these compounds makes calculations a fast and safe route for the evaluation of the properties of potential new compounds. 

Table 2 Calculated data for the decomposition of substituted phenyl pentazoles at the B3LYP level in kcal mol 1...

Quantum-chemical calculation of the decomposition of methylpentazole has shown that methylpentazole is as stable as aromatic pentazole derivatives <2003CEJ5511>. In the reaction of 1SN-Iabeled diazomethane with hydrazoic acid at — 80 °C, which leads to methyl azide, no methylpentazole intermediate has been detected <1932G716, 1958HCA1823>. Therefore, methylpentazole can only be synthesized by using a different route. Other substituents like CN, F, or NH2 lead to pentazoles with lower activation barriers (Table 5) than the known arylpentazoles. 

The question of whether metal complexes of pentazoles similar to ferrocene are stable has been discussed since extended Hilckel calculations predicted the stability of these complexes 20 years ago (Figure 3) <1985POL1721, 1988AIC171>. 

At the RHF level, the preferred site for protonation of pentazole is the N-3 position with a protonation energy of 191.6 kcal mol-1. The protonation energy for the protonation at N-2 is 170.9 kcal mol-1 <1986JPC5597>. At the more reliable B3LYP level of theory, the proton affinity of the pentazole anion was calculated to be 316.5 kcal mol-1. In dimethyl sulfoxide (DMSO) solution, this corresponds to a pAla of 2.1 <2005MP209>. 

The electron-withdrawing effect of pentazole is similar to a nitro group and the H NMR shifts of nitrophenyl and pentazolylphenyl compounds are similar <2002ZFA1933>. The 7i-electron densities calculated by the Hiickel molecular orbital (HMO) method show a linear correlation with 111, 13C, and 14N NMR chemical shifts in the azole series <1977IJB168>. The 1SN NMR chemical shifts of the following compounds have been reported cesium/tetramethyl ammonium (TMA) pentazolylphenolate (8 —81.1 (N-l), —29.7 (N-3/N-4), 1.9 (N-2/N-5)) <2002AGE3051> ... 

The solvent has a strong influence on the rate of decomposition of the arylpentazoles (Table 9). Solvents with lower polarity increase the rate of decomposition. This suggests that the transition state for the decomposition is less polar than the ground state of the pentazole, which was confirmed by quantum-chemical calculations <2003CEJ5511>. The decomposition of phenylpentazoles is not influenced by acids or bases <1957CB2914>. The rate of nitrogen evolution is also independent on the concentration of azide ions at —40 °C. 

Compounds of Group 15. Proton NMR spectra were used to characterise NO+ trapped in solutions of a 1,3-alternate bis-calix[4]arene tube.790 Ab initio calculations have been made of NMR parameters for pentazoles RN5 (R = H, F, CH3, CN).791 15N data were reported for these systems, (163).792... 

As dramatic as the tale of the pentazoles is the story of the unsuccessful attempt to detect a six-membered nitrogen ring. The first calculations were performed in the early seventies. In 1980 Vogler et al. [8] found indications that Ne is formed on UV irradiation from cis-[Pt(N3)2(PPh3)2] embedded in a matrix, but it is only stable at low temperature. At that time hexaazabenzene (9) was considered the only feasible structure for Ne. This report unleashed an avalanche of quantum chemical calculations, in particular because 9 is isoelec-tronic with benzene and thus should likewise display aromatic stabilization. The calculations over the decade produced every conceivable answer hexaazabenzene has Den symmetry. .. has Dsh symmetry. .. does not exist, because 9 is the transition structure for nitro-... 

The Cs isomer (36) containing two pentazole ring systems in Scheme 11 was calculated to be ISOkJmoA lower in energy than the C2 isomer (37) and 312 kJ moA lower than the open-chain Czv isomer (38). For the decomposition of the Czv open-chain undecanitrogen (38) a dissociation barrier of 59 kJ moA to open-chain Ng (31) and N2 was calculated. As remarked for the dissociation barrier of nonanitrogen, this activation barrier for the decomposition of (38) also seems to be too high. 

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