15N Chemical Shift Tensors of β-HMX

New aspects of initiation reactivities of energetic materials demonstrated on nitramines

A brief survey is presented of the author's results obtained from studies of the chemical micro-mechanisms of nitramines initiation from the point of view of organic chemistry. The relationships have been presented and discussed between the characteristics of impact and electric spark sensitivities, detonation and thermal decomposition, on the one hand, and (15)N NMR chemical shifts of nitrogen atoms of nitramino groups, on the other. In the case of the impact sensitivity, the said relationships involve the (15)N shifts of the amino nitrogen atoms carrying the nitro group primarily split off from the molecule. In the case of the initiation by shock, heat and electric spark, the (15)N shifts of nitrogen atoms in the primarily split off nitro groups themselves are involved. Also, the relationships are presented between the characteristics of thermal reactivity and values of the electronic charges at the nitro groups that are primarily split off. It has been stated that the chemical micro-mechanisms of primary fission processes of molecules of nitramines in the initiation by mechanical stimuli (inclusive the detonation course) and electric spark should be the same as in the case of their low-temperature thermal decomposition. It has been found that the electron structure and close neighbourhood of nitrogen atom of the primarily leaving nitro group is a dominant factor in initiation by shock, electric spark and heat. In the case of initiation by impact a key role plays characteristics of amino nitrogen atoms which are carriers of these most reactive nitro groups. Also mentioned is relevance of the modified Evans-Polanyi-Semenov relationship. On the basis of the findings presented it also has been stated that the detonation transformation itself of the nitramines should be preceded by an induction period.

Revisiting the calculation of (13)C chemical shift tensors in cadmium acetate dihydrate with EIM and EIM/cluster methods

The chemical shift tensors of the acetate anions in cadmium acetate dihydrate are calculated using a cluster approach, the embedded ion method (EIM), and a combination of the two in the EIM/cluster method. The results of these calculations are compared with those completed on the isolated acetate anion and show the need for the inclusion of intermolecular interactions. The RMS difference between experiment and theory improves from over 60 ppm when the calculation is completed on an isolated anion, to below 10 ppm when interactions to nearby atoms are included. The best cluster model includes three cadmium acetate dihydrate and gives an RMS result of 4.4 ppm. The EIM method, which uses point charges to account for the intermolecular effects, achieves an RMS of 7.7 ppm on individual anions alone. A combination of the two, the EIM/cluster method, shows that the only necessary atom to explicitly add is the nearest cadmium; this addition results in an RMS of 4.1 ppm. These results are also discussed in terms of the computational cost of the different calculations.

Solid-state 15N NMR studies of tobacco leaves

Nitrogen-containing compounds are one important class of constituents in tobacco because of various pharmacological and biological properties. Three types of tobacco leaves (burley, bright, and oriental) were studied using solid-state (15)N NMR cross polarization with magic-angle spinning, dipolar dephasing and five pi replicated magic angle turning (FIREMAT) experiments. The results show that burley tobacco leaves contain significantly more pyridinic nitrogen than that of bright or oriental tobacco leaves. The principal values of (15)N chemical shift tensors of nitrogen functional groups were obtained from the FIREMAT data. Possible assignments of solid-state (15)N NMR resonances were made using nitrogen chemical shift tensors in some model compounds or isotropic chemical shift values from liquid NMR results. To the best of our knowledge, this is the first solid-state (15)N NMR study of tobacco plant material.