Dissociation Enthalpies of Chloride Adducts of Nitrate and Nitrite Explosives Determined by Ion Mobility Spectrometry

The kinetics for thermal dissociations of the chloride adducts of the nitrate explosives 1,3-dinitro­glycerin (1,3-NG), 1,2-dinitro­glycerin (1,2-NG), the nitrite explosive 3,4-dinitro­toluene (3,4-DNT), and the explosive taggant 2,3-dimethyl-2,3-dinitro­butane (DMNB) have been studied by atmospheric pressure ion mobility spectrometry. Both 1,3-NG·Cl– and1,2-NG·Cl– decompose in a gas-phase SN2 reaction in which Cl– displaces NO3 – while 3,4-DNT·Cl– and DMNB·Cl– decompose by loss of Cl–. The determined activation energy (kJ mol–1) and pre-exponential factor (s–1) values for the dissociations respectively are 1,3-NG·Cl–, 86 ± 2 and 2.2 × 1012; 1,2-NG·Cl–, 97 ± 2 and 3.5 × 1012; 3,4-DNT·Cl–, 81 ± 2 and 4.8 × 1013; and DMNB·Cl–, 68 ± 2 and 9.7 × 1011. Calculations by density functional theory show the structures of the nitrate ester adducts involve three hydrogen bonds: one from the hydroxyl group and the other two from the two nitrated carbons. The relative Cl– dissociation energies of the nitrates together with the previously reported smaller value for glycerol trinitrate and the calculated highest value for glycerol 1-mononitrate are explicable in terms of the number of hydroxyl hydrogen bond participants. The theoretical enthalpy changes for the nitrate ester displacement reactions are in agreement with those derived from the experimental activation energies but considerably higher for the nitro compounds.