http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1292 The nitrosation reactions of both thiourea and ammonia have been investigated, owing to their interesting kinetic behaviour and their industrial importance in the sensitisation of emulsion explosives. Kinetic experiments were conducted using a well-mixed aqueous reactor, from which samples were periodically removed, quenched, and then analysed for nitrite concentration using ion chromatography. We derived a number of plausible rate equations for both the ammonia and thiourea gassing processes, which we subsequently tested against experimental results to determine the operating kinetic mechanism, under a range of conditions. The reaction between ammonia and nitrite was seen to proceed by way of the nitrosating agent dinitrogen trioxide. In the presence of thiocyanate, however, dinitrogen trioxide was replaced by the thermodynamically more powerful nitrosating agent nitrosyl thiocyanate. Both reaction mechanisms exhibited relatively similar temperature dependencies, with respective activation energies of 56 and 68kjmol⁻¹ determined between 25 and 45 °C, indicating reaction-controlled regimes. The effects of pH, temperature and ammonium nitrate concentration on the rate of thiourea nitrosation were studied. The rate of reaction was independent of ammonium nitrate concentration, while decreasing the pH of the reaction medium resulted in significant increases in the rate of reaction. The activation energy was evaluated as being 61kjmol⁻¹ in the temperature range of 25 to 45 °C. Theoretical evidence at the G2MP2 level of theory supports a mechanism of thiourea nitrosation involving rapid deprotonation of S-nitrosothiourea, followed by rate-limiting S to N migration of the nitroso group. 2010-04-27T06:54:39.699Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1238 A study has been undertaken into the chemical production of gas bubbles within a concentrated water-in-oil emulsion, typical of those used as emulsion explosives. Chemical foaming was initiated by the introduction of a concentrated sodium nitrite solution to the emulsion, and the measurement of the decreasing emulsion density with time served to estimate the rate of nitrogen production. A conversion of emulsion density to nitrite ion concentration facilitated a kinetic analysis of the data. The change in nitrite ion concentration follows a rate equation which indicates that the rate-limiting reaction step corresponds to the N-nitrosation of thiourea by ON⁺, with an apparent rate constant of 0.22 M⁻¹ s⁻¹ at 25 °C. Tests over a temperature range of 25 to 50 °C yielded an activation energy of 59 kJ mol⁻¹. A mass-transfer model describing the rate of diffusion between aqueous droplets is presented. This model suggests that chemical kinetics, rather than molecular diffusion, is the rate-limiting phenomenon in the foaming of emulsions. Supporting this finding, the kinetic experiments in emulsion returned very similar results to previous experiments performed in aqueous media under similar conditions. 2010-04-27T06:39:17.909Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:150 2010-04-27T06:00:12.482Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3501 This study investigates the reactivity of the electrophilic nitroso group towards nucleophilic aniline derivatives from various nitrosating agents. A relationship between the rate of nitrosation and Edwards' nucleophilic parameter (En) is disclosed, indicative of a transition state that is mostly product-like in nature with respect to cleavage of the nitroso bond in the nitrosating agent. A similar relationship based on the work of Marcus provides a considerably better explanation of the data. Through application of the Marcus equation, the free energy change of reaction is calculated for the nitrosation reactions studied, which in turn is applied to develop an equation linking the free energy of formation of a nitrosamine and its corresponding protonated amine. This equation accounts for the often-observed Brønsted relationships in nitrosation reactions. The intrinsic barrier to reaction is estimated to be 10 kJ mol⁻¹, indicating that the main impedance to nitrosation arises from the unfavourable reaction thermodynamics. However, for the nitrosation of aniline derivatives substituted with π-electron withdrawing groups, an unbalancing of the transition state results in an increased intrinsic barrier, of the order of 19 kJ mol⁻¹. For electronic-effect aniline derivatives, a More O'Ferrall-Jencks diagram shows the nitrosation transition state to be synchronously well balanced between reactants and products. This diagram also confirms that the reaction follows a concerted mechanism. The nitrosation of resonance-stabilised aniline derivatives is somewhat less synchronous, however, due to delocalisation of the lone electron pair on the amino group, induced by π electron withdrawing substituents. Transition states were located according to the theory of harmonic parabolic wells. The results of these calculations agreed with transition state locations predicted using linear free energy relationship techniques. A method is developed which approximates free energy profiles by treating the product and reactant free energy wells as harmonic parabola, while the free energy profile around the transition state is taken as the parabolic barrier to the product and reactant energy wells. Applying this technique, free energy profiles for electronic-effect and resonance-stabilised aniline nitrosation are predicted, utilising measured kinetic values and predicted thermodynamic properties. We couple the simulated free energy profiles with their corresponding More O'Ferrall-Jencks diagrams in order to elucidate the three-dimensional reaction path traversed between reactants and products, in terms of both structure and energy. We also demonstrate that the nitrosonium ion behaves as a soft acid, and should therefore undergo covalent frontier-orbital controlled bonding. 2010-04-27T05:30:33.091Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3491 This contribution develops a comprehensive kinetic model of the N-nitrosation reaction mechanism, consisting almost entirely of elementary reaction steps and applies it to study the nitrosation of ammonia. The reaction mechanism features 26 species and 22 reactions, with 8 parallel reaction pathways for ammonia nitrosation and a side pathway for nitrous acid decomposition. We compiled forward and reverse rate constants for each of the reactions, either from the literature sources or by correlation with known rate and equilibrium constants. The concentration of each reaction species with respect to time can be obtained for any set of initial concentrations by invoking a simultaneous solution to the system of ordinary differential equations describing the reaction mechanism. The model successfully predicts previous experimental results for ammonia nitrosation, with and without the addition of the catalyst thiocyanate. The effect of pH on the rate and mechanism of ammonia nitrosation was studied with the model. For uncatalyzed nitrosation, the results indicate that between pH 6.0 and 1.5 the reaction proceeds predominantly via reaction with N₂O₃, whereas ON⁺ nitrosation becomes the preferred pathway below pH 1.5. ONSCN is the dominant nitrosating agent across the entire pH range studied when the nucleophile thiocyanate was added in appreciable quantities. However, with the weak nucleophile Cl⁻, nitrosation by N₂O₃ and ON⁺ governed the reaction kinetics at high and low pH, respectively. We demonstrate that nitrosating agent formation is rapid and does not limit the rate of ammonia nitrosation; however, nitrosating agent formation could become the rate-limiting step for the nitrosation of highly reactive substrates. 2010-04-27T05:30:31.171Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3394 We present an ab initio procedure for accurately calculating aqueous-phase pKa values and apply it to study the acidity of nitrous acid (HNO₂, or HONO). The aqueous-phase pKa of nitrous acid was obtained from calculated gas-phase acidities and solvation free energies via a thermodynamic cycle and the solvation model chemistry of Barone et al. (J. Chem. Phys. 1997, 107, 3210). Solvation free energies were calculated at the HF/6-31G(d) level using the dielectric-polarizable continuum and the integral equation formalism-polarizable continuum solvent models (D-PCM and IEF-PCM, respectively), with the D-PCM model yielding the most accurate pKa values. For HF free energies of solvation, significant improvements in accuracy could be made by moving to the larger 6-311++G(3df,3pd) and aug-cc-pVQZ basis sets. Solvation free energies were also calculated using the density functional theory (DFT) methods B3LYP, TPSS, PBE0, B1B95, VSXC, B98 and O3LYP, with the most accurate methods being TPSS and VSXC, which provided average errors of less than 0.11 pKa units. Solvation free energies calculated with the different DFT methods were relatively insensitive to the basis set used. Our theoretical calculations are compared with experimental results obtained using stopped flow spectrophotometry. The pKa of nitrous acid was measured as 3.16 at 25 °C, and the enthalpy and entropy of nitrous acid dissociation were calculated from measurements as 6.7 kJ mol⁻¹ and −38.4 J mol⁻¹ K⁻¹, respectively, between 25 and 45 °C. The UV/visible absorption spectra of the nitrite ion and nitrous acid were also examined, and molar extinction coefficients were obtained for each. 2010-04-27T05:03:47.547Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:3422 The bonding between anionic nucleophiles and the nitroso group has been studied in the common nitrosating agents nitroso chloride (ONCl), nitroso bromide (ONBr), nitroso thiocyanate (ONSCN), and dinitrogen trioxide (N₂O₃) in aqueous solution. A variety of theoretical methods were employed, including ab initio, density functional theory (DFT), and composite theoretical techniques, with solvent effects described using the polarizable continuum model (PCM). Experimental nitroso bond heterolytic dissociation free energies were accurately reproduced with a number of composite theoretical methods, the most successful being CBS-Q and G2MP2, with average errors of 3.1 and 3.4 kJ mol⁻¹, respectively. Using the MP2 and B3LYP methods, calculations were made with correlation consistent basis sets up to quadruple-ζ, extrapolated to the complete basis set (CBS) limit. The MP2/CBS calculations were accurate to around 10 kJ mol⁻¹, while the B3LYP/CBS calculations routinely overpredicted experimental bond free energies by ca. 40 kJ mol⁻¹. It is therefore highly recommended that B3LYP energies are not used for nitroso compounds, although other results demonstrate that the B3LYP method provides a good account of nitroso compound geometries, frequencies, and entropies. Single-point CBS energy calculations using MP2/aug-cc-pVQZ geometries and frequencies showed that the MP4(SDTQ) and QCISD(T) methods provide a slight improvement over MP2 at the CBS limit, although the inclusion of triple excitations is necessary to achieve this improvement in accuracy. Enthalpy−entropy compensation was also discovered, with an average isoequilibrium temperature of 825 K. This relatively large isoequilibrium temperature indicates that enthalpic effects dominate over entropic ones. 2010-04-27T05:01:14.560Z ]]>