Nitric oxide, decomposition formation

Comparison of the observed pseudo-first-order decay of biological activity with a half-life of 30 sec at normal oxygen tensions versus decomposition via nitrogen dioxide by pseudo-second-order kinetics predicted by Reaction 4. The loss of nitric oxide through formation of nitrogen oxide is twice as fast as calculated by Reaction 4 because each nitrogen dioxide formed rapidly attacks a second nitric oxide to form nitrite. 

Possibly, cellular thiols may be oxidized by the inactive adduct of nitric oxide and oxygen to regenerate a nitrosothiol or related species with EDRF activity. Some of the inconsistent results observed in bioassay systems may be due to the secondary and nonenzymatic formation of a nitrosothiols or other species capable of regenerating nitric oxide, which are leached into perfusion cascades. Consequently, bioassay systems should not be the gold standard to distinguish whether nitric oxide is the EDRF, because secondary reactions of nitric oxide decomposition products may regenerate nitric oxide. 

Because of its relevance to the chemistry of air at elevated temperatures the homogeneous decomposition of nitric oxide has received considerable attention from gas kineticists. References to early studies are given in the more recent work discussed below. The mechanisms for the decomposition and for the reverse reaction, the formation of NO from air, are well established and good quantitative data (Table 12) are available for the rate coefficients of the elementary steps. 

For the photolysis of ferf-butyl nitrite a possible reaction mechanism (Scheme 6) consists of the production of ferf-butoxy radicals (equation 3), followed by their decomposition to give acetone and methyl radicals (equation 4). The latter are trapped by the nitric oxide liberated in the first step (equation 5). However, the absence of ethane production in the actual experiments suggested that an intramolecular formation of nitrosomethane is unlikely ". ... 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

The stable free radical nitric oxide (NO) has an important role as a biological messenger. The reaction of NO with superoxide (O2 ) forms the powerful oxidant peroxynitrite (ONOO ), and a mechanism for the reaction of ONOO resulting in the abstraction of H from C—H bonds is shown (equations 109, 110). The formation of HO from the spontaneous decomposition of peroxynitrite, and of COJ radicals from CO2 catalyzed decomposition of peroxynitrite, have been demonstrated. ... 

The formation of nitric oxide from its elements was investigated by Nernst at 1,538° and 1,737°C., the decomposition by Jellinekf between 650° and 1,750° C. Both investigations were made by the streaming method. Each of the reactions is stated to be bimolecular. 

Bodenstein, in the course of a comprehensive study of the formation and decomposition of the higher oxides of nitrogen, showed, in conjunction with Lindner, that the reaction between nitric oxide and oxygen proceeds in accordance with the equation... 

In Part II (Sections 7-10) are described experiments on the decomposition of nitric oxide which was added to the explosive mixture in advance. These experiments establish a proportionality between the rate of decomposition and the square of the concentration of nitric oxide and give the heats of activation for the formation and decomposition of nitric oxide. The similarity theory and the exact mathematical theory of a reversible bimolecular... 

The hypothesis of the thermal nature of the oxidation of nitrogen leads to definite conclusions as to the relation between the formation and decomposition of nitric oxide in an explosion and the equilibrium concentration of nitric oxide at the explosion temperature. The equilibrium concentration may be computed independently from thermochemical data. The first task confronting us was to check up on these relations. 

In the example quoted the theoretical explosion temperature Tm equals 2350° K and the equilibrium amount of nitric oxide at this temperature is [NO] = 2.78 mm. The quantities [NO] and NO are very close, as was to be expected on the assumption that formation and decomposition take place at a temperature close to Tm. At a higher explosion temperature, however, NO is considerably less than [NO]. For example with a mixture composition of 40% H2, 40% 02, 20% N2, we obtain... 

It can be assumed that in the series of experiments with the same initial pressure p0 in vessels of the same dimensions the reaction time r will be the same and will depend on the cooling rate. Experiments which are confined to an analysis of the final nitric oxide content in the explosion products can only yield in principle the product of the rate constant into the time hr, but not each of these quantities separately. By studying the dependence of kr on the explosion temperature one can also find the temperature dependence of the activation heat of the reaction. In Fig. 13 log kr is plotted as a function of 1 /Tm. In each series with constant summary content of the mixture kr was calculated by utilizing the data on the formation of nitric oxide, NO0 = 0 (circles in Fig. 13) and on the decomposition of nitric oxide NO0 = 7 or 10 mm and is 2-5 times greater than NO (triangles in Fig. 13). 9... 

According to these data the heat of activation for the decomposition of nitric oxide, to which reaction the factor k refers, is A = 82 10 kcal/mole.10 It should be especially noted that there is no systematic divergence between the data on the formation and on the decomposition of nitric oxide this fact justifies the assumption that the rate of decomposition is directly proportional to the square of the nitric oxide concentration.11 The investigation covered the temperature range from 2000°K to 2900°K in which the rate varies by a factor of 300. As appears from Fig. 13, except for the scattering due to the inevitable errors of the experiments and computations, the points actually do fall on a straight line in the coordinates lg kr, 1/Tm, i.e., the Arrhenius temperature dependence of the reaction rate holds. The thermodynamic relation between the rates of the direct and reverse reactions permits determining the heat of activation A for the formation of nitric... 

It should be recalled that k is the rate coefficient for the formation and k for the decomposition of nitric oxide. Thus, the reaction time r of formula (9.10) is the time in which the rate of the direct reaction of formation of nitric oxide decreases by a factor e. 

The experiments which we used to determine the rate coefficient of the reactions of formation and decomposition of nitric oxide referred to an oxygen concentration which varied within comparatively narrow limits near 150 mm 02 at 17°C. Substituting in place of the old expression for the rate coefficient... 

The decomposition of nitric oxide pre-mixed with the explosive mixture is investigated. The heats of activation for the formation and decomposition of nitric oxide are determined. 

The absolute values of the rate coefficients of the decomposition and formation of nitric oxide have been found. 

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