Thermal decomposition, theoretical studies

The CVD of graphite is theoretically simple and is based on the thermal decomposition (pyrolysis) of a hydrocarbon gas. The actual mechanism of decomposition, however, is complex and still not completely understood. This may be due, in part, to the fact that most of the studies on the subject of hydrocarbon decomposition are focused on the improvement of fuel efficiency and the prevention of carbon formation (e.g., soot), rather than the deposition of a coating. 

The heat of decomposition (238.4 kJ/mol, 3.92 kJ/g) has been calculated to give an adiabatic product temperature of 2150°C accompanied by a 24-fold pressure increase in a closed vessel [9], Dining research into the Friedel-Crafts acylation reaction of aromatic compounds (components unspecified) in nitrobenzene as solvent, it was decided to use nitromethane in place of nitrobenzene because of the lower toxicity of the former. However, because of the lower boiling point of nitromethane (101°C, against 210°C for nitrobenzene), the reactions were run in an autoclave so that the same maximum reaction temperature of 155°C could be used, but at a maximum pressure of 10 bar. The reaction mixture was heated to 150°C and maintained there for 10 minutes, when a rapidly accelerating increase in temperature was noticed, and at 160°C the lid of the autoclave was blown off as decomposition accelerated to explosion [10], Impurities present in the commercial solvent are listed, and a recommended purification procedure is described [11]. The thermal decomposition of nitromethane under supercritical conditions has been studied [12], The effects of very high pressure and of temperature on the physical properties, chemical reactivity and thermal decomposition of nitromethane have been studied, and a mechanism for the bimolecular decomposition (to ammonium formate and water) identified [13], Solid nitromethane apparently has different susceptibility to detonation according to the orientation of the crystal, a theoretical model is advanced [14], Nitromethane actually finds employment as an explosive [15],... 

Hundreds of cycles have been studied from the viewpoint of the feasibility of component chemical reactions in terms of conversion ratio or product separation, theoretical thermal efficiency of hydrogen production, etc. [16]. Among them, those that utilize thermal decomposition of sulfuric acid, which are categorized as "sulfur cycles," have been considered one of the most promising cycles. 

The thermal decomposition of some 3,5-disubstituted-l,2,4-thiadiazoles has been studied and some nonisothermal kinetic parameters have been reported <1986MI239>. Polarographic measurements of a series of methylated 5-amino-l,2,4-thiadiazoles show that thiadiazoles are not reducible in methanolic lithium chloride solution, while thiadiazolines are uniformily reduced at 0.5 = — 1.6 0.02 V. This technique has been used to assign structures to compounds which may exist theoretically as either thiadiazoles or thiadiazolines <1984CHEC(6)463>. The photoelectron spectrum for 1,2,4-thiadiazole has been published <1996CHEC-II(4)307>. 

In our detailed theoretical study of reaction pathways of the model A-azido-A-methoxyformamide 82b we showed that decomposition by loss of nitrogen was the energetically most favourable process with an EA of only 5.3 kcal mol-1 at B3LYP/6-31G. 36 In addition this step is exothermic by 42-44kcalmol-1. Thermal decomposition of 68b to methyl formate 67b and nitrogen has an EA of only 2.9 kcal mol-1 and is exothermic by 95 kcal mol-1. Overall, the conversion of 82b to methyl formate 67b and two molecules of nitrogen is thus predicted to be exothermic by 137-139 kcalmol-1. 

Numerous experimental studies have investigated the atomistic details of HE decomposition by examining the net products after thermal (low-pressure) decomposition (see, for example, Ref. 54). For RDX and HMX, the rate limiting reaction is most likely NO2 dissociation and a plethora of final products in the decomposition process have been isolated. Several theoretical studies have also... 

Scheme 16.5). Thus, the mechanism of the thermal decomposition of 7 has not been fully clarified, yet although the aryne contraction pathway has been established for some benzannellated derivatives of 4. A number of theoretical studies has been devoted to rearrangements on the CeKU potential energy surface. At... 

Finally, the relative rates of cleavage and rotation of 1,4-diradicals have been directly studied by thermal decomposition of the tetrahydropyridazines 9 and 10 (R = Me or D) at 415 "C.85-87 Judging from the rate constants and product distributions obtained for various processes, it is likely that the fate of the diradicals 11 and 12 is identical to those generated by thermolysis of cyclobutanes.85 - 87 Obviously, a choice in favor of the nonconcerted diradical pathway can, therefore, be made on the basis of the aforementioned theoretical as well as experimental endeavors. 

Thermolysis of l,l-difluoro-2,3-diphenylcyclopropane in supercritical CO2 has allowed the rate of geometrical isomerization [i.e. cis-( 109) to /ra/M-(109)] and racemization [i.e. (/< )-( 109) to (S)-( 109)] to be determined from O2 dependence of the trapping rate of the postulated intermediate 1,3-biradical.246 Above 150 °C, the formation of 2,2-difluoroindane and its decomposition products is reported. A similar thermally induced equilibrating series of stereomutations has been observed with the analogous non-fluorinated cyclopropane in which rate constants and deuterium exchange isotope effects are reported.247 Theoretical studies of this isomerization have focused on classical248 and quasi-classical trajectories.249... 

A detailed quantum mechanical study of the mechanism of thermal decomposition of isoxazole has been conducted since previous theoretical predictions appeared to be inconsistent with the experimental results.33 It has been concluded that the mam unimolecular decomposition is through the sequence isoxazole —> NCCH2CHO —> CH3CN + CO and that the minor products, HCN and H2CCO, probably arise via a cyclic carbene as proposed in the experimental study. 

A theoretical study of the thermal isomerization and decomposition of oxalic acid has attempted to account for the predominant formation of C02 and HCOOH from the vapour at 400-430 K.41 Transition-state theory calculations indicate that a bimolecular hydrogen migration from oxygen to carbon of intermediate dihydroxycarbene (formed along with C02) achieved through a hydrogen exchange with a second oxalic acid... 

A Sizable primary KIE s have been observed in the gas-phase thermal decomposition of 2,2-dimethyl-4-phenyl but-3-enoic acid528,529, consistent with a synchronous mechanism via a six-membered cyclic ideal TS, 462. Recently531 the mechanism of thermal decarboxylation of 461 and its derivatives HR1C=CR2CH2COOH (R1 and R2 = H, F, Me, Et and Cl) has been studied again530,531 from the theoretical point of view by ctb initio MO calculations and a twisted chair six-membered cyclic TS (463) has been constructed. 

The kinetics of the gas-phase elimination of ethyl and /-butyl carbazates have been studied in a static reactor system over the temperature range 220.3-341.7 °C and pressure range 21.1-70.0 torr.13 Theoretical calculations on the thermal decomposition of ethyl carbazate (4) suggest that the reaction proceeds by a concerted non-synchronous mechanism, through a quasi-three-membered ring transition state (Scheme 4). In contrast, the transition state structure for the thermal decomposition of /-butyl carbazate is an almost planar six-membered ring. 

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