Thermal reaction pathways

Quantum-mechanical ab initio calculations for small molecular systems are widely used these days as an instrument in studying problems in various Helds of chemistry and molecular physics . Most studies deal with ground-state phenomena, i.e. the structure and properties of compounds, thermal reaction pathways and dynamical behavior based on this information. There has been a noticeable increase in excited-state studies in recent years, however, in particular in connection with problems in molecular spectroscopy, in ionization processes or in the detailed study of photochemical reactions, such as photodissociation, energy-transfer and charge-exchange reactions. The calculations are especially powerful for small molecules (for example, for systems up to SO electrons and six atoms other than hydrogen), and hence numerous applications are found in particular in the area of atmospheric and interstellar chemistry and in the study of combustion processes. In these Helds it is often found that experimental and theoretical studies are undertaken in close conjunction and that the two yield complementary data which, taken together, are able to clarify a process. In other instances it is not uncommon that for short-lived species the values obtained from calculations are so far the only ones available. 

Thermal or photochemical decomposition of cyclopropyl azides may give rise to 1-azetines and/or stereospecific fragmentation to olefin plus nitrile. In contrast to the photochemical reaction, substituents have a pronounced effect on the thermal reaction pathway. In general, substitution in the 2-position of the cyclopropyl azide... 

Studies of the diradical (stabilized by a phenyl group) or zwitterionic intermediate have been undertaken for the Schmittel reaction of the en)me-allene with larger substituents (SiMes, /-Bu). It was reported that C2-C6 cyclization occurs via the diradical intermediate and aryl or bulky groups at the alkyne terminus trigger a general thermal reaction pathway for the enyne-allene. ... 

This book was written with the aim of providing a fundamental way of thinking about chemical reactions in the atmosphere for midergraduate and graduate students who wish to learn atmospheric chemistry. In particular, it is hoped that students in chemistry majors would understand more deeply the science hidden in each atmospheric reaction which is anchored to physical chemistry and quantum chemistry. It is also hoped that students in physics and meteorology majors would imderstand the fundamental principles of chemical reactions and kinetics. It is also hoped that it will be useful for research scientists in these fields as a reference book for consulting photochemical and thermal reaction pathways, absorption spectrum, and rate constants. 

Fig. 3 The two investigated thermal reaction pathways of the isomerization reaction of S-SPl (a), R-SPl (b), S-SP2 (c), R-SP2 (d) to MC. The free energies are calculated at the CAM-B3LYP/6-311 +G(d,p) level in the gas phase at room temperature and reported with respect to the S-SPl conformer in all four panels. Energy differences in parenthesis are computed with respect to corresponding stable SP conformer for each panel. The multistep reaction path is shown in solid line and the direct one in dotted ones. See also Table 1...

Fig. 7. Schematic showing reaction pathways by which fuel nitrogen, N, is converted to NO and N2. The bold lines indicate the key pathways (28). Thermal...

Both these 1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.12. In contrast with these thermal [L,51 sigmatropic hydrogen shifts, however, thermal [1,3 hydrogen shifts are unknown. Were they to occur, they would have to proceed by a strained antarafacial reaction pathway. 

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

In the Mobius-Hiickel approach, diagrams similar to Figure 18.4 can be drawn for this case. Here too, the disrotatory pathway is a Hiickel system and the conrotatory pathway a Mobius system, but since six electrons are now involved, the thermal reaction follows the Hiickel pathway and the photochemical reaction the Mobius pathway. 

Thus, the parameters of acoustic intensity, temperature, ambient gas, and solvent choice have strong influences on sonochemical reactions. It is clear that one can fine tune the energetics of cavitation by the use of these variables and hence exercise control on the rates and reaction pathways followed by the associated chemistry. Specific examples will be discussed shortly. Clearly, the thermal conductivity of the ambient gas (e.g., a variable He/Ar atmosphere) and the overall solvent vapor pressure provides easy mechanisms for experimental control of the peak temperatures generated during the cavitational collapse. 

In this article we have summarized the use of both photochemical and more classical thermal kinetics techniques to deduce the nature of intermediates in the ambient temperature, fluid solution chemistry of several triruthenium clusters. In some cases the photochemically generated intermediates appear to be the same as those proposed to be formed along thermal reaction coordinates, while in other cases unique pathways are the results of electronic excitation. The use of pulse photolysis methodology allows direct observation, and the measurement of the reaction dynamics of such transients and provides quantitative evaluation of the absolute reactivities of these species. In some cases, detailed complementary information regarding... 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

As reported by Padwa and coworkers, exposing a suitable nitrofuran precursor to microwave irradiation in l-methyl-2-pyrrolidone (NMP) in the presence of 2,6-luti-dine catalyst provided l,4-dihydro-2H-benzo[4,5]furo[2,3-c]pyridin-3-one as the major product in 36% yield (Scheme 6.236) [419]. In contrast, under thermal conditions, removal of the tert-butyl group was observed as the major reaction pathway. 

Eq. 17 is meant to represent the possibility for a concerted formation of oxetane product. A problem that always exist in cycloadditions is the question of whether the reaction takes place by a two-step biradical reaction pathway or through a concerted mechanism. Such questions have not even been resolved for purely thermal reactions. 4> A recent speculation on this point proposes almost universal concertedness for all cycloaddition reactions. 79> In that work, mixed stereochemistry in the products of [2+2] cycloaddition reactions is generally attributed to a mixture of two concerted reactions, suprafacial-suprafacial, and supra-facial-antarafacial. It will be seen later that the PMO calculations generally do not support this idea. A mixture of biradical and concerted reactions is in better agreement with experimental facts. 

On the basis of the data obtained from the early thermal analysis and tube furnace pyrolysis experiments performed during the initial phases of this investigation, it became apparent that in order to establish the principal reaction pathways to the generation of volatile antimony species, the volatile degradation products of the DBDPO itself would need to be characterized (24, 25). 

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. 

In summary, major challenges in the partial oxidation of methane are (1) designs to avoid excessive thermal gradients (hot spots) in the catalyst bed (2) reduction of the cost of O2 separation and (3) elucidation of the reaction pathways as a step toward improved catalyst design. 

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

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