Dark reactions steps

Photosensory Transduction Chain Photoreceptor Pigments Dark Reactions Steps in the Sensory Transduction Chain 122-7... 

V,/V-dimethylaniline, especially when those strong donors are paired with the relatively electron-poor MES derivative of the bis(arene)iron(ll) acceptor. As such, the dark reactions arise via essentially the same multistep mechanism as that for charge-transfer de-ligation, the difference arising from an adiabatic electron transfer (10) as the initial step that is thermally allowed when the driving force -AGET is sufficient to surmount... 

The reactions initiated by electromagnetic radiation are said to bephotochemi-cally activated. Note that only the initiation step may require the absorption of one or more photons (a photochemical reaction). Subsequent steps of the mechanism may be dark reactions, proceeding by thermal activation. 

On p. 407, the initial step of the dark reactions in plant photosynthesis (in the Calvin cycle) is shown at the top left. 

Water is inevitably present in the second reaction step, since it is produced during the reduction of Ru(II) to Ru(I), thereby allowing this reaction to take place, albeit at slow rate. Effectively, dark solutions incidentally obtained at the end of the abovementioned preparation and reflecting the presence of [Ru3(p-H) (CO)ii] become clearer on prolonged treatment overnight with CO at 25°C, with concomitant recovery of Ru3(CO)i2-... 

The dark reaction involves the fixation of carbon dioxide to form carbohydrates. The ATP and the NADPH produced in the light reaction drive this carbon fixation. It might be thought that the interruption of the Calvin cycle would also produce effective herbicides but this is not the case. There are two reasons why. First, the reaction is not an energetic reaction whose interruption would lead to the destruction of cellular components and second, the enzymes involved in the process are present in very high amounts. If an enzyme is to be targeted as a key step in the metabolism of a plant, it is important that it is present in small amounts and that it is not turned over very quickly. If an enzyme is abundant,... 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

The reduction of C02 -> carbohydrate, is a dark reaction which occurs in a number of enzymatic steps. 

Reactions (3-27b ) and (3-27c ) are actually terminating steps since they are much more rapid than reaction (3-28c ) until important quantities of peroxide accumulate. Therefore the reported rate constants are deduced from data obtained under the initial conditions so as to eliminate dark reaction effects. If this mechanism is the correct one then 27a / 27b 300 from the data of Huybrechts et al. or 230 from those of Schott and Schumacher. Furthermore k28, k2W = 5.7 from Huybrechts et al. or 4.0 from Schott and Schumacher. 

There are two possible initiation steps for the free radical reaction step lb and the combination of steps la and 2a. The role of step lb in the reaction scheme is an important consideration in minimizing the population of atomic fluorine. This step was first postulated by Miller et al. (9) on the basis of reaction products. As indicated in Table I, this process is exothermic at room temperature (AC2 9g° k = -5.84 k cal mole 1) although the enthalpy is slightly positive. The validity of this step has not yet been conclusively established by spectroscopic methods, and this remains an unsolved problem of prime importance in fluorine chemistry. The fact that fluorine reacts at a significant rate with hydrocarbons in the dark at temperatures lower than —78°C is an indication that step lb is a significant step that may have very little or no activation energy at room temperature. At very low temperatures there is no reaction between molecular F2 and CH4 or C2H6 when isolated in 10°K matrices (10). 

The industrial synthesis of vitamin D is a perfect replica of the biosynthesis which relies on a key photochemical step of electrocyclic ring closure/ring opening (section 5.6). In this case the photochemical process is essential, since the dark reaction is forbidden by reasons of orbital symmetry considerations. 

Sporer (45) gives conclusive evidence for the presence of a radical intermediate [electron paramagnetic resonance (EPR), and radical polymerization] but fails to describe the path by which the intermediate radical is converted to the cation. As possibilities he cites the crossing of the excited molecule to another, undescribed state from which it reacts, and reaction during the internal conversion step in which a molecule in the excited state converts to a molecule in a high vibrational ground state. Kinetic studies by Brown et al. (46) support the formation of ion-pair intermediates in the dark reactions. 

A mixture of 22 g 2,5-dimethoxypropylbenzene, 23 g POCl3 and 22 g N-methylformanilide was heated on the steam bath for 1.5 h. The hot, dark reaction mass was poured into 1 L H20, which allowed the eventual separation of 2,5-dimethoxy-4-(n)-propylbenzaldehyde as a clear yellow oil weighting 14 g. Although the homologous 4-ethyl and 4-butyl benzaldehydes were clean crystalline solids, this propyl homologue remained an oil. Gas chromatographic analysis showed it to be about 90% pure, and it was used as obtained in the nitrostyrene steps with either nitromethane (here) or nitroethane (under DOPR). 

Turning back to the definition of photochemistry and anticipating the classification of photochemical reactions of metallotetrapyrroles, it should be kept in mind that a true photochemical process is only that involving an electronically excited particle (in this review it means an excited tetrapyrrole complex). All subsequent reactions are spontaneous (in photochemistry they are familiarly called dark reactions). Exactly speaking, each classification of photochemical reactions should start with an answer to the question what is the nature of the primary photochemical step involving a complex in its photochemically reactive excited state It must be admitted that for the... 

It will be recalled that Warburg suggested the following steps in the decomposition, after carefully considering the possible secondary dark reactions ... 

According to this model, the first stage in the treatment of nitrophenols aqueous wastes was the release of the nitro group from the aromatic ring. As a consequence, phenols or quinones were formed. These organic compounds were oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Also the cathodic reaction steps were considered in the global process when the electrochemical cell was undivided at the cathode, the reduction of the nitro to the amine group and the transformation of nitrate into ammonia were observed. In alkaline media, aminophe-nols were polymerised and transformed into a dark brown solid. 

It is rather atypical that a photochemical reaction will proceed in a single molecular pathway. Thus, several elementary steps are involved. Normally, the majority of them are dark (thermal) reactions while, ordinarily, one activation step is produced by radiation absorption by a reactant molecule or a catalyst. From the kinetics point of view, dark reactions do not require a different methodological approach than conventional thermal or thermal-catalytic reactions. Conversely, the activation step constitutes the main distinctive aspect between thermal and radiation activated reactions. The rate of the radiation activated step is proportional to the absorbed, useful energy through a property that has been defined as the local volumetric rate of photon absorption, LVRPA (Cassano et ak, 1995 Irazoqui et al., 1976) or the local superficial rate of photon absorption, LSRPA (Imoberdorf et al., 2005). The LVRPA represents the amount of photons that are absorbed per unit time and unit reaction volume and the LSRPA the amount of photons that are absorbed per unit time and unit reaction surface. The LVRPA is a property that must be used when radiation absorption strictly occurs in a well-defined three-dimensional (volumetrical) space. On the other hand, to... 

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Through a rather complicated sequence of meanwhile fully understood reaction steps ethyl benzoate and lithium bis(trimethylsilyl)phosphanide form tris(l,2-dimethoxyethane-0,0 )-lithium 3-phenyl-l,3-bis(trimethylsilyl)-l,2-diphosphapropenide and 3,5-diphenyl-l,2-bis(tri-methylsilyl)-l,2,4-triphospholide. X-ray structure determinations on orange or green, metallically lustrous, crystals show the compoxmds to be ionic in the solid and to contain a 1,2-diphosphaallyl and a 2-phosphaallyl anion, respectively. Dark red tetrakis(tetra-... 

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