Chemical trapping

Tile following photochemical conversions also involve 1,2-dithietes as intermediates whose chemical trapping was reported in most cases. Tlie formation of the dithiin 249 from 250 may best be explained by the formation of the dithiete dimer 251 and the loss of S2 (73ZC424).Tlie formation of 252 and 253 from 254 (78NJC331) should be compared with the sulfuration of the acetylene 182 with elemental sulfur (93BCJ623).Tlie photolysis of 255 provides a rare example when the ejection of a nitrile was employed for the generation of a 1,2-dithiete (73ZC431). 

Singlet halo(trifluoromethyl)carbenes [33a]-[33c], which were characterized by IR and UV spectroscopies and chemical trapping with HCl, have been generated from respective 3-halo-3-(trifluoromethyl)diazirines [34a]-[34c] frozen in an argon matrix at 12 K and irradiated with UV light at... 

Chemical trapping experiments demonstrate that this silicon analog of a carbodiimide is formed in 10% yield. Several trapping experiments have been carried out. 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

A battery of sensitive techniques is being developed to probe the photofragments resulting from photolysis of metal complexes in collision free conditions. The aim is to characterize the energy content, structure and chemistry of the photoproducts. These methods rely on ultraviolet (UV) laser photolysis followed by detection methods based on UV absorption (20), chemical trapping (21,22), IR absorption (23,24.25) and ionization (5,6,26,27). 

Knowledge of the 90 chemical elements and their properties in compounds led to the construction, by man, of a unique table of elements, the Periodic Table, of 18 Groups in six periods in a pattern fully explained by quantum theory, described in Chapter 2. There is then a huge variety of chemical combinations possible on the Earth and limitations on what is observable are related to element position in this Table. It also relates to the thermodynamic and/or kinetic stability of particular combinations of them in given physical circumstances (Table 11.3). The initial state of the surface of the Earth with which we are concerned was a dynamic water layer, the sea, covering a crust mainly of oxides and some sulfides and with an atmosphere of NH3, HCN, N2, C02(C0, CH4), H20, with some H2 but no 02. This combination of phases and their contents then produced an aqueous solution layer of particular components in which there were many concentration restrictions between it and the components of the other two layers due to thermodynamic stability, equilibria, or kinetic stability of the chemicals trapped in the phases. It is the case that equilibrium... 

The composition of thermolysis products in ethanol, which acted as a chemical trap, allows the estimation of the contributions of these processes under different conditions (Scheme 24). On heating of an alcohol solution of 20a in a sealed tube, the short-lived silanethione Me2Si=S (50) and sila-thiirane 46a are trapped immediately by ethanol. The first compound forms Me2Si(OEt)2, and the second one gives silylated mercaptan Me2Si(OEt) CMe2SH (51). At 150°C these products are formed in a ratio of 1 1. The Corey-Chaykovsky type reaction becomes predominant at 245 °C. The Me2Si(OEt)2 51 ratio at this temperature is already 1 3. 

A spectroscopic detection of germathiones and silathiones, previously characterized only by chemical trapping, was described elsewhere127,128. Dimethylgermathione (33) and dimethylsilathione were generated from the corresponding trimers (Me2MS)3[M = Ge(34), Si] by pyrolysis (250-300°C, 5 x 10 2 mbar). 

Azacycloheptatrienylidene (4b), the aza analogue of cycloheptatrienylidene (4a), has on occasion been postulated to be the product of the ring expansion of lb.la In contrast to the case with 34a,lla 34b has never been directly detected. However, Iwamura and co-workers have reported chemical trapping of 4b and its o-phenyl derivative (4c) by tetracyanoethylene (TCNE), as shown in Scheme 15 49 While the product in Scheme 15 is formally the product of reaction of 4b with TCNE, the authors noted that it could also arise from reaction of TCNE with ketenimine 3b. 

The development of this technique has proceeded along several independent paths with two principal lines being elemental analyzer-IRMS and capillary gas chromatography-IRMS. In elemental analyzers, samples are combusted to CO2, N2, SO2, and H2O, which are either chemically trapped or separated on GC columns. The advantages of these techniques are an automated preparation with low costs per sample and a large sample through-put. 

Commercial combustion elemental analyzers perform a flash combustion, converting samples to CO2, H2O, N2, and SO2 simultaneously. These different gases are then chemically trapped, converted, or separated on GC columns and measured in a continuous flow mass spectrometer. This technique allows the determination... 

Parenthetically, it should be noted that there is evidence for the accumulation of an acylenzyme (i.e., a mixed anhydride with Glu-270) in the carboxypeptidase A-catalyzed hydrolysis of esters at low temperature, but this evidence does not include confirmation by chemical trapping experiments (Makinen et al, 1979 Kuo and Makinen, 1982 Suh et al., 1985). This would imply a nucleophilic, rather than promoted-water, pathway for ester hydrolysis. Sander and Witzel (1985) provided the only... 

Palladium and rhodium based catalysts, which yield methanol and ethanol from synthesis gas respectively, were selected for a mechanistic study. Chemical trapping showed a correlation between formyl species and the catalytic activity, indicating that these species probably are reaction intermediates. The role of the support on the activity and on the nature of the products was elucidated by chemical trapping of formyl, methoxy and formate species on palladium catalysts, and of formyl and acetate on rhodium catalysts. The rhodium catalysts were also studied by probe molecule experiments CH CHO) and by FT-IR spectroscopy (chemi-... 

To obtain more information about the role of the support, it is necessary to investigate the reaction mechanisms over the various catalyts more thoroughly. One approach to the study of the reaction mechanism is the identification of reactive surface species by means of chemical trapping. This method has been fully described earlier (35-36) and only an outline will be given here. 

In the chemical trapping experiments, with respect to the evolution of the formyl and methoxy species, we observed a delay between the formation of formyls and the appearence of methoxy moieties. This effect brought us to consider another intermediate between them. 

Most of the species included in this scheme have previously been characterized by chemical trapping and by I.R. spectroscopy (41) except for the adsorbed formaldehyde. 

Results of chemical trapping of the formyl species on these catalysts showed a higher concentration of formyl moieties on Rh/SiO than on cerium containing catalysts (Table II). 

Chemical trapping was shown to be a very useful tool in the study of reaction mechanisms. The results thus obtained combined with those by F.T.I.R. spectroscopy have pointed out ... 

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