Intermediates, direct observation

Nonclassical ions, a term first used by John Roberts (an outstanding Caltech chemist and pioneer in the field), were defined by Paul Bartlett of Harvard as containing too few electrons to allow a pair for each bond i.e., they must contain delocalized (T-electrons. This is where the question stood in the early 1960s. The structure of the intermediate 2-norbornyl ion could only be suggested indirectly from rate (kinetic) data and observation of stereochemistry no direct observation or structural study was possible at the time. 

Perhaps the most convincing evidence for nucleophilic attack at an unexpected ring position comes from the direct observation of intermediate Meisenheimer complexes in the NMR spectrum. When 2-chloro-3,6-diphenylpyrazine is treated with KNH2 in liquid ammonia, the intermediate (29) was observed directly (Scheme 8). It was postulated that this initially formed complex rearranges to (30) which gives the observed product by elimination of a chloride ion (73RTC708). 

Oxirene is probably a true intermediate, but is separated from ketene by only a very low barrier. Since its instability results from unimolecular isomerization rather than from attack of other molecules, the only viable current technique for its direct observation seems to be generation and spectroscopic examination in an inert matrix at temperatures near absolute zero. 

In the course of this development, knowledge about low valent (in the sense of formal low oxidation states) reactive intermediates has significantly increased [26-30]. On the basis of numerous direct observations of silylenes (silanediyles), e.g., by matrix isolation techniques, the physical data and reactivities of these intermediates are now precisely known [31], The number of kinetic studies and theoretical articles on reactive intermediates of silicon is still continuously growing... 

Experimentally, there has been but little direct observation of the reactive intermediates present in this, or any other, t5rpe of discharge, as it is extraordinarily difficult to analyze a reactive plasma without perturbing the system. 

As mentioned in Section 21.4.1, the existence of intermediate phase with slightly stiffer modulus than that of mbber matrix was reported, which was determined by finite element calculation. The author reported that there are two phases around CB—one is almost comparable with bound mbber, 2 nm-thick glassy phase (GH-phase), another is 10 nm-thick uncross-Unked one (SH-phase). The intermediate regions in this smdy were usually observed around CB regions and the Young s modulus of this region was higher as well. Thus, there is a possibility that SH-phase was directly observed in real space for the first time. 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

The goal of this work was to determine surface species present under reaction conditions, and to investigate the interactions of adsorbed NO and NO2 with CH4. Infrared spectra were collected under reaction conditions, and in various mixtures of NO, O2, NO2, and CH4. It was of particular interest to make direct observations of the factors affecting the formation of NO2 and its reaction with CH4, since NO2 has been suggested as an intermediate in the reduction of NO by CH4. A further objective was to elucidate the pathway by which N2 and CO2 are formed. 

Noguchi, H., Yoda, E., Ishizawa, N., Kondo, J. N., Wada, A., Kobayashi, H. and Domen, K. (2005) Direct observation of unstable intermediate species in the reaction of trans-2-butene on ferrierite zeolite by picosecond infrared laser spectroscopy. J. Phys. Chem B, 109, 17217-17223. 

The following representative examples of TRIR studies are not meant to be an exhaustive treatment of the various organic reactive intermediates that have been investigated by TRIR methods, but rather to demonstrate the unique insight that such studies can provide. The direct observation of organic intermediates in solution at room temperature by IR spectroscopy can reveal fundamental information related both to bonding and structure of reactive intermediates as well to mechanisms of product formation. 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

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 rectangular plates yield on immersion in oily liquids of known refractive index the values of /3 and y, but crushed fragments usually show values intermediate between a and in one direction. Observations were made in light of variable wave length, obtained by a monochromatic illuminator, at 20°. The dispersion relations were found to be as stated in Table II. 

Exploitation of time-resolved spectroscopy allows the direct observation of the reactive intermediates (i.e., ion-radical pair) involved in the oxidation of enol silyl ether (ESE) by photoactivated chloranil (3CA ), and their temporal evolution to the enone and adduct in the following way.41c Photoexcitation of chloranil (at lexc = 355 nm) produces excited chloranil triplet (3CA ) which is a powerful electron acceptor (EKelectron-rich enol silyl ethers (Em = 1.0-1.5 V versus SCE) to the ion-radical pair with unit quantum yield, both in dichloromethane and in acetonitrile (equation 20). 

The fact that both the thermal and the photochemical insertion reactions yield the same products via formation of charge-transfer complexes leads to the conclusion that the reactive ion-radical pair in equation (52) is the common intermediate for both activation processes. Such a conclusion is further verified by the direct observation of anion-radical intermediates from the thermal reaction of TCNE and DDQ with various metal hydrides.188... 

The donor-induced disproportionation in equation (91) leads to the EDA complex, i.e., [D, NO+]NO as the (first) directly observable intermediate. The critical role of the nitrosonium EDA complex in the electron-transfer activation in equation (92) is confirmed by the spectroscopic observation of the cation-radical intermediates (i.e., D+ ) as well as by an alternative (low-temperature) photochemical activation with deliberate irradiation of the charge-transfer band252 (equation 95). 

Although there is general agreement about the occurrence of ionic intermediates on the bromination pathway, there are only few direct observations of these bromocations. It is still difficult to decide whether they are bridged or open depending on the double-bond substituents. 

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