Carbonyl compounds, electron spectroscopy

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The most important application of organolithium reagents is their nucleophilic addition to carbonyl compounds. One of the simplest cases would be the reaction with the molecule CO itself, whose products are stable at room temperature. Recently, it was shown that a variety of RLi species are able to react with CO or f-BuNC in a newly developed liquid xenon (LXe) cell . LXe was used as reaction medium because it suppresses electron-transfer reactions, which are known to complicate the reaction . In this way the carbonyllithium and acyllithium compounds, as well as the corresponding isolobal isonitrile products, could be characterised by IR spectroscopy for the first time. 

It is commonly accepted that chemisorption of CO on transition metals takes place in a way that is quite similar to bond formation in metal carbonyls (4). First experimental evidence for this assumption was obtained from a comparison of the C—O stretching frequencies (5) and was later confirmed by data on the bond strength (6) as well as by valence and core level ionization potentials obtained by photoelectron spectroscopy (7). Recent investigations have in fact shown that polynuclear carbonyl compounds with more than about 3-4 metal atoms exhibit electronic properties that are practically identical to those of corresponding CO chemisorption systems (8, 9), thus supporting the idea that the bond is relatively strongly localized to a small number of metal atoms forming the chemisorption site. 

Because of the high discriminating capacity of MAD for sterically and/or electronically similar ethers, Yamamoto and Maruoka examined the affinity of the compound toward other substrates with oxygen-containing functional groups, for example various carbonyl compounds, including both aliphatic and aromatic aldehydes, amides, esters, ethers, and ketones with similar structural substituents. Binding behavior was monitored by low-temperature NMR spectroscopy of these substrates and their... 

Addition of an electron to the LUMO of the carbonyl group to form a radical anion is the first step in the reduction process. Radical anions can be characterized in aprotic solvents by electron spin resonance (esr) spectroscopy. Those derived from unconjugated carbonyl compounds are highly reactive and can only be detected in a matrix at low temperatures [3]. Decay is rapid because the excess carbonyl compound acts as a proton donor toward the basic oxygen center in the radical anion. Aromatic carbonyl compounds give less reactive radical anions in which the free electron is delocalized over the whole... 

The earlier kinetic investigation had used infrared spectroscopy and UV-VIS electronic absorption spectroscopy to monitor the concentration of metal carbonyl compounds. They had often observed the production of alkyl chloridesor olefins plus from alkyl-rhodium carbonyl... 

The distinguishing feature of this mechanism is the second step, in which an electron is transferred from the organometallic reagent to the carbonyl compound to give the radical anion of the carbonyl compound. Subsequent collapse of the ion pair gives the same product as is formed in the normal mechanism. The electron transfer mechanism would be expected to be favored by structural features that stabilize the radical anion. Aryl ketones and diones fulfill this requirement, and much evidence for the electron transfer mechanism has been accumulated for such ketones. In several cases, it is possible to observe the intermediate radical anion by EPR spectroscopy. ... 

What are the spectral characteristics of carbonyl compounds In H NMR spectroscopy, the formyl hydrogen of the aldehydes is very strongly deshielded, appearing between 9 and 10 ppm, a chemical shift that is unique for this class of compounds. The reason for this effect is twofold. First, the movement of the ir electrons, like that in alkenes (Section 11 ), causes a local magnetic field, which strengthens the external field. Second, the charge on... 

CHjCHO Acetaldehyde is the prototype species for a wide variety of aliphatic carbonyl compounds. The photochemistry and electronic spectroscopy have been reviewed by Lee and Lewis The electronic structures in the carbonyl group of acetaldehyde are similar to those of formaldehyde in that the low lying valence and Rydberg levels have about the same energy. For example, n k excitation in both species gives rise to a weak absorption at 350-250 nm. The first Rydberg n - 3s excitation is found as a line-like feature at 182 nm, compared to the 174 nm band of CHjO. 

Numerous dynamic and kinetic studies have been performed using O NMR. Line shape analysis is not straightforward because the line width itself is temperature dependent. Nevertheless, it has been shown that dynamic O NMR is a convenient alternative to NMR with, for example, methyl carbonyl compounds. A substantial number of publications have been concerned primarily with studies of the hydration (solvation) of ions in solution and the determination of rates and activation parameters for ligand substitution (a rapid development of high-pressure NMR has occurred in the last twenty years). There is an extensive literature of NMR spectroscopy used for the study of hyperfine interactions between the unpaired electrons in paramagnetic molecules and ions and the nucleus. The... 

The use of infra-red or ultraviolet spectroscopy to examine the molecular groups present in a chemical compound is familiar to any chemist. One of the main uses of this technique is to apply a range of electromagnetic frequencies to a sample and thus identify the frequency at which a process occurs. This can be characteristic of, say, the stretch of a carbonyl group or an electronic transition in a metal complex. The frequency, wavelength or wavenumber at which an absorption occurs is of most interest to an analytical chemist. In order to use this information quantitatively, for example to establish the concentration of a molecule present in a sample, the Beer-Lambert law is used ... 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

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