In Situ Nuclear Magnetic Resonance NMR

Computational and deuterium-labeling studies, isolation of model complexes such as 122 and 7.23, and in situ nuclear magnetic resonance (NMR) identification of 7.24 are consistent with the mechanism shown in Figure 7.2. 

Frei and co-workers also extended this reaction to other zeolites showing that almost identical behavior was observed in BaY, BaX, and in the K+ and Ba " forms of zeolite L [45,46]. Xiang et al. [47] have also studied the photooxidations of a series of 1-alkenes in the more acidic BaZSM-5 [48] and Ba- 3. The extensive polymerization of propylene in these zeolites demonstrates the detrimental effect of Bronsted acid sites on the reaction selectivity. These workers also used ex situ nuclear magnetic resonance (NMR) allowing more detailed... 

An extremely sensitive technique able to detect the nature of radical pairs in a photochemical reaction is called chemically induced dynamic nuclear polarization (CIDNP), which depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample, irradiated in situ, in the cavity of a NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (28). 

The azoniaspirocycles described in this chapter have mostly been synthesised in situ, and thus were not isolated. As a result, complete characterization by nuclear magnetic resonance (NMR) spectroscopy is not always available. However, in many cases, the azoniaspiro species has been detected by H NMR analysis of the reaction mixture. In addition, the formation of the ammonium salts can sometimes lead to stable solids which can be kept for significant periods without decomposition. 

Aurbach and co-workers performed a series of ex situ as well as in situ spectroscopic analyses on the surface of the working electrode upon which the cyclic voltammetry of electrolytes was carried out. On the basis of the functionalities detected in FT-IR, X-ray microanalysis, and nuclear magnetic resonance (NMR) studies, they were able to investigate the mechanisms involved in the reduction process of carbonate solvents and proposed that, upon reduction, these solvents mainly form lithium alkyl carbonates (RCOsLi), which are sensitive to various contaminants in the electrolyte system. For example, the presence of CO2 or trace moisture would cause the formation of Li2COs. This peculiar reduction product has been observed on all occasions when cyclic carbonates are present, and it seems to be independent of the nature of the working electrodes. A single electron mechanism has been shown for PC reduction in Scheme 1, while those of EC and linear carbonates are shown in Scheme 7. ... 

Nuclear magnetic resonance (NMR) spectroscopy is the most widely used spectroscopic technique in synthetic chemistry [1], One main reason for the dominance of NMR is its versatility—by variation of only a few experimental parameters, a vast number of different NMR experiments can easily be performed, giving access to very different sets of information on the substance or the reaction under investigation. Today, NMR is dominant in structure elucidation, and in situ NMR spectroscopy can conveniently give insight into chemical reactions under real turnover conditions (in contrast to, e.g., x-ray crystallography, which can only provide a solid-state snapshot of a molecular conformation). 

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

Among the techniques ideally suited for in situ studies are infrared, Raman, and nuclear magnetic resonance (NMR) spectroscopies and extended x-ray absorption fine structure (EXAFS). While still relatively new, the scanning tunneling and atomic force microscopes are expected to play an increasingly important role in catalyst characterization. Both instruments permit visualization of a catalyst surface at the atomic level and hold the potential of showing how atoms and molecules interact with a surface. 

Although they may be part of a catalyst testing [1-3] programme, investigations focused on revealing the reaction mechanism, such as in-situ Fourier transform infrared (FTIR) in transmission or reflection mode, nuclear magnetic resonance (NMR), X-ray diffraction (XRD), X-ray absorption fine-structure spectroscopy (EXAFS), X-ray photoelectron spectroscopy (XPS), electron microscopy (EM), electron spin resonance (ESR), and UV-visible (UV-vis) and the reaction cells used are not included. For the correct interpretation of the results, however, this chapter may also provide a worthwhile guide. 

Nuclear magnetic resonance (NMR) spectroscopy — Nuclear magnetic resonance (NMR) spectroscopy of atoms having a nonzero spin (like, e.g., H, 13C) is an extremely powerful tool in structural investigations in organic and inorganic chemistry. Beyond structural studies atoms observable with NMR can also be used as probes of their environment. Thus NMR may be employed for in situ spectroelectrochemical studies [i]. Cell designs for in situ NMR spectroscopy with electrochemical cells are scant. Because of the low sensi-... 

The exact chemical composition of a plant extract is not always completely known. Many articles published in recent years attempt to identify the compounds structure by coupling chromatography with spectro-metric methods. Modern densitometers are able to record the in situ ultraviolet-visible (UV-vis) spectra of a separated substance on a TLC plate [6]. Thin-layer chromatography can be also coupled with other methods in order to enhance the identification of compounds, such as mass spectrometry (MS) or nuclear magnetic resonance (NMR). There are devices able to record the in situ spectra on the TLC plate, or the separated substance is removed from the plate together with the layer, then extracted in a small volume of an adequate solvent, and the sample can be used for obtaining the spectra [6,7]. 

The urgent need to develop more efficient fuel cell anodes and cathodes has brought the electrochemical, catalytic, and surface science communities closer than ever before and has made electrocatalysis a rapidly growing field both in experimental new findings and in theoretical understanding. It is very likely that the rapid advances in catalyst and electrocatalyst nanoparticle preparation and characterization [10], together with the utilization of new powerful in situ techniques, such as electrochemical nuclear magnetic resonance (NMR) [216] in conjunction with... 

In the case of H-CBS, this catalyst could be prepared by mixing diphenylprolinol with borane-tetrahydrofurane (THF) or borane dimethylsulflde. Despite numerous efforts in many groups to isolate or even characterize H-CBS by nuclear magnetic resonance (NMR) spectroscopy, all attempts have been unsuccessful. H-CBS is used in situ and good results can be obtained in many cases. 

To bridge the gap between ideal and practical catalysts, optical spectroscopies, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and Mossbauer spectroscopy can be used. All have been reviewed recently (373, 396), and some examples have been cited earlier (107, 108). Electron spin resonance has been used in several studies of electroorganic reactions (357,371). It can detect short-lived radicals resulting from electron transfer. Recent application of Mossbauer spectroscopy in situ in electrochemical cells deserves mentioning, although it addressed only the anodic polarization and film stability of Co- and Sn-coated electrodes (397,398). Extension to electrocatalytic studies involving Mossbauer nuclides seems feasible. 

---