Time Dependence of NMR Spectroscopy

This principle is illustrated by the cyclohexane spectrum. In the chair conformation, there are two kinds of protons the axial hydrogens and the equatorial hydrogens. The axial hydrogens become equatorial and the equatorial hydrogens become axial by chair-chair interconversions. These interconversions are fast on an NMR time scale at room temperature. The NMR spectrum of cyclohexane shows only one sharp, averaged peak (at SI.4) at room temperature. 

Low temperatures retard the chair-chair interconversion of cyclohexane. The NMR spectrum at -89°C shows two nonequivalent types of protons that split each other, giving two broad bands corresponding to the absorptions of the axial and equatorial protons. The broadening of the bands results from spin-spin splitting between axial and equatorial protons on the same carbon atom and on adjacent carbons. This technique of using low temperatures to stop conformational interconversions is called freezing out the conformations. 

NMR spectroscopy is useful for determining the conformations of biological molecules such as neuro-transmitters. For example, information about the conformations of acetylcholine has been used to design rigid analogs that are used as drugs to treat Alzheimer s disease. 

Comparison of the NMR spectrum of unusually pure ethanol and the spectrum of ethanol with a trace of an acidic (or basic) impurity. The impurity catalyzes a fast exchange of the —OH proton from one ethanol molecule to another. This rapidly exchanging proton produces a single, unsplit absorption at an averaged field. 

Propose mechanisms to show the interchange of protons between ethanol molecules under (a) acid catalysis (b) base catalysis 

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Stereochemical Nonequivalence of Protons 591 13-11 Time Dependence of NMR Spectroscopy 594 Problem-Solving Strategy Interpreting Proton NMR Sectra 597... 

In the preceding decade, solid-state NMR spectroscopy has provided important and novel information about the nature and properties of surface sites on working solid catalysts and the mechanisms of these surface reactions. This spectroscopic method offers the advantages of operation close to the conditions of industrial catalysis. A number of new techniques have been introduced and applied that allow investigations of surface reactions by solid-state NMR spectroscopy under both batch and flow conditions. Depending on the problems to be solved, both of these experimental approaches are useful for the investigation of calcined solid catalysts and surface compounds formed on these materials under reaction conditions. Problems with the time scale of NMR spectroscopy in comparison with the time scale of the catalytic reactions can be overcome by sophisticated experimental... 

In Sections 2.2 and 2.3, it is described that the network structures of polymer gels, especially microscopic inhomogeneity, may be detected through the diffusion behaviour of probe molecules of different sizes, which can be obtained by using time-dependent diffusion NMR spectroscopy. In this section, it will be... 

Fig. 14 Monitoring of an acetylation reaction in a methylimidazolium-PF6 IL via time-dependent HRMAS NMR spectroscopy [70]. Reproduced by permission of The Royal Society of Chemistry...

At high pH, the stability of the cyclometalated iridium complexes 21 and 23 is limited, because they gradually decomposed when heated in 1 M NaOH at 80 °C for 24 hours. At room temperature, the stability was higher and the complexes were partly deuterated when dissolved in 1M NaOH. Time-dependent H NMR spectroscopy allowed the sequence of deuteration to be monitored, thus providing access to selectivity patterns (Figure 3.2). 

Refluxing benzene solutions of Cjq in the presence of a 20-fold excess of BujSnH leads to hydrostannylation (Scheme 6.15) [73]. Multiple additions can also take place. To maximize the yield of the monoadduct CgoHSnBuj (24), the time dependence of the reaction was followed quantitatively by HPLC. After about 4 h, the concentration of the monoadduct 24 reaches its maximum. Compound 24 can be isolated by preparative HPLC on a Cjg-reversed-phase stationary phase with CHCI3-CH3CN (60 40, v/v) as eluent. The structure of C5oHSnBu3 (24) was determined by NMR spectroscopy and other methods, showing that a 1,2-addition takes place regio-selectively (Scheme 6.15) [73]. 

An additional limitation of NMR spectroscopy is the long observation time. Depending on the technique applied, observation times of ca. 100 ms to some hours may be needed. Hence, only structures that are stable during the observation time... 

In principle, the determination of molecular uptake may be based on any experimentally accessible quantity which is a function of the amount adsorbed. Being directly sensitive to a certain molecular species, in this respect the application of spectroscopic methods is particularly suitable. IR spectroscopy has been successfully applied to studying molecular uptake by beds of zeolite catalysts [26-28] as well as—in combination with IR microscopy [29, 30]—on individual crystallites. Similarly, NMR spectroscopy has also been used to monitor the time dependence of the sorbate concentration within porous media [31]. Moreover, recent progress in NMR imaging allows the observation of concentration profiles within porous media with spatial resolution below the mm region [32-34],... 

Near-Infrared Spectroscopy. Near-infrared (NIR) spectroscopy is a technique that has been around for some time but, like NMR spectroscopy, has only recently been improved and developed for on-line applications. Near-infrared analysis (NIRA) is a nondestructive technique that is versatile in the sense that it allows many constituents to be analyzed simultaneously 112, 113). The NIR spectrum of a sample depends upon the anharmonic bond vibrations of the constituent molecules. This condition means that the temperature, moisture content, bonding changes, and concentrations of various components in the sample can be determined simultaneously. In addition, scattering by particles such as sand and clay in the sample also allows (in principle) the determination of particle size distributions by NIRA. Such analyses can be used to determine the size of droplets in oil-water emulsions. 

Because of the quantum mechanical time scale of the NMR instrument, one can study certain time-dependent phenomena that are not generally accessible to the other branches of spectroscopy. Both molecular motion and chemical exchange may affect the appearance of NMR spectra. This unique characteristic has important implications in the study of Grignard reagents. This chapter is designed to provide information to the chemist who uses NMR as an investigative tool. It is assumed that the reader has a basic understanding of the theory and practice of NMR spectroscopy. Those who do not may find it helpful to consult one of several excellent texts on NMR spectroscopy [1-13]. 

Catalytic systems are inherently multicomponent. An attractive method for determining the difFusivities in multicomponent systems is Fourier transform (FT) PFG NMR spectroscopy, which allows the simultaneous determination of the self-difFusivities of the individual components in a. mixture. If the chemical shifts of the individual species are sufficiently different, the Fourier transform of the spin echo yields separate peaks for the various adsorbates, and then similar to normal PFG NMR the attenuation of the separate peaks with increasing applied field gradient intensity yields the self-diffusivities. Because the technique can also be applied at elevated temperatures, it provides the opportunity for in-situ diffusion measurements under reaction conditions. The experiment also yields the time dependence of the relative concentrations of the reactant and product molecules and thus the intrinsic reaction rate. 

Pt NMR spectra were employed to study the nature of Pt(bipy)22+ in aqueous alkaline solutions. The data were consistent with formation of a conformationally mobile pseudo-five-coordinate complex involving OH coordination.1079 The time dependence of Pt(II) complexation with poly(amid-oamine)dendrimers was established by 195Pt NMR spectroscopy.1080 The 2H and 13C NMR spectra for solutions of cA-[L2Pt 9-MeAd(-H) ]3(N03)3 show dissociation to a mononuclear species, e.g. cA-[L2Pt 9-MeAd(-H) ]+, where L... 

Some information is available concerning the kinetics of transhydrocyanation reactions employing benzaldehyde as the substrate [173]. In this study the time dependence of the concentrations of all educts and products were determined by NMR spectroscopy. However, this complex kinetic system is still not fully understood. 

In this book, NMR is viewed from the perspective of imaging. NMR spectroscopy, relaxometry, and transport measurements are considered to be useful for defining image contrast. Clearly, such an approach is likely to be foreign to an NMR spectroscopist, who may consider NMR imaging a modification of multi-dimensional NMR. These different perspectives can be related to each other by considering the time dependence of the Larmor frequency (1.1.7). 

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