Proton NMR technique

Figure 13 shows a full NMR spectrum of PET, in TCE solvent, acquired at 100°C. Resonances a and b are from the terephthalate and ethylene glycol derived protons, respectively. Resonances c and d arise from the protons in diethylene glycol (DEG), which is formed as a side reaction in the PET polymerisation process. An expansion of the ethylene glycol-based region of the spectrum is shown in Figure 14. Proton NMR techniques have been described for characterising end groups [42-44], the methodology below being that which is used in our laboratory.

By use of proton NMR techniques Parsons and coworkers studied a similar system in more detail in terms of the threading process and molecular motion... 

Thus it is possible to study the hydrolysis reactions of esters under conditions where the substrate is completely protonated. The properties of the protonated ester, however, are more conveniently examined using more strongly acidic media, in the absence of water, where bimolecular reactions are reduced to insignificance. At sufficiently low temperatures under these conditions the rates of exchange of the added protons are slow, and the detailed structures of protonated carboxylic acids and esters can be investigated, particularly by proton nmr techniques. 

In contrast, no such invariability of mechanism apparently holds for ligand exchange at beryllium(II). Lincoln and Tkaczuk " have used the variable-temperature proton nmr technique to investigate the exchange of... 

Section 13 19 2D NMR techniques are enhancements that are sometimes useful m gam mg additional structural information A H H COSY spectrum reveals which protons are spin coupled to other protons which helps m deter mining connectivity A HETCOR spectrum shows the C—H connections by correlating C and H chemical shifts... 

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

The rates of removal of axial and equatorial protons from 4-t-butylcyclohexane in NaOD/dioxan have been measured by an NMR technique. The rate of removal of an axial proton is 5.5 times faster than for an equatorial proton. What explanation can you offer for this difference ... 

The case for the generality of the o-complex mechanism is further strengthened by numerous studies showing that benzenium ions (an alternative name for the o-complex) can exist as stable entities under suitable conditions. Substituted benzenium ions can be observed by NMR techniques under stable-ion conditions. They are formed by protonation of the aromatic substrate ... 

HETCOR (Section 13.19) A 2D NMR technique that correlates the H chemical shift of a proton to the C chemical shift of the carbon to which it is attached. HETCOR stands for heteronuclear chemical shift correlation. 

Intimate information about the nature of the H bond has come from vibrational spectro.scopy (infrared and Raman), proton nmr spectroscopy, and diffraction techniques (X-ray and neutron). In vibrational spectroscopy the presence of a hydrogen bond A-H B is manifest by the following effects ... 

Different solid-state NMR techniques CPMAS NMR, the second moment of the signal, the spin-lattice relaxation time in the rotating frame T p) were combined to reach the conclusion that in the case of por-phine H2P the double-proton transfer is followed by a 90° rotation within the crystal (see Scheme 2). 

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

Relatively simple NMR techniques have been employed to obtain useful information about dendrimers. For example, the formation of one generation from its immediate predecessor containing NH groups has been followed by NMR. As coupling proceeds, the chemical shifts of the NH protons become shifted to lower field (higher frequency), a shift that is consistent with coupling at the NH groups within the branches. 

The NMR techniques discussed so far provide information about proton-proton interactions (e.g., COSY, NOESY, SECSY, 2D y-resolved), or they allow the correlation of protons with carbons or other hetero atoms (e.g., hetero COSY, COLOC, hetero /resolved). The resulting information is very useful for structure elucidation, but it does not reveal the carbon framework of the organic molecule directly. One interesting 2D NMR experiment, INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment), allows the entire carbon skeleton to be deduced directly via the measurement of C- C couplings. 

The earliest NMR technique to gain importance in chemistry was that of proton NMR. Spectra could be obtained for compounds containing the nucleus by continuously sweeping the field at constant frequency. This... 

The development of microcoil techniques has been reviewed by Minard and Wind [24, 25] and by Webb [26]. In a more recent publication Seeber et al. reported the design and testing of solenoidal microcoils with dimensions of tens to hundreds of microns [27]. For the smallest receiver coils these workers achieved a sensitivity that was sufficient to observe proton NMR with an SNR of unity in a single scan of 10 pm3 (10 fL) of water, containing 7 x 1011 proton spins. Reducing the diameter of the coil from millimeters to hundreds of microns thus increases its sensitivity greatly, allowing analysis of pL to pL sample volumes. 

The usefulness of NMR in such analysis is because the proton spin-relaxation time constants are different for different components, such as water, liquid fat and solid fat. For example, the signal from solid fat is found to decay rapidly while the liquid signals decay much slower. This phenomenon is the basis for an NMR technique to determine the solid fat content [20], However, as the relaxation time constant of water, for example, could depend on its local environment, such as protein concentration, it may overlap with that of oil and other components. As a result, it could be difficult to formulate a robust and universal relaxation analysis. It... 

Applications Useful 2D NMR experiments for identification of surfactants are homonuclear proton correlation (COSY, TOCSY) and heteronuclear proton-carbon correlation (HETCOR, HMQC) spectroscopy [200,201]. 2D NMR experiments employing proton detection can be performed in 5 to 20 min for surfactant solutions of more than 50 mM. Van Gorkum and Jensen [238] have described several 2D NMR techniques that are often used for identification and quantification of anionic surfactants. The resonance frequencies of spin-coupled nuclei are correlated and hence give detailed information on the structure of organic molecules. 

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