NMR, proton

The molar diamagnetic susceptibility of thiazole and some derivatives was initially determined by the classical Curie-Cheneveau method (5,315,316) and later confirmed by a method (317) based on the difference of NMR proton chemical shift of a sample of tetramethylsilane immersed in the liquid to be investigated, according to the shape (cylindrical or spherical) of the sample tube (Table 1-47) (318),... 

Sadtler Research Eaboratories Standard NMR, Proton NMR, IR and EWSpectral Data, Philadelphia, Pa., 1980. 

Another example is provided by [30] anmlene. Longuet-Higgins and Salem have shown that the observed visible and UV absorption spectrum and, in particular, the NMR proton chemical shifts of this molecule are very difficult to reconcile with the symmetrical nuclear configuration (Dg ) suggested by the superposition of the Kekule-type resonance structures. The hypothesis of a bond-length alternation of symmetry removes this difficulty. This indicates that the resonance between Kekule-type structures should be very much impeded also in this molecule. 

An on-flow experiment is now carried out. 50 pi of a solution of the product mixture (5 mg in 5 mL solvent) are injected and the NMR proton signal accumulation started simultaneously. The time taken for the chromatogram is 17 min. During this time a total of 128 proton NMR spectra are recorded, each with eight scans, i.e. an FID is accumulated approximately every 7 sec. After the Fourier transformation we obtain a two-dimensional representation (Fig. 33) of the on-flow experiment. 

Results of liquid phase NMR measurements (Table 4) show that only the real substrates influences the proton shift of H3 and H9 protons of CD. These data confirmed the liquid phase interaction between the diketones and the chiral modifier. No effect of dummy substrates (ethyl acetate, acetone, etc.) was observed. No direct connection was found between kinetic data (reaction rate and optical yield) and NMR proton shift. Liquid phase NMR measurements confirmed the interaction of both 2,3-butandione and 3,4-hexanedione with the alkaloid used. 

Figure 6 NMR proton spin-echo intensity vs. time.

Distribution of protons by type and overall aromatic/aliphatic proton ratios for the original fractions and bottoms products, as determined by proton nmr. Proton distribution for fraction D and the whole coal are not included as these materials were only partly soluble and the resultant spectra were not representative of the whole material. 

NMR. Proton NMR is obviously likely to give an enormous range of signals from a typical confectionery product. An NMR instrument to analyse water in foods has to be a low-resolution instrument, whether of the original continuous form or of the later pulsed type. The aim is to discriminate between the protons in water and those in other molecules. Fortunately, this is not too difficult. 

Figure 3.38 Compounds enantiomerically recognized by 23j in H NMR. Protons marked with arrow indicate recognized ones and figures represent AA8 (ppb). Negative value indicates upfield shift.

Upon the introduction of NMR proton relaxometry in muscle studies, it was early recognised that a model based only on dipolar interactions is far too simple,8 14 as ... 

Most characteristic for the overall conformation of the cinchonan carbamates in solution is the dihedral angle of the torsion H8-C8-C9-H9, which can be inferred from the vicinal coupling constant of the NMR proton signal of H9. The experimentally measured coupling constant /hsh9 represents actually the average over the populations P(i) of the different conformers in solution according to [100]... 

Takahashi et al. (1996) described another case of cation-radical cycloreversion. Benzocyclobu-tenols undergo ring opening induced by electron transfer to generate quinodimethide intermediates, which then tautomerize to benzophenones. The reaction proceeds on photoirradiation in the presence of tetracyanoanthracene (X > 350 nm). Yields (based on NMR (proton) analyses) are quantitative... 

Table III. NMR Proton Coupling Constants for Mannitol and Sorbitol in Water...

The NMR proton spectra of the syndiotactic copolymers obtained from perdeuteropropylene with the l-d,-cis or the l-dj-trans monomer present a doublet around 16 attributed to the methylene proton of the r dyad coupled to the methine proton. In the two polymers the vicinal coupling constants are different, 4.8 Hz... 

Electron spin resonance. Structure of pyridine derivatives with unpaired electrons NMR proton signals in pyridine... 

NMR proton relaxation times for water in living tissues are found to be shorter than in ordinary water, and the lines correspondingly broader. Some workers have interpreted this fact as indicating that water in living cells may be in a semicrystalline state, rather than a liquid state the... 

Steric interactions generally predominate relative to ring current effects in carbon-13 NMR. Proton shifts of bridged [10]- and [14]annulenes, for example, clearly identify these compounds to be aromatic due to the typical deshieldings of about 2 ppm relative to comparable non-aromatic cyclopolyenes [401a]. In contrast, carbon-13 shifts of bridged annulenes (Table 4.58) are more closely related to those of comparable non-aromatic cyclopolyenes with similar steric interactions than to those of other annulenes [401 b]. 

Because of the high electron densities at these positions protonation of unsubstituted pseudoazulenes should take place at a 1- and/or 3-position of the five-membered ring. As was investigated in more detail for the cyclopen ta[/ ]pyran series (e.g., 27 and 40) by NMR protonation takes place... 

Cooke and Wien have found that the NMR proton chemical shift in muscle water is similar to that in an aqueous salt solution273. The relaxation times are different. But the activation energy of the diffusion co-efficient of cellular water in muscle... 

Grishaev, A. and Ninas, M. (2002). Protein structure elucidation from NMR proton densities. Proc. Natl. Acad. Sci. USA 99, 6713-6718. 

Keywords Heteroaromaticity, Hydrogen bonds, NMR, Proton transfer, Tautomerism... 

When this conjugation occurs, the level of active (corrosive) acid is substantially decreased. No simple quantitive correlation has been shown between the acidity (pKa) of acids in hydrocarbon formulation and low polar solvents (Coetzee, 1967). Acid-base interaction with and without proton transfer (PT) (BH+A B...(HA)m) has been related to acid and base enthalpies of reaction (Pawlak and Bates, 1982), the infrared carbonyl stretching band and gradual appearance of the asymmetric COO band (Lindeman and Zundel, 1972 Magonski and Pawlak, 1982), changes in pH (Kuna et al., 1982 Pawlak et al., 1982), NMR proton chemical shifts (Magonski and Pawlak, 1982), and dipole moments (Sobczyk and Pawelka, 1979). These parameters depend upon the acid-base strength of the partners, ApKa(PT) the difference between the pKa(acceptor) and pKa(donor) on the water scale (Sobczyk, 2001). 

Two different nuclei in TDAE-C60 have been investigated so far by NMR protons of methyl groups of the TDAE molecule and 13C nuclei of the Qq ion. The main difference between these two nuclei (in addition to their relative sensitivity) is that methyl protons experience mostly the dipolar fields of the Qq magnetic moments. On the other hand 13C nuclei on each Cgo ion will, in addition to dipolar fields, also feel the hyperfine contact field of the unpaired electron spin. Details of the 13C NMR results will be given in the next section. Here we... 

The addition of a solid to the solution to analyse induces susceptibility broadening of the NMR proton signals, which can reach several tens of Hz. This effect, in conjunction with shortened Tz relaxation for adsorbed species, may degrade the spectral resolution of aH NMR spectra beyond exploitation. As the latter effect cannot be countered by technological means, only susceptibility-broadening methods will be discussed in this section, but the reader should be aware that exclusively systems in which the solid phase, or any other matrix, does not induce severe relaxation broadening are suitable for chrom-NMR analysis. 

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