Proton-enhanced spectra

It is evident that relaxation studies in the solid state can look at the motions which are responsible for the mechanical properties of the cured epoxy systems 43). Therefore, Garroway, Moniz and Resing continued to do relaxation studies 61). Garroway, et al. looked at four epoxy polymers based on the DGEBA resin. Two of the epoxy resins were cured with amines and the other two were cured with anhydrides. Proton enhanced spectra of the epoxy systems were generated. The solid state spectra were compared to the solution spectra of the unreacted epoxy. The< epoxy resin of interest was again DGEBA which was reacted with ... 

Figure 6. Magic angle spinning, high-power proton decoupling, FT C-13 NMR spectrum of cured, carbon-black-loaded polyisoprene at ambient temperature, FT of normal FID without proton enhancement.

Selective population or polarization transfer (abbreviation SPT) was first achieved for chloroform [51] by brief irradiation of one of both 13C satellites in the proton NMR spectrum before recording the 13C—1H doublet as FID signal. In this experiment, the decoupling pulse inverts the population of the proton precession states connected by the irradiated satellite transition (Fig. 2.43(b, c)). The two SPT experiments shown for the 13C —H doublet of chloroform in Fig. 2.43 (b, c) give an impression of the signal enhancement achievable by population transfer in comparison to a normal 13C NMR spectrum... 

According to the Forster cycle, if the longest wavelength electronic transition of the deprotonated form is of lower energy compared to that of the protonated form (red-shifted electronic absorption or emission spectrum of the deprotonated form with reference to the protonated-form spectrum), the molecule has enhanced excited-state acidity (i.e., the pK a of the molecule is lower than pKa). Equation (1) provides a quick and effective method for evaluating a molecule for its ESPT behavior. 

Since the ground state of 11+1Pr is 5/2+ the (180,17N) reaction on that nucleus strongly enhances final states in llf2Nd of (ird5/ 2) 20+, 2+, 4+ and ird5/2 irh /2 3 .8 . The single proton transfer spectrum [B0N85]... 

In Figure 3.60, the conventional proton spectrum is labeled a. Spectrum b shows the result of irradiation of the 3-methyl group only H-4 is enhanced. Spectrum c shows the result of irradiation of the 5-methyl group both H-4 and H-6 are enhanced-all predictable since enhancement is proximity driven. Some of the details may seem puzzling, but remember that this is a difference experiment The conventional spectrum is subtracted from the irradiated spectrum during the subtraction and enhancement steps, the irradiated peak returns to the up position and the un-irradiated peak is not detectable because it has not been enhanced. The sensitivity... 

Two-dimensional NMR spectroscopy has proven of great value for product analysis in solution phase chemistry and by its nature should provide a platform by which enhanced data quality and ease of interpretation is obtained. Geralt demonstrated that the use of 2D CH correlated NMR spectra (Fig. 6) enhances the utility of gel phase NMR even though the proton NMR spectrum was broad (36). Since MAS NMR spectra of the swollen resins are of higher quality than gel phase NMR, it is possible to treat these samples as if they... 

The through-space distances involved in H H proximity determinations are quite small, and the effect decreases as the inverse of the sixth power of the distance, through space, between the protons. The usual observable enhancement is less than 20%. To increase the sensitivity, we use the NOE difference experiment, in which a conventional H spectrum is subtracted from a specific proton-irradiated spectrum this subtraction leaves only the enhanced absorptions. Under these conditions, a measurable effect can be expected between H nuclei over a distance of up to about 4 A (0.4 nm)t for example, the distance between 1,3-diaxial protons in the cyclohexane chair form is — 2.6 A. This procedure is a powerful tool in distinguishing among isomers, all of which should be available, if feasible, for NOE difference spectra (Fig. 4.54). 

Figure 5. Proton-enhanced static powder spectra of a, Hytrel 4056 b, Hytrel 7246 and c, polybutylene terephthalate, compared with the respective calculated spectra of the carbonyl and aromatic regions (d-f). The experimental spectra were obtained at ambient temperature on 0.3 g of sample. Each spectrum represents 8192 accumulations using a 0.5 ms contact time and a 3 s recycle delay. The calculated spectra were obtained using the values listed in Table III.

As far as the C-NMR spectrum of indole is concerned, the chemical shifts of C2 and C3 follow the trend of those in pyrrole. C2 (124.4 ppm) is much more down field in comparison with that of C2 (102.5 ppm), again thanks to the inductive effect of the N atom. The two quaternary carbon atoms Ctb and Csa show up at 135.8 and 127.9 ppm, respectively. In addition, the heights for these two quaternary carbon atoms are significantly short in comparison with the carbon atoms with a hydrogen atom adjacent to them. The reason is that the two quaternary carbon atoms lack the nuclear Overhauser effect (nOe) that made the carbon peak taller with more protons (-CH3 > -CH2 > -CH > -C), also known as proton-enhancement. 

Figure 1 shows the resolution-enhanced 500MHz proton spectrum of PVA in D2O. Tlie spectrum consists of two parts, the low-field part comes from methine protons and high-field part from methylene protons. The spectrum is very... 

Figure 1. Resolution-enhanced 500MHz proton NMR spectrum of PVA in D2O at 353K.

When we obtain a proton-decoupled spectrum, the intensities of many of the carbon resonances increase significantly above those observed in a proton-coupled experiment. Carbon atoms with hydrogen atoms directly attached are enhanced the most, and the enhancement increases (but not always linearly) as more hydrogens... 

Fig. 7. Signal assignment in high field region of 300-MHz proton NMR spectrum of neurotoxin II in D O solution at pH 7.3 and 32°C. A - Normal spectrum. B - resolution enhanced spectrum by means of a triangular function(41).

Enhanced relaxation due to dipolar interaction with paramagnetic ions leads to line-broadening in the high resolution NMR spectra of ligands. If a proton NMR spectrum of a protein molecule containing a bound Gd ion is subtracted from a... 

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