Proton-enhancement

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

Protonation enhances the electron affinity of the superoxide ion and it easily transforms into hydrogen peroxide HOO + H+ —> H2O2 (Costentin et al. 2007). 

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.

Fig. 8. Nuclear Overhauser enhancement of PEG-ethylene protons vs irradiation time in P(MAA-j -EG) gels exhibiting complexation. Proton enhancements of graft copolymer with PEG M = 400 in D20 (curve 1), graft copolymer in NaOD solution (curve 2), and polymer mixture with PEG M = 1000 in D20 (curve i). The PEG concentration was 0.01 wt%, PMAA concentration was 0.09 wt%, copolymer concentration was 0.1 wt%, and temperature was 21 °C...

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 ... 

It was noted that more refinement in spectral resolution was still needed. The combination of proton enhanced C-13 spectra combined with magic angle spinning made possible the identification of functional groups in the four DGEBA epoxy systems. A peak between 70-73 ppm were evidenced in all four systems indicative of the carboxyl-methine ether carbon of the reacted epoxide groups and adjacent methylene groups. 

Pines, A., Gibby, M. G., and Waugh, J. A. (1972). Proton-enhanced nuclear induction spectroscopy. A method for high sesolution NMR of dilute spins in sohds. J. Chem. Phys. 56, 1776-1777. 

The solid state high resolution n.m.r. spectra were run on a CXP 200 BruKer spectrometer in which the 31P nucleus resonates at 81 MHz. Samples used were finely powedered and hand-tamped in glass tubes. The spectra were recorded using the Proton Enhanced Nuclear Induction Technique (6) on the same basis of a one shot cross polarization and high power decoupling during acquisition. A capillary tube of trimethylphosphate inserted in the powder sample is used as internal reference. 

To demonstrate the importance of the golden ratio it is assumed that protons and neutrons occur in the nucleus on three-dimensional spirals of opposite chirality, and balanced in the ratio Z/N = r, about a central point. The overall ratio for all nuclides, invariably bigger than r, means that a number of protons, equal to Z — Nt, will be left over when all neutrons are in place on the neutron spiral. These excess protons form a sheath around the central spiral region, analogous to the valence-electron mantle around the atomic core. The neutron spiral is sufficient to moderate the coulomb repulsion while the surface layer of protons enhances the attraction on the extranuclear electrons. 

Pines A, Gibby MG, Waugh JS (1973) Proton-enhanced NMR of dilute spins in solids. J Chem Phys 59 569-589... 

Lippmaa et al. (86) have successfully studied solid state Si NMR of several trimethylsilyl silicate samples using proton-enhanced nuclear induction spectroscopy and magic angle spinning. They showed that the high resolution Si NMR of solids can be useful for structural studies of certain soluble and insoluble silicates, because many of these compounds with well defined molecular structures in the solid state tend to undergo very complicated rearrangements and condensation in solution. (93)... 

As described above, the reduction of imines often proceeds via iminium ions particularly in acidic media, since protonation enhances the electrophilicity of the imine carbon. Thus, as expected, preformed iminium salts generated from carbonyls and secondary amines are also readily reduced by most hydride reagents. Several examples of synthetic applications with a variety of reagents are illustrated in Table 4. Entries 9-12 illustrate the use of iminium intermediates for the reductive removal of amide carbonyls. Thus, treatment of amides with POCI3 affords the iminium derivatives (e.g. 9 Scheme 3), which are reduced by NaBHt to the corresponding amines (Table 4, entries 9, 10). Likewise, reaction of amides witii trialkyloxonium salts to give imidic esters (entry 12) or thioamides with methyl iodide to give... 

Stejskal, E.O. Schaefer, J. Waugh, J.S. Magic-angle spinning and polarization transfer in proton-enhanced NMR. J. Magn. Reson. 1977, 28 (1), 105-1121969-1992. 

Figure 1 Mean satellite anomaly frequencies in 0- and 1-days of proton enhancements in dependence on the maximal > 10 MeV flux.

The stereochemistry of rhexifoline was deduced from NOE studies, in which irradiation of H-5 enhanced H-6/3 at 2.03 ppm, but not H-6a at 2.43 ppm. Irradiation of this latter proton enhanced H-7 (3.59 ppm) but not H-5. Therefore the methine protons were deduced to be anti to each other, and rhexifoline has the stereochemistry shown in 23 (35). The stereochemistry was confirmed by semisynthesis (see Section III.G) (81). The yield of 23 was not altered depending on whether NH3 or NaOH was used for basification. A related taxon, C. rhexifolia aff. miniata, also yielded 23 as the major alkaloid, concentrated in the seeds and flowers (35). 

A somewhat more unusual case is the positive proton enhancements observed for the t-butyl groups of 2,4,6-tri-t-butylphenol in solutions containing the corresponding phenoxyl radical. In these solutions there is a proton exchange reaction—... 

---