Nuclear magnetic resonance downfield

Nuclear Magnetic Resonance. The nmr spectmm of aromatic amines shows resonance attributable to the N—H protons and the protons of any A/-alkyl substituents that are present. The N—H protons usually absorb in the 5 3.6—4.7 range. The position of the resonance peak varies with the concentration of the amine and the nature of the solvent employed. In aromatic amines, the resonance associated with N—CH protons occurs near 5 3.0, somewhat further downfield than those in the aliphatic amines. 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

Representative examples of ring proton and carbon chemical shifts of all known l,4-(oxa/thia)-2-azoles were reported in CHEC-II(1996). A special notice should be given for H and 13C nuclear magnetic resonance (NMR) spectra of both dithiazolium 5 (X = Y = S) and oxathiazolium salts 6 (X = 0 Y = S) and 7 (X = S Y = 0) <1996CHEC-II(4)489>. A S downfield shift for both 3-H and 5-H as well as C-2 and C-5 is correlated with a potential 7r-electron delocalization and thus the aromaticity of these ring systems <1996CHEC-II(4)498>. 

The sulfone group exhibits typical infrared frequencies at 1150 to 1170 and 1330 to 1350 cm"1 (350, 354). Nuclear magnetic resonance spectra have been recorded in acetone for 2- and 5-ethylsulfonylthiazoles and compared to other groups (270). The sulfonyl ethyl substituent induces a general downfield shift compared to the parent sulfides. Prot°ns in the 2-and 4-positions appear at 9.42 and 8.45 ppm. respectively, in 5-ethvlsulfonylthiazole and the protons at both the 4- and 5-positions emerge at 8.15 ppm in 2-ethylsulfonylthiazole. 

Seebach et al. published seminal studies of observable halocarbenoids <1983HCA308>. They reported that the nuclear magnetic resonance (NMR) chemical shifts of a variety of a-bromolithiohydrocarbons lie markedly downfield of those of the analogous alkyllithiums, alkyl bromides and dibromides, and hydrocarbons. Eor examples, see Table 2 <1979AGE784>. The dramatic shift was ascribed to a substantial rehybridization of the carbon atom arising from... 

Nuclear magnetic resonance spectroscopy gives precise information on complexation in solution. Equilibrium is rapidly established on an NMR time scale, hence only an average spectrum is observed and it is difficult to determine the spectrum of a pure complex. When complexation of a sugar or polyol with a diamagnetic ion occurs, all of the signals shift downfield. Equation (11.1) allows the variation of the shielding constant Ao- of the proton to be calculated when the nucleus is subjected to an electric field E whose projection on the C-H bond is... 

The last two criteria, (4) and (5), are the most unequivocal and powerful signs for the formation of the hydrogen bond. A A-H) and IRa H bCKA—H))/ IRa h(KA—H)) are the most important characteristics of H-bonding, its fingerprints or signature [33], say literally. Proton nuclear magnetic resonance ( H NMR) chemical shifts in the A H- -B hydrogen bond are shifted downfield compared to the monomer,... 

In the early proton nuclear magnetic resonance ( H NMR) studies with the O2 complexes of amide functionalized, basket-handle porphyrins (5, Fig. 2), a pronounced downfield shift of the amide C(0)—NH was observed in apolar solvent, indicating the C(0)—NH 02 hydrogen bonding (14a,16). This hydrogen bond, however, was weak, since the estimated nitrogen-02 distance is 4 A. 

The large number of histidine residues associated with the active site of bovine SOD facilitates the study of its chemical and structural properties by nuclear magnetic resonance (NMR) spectroscopy. Mainly the downfield histidine imidazole N-proton resonances were subject of a large quantity of investigations The deuteration of histidine residues is dependent only on the absence of co-ordinated metal ions and not changes in the protein structure Therefore, in apo-, zinc- and holo-super-oxide dismutase, only those histidine residues not co-ordinated to metal ions will be deuterated at the C-2 position. This can be used as a simple method for the identification of co-ordinated histidine residues in metalloproteins employing only minimal chemical modifications. 

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