Study by NMR

Carbon 12, the most abundant naturally occurring isotope, has zero spin and thus cannot be studied by NMR. On the other hand, its isotope carbon 13 has an extra neutron and can be its low natural occurrence (1.1%) nevertheless makes the task somewhat difficult. Only pulsed NMR can be utilized. 

We begm tliis section by looking at the Solomon equations, which are the simplest fomuilation of the essential aspects of relaxation as studied by NMR spectroscopy of today. A more general Redfield theory is introduced in the next section, followed by the discussion of the coimections between the relaxation and molecular motions and of physical mechanisms behind the nuclear relaxation. 

We pointed out in Section 13 3 that both H and are nuclei that can provide useful structural information when studied by NMR Although a H NMR spectrum helps us infer much about the carbon skeleton of a molecule a NMR spectrum has the obvious advantage of probing the carbon skeleton directly NMR spectroscopy is analogous to H NMR in that the number of signals informs us about the number of different kinds of carbons and their chemical shifts are related to particular chemical environments... 

All l and Aryl Complexes. CP2MR2 and CP2MRCI, [R = CH(CgH5 CH(Si(CH3)3)2] have been prepared and studied by nmr, and crystal... 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

Table 15 [S 19 56, 59 93 107 108 109, 110, 111, 112 113, 114] contams NMR data for a vanety of tnfluoromethyl compounds that do not fit into the more common categones already covered Most carbon-attached tnfluoromethyl (CF3-C) F-NMR signals fall in the 25 ppm range from -60 to -85 ppm, a notable exception being trifluoromethylacetylenes, with signals at approximately -50 ppm Trifluoromethylated compounds in which the CFj is directly bonded to an atom orlier carton are considered in Table 16 [57 59,95 109,115,116 117 118,119,120 121 122, 123 124, 125] These heteroatom contaming compounds are of interest to both organic and morganic chemists and have been studied by NMR for numerous reasons...

This reaction has been well studied by NMR. Another important exchange process is the inversion of cyclohexane between equivalent chair forms (Scheme XII), a process in which a proton is exchanged between equatorial and axial positions... 

The acylation of enamines derived from cyclic ketones, which can lead to the acyl ketone or ring expansion (692-694), was studied by NMR and mass spectroscopic analysis of the products (695,696). In a comparative study of the rates of diphenylketene addition to olefins, a pronounced activation was observed in enamines (697). Enamine N- and C-acylation products were obtained from reactions of Schiff s bases (698), vinylogous urethanes (699), cyanamides (699), amides (670,700), and 2-benzylidene-3-methylbenzothiazoline (672) with acid chlorides, anhydrides, and dithio-esters (699). 

Angeli s salt Na2N203 has been shown by vibration spectroscopy to contain the trioxodinitrate(II) anion structure (2). Its decomposition and reactions in aqueous solutions have been extensively studied by nmr spectroscopy and other techniques. 

Substituted benzofurazan oxides 9 and 11 have been studied by NMR at low temperature and were observed as a mixture of tautomers, presumably interconverting via the ortho-dinitroso intermediate 10. When R = Cl, MeO, or AcO, tautomer 9 is the more stable... 

Tire tautomeric equilibrium of azocinones 65 has been studied by NMR, IR, and UV spectroscopy (90JHC1323). Dioxo derivatives of diazocines all exist as such, as illustrated by the analgetic 66 (75MI37) and the extensively studied dibenzo[h,/][l,4]diazocines (67) [81JCS(P1)988]. 

Azide-tetrazole isomerization has been studied by NMR spee-troseopy for 2-azidothiazolo[5,4-h]- and 2-azidothiazolo[4,5-c]pyridines, and the equilibrium eonstants were determined at various temperatures (77JHC1045). 

The reaction between hydrazine hydrate and ethyl 3-oxobutanethiolate 4 has been studied by NMR spectroscopy. It was found to occur via initial formation of addition product S, cyclization to 6, dehydration to pyrazol-3-one 7, and finally tautomerization to pyrazol-3-one 8 (81CJC629) (Scheme 2). 

Tautomerism in tetrahydrotetrazines (also known as leucoverdazyls) has been studied by NMR spectroscopy, which showed that unsymmetrically substituted aryl leucoverdazyls 132 exist in solution in a tautomeric equilibrium of two stable forms (Scheme 38) (85KGS1425). 

Cations exchanged into the interlayers of expandable clays (smectites) are comparatively easy to study with NMR methods because the cations become major components of the phase and their concentrations are often several wt %. In addition to Cs Li, Na, K, and Cd have been studied by NMR. We have chosen to investigate Cs because it is a significant component of nuclear waste, because it provides an end-member case as the least electronegative cation, and because it has desirable nuclear properties (100% abundance, relatively high frequency, 65.5 MHz at H = 11.7 T, and small quadrupole moment)... 

Since that time, many studies by NMR and other techniques on the microstructure of acrylic and methacrylic polymers formed by radical polymerization have proved their predominant head-to-tail structure. 

Oxidation of either alkyl or aryl sulphoxides to sulphones in 65-90% yields may be accomplished by treatment with a nitronium salt15. In the case of aryl sulphoxides no nitration is observed (which is in contrast to the results of nitric acid oxidation). The reaction was shown to proceed through intermediate nitratosulphonium and nitritosulph-oxonium ions, as depicted in equation (7), which were studied by nmr spectroscopy. 

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

The NMR characteristics of the bromonium ion triflate prove to be interesting in that the proton NMR spectrum of a 1 1 mixture of parent olefin and bromonium ion at room temperature is an average of those of the two isolated species. This suggests that there is a rapid site exchange of the Br+ between the ion and olefin. The phenomenon can be most simply studied by NMR spectroscopy. An approximately 100 mM solution of the ion in CD2C12 held at - 80°C was treated with small aliquots of Ad=Ad, the NMR spectrum was monitored after each addition. At zero added Ad=Ad, the spectrum of the ion consists of a seven line pattern indicative of a species (eqn. 3, 5a) having two perpendicular planes of symmetry that pass, respectively, through the three heavy atoms of the bromonium... 

Several phenonium ions have been prepared as stable ions in solution where they can be studied by NMR, among them are 45, 46,and the unsubstituted 44. These were prepared by the method shown for 44 treatment of the corresponding P-arylethyl chloride with SbFs—SO2 at low temperatures. These conditions are even more extreme than the solvolysis in... 

Fig. 2.35 y-Peptides studied by NMR and shown to adopt a 2.6,4-helical secondary structure. These include y-peptides with homochiral sequences consisting of enantio-pure y" -amino acid residues (139-142), y -amino acid residues of relative configuration... 

The transformation of 4-fluorobiphenyl by the ectomycorrhizal fungus Tylosporafibrilosa was studied by NMR (Green et al. 1999) and the principal products were 4-fluorobi-phen-3 -ol and 4-fluorobiphen-4 -ol. 

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