Proton resonances

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

The reasonable stable products are characterized by an ir-absorption near 1615 cm". The 4-protons resonate near 6.2 ppm in the H NMR spectrum (23). NMR spectra exhibit a carbonyl atom signal near 173 ppm, whereas C-4 resonates near 8 108 these positions are characteristic of other mesoionic ring carbon atoms (24). In the mass spectra, decomposition with loss of CO, rupture of the 1,5 and 2.3 bonds with elimination of R NC2R 0 and cleavage of the 1,2 and 3,4 bonds with elimination of C2R 0S is observed (11)... 

D2O Those bound to carbon normally do not and so this technique is useful for assign mg the proton resonances of OH NH and SH groups... 

Acetates. Because of the significant interest in selective acetylation reactions of sucrose, the need for a convenient and unambiguous method of identification has been recognized (34,35). The position of an acetyl group in a partially acetylated sucrose derivative can be ascertained by comparison of its H-nmr acetyl methyl proton resonances after per-deuterioacetylation with those of the assigned octaacetate spectmm. The synthesis of partially acetylated sucroses has generally been achieved either by way of selectively protected derivatives such as trityl ethers and cychc acetals or by direct selective acetylation and deacetylation reactions. 

In deuterochloroform, pyrazine shows a single proton resonance at S 8.59 (72CPB2204). Vo, Vm and Vp values between pyrazine ring protons obtained from a number of pyrazine derivatives are 2.5-3, 1.1-1.4 and 0 Hz respectively, and these values do not appear to be affected by the nature of the ring substituents. Some substituent shielding parameters are shown in Table 1. 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Table 2.13. Selected applications of dynamic proton resonance ...

NMR instrumentation consists of three chief components a magnet, a spectrometer console, and a probe. While in the past much solid state NMR research was conducted on home-built equipment, the current trend is toward the acquisition of commercial systems. The magnets used for solid state NMR applications generally are superconducting solenoids with a cylindrical bore of 89-mm diameter. The most common field strengths available, 4.7, 7.0, 9.4, and 11.7 Tesla, correspond to proton resonance frequencies near 200, 300, 400, and 500 MHz, respectively. 

In nuclear magnetic resonance (NMR) spectra the protons of the oxirane ring are usually shifted out of the steroid methylene envelope. Tori et al have tabulated the relationships of the angular methyl proton resonances and the oxirane proton signals with respect to location and configuration of the oxirane group. 

The NMR study of steroidal epoxides (discussed in section II-F) parallels that of the analogous thiiranes. It is possible to relate the location and configuration of the thiirane group with the angular methyl and thiirane proton resonances. The proton NMR relationships for the intermediate thiocyanatohydrins have been included inageneral NMR study of steroids. Electronic spectra may be used in the analysis of steroidal thiiranes. Spectroscopic measurements have shown the existence of a low intensity absorption in the 240-260 m region. The regular patterns of rotatory contributions of thiiranes which are comparable with those of ketones prompted an accumulation of ORD and CD data for steroidal thiiranes. 

The salts of some enamines crystallize as hydrates. In such cases it is possible that they are derived from either the tautomeric carbinolamine or the amino ketone forms. Amino ketone salts (93) ( = 5, 11) can serve as examples. The proton resonance spectra of 93 show that these salts exist in the open-chain forms in trifluoroacetic acid solution, rather than in the ring-closed forms (94, n = 5, 11). The spectrum of the 6-methylamino-l-phenylhexanone cation shows a multiplet at about 2.15 ppm for phenyl, a triplet for the N-methyl centered at 7.0 ppm and overlapped by signals for the methylene protons at about 8.2 ppm. The spectrum of 93 ( = 11) was similar. These assignments were confirmed by determination of the spectrum in deuterium oxide. Here the N-methyl group of 93 showed a sharp singlet at about 7.4 ppm since the splitting in —NDjMe was much reduced from that of the undeuterated compound. 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

In general, physical methods have been used to study tautomerism more successfully than chemical methods, and, of the physical methods, those involving measurements of basicities and ultraviolet spectra are the most important, followed by those involving measurement of infrared and proton resonance spectra. An attempt is made here to delineate the scope and to indicate the advantages and disadvantages of the various methods. A short review by Mason of the application of spectroscopic methods appeared in 1955. Recently a set of reviews on the applications of physical methods to heterocyclic chemistry has appeared, which treats incidentally the determination of tautomeric structure. 

Infrared and proton resonance spectra indicate that the tetrazine-dithione 223a exists in the dithione form shown. ... 

The first evidence for an unsymmetrical structure for the benzo-furoxan molecule and for the tautomerism of Eq. (3) was provided in 1961 by several groups of workers," using proton resonance... 

A stable lO-As-3 species (235) underwent a ready reaction with 144 to give adduct 236, which exhibited a single ferf-butyl and ring-proton resonance in the NMR at room temperature and showed a single resonance at 8 -54.0 in the NMR (85JA1089). Tire low temperature and NMR showed a split of the room temperature resonances suggestive of the frozen structure of 236. 

The a-aminobenzofuran 92a exists in the amino form shown, as evidenced by infrared and proton resonance spectra... 

This formulation is supported by the proton resonance spectrum of the trifluoromethyl compound 101 which shows that it exists in the CH form shownd However, strong electron-withdrawing groups in the 4-position apparently lead to enolization, and compound 102, for example, gives an intense color with ferric chloride, - Other 4-acylated oxazol-5-ones are often formulated as 103 (see, e.g, reference 113). Tautomerism of the type illustrated by the equilibrium 104 103 has been discussed (see reference 115 for further references). 

Trithiatriazepine 1 is a stable colorless volatile solid. Its IR spectrum shows an asymmetric NSN vibration at 1136 cm 1 and the UV spectrum has a long wavelength absorption at 327 nm, typical of an aromatic transition. The ring proton resonates at r) = 9.0 and the... 

H1 NMR spectroscopy was found to be unsuitable for head-group analysis of HSi-PaM-eSt. The resonance associated with the Si-H proton at the head-group is broadened by multiple splitting and the resonances of the aromatic protons of the initiator fragment are buried in the aromatic proton resonances of phenyl rings of the aMeSt repeating units. 

The so-prepared compounds are very sensitive toward air and moisture, in particular in solution. H- and C-NMR spectra prove the formation of 1 1 adducts of the type dmap—M(R2)E(Tms)2- The proton resonances of the dmap molecule are shifted to higher field, as was observed for similar borane adducts... 

Proton resonances for aU residues were assigned using a combination of COSY, TOCSY, HSQC, and HMBC experiments. The large values of y(NH/H-C(/9)) and the small values of J(H-C(a)/H-C(y9) were indicative of antiperiplanar and synclinal arrangements respectively, around those bonds. In addition, medium-range NOE connectivities H-C(/ )i/NH +i, H-C(a)i/NH, + i, NHi/NH +i were consistent... 

A field of application of MD that is beginning to bear fruit is the refinement of data from nuclear magnetic resonance (NMR) and from diffraction experiments. High-resolution NMR at frequencies around 500 MHz is able to resolve individual proton resonances of biological macromolecules in solution with molecular weights exceeding 10,000... 

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