Spectra predicted proton resonance

Annulene was the first macrocyclic annulene containing (4n -j- 2) zr-electrons to be synthesized. The compound is of considerable interest, since it is the type of annulene that was predicted to be aromatic by Hiickel.10 It proved to be aromatic in practice, as evidenced from the proton magnetic resonance spectrum,8-11 the X-ray crystallographic analysis,18 and the fact that electrophilic substitution reactions could be effected.13... 

A different type of dynamic process involving a polynuclear metal system has been identified in [(CH C H,)ltRul)Sl(]2+, which has a distorted cubane-like structure with three Ru—Ru bonds in the crystalline state (Fig. 15.56). By following its methyl and ring proton resonances over a temperature range from +70 to —43 BC (Fig. 1537), the complex is shown to undergo a dynamic process involving the metal-metal bonds. At the low-temperature limit, the spectrum contains features predicted for the static structure two lines of equal intensity for the methyl protons... 

For benzene, [6]annulene, with six it electrons (4n + 2, n = 1), the theory clearly meets both tests. As we have pointed out, there is a substantial stabilization of about 37 kcal mole-1 compared with a hypothetical localized model. The familiar chemical properties also point to a strong tendency for maintenance of the six it electron unsaturated system. The proton magnetic resonance spectrum of benzene and its derivatives shows the proton resonances in the range of 8 = +7 to +8 ppm (downfield from tetramethylsilane), 1-2 ppm lower than protons attached to nonbenzenoid double bonds. Referring to Figure 1.21a, we can see that the prediction is in agreement with this result. The induced field adds... 

Figure 2. Predicted spectrum for the hydroxylic proton resonance of [(CH8)3PtOH h observed spectrum redrawn from Ref. 41...

Bradbury and Norton (1975), on the basis of the model building studies by Browne et al. (1969) and by Warme et al. (1974), were able to make assignments of specibc resonances in the proton NMR spectrum of bovine a-lactalbumin to the three His residues. These resonances, after reaction of the protein with iodoacetate under conditions that were nearly specific for the His residues, disappeared from the native frequency positions, but did so in a differential manner, consistent with differences in the degrees of exposure to the solvent. This was predicted by the model building studies, particularly those by Browne et al., which were subsequently confirmed by X-ray analysis. Thus, His-68 is the most exposed His-32, being involved in a helical region (according to Browne et al., but not to Warme et al.), is less exposed, while His-107 is the least exposed. 

Further evidence for the presence of two intact alanine residues in diabroticin B (2) was provided by acid hydrolysis (6N HCl/110 C/24 hr) followed by quantitative PTH and OPA amino acid analysis. Treatment of 2 with DMF-DMA resulted in formation of a bisdimethylamidine adduct (FABMS, miz 494 Da) as predicted. Confirmation of the 2,5-pyrazine substitution pattern in diabroticin A (1) and B (2) was facilitated by the unsymmetric nature of 2. Two chemically nonequivalent aromatic proton resonances were observed in the H NMR spectrum at 58.58 and 58.45 as 1.3 Hz doublets consistent with in mono-substituted pyrazines (6). 2-Methylp3rrazine and other mono-substituted pyrazines show long range coupling constants of J3.6 = 1.33-1.54 Hz, = 0.01-0.35 Hz and e/5-6=2.4-2.9 Hz. 

The NMR properties of (Table 1) give an indication of both the strengths and the weaknesses of analysis using it. The gyromagnetic ratio of is approximately one-fourth that of the proton and, as a result, the resonance condition for at a given magnetic field strength occurs at a radiofrequency approximately one-fourth that of the proton resonance. For a sample with equal numbers of spins and equal spin-lattice relaxation rates, the predicted sig-nal-to-noise ratio of the spectrum at a particular... 

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