Vicinal shifts

Chemical shifts are shifted to higher field when deuterium replaces hydrogen either geminally or vicinally the stereochemical dependence of the vicinal-shift isotope effect has been investigated by means of cis- and trans-[2- H]fluorocyclohexane with spectra obtained under conditions of slow ring-inversion at — 85°C. The observed F isotope shifts (/p.p.m.) for (34) and (35) are shown. Values for (36) and (37) and the norbornyl systems (38) and (39) are also shown these were determined in order to ascertain the stereochemical dependence of the isotopic shift. The smallest... 

The induced magnetic field by the circulating it electrons reinforces the applied field in this vicinity, shifting the absorption downfield to a somewhat higher chemical shift. 

A very important characteristic of spin-spin splitting is that protons that have the same chemical shift do not split each other s signal Ethane for example shows only a single sharp peak m its NMR spectrum Even though there is a vicinal relationship between the protons of one methyl group and those of the other they do not split each other s signal because they are equivalent... 

The —OH proton of a primary alcohol RCH2OH is vicinal to two protons and its sig nal would be expected to be split into a triplet Under certain conditions signal splitting of alcohol protons is observed but usually it is not Figure 13 21 presents the NMR spec trum of benzyl alcohol showing the methylene and hydroxyl protons as singlets at 8 4 7 and 2 5 respectively (The aromatic protons also appear as a singlet but that is because they all accidentally have the same chemical shift and so cannot split each other)... 

Study of the mechanism of this complex reduction-Hquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the ceUulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the Hquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

Proton chemical shifts and spin coupling constants for ring CH of fully aromatic neutral azoles are recorded in Tables 3-6. Vicinal CH—CH coupling constants are small where they have been measured (in rather few cases) they are found to be 1-2 Hz. 

Useful compilations of NMRspectra are , . The H and NMR chemical shifts and the geminal and vicinal proton-proton coupling constants for oxirane and other heterocycles are given in a very readily compared manner,... 

The shielding effect of the vicinal phenyl substituent accounts for the favored attack of the cyanide from the opposite direction, giving rise to the (S)-diastereomers. However, it should be noted that imines of aliphatic aldehydes which bear an odd number of carbons in the main side chain surprisingly give (/ )-diastereomcrs, concluded from ORD data48. This opposite stereochemical course in the formation of these compounds has not been explained48. It might simply be due to crystallization of the (R)-diastereomer which continuously shifts the equilibrium (vide supra). 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

Barium and strontium salts of polystyrene with two active end-groups per chain were prepared by Francois et al.82). Direct electron transfer from tiny metal particles deposited on a filter through which a THF solution of the monomer was percolated yields the required polymers 82). The A.max of the resulting solution depends on the DPn of the formed oligomers, being identical with that of the salt of polymers with one active end-group per chain for DPn > 10, but is red-shifted at lower DPn. Moreover, for low DPn, (<5), the absorption peak splits due to chromophor-chromophor interaction caused by the vicinity of the reactive benzyl type anions. 

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