Vicinal relationship

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... 

Usually, atoms with a vicinal relationship or more are considered to be nonbonded. Sometimes, however, only atoms with a 1-5 relationship are considered to be fully nonbonded and the atoms with a relationship have scaled down nonbonded interactions or are deleted completely from the nonbonded computations, or different parameters are used. 

Representative examples of these types are shown in Scheme 7. The range of structural types is greatly extended when acyl halides attached to any ring system in a vicinal relationship are considered. 

The problems encountered in any attempt to treat the transmission of the effects of one substituent in a disubstituted heterocycle through the heterocyclic nucleus to a reaction site in the other substituent (i.e. the side-chain) are enormous, and it is consequently not surprising that relatively little work has been done in this area. First, while in benzene derivatives there are three positions, i.e. three relations between substituent and reacting side-chain to be considered, the number of complexities is much greater in heterocycles. Thus, e.g., in pyridine alone, after elimination of the orientations involving a vicinal relationship between substituent R and the side-chain Y to which no Hammett-type relation is likely to be applicable, the following cases should be considered ... 

A further complication arises out of the fact that of all the orientations discussed only one, 5-R-3-Y, does not involve a vicinal relationship between at least two of the three structural features—substituent, side-chain, and heteroatom. In the cases of 4-R- and 5-R-2-Y the problem of vicinal relations appears not too serious, since this relation is equivalent to the problem of the constant ortho substituent. For this situation it was shown that the constant ort u) substituent, i.e., in this case the heteroatom, may make a contribution to the substituent-independent term (logA °) but generally leaves the reaction constant (p) unaffected. Where the substituent, however, is alpha to the heteroatom it appears likely that its electronic structure, and hence its <7-values, may be substantially affected. This appears particularly likely for large substituents and especially for those which can form a hydrogen bond with the heteroatom, such as CO OH. 

Consistently with empirical observations, the stabilization AE(na- 7tb ), for example, is found to be relatively negligible unless 7ta and 7tb are in a vicinal relationship, due to the fact that the (7ta F 7th ) matrix element falls off rapidly (exponentially) with distance, and analogously for the other interactions in (3.109). Furthermore (as illustrated in Example 1.4 for the nb 7ta interaction), the vicinal 7ta 7tb interaction is equivalent to a partial admixture of the alternative resonance structure ... 

To illustrate the importance of vicinal connectivity of conjugating units, we consider two dienes in non-vicinal relationships 1,5-hexadiene, 9, and allene, 10. As shown in Table 3.19, the direct diene conjugations are negligible in both species, on account of spatial separation in 9 and symmetry-imposed orthogonality of the two pi planes in 10. Consistently with the essential absence of conjugation, the unsaturated C—C bonds of 9 and 10 have calculated bond orders characteristic of ethylene or other unconjugated systems and the ficc NLMOs have essentially localized character ... 

What procedure is available for placing two deuterium a in a vicinal relationship on a molecule ... 

In cholesterol we can see nearly all of the 1,3-diaxial and 1,2 axial-equatorial NOE interactions, whether they are H-H or H-CH3 relationships. One should take great care, however, in establishing regiochemistry or stereochemistry by NOE experiments. Even in a small ring (4-6 members), the difference in the H-H distance between a cis 1,2 (vicinal) relationship and a trans relationship is small. For an ideal cyclohexane chair conformation, for example, the vicinal distances are 2.54 A for equatorial-equatorial (trans), 2.48 A for... 

Figure 5.54. The absolute-value COSY-45 spectrum of 5.1 compared with the equivalent COSY-90 spectrum. The tilting apparent on some COSY-45 crosspeaks can indicate whether the active coupling arises from protons sharing geminal or vicinal relationships which produce peaks with positive and negative slopes respectively (see text).

Constitutional isomerism is defined as that type of isomerism (i.e., different structures corresponding to the same molecular formula) resulting from differences in vicinity relationships between atoms. Examples of pairs of constitutional isomers are -butane and isobutane [CCCC versus CC(C)C in Smiles notation], ethanol and dimethyl ether (CCO versus COC), 1 - and 2-methylbutene (C=CCC versus CC=CC), and 1- and 2-propanol [CCCO versus CC(0)C]. Constitutional isomerism is adequately accounted for in chemical graph theory by the adjacency or distance matrices, which consider only the vicinity relationships. ... 

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