Hydrogen vicinal

For each molecule (isomer A and isomer B), obtain dihedral angles for the following pairs of vicinal hydrogens Hg-Hy, H Hg, Hy-Hn, and Hg-Hyaxiai- Use the Karplus equation to estimate coupling constants for each pair, and then compare your predictions to the experimental coupling constants (see above). Which molecule is artemisin acetate and which is 6-epiartemisin acetate ... 

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

Vicinal coupling constants between fluorine and hydrogen are generally between 18-20 Hz for both primary and secondary CF2 groups. On the other hand, H—H coupling constants between vicinal hydrogens are much smaller in these compounds, between 4 and 8 Hz. 

Generally, the presence of a CF3 group induces relatively mild deshielding of vicinal hydrogens (less than that from a CC13 group. 

Carbohydrates have been included in the wide range of molecules used in the parameterization of MM2 and of MM3. Alcohol and ether parameters have usually been determined from simple alcohols and ethers themselves. However, carbohydrates contain some unusual features in the acetal linkages, and in the many vicinal hydrogen-bonded hydroxyl groups. The "anomeric effect", first discovered by Edward (15) and popularized by Lemieux (16.), is best known in carbohydrates, although, of course, it occurs in other classes of compounds as well. One apparent result of this effect is that an axial alkoxy substituent is often more stable than the corresponding equatorial substituent when attached at the Cl position of a tetrahydropyranyl ring. This effect can be... 

A similar relationship between PCDF isomers retained and apparently excreted has been observed for patients with the Yusho disease, an intoxication by a rice oil contaminated with PCBs and PCDFs. The contaminated rice oil and liver samples from two of the patients were analyzed and all the major PCDFs were identified. A comparison revealed that none of the isomers retained had two vicinal hydrogenated C-atoms in any of the two... 

C-rings of the benzofuran system. Most of these isomers had all their lateral positions chlorinated. Contrary, all the PCDF isomers apparently excreted had two vicinal hydrogenated C-atoms in at least one of the two rings, and these unblocked positions are involved in the metabolism by forming epoxides, see Figure 3 (23) The data discussed here show a striking similarity between the most toxic PCDD and PCDF isomers and the isomers most efficiently retained. 

When HMBC spectra are used in conjunction with HH COSY spectra, the JcH couplings can be identified in the HMBC spectrum because the vicinal hydrogen atoms can be identified from the COSY spectrum and related to their attached carbon atoms. 

Dihydroxylation, the addition of two hydroxy groups across a C = C bond, converts fluorinated alkenes into different products depending on the presence or absence of a fluorine atom at the hydroxylated carbon. Partially fluorinated alkenes with vicinal hydrogen atoms attached to the C = C bond can be hydroxylated to vicinal diols. When the reaction is performed with a sufficiently strong oxidizing agent, the initially formed diols are oxidized to vicinal diketones as the end products. 

The formation of regioisomers is due to the presence of several sets of nonequivalent vicinal hydrogens of similar but not identical reactivity. The resulting mixture of similar products must be separated if only one of the regioisomers is desired. Since the alkene isomers are very similar in physical properties, such separations can be very difficult and certainly are not practical. 

This outcome derives from the fact that the carbon that bears the fluorine is chiral, which makes the two vicinal hydrogens diastereotopic and thus magnetically nonequivalent. In such a case, the two diastereotopic protons will not only appear as separate signals (an AB system), but they usually will also couple to vicinal fluorines (and hydrogens) with different coupling constants. Examining Figure 2.8b, which represents... 

The chemical shift differences observed for these 1-fluoro-l-methyl-f-butylcyclohexanes, the cis- and rrans-9-nuomdecalins, and for 1-fluoroadamantane provide insight regarding the significant influence of conformation upon fluorine chemical shifts in fluorocyclohexanes. The relative chemical shifts of these various cyclohexyl fluorides can be rationalized simply on the basis of what is commonly known as an anomeric effect. That is, a vicinal hydrogen that is rigidly anti to a fluorine substituent will exhibit an anomeric double-bond/no-bond... 

Three-bond F-H coupling constants between CF2 fluorines and vicinal hydrogens are usually in the 18-20 Hz range (Scheme 4.5). 

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