The exact analysis of the ABC type absorption due to the -CH20H group of 3j3-acetoxy-5j3,6[3-oxidocholestan-19-01 provides two vicinal HCOH coupling constants for dihedral angles of 80" and 160". A least squares fit of these and previous data in the literature to a "Karplus" relation provides values for the coefficients in such an equation. The theoretical justification for such a relation has been obtained by extended Hiickel molecular orbital calculations for methanol with different dihedral angles. The practical limitations in, and applications of the relation are discussed. An effect of solvent on the vicinal coupling constants is also noted and discussed.
The pKa's of 13 heterocyclic aromatic compounds have been measured in tetrahydrofuran (THF) using 13C nmr spectroscopy. The acidifying effect of a nitrogen heteroatom is seen to be quite large (4–6 pK units versus C—H) on an adjacent C—H bond but small on a more remote hydrogen. Most of these aromatic heterocycles are sufficiently acidic, pKa < 34, to be completely deprotonated by lithium tetramethylpiperidide in THF. Benzoxazole appears to be an extremely strong carbon acid, its ring-opened isomer having a pKa of less than 15.7.
38220 2 rather than benzvalene is reduced. The correspondence of quantum yields, intensity effects, and rates for aldehyde formation to those in the absence of 02, however, suggests that two molecules of benzvalene are consumed in both cases. It appears that oxygen intercepts a precursor of the cyclohexadiene without diminishing the effectiveness of the chain-carrying process.
Barriers to Rotation about the Sulfur-Sulfur Bond in Acyclic Disulfides'Sir :Although the presence of a barrier t o rotation about the S-S bond of a disulfide was established over 20 years ago, empirical estimates of its size have varied between 2 and 16 kcal/mol.2 In addition, barriers t o rotation in HzSz have recently been calculated3 to be 9.3 and 6.0 kcal/mol for the cis and trans transition states, each of which is produced by a 90" rotation about the S-S bond from the ground-state confor-mationZa (see Scheme I). These values agree well with Scheme I H,.'c-(1) This work was presented at the
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.