Charge distributions and chemical effects. XXXVIII. Correlations between nuclear magnetic resonance shifts and electronic charges of nitrogen atoms

Correlations between nuclear magnetic resonance shifts and atomic charges of nitrogen in selected alkylamines, nitroalkanes, isonitriles, and azines consistently follow the general trends observed for carbon and oxygen nuclei. In azines, any increase in total electronic population on nitrogen, resulting from a gain in π charge prevailing over a concurrent loss of σ electrons, is accompanied by an upfield resonance shift—as found for aromatic and ethylenic carbon and carbonyl oxygen atoms. On the other hand, any gain in total charge dictated by that of σ populations translates into a downfield 15N shift, which is the trend exhibited by alkylamines, nitroalkanes, and isonitriles—a situation encountered earlier with sp3-hybridized carbon, carbonyl carbon, and dialkyl ether oxygen atoms.

Époxydes α-éthyléniques et phénate de sodium: accès à des éthers phénoliques et phénols ortho-allyliques

The ring cleavage reactions of α-ethylenic epoxides by sodium phenoxide afforded a mixture of products. Problems of competitive attack by this nucleophile, at the less substituted carbon (compounds A) or at the β-ethylenic carbon atom (compounds B and C), were encountered and could be resolved by judicious choice of reaction conditions (solvents, stereochemistry of the oxiranes). The regioselectivity of the attack was dependent on the transition states, implying weak steric hindrance and a conjugation oxirane – double bond.

Photoelectron spectroscopic studies of the chemisorption of ethylene and halo-substituted alkenes on the (100) and (111) crystal surfaces of platinum and a general model for the dissociation of unsaturated molecules at surfaces

The chemisorption of ethylene and halogen-substituted alkenes has been studied by X-ray (A1 Kα) and ultraviolet (HeI and HeII) photoelectron spectroscopy. Initial dissociation of the vinyl halides at submonolayer coverage is followed by an associative regime at higher coverages; a mechanism for the surface reaction, involving oxidative addition (metallation) of carbon-hydrogen bonds and subsequent dehydrohalogenation, is proposed. The reactivity of ethylenic carbon-hydrogen bonds is shown to depend strongly on adjacent halogen substitution. An intermolecular (Langmuir-Hinshelwood) mechanism is demonstrated for hydrogen halide elimination from mixed adlayer surfaces of chlorotrifluoroethene and ethene or other alkenes with reactive carbon-hydrogen bonds. A statistical model in which dissociation requires the presence of two adjacent vacant coordination sites accounts semi-quantitatively for the dissociative-associative pattern of sorption as a function of coverage, for the differing reactivities of the alkenes at the Pt(100) and Pt(111) substrates and for the associative sorption of the vinyl halides which follows preadsorption of fractional monolayers of carbon monoxide.