Selenium-77 nuclear magnetic resonance spectroscopy

Carbon—hydrogen spin—spin coupling constants

Techniques of signal assignment of 13C-NMR spectra of organic compounds, and in particular those of natural origin, are reviewed. They include symmetry, chemical shift considerations, spin-spin coupling (both one-bond and long-range), spin relaxation (T1, NOE), isotopic enrichment and complexing agents. Special emphasis is given to the effects of oxygenated substituents on aromatic rings.

Techniques for Signal Assignment in 13C-NMR Spectroscopy of Naturally Occurring Aromatic Compounds

High resolution 1H, proton coupled 13C and broad-band decoupled 13C{1H} spectra have been recorded for pyrylium, thiapyrylium and seleninium tetrafluoroborates in CD3CN. Analysis of these spectra has provided accurate values and relative signs of all 1H,1H and 13C,1H coupling constants, as well as the magnitudes of all one- and three-bond 13C,13C couplings observable in naturally occurring doubly-labelled isotopomers. For the seleninium cation, the magnitudes and relative signs of all 77Se,1H and 77Se,13C coupling constants were also obtained. The chemical shifts and coupling data are compared, on a qualitative basis, with existing theoretical predictions.

High resolution NMR spectroscopy of heteroaromatic cations. II—pyrylium, thiapyrylium and seleninium cations†

Publisher Summary This chapter deals with thiopyrylium, selenopyrylium, and telluropyrylium salts. These compounds are considered the parent structures of an important class of six-membered heteroaromatic cations. It is observed that the reactivity of thiopyrylium salts resembles that of pyrylium salts with two notable differences— namely, the lesser tendency of 2 H adducts formed on nucleophilic attack to undergo ring opening and the ability of sulfur to accommodate more than eight electrons in its valence shell, leading, in some cases, to the formation of thiabenzene derivatives. The chapter also describes the structural and physical properties, the methods of synthesis, the reactions, and the practical uses of these pyrylium salts. There are large number of patents and applicative studies making use of a chalcogenopyrylium salt. Most applications of chalcogenopyrylium salts exploit their photophysical and photochemical properties, and principally regard photographic and reprographic technologies. Another important application of chalcogenopyrylium salts is as polymerization and crosslinking photoinitiators, especially in the preparation of photoresists, printing plates, and photosensitive compositions for laser imaging.

Thiopyrylium, Selenopyrylium, and Telluropyrylium Salts

Une serie de selenophenes mono- et bisubstitues est etudiee en resonance de 1H et 13C. Les deplacements chimiques protoniques sont analyses en termes d'effets d'anisotropie et de champ electrique des substituants et utilises a une discussion des equilibres conformationnels. Les relations entre δ1H et δ13C et les charges π calculees par la methode PPP sont examinees. Les couplages 1J(13CH), nJ(13C ∼ H) et 1J(13C77Se) se revelent de bonnes caracteristiques structurales et des relations d'additivite sont degagees pour δ13C.



Proton and carbon magnetic resonance spectra of mono-and disubstituted selenophenes are investigated. The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents, with a view to studying the conformational equilibrium of the carbonyl group. π Electronic charges, computed by the PPP method, are correlated with the proton and carbon chemical shifts. The coupling constants between 13C and 1H (1, 2 or 3 bonds) and 13C77Se are shown to be good structural parameters and a set of substituent additivity constants is calculated.

J. Morel

Papers

Etudes des sélénophènes mono et bisubstitués par Résonance Magnétique de 1H et de 13C