The origin of conformational equilibrium isotope effects for carbon

✍ Scribed by Carolyn A. Carr; Michael J.T. Robinson; David J. Young

• Publisher: Elsevier Science
• Year: 1989
• Tongue: French
• Weight: 273 KB
• Volume: 30
• Category: Article
• ISSN: 0040-4039
• DOI: 10.1016/s0040-4039(01)80753-x

✦ Synopsis

Conformational equilibrium isotope effects for carbon are dominated by orbital interactions between C-C bonds and strong acceptor (the K* in C=O) or strong donor (the nonbonding electrons on 0) groups, and show a small dependence upon solvent polarity. Conformational equilibrium isotope effects (CEIEs) for hydrogen have several origins, depending upon the structure of the molecules concerned."' Steric effects are important for strongly hindered hydrogens but the anomeric effect3 and long range analogues of it,' hyperconjugation,' and differences in polarity of C-'H and C-*H bonds (leading to solvent effects)' are usually more important in compounds with heteroatoms near the site of isotopic substitution for hydrogen.

CEIEs for carbon are expected to be much smaller than for hydrogen and as yet successful measurements of such CEIEs have been reported for two systems only, l4 and 2,' in both of