Conformational studies of nitroxide radicals by esr spectroscopy.II. Hindered rotation of the n-methylene group

✍ Scribed by Antal Rockenbauer; Miklós Gyõr; László Jókay; Ferenc Tüdõs

• Publisher: Elsevier Science
• Year: 1980
• Weight: 832 KB
• Volume: 18
• Category: Article
• ISSN: 0378-4487
• DOI: 10.1016/0378-4487(80)80053-7

✦ Synopsis

The hindered internal rotation of N-methylenegroups has been studied throughtheteqeraturedependeaxceofhyperfinecouplingconstants in ten different rrethylene-phenyl-nitroxide free radicals. tiels consisting of two or three equilibriumconformations in fastequilibriumwere applied. The quilibriumoonformationsand free~differencesobtainedhavebeenaMlysed in terms of the distortion of molecular symxtry due to ortho substitution of N-phenyl ring. Sane differences between the hindered rotation of N-methins a,1-Qi N-mathylenegroupswere explainedbythe invertedorderof non-degenerate and degenerate conformations when the phenyl ring had no ortho substituent. In Part I of the work [I] we studied the hindered rotation of the N-methinegroup invarious phenyl-methine nitroxides throughteqeraturedependenceof theN-methineprotonhyperfinecoupling. We foundtheteqeratureeffects can be well described by the node1 consisting of two conforx&ions in fast interconversional equilibrimn and torsional oscillations seen to play onlyaminor role.The structureof theseconfonnationswerefourdasindeperxdentof thenkolecular qtmetry. Inall cases theconformations canbecharacterized by the same dihedral angles of 90' and 0' between the NCH plane and the p orbital where the unpaired electron is centered. The free energy differences betwefx~confonnations, on the other hard, sensitivelydeperdon the