Conformational studies of nitroxide radicals by esr spectroscopy I. The hindered rotation of N-methine group

✍ Scribed by A. Rockenbauer; M. Győr; L. Jókay; F. Tüdős

• Publisher: Elsevier Science
• Year: 1979
• Weight: 657 KB
• Volume: 15
• Category: Article
• ISSN: 0378-4487
• DOI: 10.1016/0378-4487(79)80061-8

✦ Synopsis

The temperature dependence of proton hyperfine coupling (a$ of the methine group linked to the NO function of nitroxide radicals has been studied by ESR spectroscopy. The radicals studied have the structure R1N(O')C(R2)(R3)H, where Rl is a phenyl group substituted in the ortho positions and R2 and R3 are alkyl or phenyl groups. The symmetry of the radicals has been found to critically affect the temperature dependence of #. While ai has a small value (ca. 3 G) independent of temperature, if the Rl phenyl ring is co-planar with CNO and R2 = R3, it slightly decreases with increase in temperature when R2#R3, it markedly increases with temperature rise when the Rl phenyl group is moderately twisted from the plane of CNO, and it has a high value (ca. 20 G) and decreases with temperature rise, if the twist is strong. These trends have been interpreted by the predominance of conformers 11 and 12, where the angle between the NCH plane and the p orbital of the unpaired electron is 0' and 90°, respectively. While the twist of the Rl phenyl ring from the CNO plane yields the destabilization of I2 with respect to 11, the asymmetric substitution of N-methine group mainly causes the distortion of the I 2 conformer.