Electron spin resonance study of conformational effects in free radicals derived from aliphatic alcohols and ethers




Briggs, Alexander Gibson

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Variable temperature ESR studies of radicals generated photolytically from simple aliphatic alcohols and ethers in cyclopropane solution reveal complex linewidth effects. Isotropic modulation of the proton hyperfine splittings (hfs) through restricted rotation about C-0 and C-C single bonds is observable in the region 230>T>150K. Such effects can be distinguished from anisotropic viscosity-dependent line broadening. In spectra from alcohol radicals resolved 2nd order structure causes no ambiguity in the interpretation. Restricted rotation about Ca-0H modulates aaH and aBH out-of-phase with a0H in the series RCHOH [R= CH3, C2H5, C2H5CH2, (CH3)2CHCH2, (CH3)3CCH2]. A general model for the process is discussed. In cases three and four restricted Co-C rotation allows the diastereotopic inequivalence of the 6-protons to be manifested as a broadening of MB = 0 components. Preferred conformations consistent with all the foregoing modulation effects and with observed HB and HY splittings are presented. The analysis is supported by results for radicals RCHOR' from related ethers and by spectral simulation. The spectrum of the 1-hydroxycyclohexyl radical demonstrates previously unobserved fine structure and a low-temperature linewidth effect tentatively attributed to radical site inversion. A second series of alcohol-derived radicals R1R2R3CCHOH with an increasingly bulky Ca substituent has been studied. The Ha hfs provide evidence of a steric flattening not hitherto observed. This effect correlates well with literature values of steric parameters for the R1R2R3C substituent. In the case R1,R2 = CH3, R3 = C2H5 an observed specific y-H interaction is assigned to a locked conformation of the crowded system. A series of highly alkylated cyclic ethers has been examined. The dramatic temperature-dependent changes in the spectrum of the 5,5-dimethyl-l,3-dioxan-2-yl radical are attributed to restricted ring flipping. A fast exchange limit spectrum has been obtained for the first time in such systems, allowing evaluation of thermodynamic parameters. the 2,4,8,10-tetraoxyspiro[5,5]undecan-3-yl radical exhibits similar behaviour. The 2,2,5,5-tetramethyl and 5,5-diethyl-2,2-dimethyl-l,3--dioxan-4-yl radicals have fixed conformations which give rise to enhanced values of ayH in agreement with theoretical calculations. In the latter case a splitting of 4.27 G is assigned to a single y-methylene proton in behaviour analogous to R1R2R3CCHOH.



Electron paramagnetic resonance spectroscopy, Radicals