Application of Electron Spin Resonance to Conformational Analysis

Conformational analysis by electron paramagnetic resonance study is naturally limited to free radicals. Very interesting information on these molecules is given by e.p.r., and with short-lived radicals e.p.r. is often the only method for conformational analysis. Then, one needs a complementary hypothesis of theoretical origin. For monoradicals, it can be theoretically and experimentally verified that hyperfine splittings are geometrically dependent. Moreover, one can observe dynamic processes corresponding to jumps between conformations, the critical frequency being about iO Hz. Different examples are given for aromatic and aliphatic radicals. For biradicals, other interactions are measurable; particularly the exchange interaction J and the dipolar interaction D. The variation of the exchange interaction J in terms of geometry is only known qualitatively. On the other hand, the simple variation of dipolar interaction D, which depends only on the distance between the two unpaired electrons, permits quantitative evaluations. There are now many examples of the application of electron spin resonance (e.s.r.) to the study of conformational analysis. Geske' has summarized most of the work published up to 1966. In this paper, we shall try to show how original can be the contribution of e.s.r. to conformational analysis, cornpared to other physical methods. One can correlate the various data obtained from e.s.r. (g-value, hyperfine splitting, line width) with structural parameters. However, a model is necessary and the conclusions obtained depend on the choice of the particular model. We shall study the following problems: 1. Determination of the geometry at the radical centre. 2. Conformational analysis of monoradicals. 3. Conformational analysis of biradicals. 1. GEOMETRY AT THE RADICAL CENTRE Although the determination of a planar or distorted structure of a radical is not strictly conformational analysis2, it is important as an illustration of the methods that can be used in e.s.r. for such a determination. We should like to demonstrate the principles of the method using the CH3 radical; in a simple description (Figure 1) the unpaired electron is