Methyl radical, proton hyperfine coupling constant

When ESR spectra were obtained for the benzene anion radical, [C6II6] and the methyl radical, CH3, the proton hyperfine coupling constants were found to be 3.75 and 23.0 G, respectively, i.e. they differ by about a factor of 6. Since the carbon atom of CH3 has a spin density corresponding to one unpaired electron and the benzene anion carries an electron spin density of 1/6, the two results suggest that the proton coupling to an electron in a n-orbital is proportional to the spin density on the adjacent carbon atom ... 

A typical example is seen for 1-hydroxyethyl radical trapped in a y-irra-diated ethanol matrix at 77 K [16]. As is shown in Fig. 2, the cw ESR spectrum of the radical is composed of five lines due to hyperfine interactions with one a proton and three P protons of a methyl group. The hyperfine interaction depends on the location of the P protons with respect to the p. orbital of the unpaired electron. However, the observed hyperfine coupling constant is the same for all the p protons because of the quick rotation of the methyl group in the time scale of the cw ESR measurement. On the other hand, the ESE-detected ESR spectrum is composed of four lines due to the hyperfine interactions with... 

The fast motion spectrum of the /-PMMA radical consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. There is coupling to the methyl group to form a quartet (22.9 G) that is then split further into a triplet from one set of p-methylene protons (16.4 G) and another triplet from the other set (11.7G). Theoretically, this should lead to 36 lines (4 x 3 x 3), but a fortuitous degeneracy exists because one of the fast motion p-methylene couphng constants is almost exactly... 

Spectroscopic data can be used to distinguish between planar and nonplanar rapidly inverting radical centers. The hyperfine coupling constant a in the methyl radical is 23.0 G, which is a typical value for the splitting of an EPR signal by protons attached to a radical center. Theoretical analysis of the spectrum suggested that the methyl radical is probably flat, although a deviation from planarity of 10-15° could not be ruled out There is also spectroscopic evidence that the methyl radical in the gas phase is essentially planar. Thus, the methyl radical is conveniently described by sp hybridization with the unpaired electron located primarily in the p orbital. 

Experimental remedies to obtain these numbers are the TRIPLE CW and pulsed ENDOR methods described in this chapter. In some cases the number of equivalent nuclei belonging to a particular hyperfine coupling constant can be determined visually from a sufficiently resolved ESR spectrum. Thus, the main structure with or = 22.18 MHz in the ESR spectrum of the 9,10-dimethylanthracene cation radical in Fig. 2.3 is a septet due to the six equivalent of the methyl groups. Assignment of the couplings of the ring protons is often made, experimentally by selective deutera-tion, and theoretically by quantum chemistry. Simulations are particularly employed for the interpretation of ENDOR spectra of disordered solids discussed later in this chapter and in Chapter 3. 

ESR spectra of free radicals in solution often show hyperfine structure due to several nuclei, particularly H, with the same coupling constant. The ESR spectrum of the methyl radical in Fig. 3.1 shows a line pattern with three equivalent protons (/ = Vi). The analysis by the diagram under the spectrum shows the construction of a stick pattern with an intensity ratio 1 3 3 1. 

It is not immediately obvious how hyperfine splitting arises in the case of the benzene anion in which the electron occupies a r-orbital that has a nodal plane coincident with the plane of the aromatic ring. Similarly, it is not obvious as to why in the ethyl radical (CH3CH2") the methyl (or fi) protons show a larger coupling constant that the a-protons. This is rationalized below. 

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