Electron Magnetic Resonance of an Oriented Allyl-Type Free Radical

It has been found that x-irradiation of a single crystal of glutaconic acid (HOOC)CH=CH–CH2(COOH) produces the oriented allyl-type free radical (HOOC)C(1)H=C(2)H–C(3)H(COOH). A detailed analysis of the electron magnetic resonance spectra of this radical confirms that the spin density on carbon 2 is opposite in sign to the spin densities on carbons 1 and 3. Additional structural features determined are: (a) The unpaired electron occupies a delocalized π molecular orbital extending over carbon atoms 1, 2, and 3; (b) spin densities ρii for carbon p orbitals on carbon atoms 1, 2, and 3 are ρ11=ρ33=+0.57, ρ22=−0.19; (c) principal values of the hyperfine tensors for protons 1 and 3 are −53, −36, and −18 Mc, and for proton 2 are +17, +12, +7 Mc; (d) all C–H bonds in the radical are nearly parallel. The parallelism of all C–H bonds indicates that the undamaged parent molecule is the trans isomer of the acid.