Molecular oxygen spin orientation

Figure 2.14. The molecular orbitals of gas phase carbon monoxide, (a) Energy diagram indicating how the molecular orbitals arise from the combination of atomic orbitals of carbon (C) and oxygen (O). Conventional arrows are used to indicate the spin orientations of electrons in the occupied orbitals. Asterisks denote antibonding molecular orbitals, (b) Spatial distributions of key orbitals involved in the chemisorption of carbon monoxide. Barring indicates empty orbitals.5 (c) Electronic configurations of CO and NO in vacuum as compared to the density of states of a Pt(lll) cluster.11 Reprinted from ref. 11 with permission from Elsevier Science.

Because of the unique orientation of the oxygen mole-cule-with-unpaired spin (—O2"—with respect to the PE molecular axis, the peroxy radical produces (theoretically) asymmetric tines in the ESR spectrum with two g-values, gii and gj. Ranby and Rabek presented a discussion on the dependence of the ESR spectrum on the orientation of the peroxy end in polytetrafluoroethylene (Pl EE) radical [21]. Rabek provides an ESR spectrum of the peroxy radical in polypropylene and also in polyethylene (Chapter 3 [2]). The polypropylene-peroxy radical presented is a clear ESR spectrum, which can be considered as the fingerprint for the peroxy radical the ESR spectrum of the polyeth-ylene-peroxy (low density polyethylene) is not as well defined. Tor a better understanding, readers are referred to a survey of the mechanisms for production, propagation, and termination of the peroxy radicals in polyolefins, including PE, compiled by J. F. Rabek (Chapter 3 [2]) and by Dole (Chapter 13 [1]). 

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