ESR spin density distribution

However, the nuclear spins are envisioned as point-like spectators of the surrounding electron clouds, with each nucleus signahng its unique chemical bonding environment through a pronounced resonance frequency shift, whereas the electron spins are spatially dispersed over multiple nuclei and exhibit only subtle shifts in resonance frequency. The ESR signals reflect the much more active participation of electron spins in the chemical interactions under study. 

Given the limitations of the experimental ESR spectrum and theoretical concepts underlying its interpretation, the primary objective of ab initio investigation is usually to provide details of the electronic spin-density distribution. At the crudest level, one may merely wish to characterize the radical as cr-type or re-type, and for this purpose the NPA orbital spin density assignments (cf. FO-3.5) are useful. For the distribution of spin density among atomic centers, the NPA summary table (cf. 1/0-3.10) can be employed. 

As a simple illustration, let us examine the spin density distribution in Tempone (2,2,6,6-tetramethylpiperidone-A -oxyl) 

Although Tempone is often depicted as having the unpaired electron localized on the nitroxide oxygen atom, the calculated natural spin density (NSD) distribution presents a more complex picture, as summarized in Table 7.3. 

How can we understand the chemical origins of these spin density patterns The starting point is the different Lewis structures for different spins NBO description of open-shell systems (cf. Section 4.5), which leads to the two distinct spin NBO representations of the Tempone nitroxide bonding pattern, as shown in (7.6a,p)  

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