Contact shifts sign alternation

The sign of the contact shift usually alternates along a chain if the spin-delocalization is mainly via n orbitals a delocalization gives like signs, and sharper attenuation with distance from the spin source. Simple considerations of electron spin correlation, such as the Pauli principle and Hund s rule, give overall guidance, but the reality is often complex (e.g.. Ref. 176). 

The authors reasoned that for these complexes, a dominant it mechanism would result in positive spin density at the pyridine nitrogen with the sign of the contact shift of the carbons alternating around the ring. Dominant a de-localization would cause an increase in negative spin density (jS) on the metal and an excess of positive spin density ([Pg.397]

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