Intramolecular Spin Coupling

1) Orbital degeneracy occurs in molecules having at least a threefold rotational axis as in triphenylene (1 ) or possessing symmetry [382, 386]. Very 

Some rules have been put forward to design molecules with degenerate NBMO s. Following the proposal of Longuet-Higgins, an alternant hydrocarbon has at least N-2T singly occupied NBMO s, where N is the number of carbon atoms and T is the maximum number of double bonds occurring in any resonance structure [383, 396]. These rules have later been unproved on the 

One spin is localized in the cr-orbital, while the other is delocalized over the phenylene unit. The carbenes, however, are problematic in some way because 1) they are very reactive and unstable at temperatures above 100 K 2) the complete photolysis of all diazo precursors becomes difficult with increasing chain length, while a high spin multiplicity is only reached if all sites are intact, and 3) there are no polymerization reactions leading to this skeleton [410]. 

Even in the meta-phenylenevinylenes 22 the neighboring units can be charged to the biradical form. This is in line with the observation of spin localization in the monoradical, where the unpaired electron is distributed in one stilbene unit only. Upon further charging of e.g. 22b the biradical formation is obtained [44, 434]. 

3 Polymers with Radical Caiters in the Side Chain 

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The observation that XmT of (bpym, Se) is temperature-independent between 50 and 110 K and that no maximum in the susceptibility occurs at temperatures below 50 K suggests that no intramolecular antiferromagnetic coupling in pairs remains in this temperature regime. In fact the magnetisation curves [9] of (bpym, S) and (bpym, Se) at 1.9 K clearly indicate the different nature of the spin pairs involved in each compound in the ground state (Fig. 8). 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

A new method to use proton relaxation data has been proposed which can be applied even in cases where protons are spin coupled to other protons and thus the relaxation is not exponential. The method consists of two experiments taking the ratio of initial decay after a conventional 180° pulse to the initial decay after a selective 180° pulse gives a more accurate figure of intramolecular dipole-dipole relaxation (Hall and Hill, 1976). Application of this method to the H Tj measurement of 3,4,6-tri-0-benzoyl-2-chloro-2-deoxy-D-glucopyranose [13] is described. 

V(t)intra describes all spins coupled intramolecularly, whereas V(t)inter takes into account the signal decay caused by intermolecular interactions between homogeneous distributed spins in the sample. F(t)intra can be described by Eq. (16.2) ... 

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