Triplet state paramagnetism

Paramagnetic molecules K)a and 2NO are very efficient quenchers of singlet and triplet states. Oxygen may form CT complex and lead to peroxide formation in the triplet state. Paramagnetic ions of the transition series and lanthanides also quench the triplet states. 

The paramagnetism of the triplet state can be observed by electron spin resonance spectroscopy. This is perhaps the most reliable means of determining the existence of a triplet state since the ESR signals can be predicted using the following Hamiltonian operator ... 

As seen in the previous section, one characteristic of the triplet state is its paramagnetism. This alone would of course not suffice as a definition of the triplet since there are many odd-electron species that also exhibit paramagnetism but do not exist as triplets. Thus we might state that a triplet is a paramagnetic even-electron species. This still does not constitute a limiting definition since compounds containing even numbers of electrons may exhibit two, three, or even five distinct electronic levels. For example, when in a biradical the radical centers are separated by several carbon atoms as below, no interaction between the electron spins occurs and the radicals appear as two doublet states ... 

Since we have two unpaired electrons, 5=1, and the number of states is three. If we should have other even numbers of unpaired electrons, such as four or six, we could have three, five, or even as many as seven states. Thus we arrive at a definition for the triplet as a paramagnetic species possessing an even number of unpaired electrons and existing in a set of three energetically similar electronic levels which result from interaction of the electronic spin. Generally these three distinct electronic levels, between which transitions may be observed under certain conditions, are collectively referred to as the triplet state. 

A diradical is an atom or molecule containing two impaired electrons. The properties of diradicals are for the most part like those of monoradicals. They are paramagnetic and show paramagnetic resonance absorption. Although they are very reactive chemically, this is not a reliable criterion for the diradical state. Spectroscopically the diradical will probably be a triplet state if a double bond structure coupling the two electrons is geometrically possible. But when the two electrons are fairly well isolated from each other the state is probably a double doublet, like two independent radicals. 

Any unsaturated molecule has diradical as well as singlet (nonradical) states. Usually one of the non-radical states will have a decisively lower free energy than the most stable of the diradical states, in which case the substance is not paramagnetic. Special circumstances can make the diradical state the ground state then the singlet state is an excited one. Some substances have detectable amounts of both forms in equilibrium. It is unknown whether the radical-like reactions of some compounds are characteristic of their singlet states directly or due to an undetectable amount of a more reactive triplet state in equilibrium. 

The triplet-state splittings of tyrosine were first observed by electron paramagnetic resonance (EPR) more than two decades ago.l30 32) The initial characterization of the splittings was limited to a measurement of >, a root-mean-square zfs defined by... 

T. Shiga and L. H. Piette, Triplet state studies of flavins by electron paramagnetic resonance—II, Photochem. Photobiol. 3, 223-230 (1964). 

J. Zuclich, Triplet-state electron paramagnetic resonance of the aromatic amino acids and proteins, J. Chem. Phys. 52, 3586-3591 (1970). 

The lowest excited states of paramagnetic metal complexes are described by configuration interactions of the porphyrin (7T,tt ) excited singlet and triplet states and the "porphyrin-to-metal" or metal-to-porphyrin charge-transfer excited states (35,36). Thus T (phosphorescence) emission of paramagnetic metal complexes decays... 

Electron Paramagnetic Resonance Study of Dimerization Effects on Porphyrins in the Photoexcited Triplet State... 

The triplet state of some of the bipyridines has interested several groups, sometimes in conjunction with electron paramagnetic resonance (EPR)... 

While the possibility of a very rapid electron reorganization from a low energy triplet state is ruled out by the observed insensitivity of the photo-Fries rearrangement to paramagnetic quenchers (vide supra) and the usual rate of intersystem crossing (kST = 10s—1011 sec-1 43a), the remaining alternative to singlet processes for both Path A and Path B, i.e., reactions from... 

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