Triplets electron spin

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

Wrachtrup J, von Borczyskowski C, Bernard J, Brown R and Orrit M 1995 Hahn echo experiments on a single triplet electron spin Chem. Phys. Lett. 245 262-7... 

Quite often, spin dephasing times are orders of magnitude shorter than the decay times measured in the (incoherent) population relaxation experiments. For a few transition metal chelates in the excited triplet state we will argue later that the homogeneous spin dephasing is determined by hyperfine interactions of the triplet electron spin with randomly flipping ligand nuclear spins. 

This form is less useful than that employing, T, and Z since some ambiguity regarding the identification of the principal axes of D frequently occurs. The triplet electron spin wavefunctions that form the basis for a diagonal representation of Jfps are of the form (Hameka and Oosterhoff, 1958)... 

The introduction of a laboratory magnetic field gradually decouples the triplet electron spins from the molecular framework. The energies of the spin sublevels then become dependent on the strength of the field as well as its orientation with respect to the fine-structure (and other magnetic) axes. This introduces new features into the interpretation of the data, which were first explored in detail by Hutchison and Mangum (1961) for the case of molecular triplet states. We wish to illustrate some of these effects by reference to recently published high-field (hf) ODMR experiments on the lowest (nn )... 

Fig. 6 compares the lineshapes of the ( T>- Z transition for a single molecule and an ensemble of about 10 molecules. The ensemble spectrum was obtained with the laser in resonance with the fluorescence excitation line of the Oi ensemble. The transition shows an asymmetric hneshape with a steep decrease towards higher microwave frequencies for both the single molecule and the ensemble case. The line-shape results from the hyperfine interaction of the triplet electron spin with the pen-tacene proton spins (/ = 1/2). Each proton can exist in one of its two nuclear spin states which yields 2 nuclear spin configurations. The h3fperfine interaction of each of these nuclear configurations causes a slight shift of the resonance. As pointed out in Section 4.1. In zero-field the hyperfine interaction is a second-order effect which leads to the observed opposite asynunetric lineshapes for the ((T>- Z and the ( T>- Z transitions (see Fig. 5). 

Fessenden R W and Verma N C 1976 Time resolved electron spin resonance spectroscopy. III. Electron spin resonance emission from the hydrated electron. Possible evidence for reaction to the triplet state J. Am. Chem. Soc. 98 243-4... 

Atkins P W and Evans G T 1974 Electron spin polarization in a rotating triplet Mol. Phys. 27 1633—44... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

The advantages of INDO over CNDO involve situations where the spin state and other aspects of electron spin are particularly important. For example, in the diatomic molecule NH, the last two electrons go into a degenerate p-orbital centered solely on the Nitrogen. Two well-defined spectroscopic states, S" and D, result. Since the p-orbital is strictly one-center, CNDO results in these two states having exactly the same energy. The INDO method correctly makes the triplet state lower in energy in association with the exchange interaction included in INDO. 

Derivation of an energy level diagram shows that it consists of two sets of energy levels, one corresponding to the single lines and the other to the double lines, and that no transitions between the two sets of levels are observed. For this reason it was suggested that helium exists in two separate forms. In 1925 it became clear that, when account is taken of electron spin, the two forms are really singlet helium and triplet helium. 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

In the limit of infinite atom separations, or if we switch off the Coulomb repui. sion between two electrons, all four wavefunctions have the same energy. But they correspond to different eigenvalues of the electron spin operator the first combination describes the singlet electronic ground state, and the other three combinations give an approximate description of the components of the first triplet excited state. 

Postulate (i) follows from the fact that when two radicals, produced by whatever means, encounter each other, the interaction of the electron spin of one radical with that on the other radical can give rise to two mutually exclusive spin states, triplet and singlet. Random combination of the two possible electron spin states for the two electrons yields the three components of the triplet state, represented as T+i, To, and T i, and the singlet state, S. Throughout this article, S is assumed to be the singlet state of lowest energy. 

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