Electron/nuclear spin effects

Spin labels contain unpaired electrons that by highly efficient electron-nuclear spin dipolar coupling lead to accelerated transverse or longitudinal relaxation. The effect is rather far-reaching (at least up to 10 A), and its general use is described in Chapt. 15. 

Nuclear Spin Effects on Rotation. There is an interesting effect on the rotational partition function, even for the hydrogen molecule, due to nuclear spin statistics. The Fermi postulate mandates that the overall wavefunction (including all sources of spin) be antisymmetric to all two-particle interchanges. A simple molecule like (1H1)2, made of two electrons (S = 1/2) and two protons (spin 7=1/2), will have two kinds of molecule ... 

We have now completed our derivation of the electronic Hamiltonian when external fields and nuclear spin effects are absent. In summary, the Hamiltonian is as follows ... 

In conclusion we summarise the total Hamiltonian (excluding nuclear spin effects), written in a molecule-fixed rotating coordinate system with origin at the nuclear centre of mass, for a diatomic molecule with electron spin quantised in the molecular axis system. We number the terms sequentially, and then describe their physical significance. The Hamiltonian is as follows ... 

The first radiofrequency/optical double resonance studies of molecules were published almost simultaneously. Observations of OH and OD were described by German and Zare [10] late in 1969, and will be discussed in detail in the next subsection. A few months later studies of the CS molecule in its excited A 1 n were reported by Silvers, Bergeman and Klemperer [11], with more detailed results described later by Field and Bergeman [12], We now describe these investigations, which are in some ways simpler than those of OH because of the absence of electron and nuclear spin effects in the CS 1n state. 

Although the low field effects on chemical reactions through radical pairs had been explained by the LCM, Timmel et al. [14] proposed that the so-called low field effects arose also fi om coherent superpositions of degenerate electron-nuclear spin states in a radical pair in zero field. They made some model calculations for their mechanism. At first, let us consider the case of a radical pair with a single spin-1/2 nucleus, e.g., a proton. When the exchange term is not included (J= 0 J), its spin Hamiltonian (H) can be expressed from Eq. (3-3) as... 

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

The data reported for the molecules in the X, X, and states have been analysed in terms of an effective Hamiltonian which refers to the rotational, spin and hyperfine levels of a particular vibronic state. The Hamiltonian is formulated in terms of the various angular momenta involved, namely TV, L, S, G, J, /, and F which are respectively the rotational, orbital, electron spin, vibrational, nuclear plus electronic, nuclear spin, and total angular momenta (strictly speaking, N = R + L where R is the angular momentum of the nuclear framework). The effective Hamiltonian can be written... 

MW spectroscopy. Conversion to mT effected with s-value of [66Col]. 1 T is the electron-nuclear spin dipolar coupling constant in MHz. Hyperfine interaction with outer C nucleus not detected. ... 

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