Hyperfme mechanisms

For instance, nitration of naphthalene, azulene, biphenylene, and triphenylene proceeds preferentially in positions with the greatest constant of hyperfme splitting at the hydrogen atom in ESR spectra of the corresponding cation radicals. The constant is known to be proportional to the spin density on the carbon atom bearing the mentioned hydrogen. It is important, however, that the same orientation is also observed in the classical mechanism of nitration in the cases of naphthalene, azulene, and biphenylene but not of triphenylene (see Todres 1985). 

The dominant line-broading mechanism of Co-deuterolysin was unresolved hyperfme splitting of cobalt nucleus and g-strain. The line width from a g-strain mechanism increases with increasing microwave frequency, but the contribution from hyperfme splitting is independent of the microwave frequency. 

On June 11, 1965, the author (H. Hayashi) and Dr. K. Itoh visited Dr. Y. Kurita at his office in The Basic Research Laboratory of Toyo Rayon Company, Ltd. and saw his beautiful ESR spectra of radical pairs ( J and K ) in single crystals of dimethylglyoxime irradiated by X-rays at 77 K [2]. Here, the radical pairs J and K are symmetric and asymmetric pairs, respectively, as shown in Fig. 4-2. The typical ESR spectra observed for the radical pairs J and K are shown in Fig. 4-3. The author noticed from Fig. 4-3(b) that the central three lines of the nine hyperfme (HF) lines due to two nitrogen atoms of K were not equally spaced [3], but that there is no anomaly in the HF lines of J as shown in Fig. 4-3(a). We found that the anomalous HF lines of K could be explained by the mixing of the singlet and triplet states of a radical pair in the complete Spin Hamiltonian of the pair developed by Dr. Itoh [3]. This theory has been called "the radical pair mechanism". 

We have introdueed the RP spin Hamiltonian, the mechanism of nuclear spin seleetive intersystem erossing, and the reaction scheme for RPs has been explained. We now possess all the tools we need to explain qualitatively how nuclear spin polarization arises and manifests itself in a CIDNP speetrum. The RPM may appear in a CIDNP spectrum as a net effect, a multiplet effect, or a combination of both. The net effeet is observed when the g factor difference between radieals and S) radieal pair is large eompared with the hyperfme interaction. The simplest example involves a RP with just one hyperfme interaetion, as shown in figure Bl.16.5. In this example we will set the following conditions (1) the RP is initially a singlet ... 

Suppose B is increased slowly to 10 T. Each hyperfme level F is found to split into 2F + 1 levels, since the quantum mechanical rule of permitted projections on an external field vector comes into operation. These permitted projections can vary from zero to a maximum value of HF for F = 3 seven new lines are obtained. Further increases in F to 10 T leads to a decoupling of F into its components J and I (Fig. 11.10). For each projection of J on the field vector there are 2/ + 1 lines (—/...0...+7), giving altogether (2/ + 1)(27 + 1) lines. The splitting of the spectral lines in a weak magnetic field is called the Zeeman ect. 

In practice, most of the collisions of interest in cold molecule studies involve collision partners with internal structure. Many of the atoms and molecules of interest have unpaired spins, and molecules have vibrational and rotational levels. Even nuclear hyperfme splittings, which are small enough to be neglected in many areas of chemistry, are large enough to be important in cold molecule studies. It is thus crucial to be able to handle collisions between atoms and molecules with internal structure. Although methods based on classical trajectories have had considerable success for inelastic and reactive collisions at room temperature or higher [3], for cold atoms and molecules it is almost always necessary to treat the collisions quantum-mechanically. 

Sanchez, F.H., Torres, C.E.R., Van Raap, M.B.F. and ZeUs, LM. (1998) Tool induced contamination of elemental powders during mechanical milling. Hyperfme Interactions, 113, 269 77. 

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