Protons interaction with electrons

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Exposure of naphthalene dissolved in liquid ammonia to europium metal immediately results in the characteristic green color of naphthalene anion-radical. ESR analysis reveals a signal that comes from an unpaired electron interacting with Eu and Eu nuclei. No hyperfine coupling with naphthalene protons is observed, although treatment with water leads to 1,4-dihydronaphthalene (Stevenson et al. 1999). This means that naphthalene has indeed been reduced to its anion-radical and undergone a normal Birch reaction. These results are consistent with the initial donation of two... 

The similarity of the preceding first-order ESR treatment to the first-order NMR treatment of two coupled protons is evident. For an unpaired electron interacting with n equivalent nuclei of spin the hyperfine coupling term in the spin Hamiltonian is... 

For an unpaired electron interacting with both a set of m equivalent protons and a set of n equivalent protons, the first-order analysis is carried out in two stages, like the treatment in Section 8.5 of the CH2 portion of... 

Adenosine is similar to cytosine in its acid-base chemistry N-l, adjacent to the -NH2 group, is the principal site of protonation. A tautomer of the cation protonated at N-3 is formed in smaller amounts. Guanosine is electronically more complex, being protonated mainly at N-7 and to a lesser extent at N-329. This can be understood in terms of electronic interaction with the adjacent oxygen as indicated in the resonance structure to the right in the following diagram ... 

In the simple case of the electron interacting with a proton in a one-electron atom, the electric field at the electron due to the proton is... 

It is useful to discuss some preliminaries of quantum mechanics before the discussion of /-orbitals in lanthanides. Consider the simplest system namely, the hydrogen atom consisting of a single electron interacting with a proton. The Schrodinger equation for a particle of mass m, the electron in a central field produced by the nucleus is... 

The nuclear Zeeman term in Eq. (4) can be omitted in the following presentation, since it does not change for levels involved in an ESR transition (AM/ = 0 if AM5 = 1). Thus, for example, the energy levels for an unpaired electron interacting with two different protons can be written as... 

FIGURE 6.36 Two hydrogen atoms approach one another. The protons are separated by the distance Rab-(a) At large values of Rab each electron interacts only with the proton to which it is bound, (b) As the atoms approach closer, both electrons interact with both protons. The distance of electron 1 from nuclei A and B is given by /"lA, /"ib the distance of electron 2 from nuclei A and B is given by r2B, the distance between the electrons is given by... 

As the atoms begin to interact strongly, we cannot determine whether electron 1 arrived with proton A and electron 2 with proton B, or vice versa. (The electrons are indistinguishable.) Therefore, the wave function must allow for both possibilities ... 

If the nuclei were equivalent protons with aA = aB, then the resultant energy levels would be as shown in Fig. 6. The observed spectrum reveals a triplet of intensities 1 2 1. The extra intensity of the middle line, which arises from the transition (Af = — M, = 0) to (Af, =, Mt = 0), is because the transition is between doubly degenerate energy levels (AfA = +MB = -, MA = —, Mb = +whereas the outer lines represent transitions between non-degenerate nuclear energy levels. Generally, for an electron interacting with n nuclei of spin, the number of ESR transitions observed is equal to... 

IX. SCALAR INTERACTIONS WITH PROTONS In the earlier Sections we indicated that most measurements of the proton electron Overhauser effect that have been reported may be interpreted in terms of dipolar interactions between the proton and the free-radical electrons the proton polarization is inverted. On the other hand, the results with nuclei show that there exists a dominant scalar interaction that often results in positive enhancement. 

Cluster ions can be formed by photon or electron interaction with a neutral cluster produced in a supersonic expansion [1321. Another process restricted to clusters is ligand-switching or the replacement of one ligand for another. Often exothermic ligand-switching reactions take place at rates near the gas kinetic limit, especially for small values of n [72, 133]. Chemical-reactivity studies as a function of cluster size show a variety of trends [93, 127. 133]. Proton-transfer reactions are often unaffected by solvation, while nucleophilic-displacement reactions are often shut down by as few as one or two solvent molecules. 

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