Electron-proton interactions

The multiplet hyperfine pattern of ESR lines in organic radicals is most frequently due to electron-proton interactions, but other nuclei with nonzero spin may also cause hyperfine structure. Weak satellite lines arising from interactions between the unpaired electron and C13 nuclei are sometimes observed C13 has and the analysis is straightforward. Nitrogen-containing radicals may show hyperfine splittings due to N14, which has 7 1. The possible Mj values are 1, 0, and -1, so that an electron... 

The powder EPR signal is dominated by a hyperfine doublet due to the interaction between the trapped electron and a single proton ( H, I = 1/2). The H hyperfine couplings can be more precisely determined by ENDOR, with values of A] = 2.07G, A2 = 2.00G, A3 = 0.31 G [23]. These hyperfine parameters indicate that the local symmetry of the site is lower than axial for a purely axial system, the hyperfine parameters should take the form A] = A2 = Aj and A3 = Ay. Although the difference between A] and A2 is small, the slightly rhombic nature of the parameters is very important and extremely informative. The magnitude of these hyperfine couplings also indicates that the electron-proton interaction is weak. 

Originally the three-spin effect was postulated to explain the positive enhancements observed for certain fluorine nuclei in solvents also containing protons.The possibility arose that the fluorine nuclei were being influenced by the proton polarization as well as by the direct coupling to the electron spins. The basic idea behind the three-spin effect can be seen by considering the transitions among the Zeeman energy levels shown in Fig. 20. Since the electron—proton interaction is of the dipolar kind, the predominant coupled transition will be an... 

Fig. R.2. The hydrogen molecule in the simplest basis set of two Is STOs. (a) The overlap integral 5 as a function of the internuclear distance R (b) the penetration energy represents the difference between the electron-proton interaction calculated assuming the electronic charge distribution and the same energy calculated assuming the point charges (the electron is located on the nucleus c) (c) the energies + and E- of the bonding (lower curve) and of the antibonding (upper curve) orbitals, respectively.

Electrons, protons and neutrons and all other particles that have s = are known as fennions. Other particles are restricted to s = 0 or 1 and are known as bosons. There are thus profound differences in the quantum-mechanical properties of fennions and bosons, which have important implications in fields ranging from statistical mechanics to spectroscopic selection mles. It can be shown that the spin quantum number S associated with an even number of fennions must be integral, while that for an odd number of them must be half-integral. The resulting composite particles behave collectively like bosons and fennions, respectively, so the wavefunction synnnetry properties associated with bosons can be relevant in chemical physics. One prominent example is the treatment of nuclei, which are typically considered as composite particles rather than interacting protons and neutrons. Nuclei with even atomic number tlierefore behave like individual bosons and those with odd atomic number as fennions, a distinction that plays an important role in rotational spectroscopy of polyatomic molecules. 

Chemical ionization (Cl) The formation of new ionized species when gaseous molecules interact with ions. This process may involve the transfer of an electron, proton, or other charged species between the reactants in an ion-molecule reaction. Cl refers to positive ions, and negative Cl is used for negative ions. 

This is the beauty of this quantity which provides specifically a direct geometrical information (1 /r% ) provided that the dynamical part of Equation (16) can be inferred from appropriate experimental determinations. This cross-relaxation rate, first discovered by Overhau-ser in 1953 about proton-electron dipolar interactions,8 led to the so-called NOE in the case of nucleus-nucleus dipolar interactions, and has found tremendous applications in NMR.2 As a matter of fact, this review is purposely limited to the determination of proton-carbon-13 cross-relaxation rates in small or medium-size molecules and to their interpretation. 

Clearly, the potential to use spin labels as a means to reduce protein concentration for detection of protein-ligand interactions, given by the factor of 6582, is tremendous. The sixth-power dependence on electron-proton distance underlines the need to carefully design the residue type which is to be spin labeled. Residues that can be spin labeled include lysine, tyrosine, cysteine, histidine, and methionine [7, 14]. At least one residue of... 

Figure 2A shows a pulsed ENDOR spectrum of an oxo-Cr(V) complex where the unpaired electron is interacting with a 1H nucleus with principal hyperfine values Hax = Hay = — 2 MHz and 1 a- 5 MHz. In this case, the isotropic hyperfine value is Haiso = (%+ % + V)/3 = 0.33 MHz ( 0.11x10 4 cm ), a value that is not resolved in the CW-EPR spectrum. The hyperfine contribution of this proton is, however, clearly resolved in the ENDOR spectrum. The ENDOR spectrum is centered around the proton Larmor frequency (vH), identifying the contribution as stemming from an interaction with a H nucleus. The principal values can be read directly from the spectrum, as indicated in Fig. 2A. 

Alkyl Free Radicals. The primary alkyl free radical, RCH./, has been postulated to exist in gamma-irradiated polyethylene (23), but its presence has never been unambiguously demonstrated. If all the a and p protons interacted equally with the unpaired electron, the primary alkyl free radical ESR spectrum should consist of five lines, in contrast to those of the secondary and tertiary alkyl radicals whose ESR spectra should consist of six and eight lines, respectively (the latter calculated for the tertiary free radical in polypropylene). The existence of the secondary... 

Another kind of particle and another kind of interaction were discovered from a detailed study of beta radioactivity in which electrons with a continuous spectrum of energies are emitted by an unstable nucleus. The corresponding interactions could be viewed as being due to the virtual transmutation of a neutron into a proton, an electron, and a new neutral particle of vanishing mass called the neutrino. The theory provided such a successful systematization of beta decay rate data for several nuclei that the existence of the neutrino was well established more than 20 years before its experimental discovery. The beta decay interaction was very weak even compared to the electron-photon interaction. 

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