Hyperfine transfer

Collisions between I and closed-shell collision partners (He, Ar, and N2) did not relax the nonequilibrium hyperfine distribution to any measurable degree. But, in accord with the theoretical picture outlined above, hyperfine relaxation was observed for collisions with 02(A). For example, the shift towards equilibrium can be seen in Fig. 9 by comparing traces recorded in the absence and presence of O2. Hyperfine transfer induced by collisions with O2 was examined at the temperatures T = 295, 150, and 10 K. For T > 150 K, I quenching and hyperfine transfer occurred at simUar rates. Hence, both processes were considered in the kinetic model used to extract the rate constants. At the lowest temperature (10 K) quenching was negligible. The hyperfine transfer rate constants were found to be independent of temperature, within the experimental errors. Values oi k F = 2 F = 3) = (2.3 0.5) X 10- and fc(F = 3 -> F = 2) = (1.6 0.5) x 10" cm s... 

The measured hyperfine transfer rate constants were smaller than previous estimates. Cerny et al reported a value for k F = 3 F = 2) of 1.5 X cm s (T = 300 K), derived from an analysis of spectral... 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

The above-mentioned set of Ki values are deduced from analyses based on the hyperfine constants of mononuclear FeS4 centers. However, we have already stressed that local parameter values are not necessarily transferable from one type of iron—sulfur center to an-... 

EPR techniques were used to show (Polyakov et al. 2001a) that one-electron transfer reactions occur between carotenoids and the quinones, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), and tetrachlorobenzoquinone (CA). A charge-transfer complex (CTC) is formed with a -values of 2.0066 and exists in equilibrium with an ion-radical pair (Car Q ). Increasing the temperature from 77 K gave rise to a new five-line signal with g=2.0052 and hyperfine couplings of 0.6 G due to the DDQ radical anions. At room temperature a stable radical with y=2.0049 was detected, its... 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

A detailed study of the C02- species on MgO has been carried out by Lunsford and Jayne 26). Electrons trapped at surface defects during UV irradiation of the sample are transferred to the CO2 molecule upon adsorption. By using 13C02 the hyperfine structure was obtained. The coupling constants are axx - 184, am = 184, and a = 230 G. An analysis of the data, similar to that carried out in Section II.B.2 for N02, indicates that the unpaired electron has 18% 2s character and 47% 2p character on the carbon atom. An OCO bond angle of 125° may be compared with an angle of 128° for CO2- in sodium formate. 

Electron spin resonance, nuclear magnetic resonance, and neutron diffraction methods allow a quantitative determination of the degree of covalence. The reasonance methods utilize the hyperfine interaction between the spin of the transferred electrons and the nuclear spin of the ligands (Stevens, 1953), whereas the neutron diffraction methods use the reduction of spin of the metallic ion as well as the expansion of the form factor [Hubbard and Marshall, 1965). The Mossbauer isomer shift which depends on the total electron density of the nucleus (Walker et al., 1961 Danon, 1966) can be used in the case of Fe. It will be particularly influenced by transfer to the empty 4 s orbitals, but transfer to 3 d orbitals will indirectly influence the 1 s, 2 s, and 3 s electron density at the nucleus. 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

The shifts in the 113Cd spectra due to the electron hyperfine interaction from an Mn++ ion two bonds distant could then be used to obtain a transferred hyperfine interaction constant A that could be compared to the value obtained from ESR measurements. For 125Te, the nearest neighbor Mn++ apparently broadened the NMR signal beyond detectability the transferred hyperfine interaction constant A for Mn++ ions in the second shell was calculated from the 125Te shifts. 

The 113Cd Ti values estimated for the various peaks varied from 10 to 50 ms and obeyed the qualitative dependence upon 1/R6 (R = Mn-Cd distance) of the dipolar relaxation mechanism expected to be operative. The broad line widths were also shown to have significant contributions from the T2 relaxation induced by Mn++, with both dipolar and contact terms contributing. The 113Cd shifts of the peaks assigned to different shells were measured as a function of temperature, and observed to follow a linear 1/T dependence characteristic of the Curie-Weiss law, with slopes proportional to the transferred hyperfine interaction constant A. 

The electron hyperfine interaction thus has important effects on both NMR relaxation and frequency shifts, and can provide valuable information on the incorporation of magnetic ions into semiconductor lattices and the resulting electronic structure as characterized by transferred hyperfine constants. Examples in Sect. 4 will show how the possible incorporation of magnetic ions into semiconductor nanoparticles can be studied by NMR. 

Superoxide ions, 02, are readily formed by the transfer of electrons from Fs centers on MgO or from Mo(V) on Mo/Si02 to molecular oxygen (7, 9). The value of g3 for 02 is particularly sensitive to the crystal field gradient at the surface and thus varies from one metal oxide to another (10). In fact, the spectrum of 01 on MgO indicates that the ions are held at four distinctly different sites (11,12). The oxygen-17 hyperfine splitting (Table I) for 170170- on MgO confirms that both oxygen atoms are equivalent, on supported molybdenum the atoms are nonequivalent, suggesting a peroxy-type bond to the metal (7,13). 

Excited State Magnetic Moment. The magnetic moment of the Te 35.6-k.e.v. level has been measured by Shikazono (13) and Hunt-zicker et al, (8). Sources were prepared by diffusing Sb into metallic Fe foils. The Te impurities produced by the decay of Sb experience a hyperfine field at the Te nucleus transferred from the ferromagnetic... 

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