Spin population distribution, determination

To determine the spin population distribution, we (32) performed semi-empirical (MNDO) calculations on the radical anion of FN with and without two water molecules hydrogen bonded to the nitro group. The optimized bond lengths are shown in Figures 2-4 (the geometry of the water molecules was not optimized and the NO HOH separation was arbitrarily set to 2.0 A). The MNDO coefficients of the SOMO on C-9 and C-a are given in Table I. Geometry optimization of FN (the neutral molecule) was performed with the aromatic portion of the molecule kept planar. The results show that the rest of the molecule is coplanar (within 1°) with the aromatic moiety. No... 

Mouesca JM, Rius G, Lamotte B. 1993. Single-crystal proton ENDOR studies of the [Fc4S4] cluster determination of the spin population-distribution and proposal of a model to interpret the H-NMR paramagnetic shifts in high-potential ferredoxins. J Am Chem Soc 115 4714 731. 

Considering a five-point spin density distribution (central ion and four nitrogens) for the determination of the dipolar proton hfs tensors in Ag(TPP) (5.5), the computed ADD principal values are found to be close to the experimental results. It should be noted that in Ag(TPP) the Mulliken population, UN, on the nitrogen nearest to the pyrrole proton provides a larger contribution to ADD along the Ag-H direction than the population UAg. 

Anodic oxidation of 4-dimethylaminoantipyrene in acetonitrile has yielded a cation-radical (165), the ESR spectrum of which indicates the spin population is mainly exocyclic on the tertiary amino group. The second-order rate constant for the decay of 165 was determined HMO calculation of the spin distribution indicates twist in the C—NMe2 bond.551... 

The relative population of each spin state is determined by the Boltzmann distribution, Eq. (2.8). Under conditions of a typical NMR experiment the ratio of spin state populations is near unity, differing only by a few parts per million. 

The X-ray and neutron diffraction data mentioned previously have been used in conjunction with the technique of polarized neutron diffraction (at 4.2 K) to deduce spin-density distributions in [MnPc].519,520 In further investigations514 it proved possible to determine individual 3d and 4s orbital populations on the manganese ion together with an estimate of 24% for the d-orbital electron density delocalized in the macrocyclic ring. From these studies it appears that the charge on the manganese is approximately +1 this charge appears to be achieved primarily by the loss of a 3d electron rather than a 4s electron. 

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

One such analysis in the review period involves the characterization of the rotation of the methyl groups in pyridoxine (vitamin B6).49 The temperature dependencies of the 1H spin-lattice relaxation time T, and Tld (the relaxation time constant characterizing the relaxation of dipolar order, a population distribution over the Zeeman spin levels, which corresponds to a density operator component T20, he. I z - Ii. I2, to equilibrium) at three different applied field strengths and for a variety of temperatures were determined, yielding the curves in Fig. 30. The only motion that could affect... 

Very interesting information from high-resolution ESR or from ENDOR spectroscopy concerns the distribution of the unpaired electron in the paramagnetic molecular cation the spin population can be determined experimentally from the electron/nuclear spin coupling, whereas the charge distribution has to be approximated by (open-shell) calculations. As concerns Me3SiCH2-substituted n radical cations, the spin population pn... 

Two ongoing experiments are described. In both experiments optical pumping is employed as a means of creating non-equilibrium spin distributions. In both cases the spins couple with one another via either nuclear or electron spin-spin interaction. The first experiment is to measure spatial transport of spin orientation in order to determine the mean spin-spin cross-relaxation rate between Tm" ions in SrF2. In the second experiment transient changes in spin population are observed as Pr" "3 spin levels come into resonance with neighboring F spin levels. This monitors the cross-relaxation rate between Pr" "3 and F and provides a means for verifying the spectroscopy of the Pr" "3 ions. 

Fig. 15. Population distribution of the instant orientation of the effective nitroxide rotational tensor symmetry axis with respect to the director as characterized by the ordering potential parameters c, C, and determined from the ESR spectra of spin-labeled PHEMA in methanol at 50 wt% concentration measured at (a) 191 K, (b) 221 K, and (c) 249 K. The magnitude of the ordering potential decreases with increasing temperature as seen in Fig. 14. (From Ref. 44, with permission.)...

Direct reading of the line positions shows fliat A = [4 /2, and flierefore 0. Numerical simulations that reproduce flie observed linewidth yielded the following hfi parameters fliso < 0.01 MHz T = -2.70 0.01 MHz. In order to reproduce the ENDOR linewidth, the anisotropic hfi constant was Gaussian-distributed with a width between the maximum slope points of ATi = 0.35 MHz. The spectrum simulated using these parameters is shown by open circles in Figure 1 lb. Assuming that the anisotropic hfi is fully determined by the spin population on the central Gd(lll) ion, one can estimate flie Gd-proton distance as... 

It should be noted that the decomposition shown in Eq. 3.7.2 is not necessarily a subdivision of separate sets of spins, as all spins in general are subject to both relaxation and diffusion. Rather, it is a classification of different components of the overall decay according to their time constant. In particular cases, the spectrum of amplitudes an represents the populations of a set of pore types, each encoded with a modulation determined by its internal gradient. However, in the case of stronger encoding, the initial magnetization distribution within a single pore type may contain multiple modes (j)n. In this case the interpretation could become more complex [49]. 

Thermal distributions of NO(u 2, J, A Ej,) states were observed, wherein the population in any level was determined by the internal energy and the parameter Tr. and independent of spin-orbit state or lambda doublet species. This is in contrast to the rotational rainbows, the propensities for preferential population in the Il(A ) lambda doublet species and the Fj spin orbit state which were observed in direct inelastic scattering of NO/Ag(l 11). 

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