Spin species

BalKSO Balasubramanian, K. Graph theoretical characterization of NMR groups, non-rigid nuclear spin species and the construction of symmetry adapted NMR spin functions. J. Chem. Phys. 73 (1980) 3321-3337. 

BalK83a Balasubramanian, K. Operator and algebraic methods for NMR spectroscopy I. Generation of NMR spin species. J. Chem. Phys. 78 (1983) 6358-6368. 

In high-spin species (to which the theorem is not restricted), perusal of the appropriate configurations for octahedral complexes across the transition period (see Fig. 5-1) shows that d" and configurations are candidates for the Jahn-Teller... 

Figure 8-5. Electron configuration for octahedral, high- and low-spin species.

Alkyliron(lll) porphyrin complexes are air. sensitive, and when exposed to oxygen under ambient conditions the products are the very stable iron(IIl) /t-oxo dimers, [Fe(Por)]20. A more careful investigation revealed that the reaction of the alkyl complexes with oxygen proceeds via insertion of O2 into the Fe—C bond. " When a solution of Fe(Por)R (R = Me, Et, i-Pr) is exposed to O2 at —70 C, the characteristic H NMR spectrum of the low spin iron alkyl complex disappears and is replaced by a new, high spin species. The same species can be generated from the reaction of an alkyl hydroperoxide with Fe(Por)OH, and is formulated as... 

Co(III) is capable of oxidising Hg2 , a result which has led Rosseinsky to speculate on the ability of Co(III) to react via a high-spin species which can t2g-overlap with Hg2 . 

The "decrease of the spin temperature means an increase of population difference between the upper and lower energy spin states and consequently an increased sensitivity of the NMR experiment. From Equation (25), the temperature of dilute spins has been lowered by a factor 7x/y1 h, that is, V4 when X = 13C. This means an increased sensitivity of the FID resonance experiment equal to about 4 for the 13C nuclei. Because the X signal is created from the magnetization of dilute nuclei, the repetition time of NMR experiment depends on the spin-lattice relaxation time of the abundant spin species, protons, which is usually much shorter than the spin-lattice relaxation times of the dilute nuclei. This, a further advantage of cross polarization, delay between two scans can be very short, even in the order of few tens of milliseconds. 

Because the d5 configuration is spherically symmetric, high-spin Mn(ii) and Fe(m) usually have nearly isotropic -matrices and Mn(ii) usually has a nearly isotropic -matrix. This means that there usually is not much information in the ESR spectrum of these high-spin species. Indeed, high-spin Mn(n) is usually an unwanted interference for those interested in low-spin Mn(n) the ESR spectrum is very characteristic with six hyperfine lines with a coupling constant of 80-100 G. Because the g- and -matrices are nearly isotropic, the six-line spectrum persists in frozen solutions. 

If we were to have an isolated polymer chain with a single nuclear spin attached to each segment (the marked chain) crosslinked into an unmarked network, the second moment of the NMR line of that spin species would carry information relating to the separation of chain segments, and to their relative orientation with respect to the field direction. If the network were to be subjected to a bulk deformation, these geometrical parameters would be altered, and hence we would expect a corresponding change in the value of the experimentally measured... 

The REDOR experiment (Fig. la), introduced by Gullion and Schaefer in 1989 [22], invokes a strong n pulse for one of the spin species (e.g., the I spin in an I-S spin system) in the middle of each rotor period xr - in addition to one n pulse at each rotor echo on the 5-spin channel to refocus isotropic chemical shift effects and... 

Finally, we should also mention that assisted spin recoupling is by no means restricted to 1H, 13C, and 15N spin species but may also be used to facilitate H and 2H to low- spin transfers. This may be exploited in extensively deuterated samples using rf fields on JH and 2H which are matched to a rotary resonance condition to facilitate spin-diffusion between 13C spins in a longitudinal mixing process. This was recently demonstrated by van Rossum and coworkers in terms of the doublenucleus enhanced recoupling (DONER) experiment [143]. 

For the high spin species formed on the loss of water, abrupt transitions to the low spin state are observed below room temperature. For the anhydrous iodide and tetrafluorob orate and for the triflate monohydrate these... 

With a single methyl substituent 66 (Dq(Ni2+)=1060 cm l) yields a completely high spin species, as does the system with two substituents 67 (Dq(Ni2+)=900 cm 1) [106]. 

These indices have been used to study the reactivity for a series of chlorobenzenes and a good correlation is observed, for example, between W and toxicity of chlorobenzene [41]. For a detail discussion of this concept and its applications, we refer the readers to a recent review [41,42]. For studying intramolecular reactivity, these philicity indices and local softness contain the same information as obtained from the Fukui functions, because they simply scale the Fukui functions. In some cases the relative electrophilicity and relative nucleophilicity may be used although they provide similar trends as s(r) and co(r) in most cases [43]. In the same vein, the spin-donicity and spin-philicity, which refer to the philicity of open-shell systems [44], could also be utilized to unravel the reactivity of high-spin species, such as the carbenes, nitrenes, and phosphinidenes [45]. 

This formalism allows us to write easily the equilibrium conditions in the case of assemblies of several spin species. Let us consider for instance the case of two magnetic ingredients. The Hamiltonian is... 

Fe(III)-peroxo porphyrin complexes comprising ligands of different electronic properties were extensively investigated by Valentine et al. (4,58) as synthetic analogues of intermediates that might occur during enzymatic reactions. All these complexes are high-spin species. 

Transient bleaching of low spin species followed by thermal relaxation t s 46-192 ns at 255-205 K... 

In this type of spin traps, 5,5-dimethyl-l-pyrroline-Af-oxide (DMPO) deserves particular mention. DMPO is widely employed as a spin trap in the detection of transient radicals or ion-radicals in chemical and biological systems (see, e.g., Siraki et al. 2007). Characteristic ESR spectra arising from the formation of spin adducts are used for identification of specific spin species. In common opinion, such identification is unambiguous. However, in reactions with superoxide ion (Villamena et al. 2004, 2007b), carbon dioxide anion-radical (Villamena et al. 2006), or carbonate anion-radical (Villamena et al. 2007a), this spin trap gives rise to two adducts. Let us consider the case of carbonate anion-radical. The first trapped product arises from direct addition of carbonate anion-radical, second adduct arises from partial decarboxylation of the first one. Scheme 4.25 illustrates such reactions based on the example of carbonate anion-radical. 

The resting state of HRP C is high-spin Fe(III) (d ) with a magnetic susceptibility of 5.23 Bohr magnetons (96). However, it cannot be described adequately by the spin quantum number S =, which would correspond to a high-spin species with five unpaired d electrons and a... 

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