Spin polarization pattern

Takamori et al The CIDEP spectra observed in the case of 1,4-naphthoquinone in ot- and y-cyclodextrins show the main formation of naph-thaquinone radical-anions whereas in the case of p-cyclodextrins the neutral naphthosemiquinone radicals dominate the EPR spectra. The carbon-centred radicals from the cyclodextrins are also identified in all cases. The spin polarization patterns of all spectra prove that the reaction takes place via the excited triplet state of naphthaquinone, and the hydrogen abstraction reaction from the inside of the framework of the cyclodextrins is the initial step of the photoreduction of 1,4-naphthoquinone. Similar results were obtained for 2-methyl-1,4-naphthaquinone. 

Figure Bl.16.22 shows a stick plot siumnary of the various CIDEP mechanisms and the expected polarization patterns for the specific cases detailed in the caption. Each mechanism clearly manifests itself in the spectrum in a different and easily observable fashion, and so qualitative deductions regarding the spin multiplicity of the precursor, the sign of Jin the RP and the presence or absence of SCRPs can innnediately be made by examining the spectral shape. Several types of quantitative infonnation are also available from the spectra.

For more complex spin systems, a computer program PHIP+ has been developed [13, 45] which allows the expected PHIP spectra to be calculated from the chemical shifts and coupling constants of the products. Depending upon which proton pair in the product molecule stems from p-H2, different - but characteristic - polarization patterns result [14]. The patterns also depend on the sign of the coupling constants. Simple sign rules governing the relative sequence of the emission and absorption lines in the PHIP spectra (i.e., their phase ) can be formulated in similar manner to the Kaptein Rules of chemically induced dynamic nuclear polarization (CIDNP) [15]. 

The effects observed for 23 (Figure 8) are particularly clear-cut, since the spectrum is fully resolved." The key to the structure of 23 lies in the prominent enhanced absorption signals of H3 (5.2 ppm) and (-0.2 ppm) and the strong emission of (2,4 ppm), H4 (2.2 ppm), and H5 (1.5 ppm). This polarization pattern supports a spepies with spin density on C3 and C6, indicating the delocalization of spin and charge into the lateral cyclopropane bond. Weakly enhanced absorption observed for H2 (5.8 ppm), (0.8 ppm), and Hi (1.75 ppm), and weak emission for H5 further support this structure type. ... 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

In the spin-correlated RP the two radicals interact via electron-electron dipolar and exchange interaction which leads to line splitting. The ET process creates the RP in a strongly spin-polarized state with a characteristic intensity pattern of the lines that occur either in enhanced absorption (A) or emission (E).144 145 The spectrum is therefore very intense and can directly be observed with cw EPR (transient EPR) or by pulse methods (field-swept ESE).14 To study the RPs high field EPR with its increased Zeeman resolution proved to be very useful the first experiment on an RP was performed by Prisner et al. in 1995146. From the analysis of the RP structure detailed information about the relative orientation of the two radicals can be extracted from the interaction parameters. In addition kinetic information about the formation and decay of the RP and the polarization are available (see references 145,147). 

The case of the hexa(pyridine) nickel(II) complex has been extensively debated in the early literature of NMR of paramagnetic complexes [17-21]. The shift pattern with a-H > y-H > (i-H (Fig. 2.11 and Table 2.3) was soon recognized to be predominantly of a-type. The ligand has a a MO system which has the correct symmetry to overlap with the dx2 y2 and dz2 orbitals. However, spin polarization can induce V2 spin density in the it system. Once some unpaired spin density is in a p orbital, it spin-polarizes the electrons of the C—H a bond, thus producing a further mechanism for transferring spin density on the proton. The proton A/h value from this mechanism is proportional to the spin density on the carbon pz orbital, p , through a proportionality constant >ch ... 

The ease of cyclobutane cleavage and the detailed mechanism can be affected by the nature of the substituents and the substitution pattern. While the above cyclobutanes are cleaved without a discernible intermediate, the n//-head-to-head dimer of dimethylindene shows a significantly different behavior. This substrate is cleaved in an apparent two-step process, involving a ring-closed radical cation (with spin and charge localized on one indan system) and a ring-opened 1,4-bifunctional radical cation. Apparently, the cleavage of the doubly benzylic cyclobutane bond is reversible. The involvement of more than one dimer radical cation is indicated by a unique polarization pattern (Fig. 13), which is incompatible with any one intermediate, but can be simulated on the basis of two successive radical cations (see Sect. 5.2) [256]. 

The first structure type to be established for a hexadiene radical cation was one in which cleavage is achieved without bonding, i.e. a representative of the dissociative mechanism. Dicyclopentadiene and several derivatives can be oxidized to radical cations (137) in which one of the bonds linking the monomer units is cleaved. The unique spin density distribution of 137 is reflected in an unmistakable polarization pattern [386-389]. 

Extensive studies of the sensitizer dependence and the solvent dependence of the polarization patterns led to the identihcation of two parallel pathways of that deprotonation. One is a proton transfer within the spin-correlated radical pairs, with the radical anion A acting as the base. The other is a deprotonation of free radicals, in which case the proton is taken up by surplus starting amine DH. Furthermore, evidence was obtained from these experiments that even in those situations where the polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6 these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by deprotonation of the radical cation by either of the described two routes. The whole mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited for an analysis is the product V. 

These case studies illustrate the power of CIDNP spectroscopy. Short-lived paramagnetic intermediates can be identified because their EPR spectrum remains frozen in as a polarization pattern of the nuclear spins in much longer-lived secondary species, and the pathways of their subsequent reactions can be traced out because these polarizations behave as nearly ideal labels. As the examples have shown, transformations of radical pairs into other radical pairs, with or without the participation of a third molecule as a scavenger, and transformations of biradicals can all be investigated by this method, which yields information that is often inaccessible by other techniques. 

The spectra presented in Fig. 12.7 and 12.8 demonstrate that a polarization pattern is preserved during a fast addition of radicals to a monomer. Absorptive (A) polarization observed under direct photolysis of Pis (Fig. 12.2) results in the A pattern of a spectrum of the spin adduct (Fig. 12.7a), and the E/A pattern observed under sensitized photolysis (Fig. 12.3) results in an E/A pattern of a spectrum of the... 

In CIDNP studies the pattern of nuclear spin polarizations is used to deduce the structures of free radical intermediates, and if there is a simultaneous NOE this can affect the conclusions drawn. (344) In the photolysis of [10] to yield [12] the biradical intermediate [11] should not cause significant polarization of the olefinic protons of [10] but in fact these are observed to have weak emission. This raises the possibility that the intermediate is really the biradical [13], but a homonuclear double resonance experiment which destroyed the cyclopropyl spin polarization of [10] eliminated the olefinic emission which was evidently solely due to the NOE. Thus it is confirmed that [11] is indeed the intermediate in the reaction. (344)... 

The effects observed for bicyclo[3.1.0]hex-2-ene, 86, were particularly clear-cut, because the H spectrum is fully resolved [218]. The polarization pattern supported a species with spin-density on C3 and C6, indicating the delocalization of spin and charge into the lateral cyclopropane bond. The bicyclohexene system has limited mobility, enabling more significant orbital overlap of the lateral cyclopropane bond with the alkene p-orbitals (86 +). The participation of the lateral bicyclohexene bond is supported by ab initio calculations, carried to the MP2/6-31G level of theory. The lateral cyclopropane bond is lengthened (C1-C6 = 1.748 A), and carbons C3 and C6 carry prominent spin densities, whereas lower spin densities were found at C2 and Cl [220]. 

In all covalent electron donor-acceptor systems produced earlier, triplet states observed by EPR were formed via a spin-orbit intersystem crossing (SO-ISC) mechanism. Another possible mechanism of triplet formation is RP-ISC, mentioned above, which results from radical ion pair recombination, and which had been observed previously by time-resolved electron paramagnetic resonance spectroscopy (TREPR) only in bacterial reaction centers and in the green plant Photosystem I and II reaction centers. These two mechanisms can be differentiated by the polarization pattern of the six EPR transitions at the canonical orientations. In SO-ISC,... 

A normal emission/absorption/emission/absorption pattern that did not vary in the time interval 0.7-5 ps was observed for the radical pairs from 35 and 36, while the spectrum from 34 was initially totally absorptive (0.4-0.6ps), then rapidly changed to emissive/absorptive (1.9-2.1 ps), and eventually became totally emissive (4.6-5 ps).37 It is suggested that for ZP4V, due to the smaller spacer chain, the time elapsed between laser excitation and radical-pair formation was shorter than for ZP6V and ZP8V and the spin polarization in the porphyrin triplet was retained to a larger extent before electron transfer took place.37... 

Time-resolved EPR spectroscopy is particularly useful for selective observation of triplet species. By the use of the zero-field splitting parameters D and j obtained from the spectral pattern of each triplet species, the actual conformation can be predicted reasonably well, when those parameters are simulated by theoretical calculation. The time-dependent changes in the spin polarization along the x, y, and z principal axes can provide information concerning the spin-orbit coupling in each direction. 

This model has been used for the description of Arrhenius curves of H-transfers as described in more detail in Chapter 6. The next two chapters show applications of these symmetry effects. First the para-hydrogen induced polarization (PHIP) experiments are discussed. There the symmetry induced nuclear spin polarization creates very unconventional NMR lineshape patterns, which are of high diagnostic value for catalytic studies. Then in Section 21.4 symmetry effects on NMR line-shapes and relaxation data of intramolecular hydrogen exchange reactions are discussed and examples from iH-liquid state and H-solid state NMR are presented and compared to INS spectra. The last section gives an outlook on possible future developments in the field. 

The transformation of this molecular rotational order into nuclear spin order during the hydrogenation reaction leads to typical polarization patterns in the NMR spectra of the hydrogenation products. Depending on whether the experiment is performed inside or outside of a magnetic field (see Fig. 21.6), these types of experiments have been referred to under the acronyms PASADENA (Para-hydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment) or... 

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