Nuclear hyperfine reaction

It has been shown that triphenyl(p-cyanobenzyl)phosphonium tetrafluoroborate (16), which exhibits a a LUMO level localized predominantly on the heteroatom and benzylic carbon, gives products derived from out-of-solvent cage chemical reactions on direct irradiation (reaction 6). This behaviour is connected with the nuclear hyperfine coupling constant of the heteroatom in triphenylphosphine radical-cation171. 

Ammonium, phosphonium and arsonium salts may also possess a a LUMO and undergo PET bond cleavage reactions to provide products that depend largely upon the heteroatom nuclear hyperfine coupling constant [93]. 

In addition to the spatial structure of the reaction center (RC) of Rhodopseudomonas (Rps.) viridis obtained from X-ray crystallography (1) a knowledge of the electronic structure is required for a basic understanding of the functional details of the RC. This can be obtained for the radical ions formed in the charge separation process by EPR and ENDOR techniques (2) which yield electron-nuclear hyperfine couplings (hfc s). From the hfc s a map of the valence electron spin distribution over the molecule is obtained. In this work we studied the intermediate electron acceptor radical anion I", a monomeric bacteriopheophytin (BPh) b (3)> Fig. 1,that was trapped at low temperature (77 K) in the RC ( ). EPR and ENDOR results on I" had been reported earlier (5). Improved instrumental design enabled us to measure additional hfc s with higher accuracy. 

Thus far, we have established that the nuclear hyperfine parameters of the spin Hamiltonian are desirable for assessing the chemically interesting problem of structure-funetion eorrelation and reaction control. The advanced EMR methods known as ENDOR and ESEEM best recover this information from samples in which the ehemieal agent of interest is paramagnetic, and, in principle, there are methods that enable the spectroscopist to cope with the sometimes pathologieal behavior of spin systems, in other word, coax a spectrum out of a sample. In this section, however, we shall address the question of whether there is neeessary and sufficient information in a single ENDOR or ESEEM spectrum and how to design an experimental approaeh that enables one to fully parameterize the spin Hamiltonian. 

The assignment of the specific peaks to specific types of free radicals is not always obvious it sometimes involves sophisticated techniques and the knowledge of related spectra and of chemical reactions to be expected. The main obstacles to be overcome are the broad widths of resonance lines from polymers and the high rates for many radical reactions. A broad linewidth often prevents an effective resolution of nuclear hyperfine structure. Other limitations have been cited [12-15]. The ESR technique nonetheless can offer an incisive insight into mechanochemistry. 

The rates of the electron transfer processes in reaction centers (RC s) of photosynthetic bacteria are controlled both by the spatial and the electronic structure of the involved donor and acceptor molecules. The spatial structure of bacterial RC s has been determined by X-ray diffraction for Rhodopseudomonas (Rp.) viridis and for Rhodobacter (Rb.) sphaeroides,- The electronic structure of the transient radical species formed in the charge separation process can be elucidated by EPR and ENDOR techniques. The information is contained in the electron-nuclear hyperfine couplings (hfc s) which, after assignment to specific nuclei, yield a detailed picture of the valence electron spin density distribution in the respective molecules. 

The low-temperature EPR experiments used to determine the DNA ion radical distribution make it very clear that electron and hole transfer occurs after the initial random ionization. What then determines the final trapping sites of the initial ionization events To determine the final trapping sites, one must determine the protonation states of the radicals. This cannot be done in an ordinary EPR experiment since the small hyperfine couplings of the radicals only contribute to the EPR linewidth. However, detailed low-temperature EPR/ENDOR (electron nuclear double resonance) experiments can be used to determine the protonation states of the low-temperature products [17]. These proto-nation/deprotonation reactions are readily observed in irradiated single crystals of the DNA base constituents. The results of these experiments are that the positively charged radical cations tend to deprotonate and the negatively charged radical anions tend to protonate. 

Nuclei provide a large number of spectroscopic probes for the investigation of solid state reaction kinetics. At the same time these probes allow us to look into the atomic dynamics under in-situ conditions. However, the experimental and theoretical methods needed to obtain relevant results in chemical kinetics, and particularly in atomic dynamics, are rather laborious. Due to characteristic hyperfine interactions, nuclear spectroscopies can, in principle, identify atomic particles and furthermore distinguish between different SE s of the same chemical component on different lattice sites. In addition to the analytical aspect of these techniques, nuclear spectroscopy informs about the microscopic motion of the nuclear probes. In Table 16-2 the time windows for the different methods are outlined. 

Let us first discuss a system which is traditional for optical pumping in the Kastler sense [106, 224, 226], namely an optically oriented alkali atom A (see Fig. 1.1) in a noble gas X buffer surrounding. It is important to take into account the fact that in alkali atoms, owing to hyperfine interaction, nuclear spins are also oriented. However, in a mixture of alkali vapor with a noble gas alkali dimers A2 which are in the 1SJ electronic ground state are always present. There exist two basic collisional mechanisms which lead to orientation transfer from the optically oriented (spin-polarized) atom A to the dimer A2 (a) creation and destruction of molecules in triple collisions A + A + X <—> A2 + X (6) exchange atom-dimer reaction... 

Technetium(II) complexes are paramagnetic with the d5 low-spin configuration. A characteristic feature is the considerable number of mixed-valence halide clusters containing Tc in oxidation states of +1.5 to + 3. This area has been reviewed (42). For convenience, all complexes, except those of [Tc2]6+, are treated together here. EPR spectroscopy is particularly useful in both the detection of species in this oxidation state and the study of exchange reactions in solution. The nuclear spin of "Tc (1 = f) results in spectra of 10 lines with superimposed hyperfine splitting. The d5 low-spin system is treated as a d1 system in the hole formalism (40). 

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