Triplet polarization

With the establishment of the primary phototriplet reduction mechanism we now turn to the explanation for the effect of flow rates and the formation of polarized phenoxy radicals. Since reaction [2] is a relatively fast secondary process it is readily understood that the observation of the primary ketyl radicals will be dependent upon flow rate. The triplet polarization (E) of the secondary phenacyl radical should not have been affected but the increased contribution of the E/A Radical-Pair polarization altered the overall appearance of the polarization pattern. The diffusion model of the Radical-Pair theory relates the E/A polarization magnitude to the radical concentration within the reaction zone. Since the phenacyl radical is considered to be very chemically reactive, and the product phenol "accumulated" within the reaction zone is also a much better hydrogen donor, the following reactions will proceed within the reaction zone ... 

The rate constant of electron transfer from 2,3-dihydrofuran to the an-thraquinone-2,6-disulfonate triplet was determined from the 2,6-AQDS triplet polarization depending on the donor concentration, and its value was estimated to be kei = (1.6 + 0.2) x 10 M s k The formation of the successor radicals from the primary 2,3-dihydrofuran radical-cation was directly observed during the first 100 ns after the laser pulse, and the first-order rate constant of the generation of all the successor radicals observed was determined to be ksuc = (5 + 1) X 10 s , which clearly differs from the escape rate constant of the radical-anion 2,6-AQDS from the primary radical-ion pair. 

The rate constants ket for the electron-transfer reaction determined from the triplet polarization are ket = 3.9 x 10 M s for a-glycine, ket = 3.3 x 10 M s for L-a-alanine and ket = 2.3 x 10 M s for a-aminoisobutyric acid. The decarboxylation reaction is a first-order decay with the rate constants kdecarb =... 

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... 

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

The tliree-line spectrum with a 15.6 G hyperfine reflects the interaction of the TEMPO radical with tire nitrogen nucleus (/ = 1) the benzophenone triplet caimot be observed because of its short relaxation times. The spectrum shows strong net emission with weak E/A multiplet polarization. Quantitative analysis of the spectrum was shown to match a theoretical model which described the size of the polarizations and their dependence on diffrision. 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

Atkins P W and Evans G T 1974 Electron spin polarization in a rotating triplet Mol. Phys. 27 1633—44... 

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

Koga T, Ohara K, Kuwata K and Mural H 1997 Anomalous triplet mechanism spin polarization... 

Nanosecond flash photolysis studies of Ct-nitronaphthalene in polar and nonpolar solvents led to the detection of the trip let-triplet absorption spectrum of this nitrocompd (Ref 26). 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

Electron-electron repulsion integrals, 28 Electrons bonding, 14, 18-19 electron-electron repulsion, 8 inner-shell core, 4 ionization energy of, 10 localization of, 16 polarization of, 75 Schroedinger equation for, 2 triplet spin states, 15-16 valence, core-valence separation, 4 wave functions of, 4,15-16 Electrostatic fields, of proteins, 122 Electrostatic interactions, 13, 87 in enzymatic reactions, 209-211,225-228 in lysozyme, 158-161,167-169 in metalloenzymes, 200-207 in proteins ... 

Consider now an encounter (F) radical pair formed from two free radicals. Since there are three components to the triplet state, T+i, To and T j, and only one singlet component, S, the encounter of two free radicals having uncorrelated spins leads to a statistical distribution of T and S radical pairs. However, some of the S radical pairs will react without undergoing T-S mixing, and this has the effect of increasing the relative number of T radical pairs. Consequently the F-pairs will give the sam e type of polarization as the T-pairs, but the degree of polarization will be less. 

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