Zero-field parameters

Optical detection of magnetic resonance (ODMR) was attempted for measurements of the pH effects on the triplet state of purine to investigate the protonation site of purine at low temperatures (78JA7131). The ODMR spectrum did not show the presence of more than one triplet state at liquid helium temperatures. Since the protonated tautomers 1H,9H (3a) and H,1H (3b) have similar bond structures, their triplets should have similar zero-field parameters and are thus not easy to distinguish by ODMR. 

FIGURE 12.4 S = 7/2 EPR of the [8Fe-7S] P-cluster in Azotobacter vinelandii nitrogenase. The experimental spectrum (trace A) has been simulated in the absence (trace B) and the presence (trace C) of D-strain modeled as a correlated distribution in the zero-field parameters D and E. 

The ESR spectrum of triplet anthronylidene lp was recorded after UV irradiation of 2p at 77 K.95-97 At 0.365 cm-1 the zero field parameter D in lp is larger than that in la, which indicates that the delocalization of the unpaired n -electron in lp is less efficient — despite the larger 7t-system — than in la. Obviously the phenoxyl resonance structure in la is more favorable than in lp. 

Carbenes with triplet ground states are, of course, paramagnetic. Thus they exhibit characteristic epr spectra which can be used to confirm their presence, to provide some data on their structure, and, at least in principle, to reveal AGST in favorable cases. The first epr spectrum of a randomly oriented triplet carbene was observed for DPM (Murray et al., 1962). In subsequent years spectra of many carbenes have been recorded (Table 3). These spectra are commonly interpreted by fitting the results to a spin Hamiltonian H (3), to give the zero field parameters D and E (Stevens, 1952). In the usual... 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Maloney and co-workersstudied diphenoxyphosphorylnitrene by direct observational techniques. Irradiation (75 s, 254 nm) of 25 in EPA glass at 77 K produced persistent triplet EPR signals with zero-field parameters D/hc = 1.5148 cm and E/hc = 0.00739 cm which are rather similar to those of triplet carboethoxy-... 

It sometimes is possible to measure the same parameter by several different physical methods. This helps to improve the accuracy by which the variable is known, and can also serve as a check on the models being used to interpret the data. For example, data for the zero field parameters of MS-325 in HEPES buffer and in HSA solutions are given in Table 4. 

In most of these glasses the complex apparently assumes several conformations which are distinguishable by their zero-field parameters.221... 

In the triplet model the spin polarization is with respect to the internal molecular states, TjJ>, Ty>, and T > of the triplet and evolves with time according to the time-dependent Schrodinger equation into a spin polarization with respect to the electron spin Zeeman levels Ti>, Tq>, and T i> in an external magnetic field Bq. Consider a simple case of axially symmetric zero-field splitting (i.e., D y 0 and E = 0 D and E are the usual zero-field parameters). Tx>, [Ty>, and TZ> are the eigenstates of the zero-field interaction Hzfs, where Z is the major principal axis. The initial polarization arising from the population differences among these states can be expressed as... 

Zero-Field Parameters of Some Intermediate Ground-State Triplet Carbenes Produced by Photolysis3... 

Kottis and Lefebvre (322) have suggested that if polarized light is used to excite randomly oriented molecules to the triplet state, observation of the changes in the AMg = +1 ESR spectrum can reveal the correlation of the polarization properties of the excitation with the principal axis system of the triplet zero-field tensor. Such photoselection experiments have been carried out successfully by Lhotse and coworkers (323) and El-Sayed and Siegel (324) on a number of aromatic systems. Piette and collaborators (325) have studied the effect of metal complexation on the zero-field parameters and lifetimes of the phosphorescent triplet of aromatic-metal complexes with similar photoselection technique. The changes in... 

Triplet signals were first noted by Kosower and Waits in concentrated solutions of stable pyridinyl radicals (4 ) and confirmed for (CHs) and 4 in MTHF glasses at 77 K K Three pairs of shoulders around the g = 2 signal at 3295G for the monomeric pyridinyl and a AM = 2 transition at 1645G suggested the presence of triplet pair. The zero-field parameters were D == 0.0098 cm and E = 0.0011 cm , for which a radical-radical separation of 6.5 A was estimated from the relationship for spin-spin dipolar interaction (Eq. 12). 

Fig. 1. Schematic drawing of the splitting of the electron spin levels for a total spin of S = 5/2 (BqIIz) showing the dependence on the size of the zero-field parameters D and E (si) D = 0 (b) D< hv and (c) D hv.

III. Electronic dipole-dipole interaction tensor D Fine structure or zero-field parameters D,E... 

The basis of the concept of a dimer of chlorophyll a in P-700 rests on evidence obtained with optical or ESR spectroscopy. In the P-700 redox difference spectrum, although very similar to that obtainable upon chemical oxidation of chlorophyll a, there are significant differences in the red region, which presents a splitting of the peak (at 685 and 700 nm) which is absent in chlorophyll a [53]. Moreover, the ESR and ENDOR spectra also present characteristics that have been interpreted as due to a dimeric arrangement [16]. Alternative interpretations have been offered suggesting that the spectral distortions are caused by a modification of the chemical environment of chlorophyll a in the RC complex. Definitively not in line with the dimer hypothesis is the spectrum of the light-induced triplet state of P-700, that can be observed when the intermediate acceptor is prereduced chemically in this spectrum the zero field parameters are the same as those of chlorophyll a monomers [54]. It is not clear, however, whether the triplet state resides on P-700 or on other chlorophylls of the RC complex. 

In a later EPR investigation of single crystals of 38 irradiated at 223 K at 366 nm, the dependence of the spectral intensity on temperature was studied, and the values of D and E were redetermined together with their signs as D = — 96.2 G and E— — 2.7 G.42 On the basis of these results and considering the two small zero-field parameters, the possibility that the triplet spectrum could be attributed to the molecular triplet state of P-TKN was excluded. Also apparently to be excluded... 

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