Zero field parameters D and

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. 

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... 

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.

The ZFS of the Trp triplet state are shown in Fig. 1, along with the location of the principal axes. Magnetic resonance transitions occur at the three frequencies, v, of ( D E )lh and 2 E /h. Assignment of any two of the transitions allows determination of the absolute values of the zero-field parameters, D and JE]. Because the ZFS is produced by the... 

Fig. 13A-D. Influence of zero field parameters D and E on the effective g values of a Cr +-ion (reprinted from reference [63], Copyright 1996 The Royal Society of Chemistry A effective g-values as a function of D B effective g-values as a function of D and E...

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-... 

Mossbauer spectroscopy can also be used to obtain information about fine structure parameters like zero-field splitting D and rhombicity parameter E D. The zero-field splitting is a second-order effect, which can be classically visualized as resulting from the circular currents generated in the atomic shell by the electron spin. In Section 3.10.5, we describe how D and E D can be calculated. 

Hutchison and Mangtim pioneered the technique with their studies of aromatic chromophores doped into single crystals 57,58). The initial investigations 55,56) were limited to observations of J s = 2 for-ladden transitions but the development of equipment with improved sensitivity rapidly led to the observation and interpretation of Ants = 1 transitions of the triplet state in frozen solutions 59,60). Observation of at least two dwg = 1 transitions from frozen solutions allows the ciromatic chromophore to be characterized by the parameters D and E which specify the zero-field sphtting (ZFS) between the triplet sublevels i>62). The dws = 2 transition by itself will yield only a root square (rms) zero-field parameter, D =]/D2 -)-3 2. 

Comparable ESR measurements made on high-spin iron (III) porphyrin model systems and mammalian liver microsomal cytochromes P450, all of which exhibit approximately the same degree of rhombicity, show a discrepancy between P450 and the model thiolates with respect to the size of the zero field parameter D (Sato et a/., 1977). These observations indicate that P450 heme has essential structural features similar to the thiolate porphyrins, but the Fe-S bond of the P450 cytochromes may be modified from its normal orientation in model thiolates, probably as a result of constraints imposed by the protein structure. 

The zero Field splitting (ZFS) parameters D and of the triplet exciton are ... 

The zero-field spin Hamiltonian parameters, D and E, are assumed to be distributed according to a normal distribution with standard deviations oD and aE, which we will express as a percentage of the average values (D) and (E). -Strain itself is not expected to be of significance, because the shape of high-spin spectra in the weak-field limit is dominated by the zero-field interaction. 

The transitions in the X-band ESR spectra of triplet species occur in two regions. The so-called Anis = 1 region represents transitions between energetically adjacent pairs of the three triplet sublevels. These are characterized by two so-called zero-field splitting parameters, D and E. The parameter D is inversely proportional to the cube of the average separation of the electron spins, and E is related to the molecular symmetry. The number of lines depends on the molecular symmetry. If all three magnetic axes of the molecular carrier of the spectrum are distinct, the spectrum in the Anis = 1 region will show six major resonances, plus any hyperfine lines that may be visible. If two of the principle axes are equivalent by symmetry, only four lines will be observed. In the latter case, the parameter E has the value of... 

EPR Spectroscopy. The structure of triplet carbenes is unequivocally characterized by EPR zero-field splitting (ZFS) parameters, which are analyzed by D and E values. In a simple model, the ZFS parameters D and E for a triplet depend on the distance between electrons with parallel spins as given by... 

Applications to Small Molecules. - 2.2.1 Intensity of Half-field Transition or Resolved Dipolar Splitting. In the pH range 8-10 the dominant species in solutions of Cu(II) and 2,2 -bipyridine (bpy) is [Cu(bpy)(OH)]22+.30 Well-resolved EPR spectra were obtained. The value of the zero-field splitting parameter D and the relative intensity of the half-field transition gave r = 3.4 A. 

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... 

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). 

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