Complexes axially symmetrical

For axially symmetric complexes, the parameter E is zero, and the spin functions S,ms) are eigenfunctions of the spin Hamiltonian ... 

The dipolar shift for an axially symmetric complex may be written as... 

It might be worthwhile to point out that in the wide variety of crystal fields possible, the symmetry at the metal site can be orthorhombic which has fewer symmetry operations than an axially symmetric complex for example. However, the strength of the axial distortion might disturb the local environment more severely (as in the heme complexes) in contrast to the weak rhombic departures from cubic symmetry in the sideramines discussed later. The electronic wave functions which determine the hyperfine interactions depend on the parameters X and fi which reflect the degree of asymmetry in the local environment. 

For the case of an axially symmetric complex the dipolar shift (in frequency units) can be expressed as a product of several factors ... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Since many planar metal complexes have nearly axially symmetric g and AM tensors, two-dimensional powder ENDOR spectra can easily be obtained from such compounds oriented in nematic glasses84. As mentioned, interpretation of this type of spectra will be discussed in Sect. 4.3. 

Nitrogen ENDOR. The nitrogen hfs tensor is nearly axially symmetric with the largest principal axis oriented approximately along the Cu-N bond direction (<(Cu-N, A ) = 7°). A very similar result has been reported by Moores and Belford168 for the planar copper complex Cu(msal)2. 

The EPR spectra of various Tc complexes have been investigated in detail. The EPR of the six-coordinate, deep blue complex [AsPh4]2[Tc(NO)(NCS)5], prepared from [NH4][Tc04] in dmf and in the presence of KNCS and H2N0H HC1, has been studied in nonaqueous solution in the liquid and in frozen glass phases. With gx = (+)2.045 and gn = (-I-) 1.928, and A = 0.0236 and x = 0-0095 cm , the spectrum is characteristic of a low-spin 4quadrupole moment observed is solvent dependent. " When Hacac is... 

The diaqua and aqua (hydroxo) hemin complexes encapsulated in the micelles [20] are found to be high-spin (peff = 5.7 — S.Sps). Their high-spin nature is further confirmed from the ESR spectra at 4.2 K (Fig. 4). The spectra are characteristic of high-spin ferric porphyrins with a large zero-field-split Ai ground state with Mg = 1/2 lying lowest. The spectra are axially symmetric (gf = 2.05, = 6.0) for the diaqua complex, while for the aqua (hydroxo)... 

The numerical computations of the cross sections for molecular collisions in fields are demanding. Part of the complexity is due to the fact that most molecules of interest are open-shell radicals, so the basis for the coupled channel calculations must include multiple angular momentum states. In addition, the interactions of the molecules with an external field couple states with different total angular momenta. Note that in the presence of a single, axially symmetric field, the... 

The reaction of cobalt(II) porphyrins with CO has been studied by Wayland et al. (153-156) in some detail by electron paramagnetic resonance (EPR) in frozen solutions. They conclude that Co(II)TPP forms an axially symmetric weakly bonded 1 1 adduct with CO (153-156). To the best of our knowledge, there are no other studies on CoP-CO complexes, neither experimental nor theoretical. 

AsPh4]2[Mo S2C2(CN)2 3] was fitted by an axially symmetric spin Hamiltonian with g, = 2.000, g l = 1.974, Att = 10 G and AL = -100 G, interpreted in terms of a 1A1 ground state in D3 symmetry.49 The unpaired electron has predominantly metal dzi character. A molecular orbital scheme (Figure 4) was proposed for the dithiolene complexes.35 This scheme has been used in the discussion of ESR results,49 and electronic and resonance-Raman spectra.34... 

Fig. 5. 75.4-MHz l3C cross-polarization spectra of the acetylium ion 1 on TaCl5 acquired at 298 K. The nonspinning spectrum shows a broad and axially symmetric powder pattern. The MAS spectrum shows that ca. 80% of the starting material (acetyl-f-13C chloride) was ionized to form the acetylium ion (<5lso = 153 ppm), and the rest formed the donor-acceptor complex with TaCls ( lso = 189 ppm). The nonspinning spectrum requires 20 times more scans to acquire than the MAS spectrum. The principal components of the l3C shift tensor of 1 were measured from both spectra, and the results are in very good agreement.

To solve eqn. (114) with the boundary condition above, even for the steady state, is a task of considerable complexity and tedium. Sole and Stockmayer [256] considered the case of axially symmetric molecules and fixed the laboratory framework with 0 = 0. Hence, only 0, 0A and 0B appear in eqn. (115) and thence in the diffusion eqn. (114). By analogy to eqn. (19), the rate coefficient is... 

A further reduction in complexity, letting one reactant, B, be isotropically reactive while the other, A, remains axially symmetric with a cap or spot of reactivity, allows approximate analytic expressions to be derived. In the limit, as the radii of the reactants, rA and rB, are very different, rB R, rA 0... 

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