Rhombic state

Figure 6.6 Schematic representation of the two MbNO forms identified using EPR. The rhombic state I is dominant at very low temperature (<30 K), whereas the axial state H dominates the EPR spectra at higher temperature.

Elemental composition, ionic charge, and oxidation state are the dominant considerations in inorganic nomenclature. Coimectivity, ie, which atoms are linked by bonds to which other atoms, has not generally been considered to be important, and indeed, in some types of compounds, such as cluster compounds, it caimot be appHed unambiguously. However, when it is necessary to indicate coimectivity, itaUcized symbols for the connected atoms are used, as in trioxodinitrate(A/,A/), O2N—NO . The nomenclature that has been presented appHes to isolated molecules (or ions). Eor substances in the soHd state, which may have more than one crystal stmcture, with individual connectivities, two devices are used. The name of a mineral that exemplifies a particular crystal stmcture, eg, mtile or perovskite, may be appended. Alternatively, the crystal stmcture symmetry, eg, rhombic or triclinic, may be cited, or the stmcture may be stated in a phrase, eg, face-centered cubic. 

Compiled from Daubert, T. E., R. R Danner, H. M. Sibiil, and C. C. Stebbins, DIPPR Data Compilation of Pure Compound Properties, Project 801 Sponsor Release, July, 1993, Design Institute for Physical Property Data, AlChE, New York, NY and from Thermodynamics Research Center, Selected Values of Properties of Hydrocarbons and Related Compounds, Thermodynamics Research Center Hydrocarbon Project, Texas A M University, College Station, Texas (extant 1994). The compounds are considered to be formed from the elements in their standard states at 298.15 K and 101,325 P. These include C (graphite) and S (rhombic). Enthalpy of combustion is the net value for the compound in its standard state at 298.15K and 101,325 Pa. 

Fig. 9. Zero-field splitting of the S = 2 ground state of [Fe3S4] clusters. Constructed for D = -2.5 and E/D = 0.23, where D and E are the axial and rhombic zero-field splitting parameters, respectively.

Fig. 14. Plot of the g values g,g ) and of the average g value g vs rhombicity (UJ of (a) wild type (open symbol) and variant forms (closed symbols) of the Rieske protein in yeast bci complex where the residues Ser 183 and Tyr 185 forming hydrogen bonds into the cluster have been replaced by site-directed mutagenesis [Denke et al. (35) Merbitz-Zahradnik, T. Link, T. A., manuscript in preparation] and of (b) the Rieske cluster in membranes of Rhodobacter capsulatus in different redox states of the quinone pool and with inhibitors added [data from Ding et al. (79)]. The solid lines represent linear fits to the data points the dashed lines reproduce the fits to the g values of all Rieske and Rieske-type proteins shown in Fig. 13.

In the native state FDH displays rhombic EPR signals, observable at low temperature (10 K), with g values typical or ferric low-spin hemes, around gmax = 3. 

Fig. 13. Transition-state structures for pseudorotation by the trigonal twist (M3) and rhombic twist (M3) mechanism. According to Ref. [93]...

Hydroxylase in the mixed-valent Fe(II)Fe(III) oxidation state (Hmv) is readily accessible by one-electron reduction of the dinuclear center. Mossbauer data indicate the presence of one Fe(III) and one Fe(II) (39). Hmv has a rhombic EPR signal with gav = 1.83 (27, 37) and J -30 cm1 (38,39), properties characteristic of other mixed-valent nonheme carboxylate-bridged diiron centers such as that in semimet hemerythrin (J = -15 cm-1) (32). ENDOR spectroscopic studies of Hmv... 

The low-spin Nini06 compound [Ni(bipy02)3]3+ (bipy02 = 2,2 -bipyridyl-1,1 -dioxide) is the product of constant-potential electrolysis of the Ni11 salt.176 Its violet solutions show a rhombic EPR spectrum indicative of a Jahn-Teller-distorted (dz )1 ground state. 

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

The main structural difference between trigonal- and tetragonal-prismatic clusters [12,77] is that not all technetium atoms of the latter are structurally equivalent a pair of technetium atoms situated on the long diagonal of the rhombic base in tetragonal-prismatic clusters have the 2.0 + formal oxidation state, whereas the technetium atoms situated on the short diagonal of these rhombs have the formal oxidation state 1.0 +. However, X-ray photoelectron... 

There are two basic crystallisation forms for sulphur - monoclinic and rhombic. Rhombic is the most stable form, at least up to 96 °C the other types revert to this stable form at a rate dependant upon temperature. When sulphur solidifies from the molten state (melting point 114 °C) the crystalline form which occurs is monoclinic (needle-like crystal structure). Below 96 °C, the monoclinic form becomes metastable and changes into the rhombic form. 

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