Valence state transition

Valence state transitions, x-ray photoemission spectroscopic study of... 

The results of thermodynamic analysis and experiments led to the UTC shown in Figure 1 that uses valence state transitions and the formation of oxidised and reduced uranium species to split H20 for the production of H2. Some of the chemical reactions are new but most of the chemical reactions are used industrially within the uranium industry. 

Farver O, Hwang HJ, Lu Y, Pecht I. (2007) Reorganization energy of the Cu center in purple azurin Impact of the mixed valence-to-trapped valence state transition. J Phys Chem Sill 6690-6694. 

Transitions of core electrons into the antibonding ir orbital are called valence transitions. Examples of valence excited states are a n, b 1., B H. It is noted that excitation into Rydberg states moves the antibonding electron away from the nuclei, while in valence state transition a bonding electron is moved to an antibonding orbital. Therefore, one expects the intemuclear distance to decrease for Rydberg excitation and increase for valence state excitation. These expectations are borne out by experiment (cf. Fig. 1). 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

The other group of transition metals comprises those metals that retain d electrons in their normal valence states, eg, Co " and Pp". These metals form peroxides from dioxygen or from hydrogen peroxide. Their colors result from d—d transitions. These peroxo species act as nucleophiles. 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

Vanadium, a typical transition element, displays weU-cliaractetized valence states of 2—5 in solid compounds and in solutions. Valence states of —1 and 0 may occur in solid compounds, eg, the carbonyl and certain complexes. In oxidation state 5, vanadium is diamagnetic and forms colorless, pale yeUow, or red compounds. In lower oxidation states, the presence of one or more 3d electrons, usually unpaired, results in paramagnetic and colored compounds. All compounds of vanadium having unpaired electrons are colored, but because the absorption spectra may be complex, a specific color does not necessarily correspond to a particular oxidation state. As an illustration, vanadium(IV) oxy salts are generally blue, whereas vanadium(IV) chloride is deep red. Differences over the valence range of 2—5 are shown in Table 2. The stmcture of vanadium compounds has been discussed (6,7). 

Cobalt exists in the +2 or +3 valence states for the majority of its compounds and complexes. A multitude of complexes of the cobalt(III) ion [22541-63-5] exist, but few stable simple salts are known (2). Werner s discovery and detailed studies of the cobalt(III) ammine complexes contributed gready to modem coordination chemistry and understanding of ligand exchange (3). Octahedral stereochemistries are the most common for the cobalt(II) ion [22541-53-3] as well as for cobalt(III). Cobalt(II) forms numerous simple compounds and complexes, most of which are octahedral or tetrahedral in nature cobalt(II) forms more tetrahedral complexes than other transition-metal ions. Because of the small stabiUty difference between octahedral and tetrahedral complexes of cobalt(II), both can be found in equiUbrium for a number of complexes. Typically, octahedral cobalt(II) salts and complexes are pink to brownish red most of the tetrahedral Co(II) species are blue (see Coordination compounds). 

Copper compounds, which represent only a small percentage of ah copper production, play key roles ia both iadustry and the biosphere. Copper [7440-50.8] mol wt = 63.546, [Ar]3/°4.t is a member of the first transition series and much of its chemistry is associated with the copper(II) ion [15158-11-9] [Ar]3i5. Copper forms compounds of commercial iaterest ia the +1 and +2 oxidation states. The standard reduction potentials, for the reasonably attainable valence states of copper are... 

Cobalt. Without a doubt cobalt 2-ethyIhexanoate [136-52-7] is the most important and most widely used drying metal soap. Cobalt is primarily an oxidation catalyst and as such acts as a surface or top drier. Cobalt is a transition metal which can exist in two valence states. Although it has a red-violet color, when used at the proper concentration it contributes very Httie color to clear varnishes or white pigmented systems. Used alone, it may have a tendency to cause surface wrinkling therefore, to provide uniform drying, cobalt is generally used in combination with other metals, such as manganese, zirconium, lead, calcium, and combinations of these metals. 

Under polymerisation conditions, the active center of the transition-metal haHde is reduced to a lower valence state, ultimately to which is unable to polymerise monomers other than ethylene. The ratio /V +, in particular, under reactor conditions is the determining factor for catalyst activity to produce EPM and EPDM species. This ratio /V + can be upgraded by adding to the reaction mixture a promoter, which causes oxidation of to Examples of promoters in the eadier Hterature were carbon tetrachloride, hexachlorocyclopentadiene, trichloroacetic ester, and hensotrichloride (8). Later, butyl perchlorocrotonate and other proprietary compounds were introduced (9,10). 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

In a combined experimental/computational study, the vibrational spectra of the N9H and N7H tautomers of the parent purine have been investigated [99SA(A) 2329]. Solvent effects were estimated by SCRF calculations. Vertical transitions, transition dipole moments, and permanent dipole moments of several low-lying valence states of 2-aminopurine 146 were computed using the CIS and CASSCF methods [98JPC(A)526, 00JPC(A)1930]. While the first excited state of adenine is characterized by an n n transition, it is the transition for 146. The... 

Prediction of the energy level structure for Pu2+ (5f ) is of particular interest since no spectra for this valence state of Pu have been reported. On the basis of what is known of the spectra of Am2+ (26), Cf2" (27), and Es2+ (28), there appears to be evidence for a very small crystal-field splitting of the free-ion levels. Such evidence encourages use of a free-ion calculation in this particular case. The parameter values selected are indicated in Table V. Based on the systematics given by Brewer (19), the first f- d transition should occur near 11000 cm-, so the f- -f transitions at higher energies would be expected to be at least partially obscured. A... 

The exhibition of variable valency is indeed a characteristic of transition metals. Main group metal ions such as those of groups 1 or 2 exhibit a single valence state. Other main group metals may show a number of valencies (usually two) which are related by a change in oxidation state of two units. This is typified by the occurrence of lead(iv) and lead(ii) or thallium(iii) and thallium(i). However, all the transition metals exhibit a range of valencies that is generally not limited in this manner. 

Sodium has 1 valence electron, and 10 bound electrons. The first two excited states are the 3 Pi/2 and the 3 P3/2 states. Transitions to these levels give rise to the Di and D2 transitions respectively. There are two h)q)erfine levels in the 3 ground state, and four h)q)erfine levels in the 3 Pa/2 excited state (Fig. 3). There is no significant energy difference between the h)q)erfine levels in the 3 Pa/2 state. Thus, the six permitted fines appear in two groups, producing a double peaked spectral distribution, with the peaks separated by 1.772 GHz. 

Raimondi M, Cooper DL (1999) Ab Initio Modern Valence Bond Theory. 203 105-120 Rao CNR, Seikh MM, Narayana C (2004) Spin-State Transition in LaCoOj and Related Materials. 234 1-21... 

The cations in transition metal oxides often occur in more than one oxidation state. Molybdenum oxide is a good example, as the Mo cation may be in the 6-r, 5-r, and 4+ oxidation states. Oxide surfaces with the cation in the lower oxidation state are usually more reactive than those in the highest oxidation state. Such ions can engage in reactions that involve changes in valence state. 

To summarize, if the low-lying states connected to the ground state by allowed dipole transition are not valence states but present a predominant Rydberg character, we have to introduce a lot of n) states if not, the value of dynamic polarizability near the first resonance is poor. 

Vertical excitation energies to states of B symmetry, calculated at the level using the orbitals optimized for the neutral molecule with the MCSCF/6422 expansion, are reported Table 12. The I Bi valence state and 2 B (3p) Rydberg state of C3H2 are respectively 5.2 eV and 7.5 eV above the ground state with large transition moments of... 

The recoil-free fraction depends on the oxidation state, the spin state, and the elastic bonds of the Mossbauer atom. Therefore, a temperature-dependent transition of the valence state, a spin transition, or a phase change of a particular compound or material may be easily detected as a change in the slope, a kink, or a step in the temperature dependence of In f T). However, in fits of experimental Mossbauer intensities, the values of 0 and Meff are often strongly covariant, as one may expect from a comparison of the traces shown in Fig. 2.5b. In this situation, valuable constraints can be obtained from corresponding fits of the temperature dependence of the second-order-Doppler shift of the Mossbauer spectra, which can be described by using a similar approach. The formalism is given in Sect. 4.2.3 on the temperature dependence of the isomer shift. 

An interesting aspect of Au oxidation states is provided by the investigation of the pressure-induced transition from the mixed-valence state of Au(l)/Au(lll) to the single valence state of Au(ll) as described for M2[Au(l)X2][Au(lll)X4] (M = Rb, Cs X = Cl, Br, 1) [385, 386]. The valence states of Au(l) and Au(lll) at ambient pressure were clearly distinguishable. With increasing pressure, the doublets gradually increase their overlap. Finally, the Au Mbssbauer spectrum of CS2AU2I6 shows, at 12.5 GPa of applied pressure, only one doublet which was associated with Au(II). 

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