T* transition

Aminoboranes have been used as ligands in complexes with transition metals (66) in one instance giving a rare example of two-coordinate, non-t/ transition-metal complexes. The molecular stmcture of the iron complex Fe[N(Mes)B(Mes)2]2 where Mes = is shown in Figure 1. The... 

Figure 4 demonstrates that in order to variationally describe a realistic barrier shape (Eckart potential) by an effective parabolic one, the frequency of the latter, should drop with decreasing temperature. At high temperatures, T > T, transitions near the barrier top dominate, and the parabolic approximation with roeff = is accurate. 

The transition obtained under stress can be in some cases reversible, as found, for instance, for PBT. In that case, careful studies of the stress and strain dependence of the molar fractions of the two forms have been reported [83]. The observed stress-strain curves (Fig. 16) have been interpreted as due to the elastic deformation of the a form, followed by a plateau region corresponding to the a toward [t transition and then followed by the elastic deformation of the P form. On the basis of the changes with the temperature of the critical stresses (associated to the plateau region) also the enthalpy and the entropy of the transition have been evaluated [83]. 

Perez-Cadenas, A. Lopez-Garzon, R. Godino-Salido, M. L. Arranz-Mascaros, P. Gutierrez-Valero, M. D. Carillo-Marquez, T. Transition Met. Chem. 2002, 27, 184-190. 

How these functions can be perturbed so as to make S -T transitions possible will be discussed in another chapter. 

In carbonyls and heterocycles possessing both n-> tt and 77 - 77 excited states one would expect S - T n. transitions to be 102-103 times more efficient than S -> T . transitions/1425 We saw in Section 5.6a that the contribution of the overlap integral (J X1X2 dTo) in the transition probability is greater for transitions occurring between states of different type (n, 77 77,77 ) than for transitions between states of the same type (ir, tt -n,... 

Not all mutagenesis in IS. coli is dependent on SOS-processing. Mutations may arise quite simply during DNA replication if a base is substituted by or converted to another, incorrect, base. Consider the consequence of oxidative deamination of the base 5-methylcytosine to thymine. Replication followed by daughter strand segregation will result in a G C base pair having been mutated to an A T base pair. Sites containing 5-methylcytosine are hotspots for G C to A T transitions in 12. coli (24). 

Frank deletions of the gene are not common. A frequent mutation among northern Europeans (=40%) is a G to A transition at the 5 donor splice site in intron 12, resulting in absence of the C terminus. Another relatively common (=20%) mutation in northern Europeans involves a C to T transition in exon 12, resulting in substitution of a tryptophan for an arginine residue [21]. Over 70 different mutations have been described to date in the American population [22]. 

Molten aluminium and gallium fluxes have shown themselves to be excellent routes to complex ternary and quaternary intermetallic phases such as R-T-Al and R-T-Al-Si, R-T-Al-Ge (when Al is used as solvent) or R-T-Ga, R-T-Si, R-T-Ga-Si, R-Ga-Ge (when Ga is used as solvent), where R = rare earth metal, T = transition metal. 

The ternary variant TiNiSi type is also called E-phase structure. Many ternary compounds belonging to a MeTX formula (Me = rare earth metal, Ti, Hf, V, etc., T = transition metal of the Mn, Fe, Pt groups, X = Si, Ge, Sn, P, etc.) have this structure. Strukturbericht designation C23. 

Several routes are possible to populate the triplet state. The triplet excited state can, in principle, be directly excited from the ground state, but a low extinction coefficient associated with the S0 to T, transition (reflected in the long lifetime) makes direct excitation an inefficient process for tryptophan. The triplet state can be thermally populated, but for tryptophan the large energy gap between the ground state and the triplet state makes this process unfavorable. Energy transfer from a higher state can also populate the... 

Initially, investigators determined that the ABCBl 3435C>T transition was associated with lowered ABCBl expression and higher digoxin levels in vivo (65). The association was stronger when the ABCBl 2677G>T/A and 3435C>T... 

As a general rule the c.t. bands shift to lower energies with increasing oxidation state, whereas Rydberg transitions (such as 4f- -5d transitions) shift to higher energies. It may, therefore, be expected that the lowest absorption bands of the tetravalent lanthanide ions will be due c. t. transitions and those of the divalent lanthanide ions to Af- -5d transitions. 

Let us consider the tetravalent lanthanide ions first. Because this valency is hard to realize, the number of investigations is restricted. Jorgensen and Rittershaus 3) described the diffuse reflection spectra of Pr + and Tb + in Th02 and Y2O3 and Blasse and co-workers those in Zr02 [4). These spectra show strong absorption bands in the visible which were ascribed to c.t. transitions. 

Since the Cg orbitals are stabihzed by interaction with the d-orbitals of the central ion, he ascribed the first c.t. transition to the parity-allowed hg- -f. Since neither the hg nor the / level are strongly influenced by the bonding in the complex, the position of the c. t. transition is expected to be host-lattice independent as observed experimentally for six-coordination. 

In passing we mention that the c.t. transitions of Ti + in these host lattices are crystal-structure dependent (7). These absorption bands must be ascribed to the 2tiu- d transition (5). Indeed these levels depend on the nature of the bonding... 

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