I „ clusters

I assume that in nuclei the nucleons may. as a first approximation, he described as occupying localized 1. orbitals to form small clusters. These small clusters, called spherons. arc usually hclions, tritons, and dincutrons in nuclei containing an odd number of neutrons, an Hc i cluster or a deuteron may serve as a spheron. The localized l.v orbitals may be described as hybrids of the central-field orbitals of the shell model. 

Under special conditions sulfur cations with up to 56 atoms have been observed [209]. Evaporation of liquid sulfur and cooling the vapor in an atmosphere of a cold buffer gas (He) at low pressures followed by adiabatic expansion into the vacuum of a mass spectrometer and El ionization produced mass spectra of clusters of sulfur molecules with m/e ratios up to ca. 1800. The intensity pattern shows that the species (Ss)h are most abundant n = 1-7) followed by (Sy)(S8)n-i clusters and (S6)(Ss)h-i clusters. The latter have the same mass as (Sy)2(S8) -2 clusters see Fig. 34. Thus, the composition of the clusters reflects the composition of hquid sulfur near the melting point which contains Sg, Sy and Se molecules as the majority species [34, 210]. 

Tetranudear gold(I) amidinate complexes are synthesized by the reaction of Au(THT)Cl with the potassium or sodium salt of the amidinate ligand in THF, Figure 1.4. Syntheses involving various substituted amidinates resulted in tetra-nuclear gold(I) clusters, [Au4(ArNC(H)NAr)4]. The C-functionalized substituted amidine ligands, ArNC(Ph)NHAr and ArNC(Me)NHAr, Ar=-QH5, were synthesized and reacted with Au(THT)Cl after deprotonation. Only tetranudear clusters were isolated. 

In addition, the long-known cubane-type Re(I)-cluster [Re(/r3-OH)3(CO)i2]... 

Investigations on the thermal decomposition of 32 revealed the continuous loss of CO with concomitant formation of the di- and tetranuelear Tc(I) clusters [TcX(CO)4]2 (62) and [TcX(CO)3]4 (63) respectively. The latter is assumed to have cubane structure comparable to [Tc(OHXCO)3]4 [76,77,78]. The thermal decomposition reactions are depicted in Scheme 13. 

The trans-bent structure was unexceptional and found to be energetically favored compared to the linear arrangement, but even more stable forms such as vinylidene, mono-bridged or double-bridged isomers exist. Compounds 392-395 are unexceptional monomeric in solution, as is typical for M(i) cluster compounds.297... 

By this time it was demonstrated that the [3Fe-4S]W+ form of aconitase is inactive, while the [4Fe-4S]2+ form is active. How is the activity of the enzyme affected by the oxidation state of the [4Fe-4S] cluster Because the active enzyme contains a [4Fe-4S]2+ cluster, either the 3+ or 1+ oxidation states may also be stable. The 3+ state is unstable since oxidation of the [4Fe-4S]2+ resulted in the immediate loss of a ferrous ion and conversion to a [3Fe-4S]i+ cluster (46,47). However, reduction of active aconitase by sodium dithionite or photoreduction in the presence of deazaflavin produced in high yields an EPR signal characteristic for [4Fe-4S]l+ clusters (47). When active enzyme within an anaerobic assay cuvette was photoreduced, the activity of the enzyme dropped to 1/3 of its initial value. Further photoreduction resulted in cluster destruction. Then, if the enzyme is reoxidized with air, the activity returned to its original value. This demonstrated that the redox state of the cluster can modulate the enzyme activity. A scheme summarizing the cluster interconversions and various redox states of the Fe-S cluster of aconitase is shown below. 

Mo(CO)6 and Co(CO)3NO NaY zeolite Adsorption from vapor phase and H2S treatment Intrazeolite Co2Mo2S i clusters, model hydrodesulfuration catalyst [25]... 

Building on the initial findings described in Section 4, we will acquire photoelectron images of 0CS (H20)i, cluster anions at different wavelengths in the visible and UV and investigate the dynamics of hydration and hydration-induced stabilization of... 

With 13 carbonyls, the metal atoms can adopt the usual tetrahedral arrangement although considerable steric crowding occurs with the smallest metal atoms, as shown in the short contacts present in the dianion [Fe4(CO)i3]2 (6). (The shortest van der Waals contacts between the carbon atoms have been found experimentally to depend strongly on the relative inclination of the carbonyls parallel carbonyls present a minimum distance of 3.0-3.1 A, while this distance decreases to 2.5-2.6 A at the relative angle of 90°-l 10° this effect is clearly related to the expected oval shape of the ligand (I).) Cluster opening occurs by... 

Polar amino acids I ( ) cluster on the surface of soluble proteins. 

Why do we separate clusters into two classes rather than deal with them as u single group of compounds It is primarily because they have unrelated chemistry. Metal atoms in class I have low formal oxidation states, -1 to +1. while those in class II are found in higher formal oxidation stales. +2 to +3. The transition metals on the right side of the periodic tabic (late transition metals) typically form class I clusters, while those on the left-hand side (early second and third row transition metals) tend to form class II clusters. 

I. Clusters Containing More Than Two Different Metals... 

Due to the nature of the density of states for a chromophore/diatomic molecule cluster, and the pivotal importance of the density of states for cluster vibrational dynamics, the results for the aniline(N2)i cluster fall intermediately between those for aniline(Ar)j and aniline(CH4)1. This can be seen in emission spectra for various aniline(N2)1 excitations (Figure 5-7). First, 6a1 excitation does... 

Figure 5-7. Dispersed emission spectra of aniline(N2)i clusters following excitation to several vibrational states of St. Relative energy is the shift, in wavenumbers, from the excited transition. The top spectrum (TJ excitation) shows an inset trace for an expanded scale about the 0° intense feature 10b, 0 J, and JJ emission can be observed. Note that the relaxed cluster emission from 0 J (following IVR) is broad as expected (compare with 6aJ + 55 cm -1 and 6aJ excitation).

Figure 5-9. TRSEP signal for the aniline(N2)i cluster. Excitation laser is tuned to the TJj transition, and the probe laser is tuned to the I 6a transition. This plot shows the extent to which the probe pulse diminishes the total fluoresence. The time axis is the difference between the arrival times of the pump and probe pulses. The maximum diminution of the fluorescence is about 30%. The smooth curve is generated using the results of a nonlinear fitting routine. The fast component time constant is 200 + 50 ps.

Figure 5-12. A typical fit of an observed TCSPC decay curve. Data correspond to the 4EA(N2)i cluster pumped at F and observed at 0 . The fit parameters are rise time 585 ps and decay time 7200 ps.

Figure 5-19. Two-color MRES of benzyl radical(C2H6)i cluster obtained from photolyzing benzyl chloride and expanding with 1% ethane in helium. Note the loss of intensity beyond 22,600 cm.

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