Four-fold cluster

Another application is concerned with highly excited rotational levels of the PH3 molecule [13,14]. The calculations show that these rotational levels form near-degenerate six-fold energy clusters analogous to the four-fold clusters formed in triatomic dihydrides H2X (see, for example, Refs. [3,15,16] and references therein). 

Fig. 3.8 Histograms showing the proportion, p(n), of Pe + ions having 0-n neighboring detects (Pe + or cation vacancies) calculated for single-cluster model (dashed line) and the four-fold-cluster-cluster model (full line) for two typical compositions. The effect of disproportionation on defect distribution is shown by the bottom line in the bottom histogram...

Figure 2. The BeisXe D3/1 cluster used to model chemisorption into the four-fold hollows. The Be 13X3 cluster used to model chemisorption into the three-fold hollows is generated by a 60° rotation of the six adsorbate atoms about the vertical axis. The D3d metal cluster is formed by rotating the bottom three metal atoms by 180° about the vertical axis.

Within a single folded chain or subunit, contiguous portions of the polypeptide chain often fold into compact local units called domains, each of which might consist, for example of a four-helix cluster or a barrel or an antiparallel )S sheet (fig. 4.17). Sometimes the domains within a protein are very... 

Fig. 8.11. 600 MHz NOESY spectra of the four-cobalt cluster of cobalt-substituted metalloth-ionein. The spectra have been recorded with tm = 7 ms using a specially built probe featuring a 90° pulse of 3.7 p,s. (A) Spectrum obtained with a 125 kHz 16 bit ADC by folding the spectrum in both dimensions cross peaks g-b and c-i are folded. (B) Spectrum recorded using a fast ADC over a 250 kHz spectral width. The nine observed connectivities between geminal protons of cobalt-coordinated cysteine P-CH2 are summarized in the inset. Cross peaks marked with numbers are EXSY cross peaks to a minor species labeled Y in the inset [12].

Tobe s group also succeeded in applying the [2+2]cycloreversion process to the formation of smaller carbon cages, notably C36 [40]. Macroscopic quantities of C35 have been produced before [41, 42] and were shown to contain carbon cages that are covalently connected to form polymeric clusters of overall D6h-symmetry. In their efforts to obtain C36 from acetylenic precursors, Tobe et al. prepared cyclophynes 19 and 20 [40]. LD time-of-flight mass spectra of 19 depict a signal for the anion of cyclophyne C3(3H8, generated from 19 by four-fold... 

This ingenious parameterization reflects the fact that an electron-electron interaction is of two-body type and hence four-fold and six-fold excitations are indeed dominated by two and three simultaneous two-fold excitations. The cluster amplitudes in T< ,) are determined by substituting 4c nt0 the Schrodinger equation and projecting it onto the determinant manifolds reachable by acting f(0) on [Pg.51]

This example illustrates the problems of interpreting photoemission spectra, since the location of CO on Cu(100) is found to be on a top-site by LEED, while Xa-SW calculations either favor a 4-fold hollow site or are inconclusive. Yu/130/ has carried out calculations for CuCO and Cu5CO clusters with CO bonded to the Cu surface through the carbon atom. In CuCO the CO molecule was put directly on top of a Cu atom and in Cu5CO it was located in the four-fold coordinated hollow site, simulating CO adsorbed perpendicular to the (100) surface. The calculated valence density of states for the four-fold adsorption site gives a satisfactory interpretation of the two main peaks observed below the copper d-band in the UV photoemission spectrum (UPS)./131/ An observed weak third peak can be correlated with a minority of CO molecules adsorbed directly on top of a copper atom. 

Messmer and Lamson later calculated core and valence photoemission spectra using the Xa-SW method and offered another explanation for the CO-Cu (100) UV photoemission spectra./132/ They pointed out that, in addition to the main photoionization transitions, the outgoing electron can also excite valence electrons. Assuming that a Cu5 cluster with CO bonded in a four-fold coordinated hollow-site models the Cu (100) surface, they obtained theoretical DOS to compare with the experimental UPS of Norton et al.133... 

Another interesting example of MCSCF calculation is the determination of the oxygen binding site on Si (100). Batra et al performed this work for a few model clusters for oxygen chemisorbed at one, two and four-fold coordinated sites on Si3H6, Si6H12, and Si7Hg clusters./172/... 

Figure 2.6.6 The isomer ri -lSiyWICO) ], a bicapped square antiprism (the pseudo four-fold axis is vertical), where the transition metal is a part of the square prism and is five-coordinate with the cluster...

Ni(l 10), Ni(l 11) gave peaks at 1350 cm (a-top), 950 (bridge, two-fold), 600 (three- or four-fold). [30]. Experimental INS peaks at 940 and 1130 cm were then assigned to hydrogen in three-fold sites and a peak at ca 600 cm to hydrogen in four-fold sites. These assignments were not entirely in agreement with those from ab initio caleulations on nickel clusters of up to 28 atoms 1420, two-fold 1212, three-fold 592 cm, four-fold [31]. 

The MoFe proteins from Clostridiumpasteurianum and from Azotobacter vinelandii have been crystallized. For the former protein, crystals of space group P2i are obtained, with two molecules per unit cell of dimensions 70 X 151 X 122 A. There is good evidence for a molecular two-fold axis, which presumably relates equivalent sites in the two a/3 dimers that make up the protein molecule.Preliminary refinement reveals that the two FeMoco units per protein are about 70 A apart and the four P clusters are grouped in two pairs. 

For the special case of a model Pd4 cluster situated in pseudomorphic fashion on MgO(OOl), we considered two adsorption sites for CO, on-top and in the four-fold hollow. The interaction with the support reverses the order of stability of the sites, with CO being more stable on the hollow site of the free cluster (262 vs. 130 kJ/mol) and on the on-top site of the MgO supported cluster (192 vs. 159 kJ/mol). This very important result shows that relatively small perturbations due to the metal-oxide interface can suffice to change the order of stability of adsorption sites of a metal particle even when there is no large electron density transfer from or to the support. This implies that the catalytic activity and selectivity of a deposited metal particle can be modified compared to the free particle even by a relatively inert support. 

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