C* structure

FIG. 6 Morphology diagram. Plotted is the dimensionless supercooling A versus the crystalline anisotropy e. Compact dendritic (CD) and compact seaweed (CS) structure comprise most of the diagram, under the influence of noise, fractal seaweed (FS) and fractal dendrites (FD) are encountered. For details see text and Ref. 108. 

The Cs structure and dimensions (Fig. 17.26b) were established by microwave spectroscopy which also yielded a value for the molecular dipole moment p. 1.72D. Other physical properties of this colourless gas are mp -115° (or -123°), bp -6°, A//f(g,298K) —34 10kJmol [or — 273kJmol when corrected for A//f(HF, g) ]. FCIO2 is thermally stable at room temperature in dry passivated metal containers and quartz. Thermal decomposition of the gas (first-order kinetics) only becomes measurable above 300° in quartz and above 200° in Monel metal ... 

An X-ray study of this latter complex reveals a C2v anion as in the chlorine analogue (Fig. 17.28a). This is closely related to the Cs structure of the neutral molecule F3IO (Fig. 17.28b). 

The meso carbon atom should present a carbenium (Cs+) structure with a low 77 electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C8-) with a high ir electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

There are two possible Cs conformations of the Sy homocycle exo and endo. The exo global minimum (Fig. 2) lies 15 kJ mol below the endo-form. Both conformers undergo facile pseudorotation through C2 transition states, with barriers of less than 4 kJ mol [54]. The exo-conformer possesses the geometry found in the sulfur allotropes y-Sy and 5-Sy [72]. This Cs structure has four bonds near the length of a normal S-S bond and one rather long bond of 215 pm with a dihedral angle of 0°. 

DesJarlais RL, Seibel GL, Kuntz ID, Furth PS, Alvarez JC, Ortiz de Montellano PR, DeCamp DL, Babd LM, Craik CS. Structure-based design of nonpeptide inhibitors specific for the human immunodeficiency virus 1 protease. Proc Natl Acad Sci USA 1990 87 6644-8. 

There is an obvious analogy in orbital energy and symmetry between D3h and C4v structures since the Cs structure also has much the same pattern, it is evident that any interconversion of PHS is a symmetry-allowed process. The high-lying non-bonding molecular orbital (see Fig. 26) has its electron density on the exterior of the molecule, making it prone to attack by Lewis acids. 

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

We note that such a species is expected to be a highly reactive oxidant,43 allowing these addition reactions to be safely carried out only in a computer ) As shown in Fig. 4.48, the equilibrium Cs structure of [PtF4]2+ is slightly distorted from... 

JA4155). However, when the 6-31G basis set is employed, the Cs structure with the pyramidalized carbon atom turns out to be a local minimum on the PES (90JA4155). 

Boratetrahedrane (121) that has the triplet A2 ground state and the carbene Cs structure (122) possess higher energies than that of structure (119e). Similar to borabenzene, the stabilization of boracyclobutadiene (119e) can be achieved through the formation of a complex with pyridine. 

The values calculated for nonplanar Cs structure of cyclopentadienylidene are given in parentheses (75JA6941). 

In addition to the C4V structure of the M(CO)3E9 ions shown in Fig. 5, a second isomer has been characterized for both the tin and lead clusters. Sn9W(CO)3" and Pb9Mo(CO)3 have both been isolated in a Cs structure in which the M(CO)3 fragment occupies a 5-coordinate waist vertex site instead of the 4-coordinate capping site shown in Fig. 5 [55, 60]. Both of these isomers give the same NMR spectra, which is consistent with the higher symmetry 4 structures. We interpret these data to indicate that there is local exchange between the 4-coordinate and 5-coordinate sites, but this interpretation is controversial [34, 55]. 

EM plays a crucial role in the development of thermodynamic data, especially for defective solids, multi-phase solids and solids with coexisting intergrowth structures. These microstructural details, which are essential to catalytic properties, cannot be revealed readily by other diffraction methods which tend to average structural information. The formation of anion vacancies in catalytic reactions and the resulting extended defects are described here, from which an improved understanding of the formation of CS planes and their role in catalysis can be obtained. These general results are applicable to other CS structures. 

FIGURE 9. Optimized structures of the silolyl anion and its lithium silolide. Bond lengths in A, bond angles in degrees, (a) The Cs structure of 67a at MP2/6-31+G (b) the C2v structure of 48a at MP2/6-31+G (c) the structure of the lithium silolide 68 at MP2/6-31G. Reprinted with permission from Reference 88. Copyright (1995) American Chemical Society... 

The singlet and triplet states of the antiaromatic D5h SisH5+ (153) are not minima on the Si5H5+ PES. When the D5h symmetry of the singlet is released, it first collapses to the C2V isomers 154 and 155 which are also not minima on the PES, and upon release of the C2v constraints they further collapse to the Cs structure 156, which is the only minimum on the Si5H5+ PES. The pentasila-allylic-type cation 156, which has a structural... 

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