Carbon crystal structure

Most diamonds are formed about 90 miles (145 km) underground, where extreme heat and strong pressure allow carbon crystal structures to grow large. Over time (some say as long as 50 million years), these diamond structures make their way to the surface of Earth and are mined from the rock by humans. About 25 countries operate active diamond mines today, and diamonds are known to exist on every continent except Europe and Antarctica. 

The production of further comprehensive compendia of X-ray and neutron diffraction results has been precluded by the steep rise in the number of published crystal structures, as illustrated by Figure A.l. Printed compilations have been effectively superseded by computerized databases. In particular, the Cambridge Structural Database (CSD) now (October 1992) contains bibliographic, chemical, and numerical results for over 100,000 organo-carbon crystal structures. This machine-readable file fulfils the function of a comprehensive structure-by-structure compendium of molecular geometries. However, the amount of data now held in CSD is so large that there is also a need for concise, printed tabulations of average molecular dimensions. 

Aspartic proteases form a group of proteolytic enzymes that catalyze peptide bond cleavage by acid-base catalysis and activation of a water molecule for nucleophilic attack on the amide carbon. Crystal structures of mammalian and fungal enzymes are known. In pepsin, the best studied aspartic protease, catalysis proceeds by water activation and leaving group protonation. Both involve an aspartate, which explains the low pH optimum of around 4. The two aspartic acid residues are situated around a hydrophobic deft that can accommodate seven amino acids (the S4-S3 subsites). The active site is covered by a flexible flap, which contributes to SI subsite specificity. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

Polyethylene. The crystal structure of this polymer is essentially the same as those of linear alkanes containing 20-40 carbon atoms, and the values of Tjj and AHf j are what would be expected on the basis of an extrapolation from data on the alkanes. Since there are no chain substituents or intermolecular forces other than London forces in polyethylene, we shall compare other polymers to it as a reference substance. 

Crystal Structure. Diamonds prepared by the direct conversion of well-crystallized graphite, at pressures of about 13 GPa (130 kbar), show certain unusual reflections in the x-ray diffraction patterns (25). They could be explained by assuming a hexagonal diamond stmcture (related to wurtzite) with a = 0.252 and c = 0.412 nm, space group P63 /mmc — Dgj with four atoms per unit cell. The calculated density would be 3.51 g/cm, the same as for ordinary cubic diamond, and the distances between nearest neighbor carbon atoms would be the same in both hexagonal and cubic diamond, 0.154 nm. 

Thiadiazole-2-suifonamide, 5-acetamido-carbonic anhydrase inhibitor, 6, 576 crystal structure, 6, 548... 

In order to answer these questions as directly as possible we begin by looking at diffusive and displacive transformations in pure iron (once we understand how pure iron transforms we will have no problem in generalising to iron-carbon alloys). Now, as we saw in Chapter 2, iron has different crystal structures at different temperatures. Below 914°C the stable structure is b.c.c., but above 914°C it is f.c.c. If f.c.c. iron is cooled below 914°C the structure becomes thermodynamically unstable, and it tries to change back to b.c.c. This f.c.c. b.c.c. transformation usually takes place by a diffusive mechanism. But in exceptional conditions it can occur by a displacive mechanism instead. To understand how iron can transform displacively we must first look at the details of how it transforms by diffusion. 

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5]. Diamond is most frequently found in a cubic form in which each carbon atom is linked to fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm. ... 

Fig. 3. Crystal structure of the compound C o(S8)2CS2 projected normal to the a-axis. Large cireles denote Coo, small eireles denote sulfur, black balls denote carbon. In this structure, the Coo-Ceo distanee is nearly 11 A, and the diameter of the Ceo molecule has been reduced relative to the other atoms for clarity [54].

Tanuma, S., Synthesis and structure of quasi-one-dimensional carbon crystal carbolite and intercalation with alkali metals and halogens. In Supercarbon, Synthesis, Properties and Applications, ed. S. Yoshimura and R. P. H. Chang, Springer-Verlag, Heidelberg, 1998, pp. 120 127. 

Now in the case of chromium carbide separation from the steel, three possible crystal structures may be taken up, those of CrjC, (or CrjsCJ, Cr7C3 and CrjCj. It is necessary first to calculate the free energies of formation of the compounds from pure chromium and carbon. The results are ... 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

The importance of the proximity effect in cyclodextrin catalysis has been discussed on the basis of the structural data. Harata et al. 31,35> have determined the crystal structures of a-cyclodextrin complexes with m- and p-nitrophenols by the X-ray method. Upon the assumption that m- and p-nitrophenyl acetates form inclusion complexes in the same manner as the corresponding nitrophenols, they estimated the distances between the carbonyl carbon atoms of the acetates and the adjacent second-... 

Yu.E. Gorbunova Investigation of crystal structure of some carbonate containing hydroxocompounds of zirconium and oxyfluoroniobates -Abstract of dissertation, Moscow 1974 (in Russian). 

The crystal structure of graphite and amorphous carbon is illustrated by the schematic representations given in Fig. 1. 

Figure 1. Crystal structure of (a) graphite (b) amophous carbon.

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