Anomalous-preservation

Shimada, W. Takeya, S. Kamata, Y. Uchida, T. Nagao, J. Ebinuma, T. Narita, H. (2005). Texture Change of Ice on Anomalously Preserved Methane Clathrate Hydrate. J. Phys. Chem. B, 109, 5802-5807. 

As shown in Figure 3.36, the anomalous preservation region is observed over the temperature range 242-271 K on rapid depressurization to 0.1 MPa. This anomalous behavior has been also called anomalous self-preservation. The latter consists of a short rapid dissociation phase with a release of 5-20 vol%... 

FIGURE 3.36 Average rates for methane hydrate samples reaching 50% dissociation at 0.1 MPa, following destabilization by rapid release of P. The anomalous preservation regime is between 242 and 271 K. Square symbols experiments in which P is maintained at 2 MPa, Diamonds 0.1 MPa rapid depressurization tests on sll methane-ethane hydrate, showing no comparable preservation behavior at 268 K. (Reproduced from Stern, L.A., Circone, S., Kirby, S.H., Durhan, W., Can. J. Phys., 81, 271 (2003). With permission from the National Research Council.)... 

Methane hydrate is a clathrate compound of water molecules surrounding a methane molecule. Natural methane hydrate is found in permafrost and deep-sea sediment, and has recently attracted much attention as a potential new resource because of the large amount of deposits. Methane hydrate is also expected as new materials for gas storage and transportation due to its unique properties called anomalous preservation, quite slow dissociation from -40 to -10°C at atmospheric pressure, despite of its dissociation over -80"C. ... 

Methane hydrate has an unclear property called the self-preservation or anomalous preservation effect, i.e., slow dissociation of the hydrate under thermodynamically unstable conditions. This slow dissociation is thought to be caused by a layer of ice covering hydrate surfaces that forms from the dissociated hydrate. However, little is known about this preservation mechanism. For example, it is still unclear that this effect is not observed between 193-240 K but above 240... 

An alternative method of proof uses repeated applications of a local transformation, the duplicate operation, which also preserves graph homomorphic images. Call a direct connection in P from n to m anomalous if n m and n... 

The most popular processing programs in current use are probably DENZO/SCALEPACK (Otwinowski and Minor, 1997) and MOSPLM (Project, 1994). Careful scaling is particularly important to preserve the relatively weak anomalous signals, and minimize the consequences of... 

Edge Spectroscopy (XANES) can be used to estimate the expected absorption energy of the anomalous atoms. Eor example, SGX has determined the energy that corresponds to the maximum absorption of X-rays by pure selenomethionine. This value is used for all experiments that utilize selenomethionine to determine the required phases. With this approach, the limited X-ray lifetime of protein crystals can be devoted to determination of diffraction. In addition to preserving crystals, this approach maximizes the time devoted to data-collection in high-throughput mode. 

A number of cheletropic reactions also appear to be anomalous, including the best known of all cheletropic reactions, the stereospecific insertion of a carbene into a double bond, as in the reaction of dichlorocarbene 2.173 with alkenes. Here we have a reaction involving only four electrons, which is known to be suprafacial on the alkene, preserving the geometry of the substituents in the starting alkenes in the cyclopropanes 2.174 and 2.175. Furthermore, the [2+2] reaction takes place even with a diene, which could. undergo an allowed [4+2] reaction, but chooses not to. 

Self-preservation is the phenomenon where hydrates can remain stable for extended periods outside the hydrate stable region (Figure 3.36). Self-preservation or anomalous self-preservation has been experimentally observed by a numerous researchers (Davidson et al., 1986 Yakushev and Istomin, 1992 Stern et al., 2001a, 2001b, 2003 Takeya et al., 2002 Kuhs et al., 2005 Shimada et al. 2005). However, little is understood of this phenomenon. The ability to increase and prolong the stability of gas hydrates is desirable for gas storage applications. 

Anomalous self-preservation stabilizes methane hydrate and carbon dioxide hydrate particles at atmospheric pressure at 242-271 K for up to 2-3 weeks. This phenomenon can have implications for natural gas storage. 

The anomalous isotope ratio observed for the noble gases cannot be explained by any chemical process, and isotope mass effects associated with physical processes like diffusion, distillation and absorption-desorption are too small to explain what is observed. On the other hand, the carbon carrier phase is very abnormal, at least for the carbon-p phase. These facts can be explained if we accept that macroscopic amounts of interstellar carbon have survived unchanged, or at least preserved from isotopic exchange with solar system carbon. It is important to observe that a S13C = +1100%o, which corresponds to 12C/13C = 42, can be compared to the low end of the range observed for carbon in molecular clouds (60 8 or 67 10)67 K Moreover, the galactic ratio observed is now probably lower than it was 4.5 Gyr ago owing to the stellar production of 13C. 

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