Metastable species stability

The reason for the lack of progress on this allotrope, which was first reported in 1939, and on which over a dozen publications have appeared, is often misunderstood. The difficulty is intrinsic. The existence of a great variety of intramolecular and intermolecular allotropes proves that many species must have comparable stability, i.e., that they can coexist. Thus, many contradictory allotropes are mixtures. We know now that the challenge of preparing and identifying a new allotrope does not end with its sjmthesis, but includes isolation of the metastable species from a mixture of equally metastable compounds, a process that... 

Naumov P, Yu P, Sakurai K (2008) Electronic tera-order stabilization of photoinduced metastable species structure of the photochromic product of spiropyran determined with in situ single crystal X-ray photodiffraction. J Phys Chem A 112 5810-5814... 

Tetrahedral N4 is expected to dissociate into two N2 molecules, but this reaction is forbidden by orbital symmetry. Dunn and Morokuma [33] characterized a transition state for the exothermic dissociation of tetrahedral N4 into two N2 and estimated the activation barrier to be 63 kcal/mol at the CASSCF(12e,12o) level, which indicates that N4 is a metastable species with significant kinetic stability. The calculated potential energy surface of N4 suggests that the low-lying triplet state might cross with the singlet surface (Fig. 3), which could reduce the activation energy barrier to about 30 kcal/mol [29,31,32],... 

Fig. 12.18, which also ignores elemental sulfur. It shows that bisulfite and sulfite ions are the major metastable species in oxidized environments. Predominant in a narrow range of intermediate Eh values with increasing pH, are tetrathionate (S4OI") and the thiosulfate species. Hydrogen sulfide and HS ion have roughly the same stability range as shown in Fig. 12.16. 

An obvious candidate for a stable noncyclic carbenium ion is the tert-butyl cation observed in superacidic media. Even if the proton affinity of isobutene (Table 22.1) does not make it very likely that tert-butyl cations will exist in zeolites, several quantum chemical studies have localized stationary points for tert-butyl cations in zeolite and found that they are less stable than the adsorption complex, but are similar in stability to surface butoxides. Because of technical limitations vibrational analysis, which could prove that this cation is a local minimum on the potential energy surface, that is a metastable species, have only recently been made. Within a periodic DFT study of isobutene/H-FER a complete vibrational analysis for all atoms in the unit cell was made [48], and as part of a hybrid QM/MNDO study on an embedded cluster model of isobutene/H-MOR a vibrational analysis was made with a limited number of atoms [49]. Both reached the... 

A template approach has been invoked by Stack et al. to immobilize a metastable [Fe"(OTf)2(phen )2] (phen , substituted 9,10-phenanthroline) complex 12 in a preformed binding pocket shaped by a thermodynamically stable [Cu(phen )2] template. After replacing Cu+ by Fe +, the iron(II)-containing polymer 12 was obtained. A [Fe (OTf)2(phen )2] intermediate is unstable in a homogeneous solution and rapidly leads to the tris(chelate) [Fe(phen )3] + complex and Fe + ions [21]. Thus, the matrix prevents ligand scrambling and again kinetically stabilizes a metastable species by site isolation. 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

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