Ammonia metastable

Acetamide [60-35-5] C2H NO, mol wt 59.07, is a white, odorless, hygroscopic soHd derived from acetic acid and ammonia. The stable crystalline habit is trigonal the metastable is orthorhombic. The melt is a solvent for organic substances it is used ia electrochemistry and organic synthesis. Pure acetamide has a bitter taste. Unknown impurities, possibly derived from acetonitrile, cause its mousy odor (1). It is found ia coal mine waste dumps (2). 

Here, I designates the ion core (NHJ in the case of ammonia) and L the clustering ligand (e.g. NH3). The intensity and width of the metastable ion peaks carry information on the internal energy of the parent cluster ions. 

An important example of the application of this method is seen for the case of ammonia. Referring to Figure 13, the measured average kinetic energy release of metastable (NH3)nH+ (n = 4-17) is seen to display a maximum value of... 

Two other molecules considered in the same context, but with proton affinities larger than ammonia, were pyridine (C5H5N) and trimethylamine (TMA) (CH3)3N whose proton affinities are 220.8 cal mol-1 and 225.1 cal mol-1, respectively.190 Metastable decomposition studies of NH3(C5H5N)mH+ (m = 1-5) yield the following results ... 

Other studies conducted on mixed protonated clusters of ammonia bound with TMA showed that the ion intensity distributions of (NH3)n(TMA)mH+191 display local maxima at (n,m) = (1,4), (2,3), (2,6), (3,2), and (3,8). Observation that the maximum ion intensity occurs at (n,m) = (1,4), (2,3), and (3,2) indicates that a solvation shell is formed around the NHJ ion with four ligands of any combination of ammonia and TMA molecules. In the situation where the maximum of the ion intensity occurs at (n,m) = (2,6) and (3,8), the experimental results suggest that another solvation shell forms which contains the core ions [H3N-H-NH3]+ (with six available hydrogen-bonding sites) and [H3N-H(NH2)H-NH3]+ (with eight available hydrogen-bonding sites). The observed metastable unimolecular decomposition processes support the above solvation model. 

When the amino group is fully deprotonated, the rate of the H-transfer is 1.5 x 10s s4, but also around pH 7 the reaction is still fast, at the ms timescale (for a quantum mechanical study see Rauk et al. 2001). Upon the decay of the amnioal-kyl radicals formed in reaction (35) ammonia as formed in a yield that points to disproportionation as the major process (Zhao et al. 1997). The fact that the ami-noalkyl radical is the thermodynamically favored species does not mean that the repair of DNA radicals by GSH (Chap. 12.11) is not due to its action as a thiol. As with many other examples described in this book, the much faster kinetics that lead to a metastable intermediate (here the formation of the thiyl radical) rather than the thermodynamics as determined by the most stable species (here the aminoalkyl radical) determine the pathway the the reaction. In fact, the C-H BDE of the peptide linkage is lower than the S-H BDE and repair of DNA radicals by peptides, e.g., proteins would be thermodynamically favored over a repair by thiols but this reaction is retarded kinetically (Reid et al. 2003a,b). 

Tolbutamide exists in two polymorphs, one was obtained either by crystallization of tolbutamide from benzene solution after addition of hexane, or by precipitation from solution in aqueous ammonia by addition of acetic acid, and the other, metastable form was obtained from ethanolic solution after addition of water. The two forms were characterized by infrared spectroscopy, x-ray diffraction and d.t.a.(l)... 

For the para-Cl-F-benzene, the efficiency of reaction with ammonia is found to be lower the fluoroaniline+ product appears in the mass spectrum as a metastable peak. However, both substitution channels are open for the 1-1 complex the Cl abstraction is the most efficient—leading to 4-fluoroanilinium. 

The excess energy within the cluster ion owing to the above exothermic reaction and the relaxation around the newly formed ion, as well as possible further multiphoton excitations, contributes to heating of the cluster ions and concomitant evaporative dissociation. As the cluster ions cool evaporatively, the dissociation extends to longer times, and the present study is directed to an investigation of the metastable dissociation processes of cluster ions (NH3) H+ in the field-free region (where the time window is about one to a few tens of microseconds). In the case of the ammonia cluster ion system, the dissociation process can be expressed as... 

A typical hard reflection time-of-flight mass spectrum of ammonia clusters is shown in Figure 6-8(a). When the reflecting voltage is reduced, only the daughter ions are reflected, as shown in Figure 6-8(b). To precisely measure the metastable decay fractions, the soft reflection mode is used for a number of reasons discussed elsewhere (Wei et al. 1990a,b). 

In the saturator process (see Figure 12.7), neutralization and crystallization are carried out in the same vessel. The sulfuric acid is delivered to the suction side and the ammonia to the pressure side of the forced circulation pump. Crystallization of the metastable solution gives particle sizes generally between 0.5 and 3 mm. The salt is continuously discharged at the lower end of the saturator. The salt is separated in centrifuges, dried, and cooled. The mother liquor is returned to the saturator. Impurities in the sulfuric acid can adversely affect crystallization. Small quantities of phosphoric acid, urea, or inorganic salts are added to promote crystal growth295. 

The application of the fusion process can lead to a control over structure-sensitive reactions for unsupported catalysts. The prototype example for such a catalyst is the multiply-promoted iron oxide precursor used for ammonia synthesis. In Section B.2.1.1 a detailed description is given of the necessity for oxide fusion and the consequences of the metastable oxide mixture for the catalytic action of the final metal catalyst. 

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