Ammonia models

Recently Arnold and Patterson (1) and Symons (22) have introduced new species for their metal-ammonia models. The Arnold and Patterson model employs an equilibrium between a monomer and an electron. 

Formation mechanisms of imidazoles in the Maillard reaction are not as well understood as those of other heterocyclic compounds. The role of a-amino carbonyl fragments as intermediates in imidazole formation was suggested in the reaction of sucrose and ammonia (43). In a study of a L-rhamnose/ammonia model system, which produced fifty-two imidazoles, it was proposed that an amino-hydroxy fragment was responsible for imidazole... 

T. Shibamoto and R. A. Bernhard, Investigation of pyrazine formation pathways in glucose-ammonia model systems, Agric. Biol. Chem., 1977, 41, 143-153. 

Figure 25 Energy diagram for the suggested mechanism for 0-0 bond cleavage in man-gemese catalase. The results using the imidazole model ue mmked with the thick line and those for the ammonia model with the thin line.

Takken (2) identified thiazoles and 3-thiazolines from the reaction of 2,3-butanedione and 2,3-pentanedione with ammonia, acetaldehyde and hydrogen sulfide at 20 °C. Study of tetramethylpyrazine (5) also showed that it can be readily formed in 3-hydroxy-2-butanone and ammonia model reaction at 22 C. Recent study of the model reaction of 3-hydroxy-2-butanone and ammonium acetate at low temperature revealed an interesting intermediate compound, 2-(l-hydroxyethyl)-2,3,4-trimethyl-3-oxazoline, along with 2,4,5-trimethyloxazole, 2,4,5-trimethyl-3-oxazoline, and tetramethylpyrazine were isolated and identified 4,5). We hypothesized that with the introducing of H2S, replacement of oxygen by sulfur could happen and sulfur-containing heterocyclic compoimds such as thiazoles and thiazolines could be formed along with oxazoles, oxazolines and pyrazines. 

Ferretti and Flanagan (1971a) found it in a lactose/casein browning system, Shibamoto et al. (1979) in a i>glucose/ammonia model system, Baltes and Bochmann (1987b) in serine/threonine/sucrose systems (and in coffee). 

Mulders (1973c) identified 1-furfurylpyrrole in a cysteine/cystine ribose browning system, Shibamoto et al. (1979) in a D-glucose/ammonia model system, Ho and Chen (1999) in a Maillard reaction of threonine with ribose as a main volatile product, Baltes and Bochmann (1987b) when heating serine and/ or threonine with sucrose (and in coffee). According to Tressl et al. (1981c), this compound was also formed from 4-hydroxyproline and 2-furaldehyde. 

Shibamoto et al. (1979) identified it in a D-glucose-ammonia model system and Baltes and Bochmann (1987b) after heating serine and/or threonine with an equimolar amount of sucrose (as well as in coffee). 

Shibamoto T. and Bernhard R.A. (1977) Investigation of pyrazine formation pathways in sugar-ammonia model systems. J. Agric. Food Chem. 25, 609-14. 

A cone can be considered as a deformation of a cylinder, in which the poles of the uniaxial direction are no longer equivalent. The symmetry group is reduced accordingly to Coov, where only the vertical symmetry planes remain. Conical symmetry is exemplified by hetero-nuclear diatomic molecules, but it is also the symmetry of a polar vector, such as a translation in a given direction, or a polarized medium or an electric held, etc. Conical molecules have C v symmetries, as was the case for the ammonia model. Again, the smallest trivial member of this series is Civ, which is fully anisotropic. This is the point group of the water molecule. 

Inspection of Table 13-2 reveals that it is not symmetric across its principal diagonal. For example, CF = B, and EC = A, this is not an abelian group. One might inquire whether this group is the smallest that we can set up for the ammonia model. Inspection of the multiplication table should convince the reader that the following subsets meet the requirements for a group E, D, F E, A E, B E, C. These subgroups can be... 

In discussing the concept of class, it is unnecessary to postulate parallel universes, and the reader should not be disturbed by this pedagogical device. The people in the other universes are merely working with ammonia models that have been reflected or rotated with respect to the model orientation that we chose in Fig. 13-3. Operations in the same class are simply operations that become interchanged if our coordinate system is subjected to one of the symmetry operations of the group. 

There are five kinds of symmetry operations that one can utilize to move an object through a maximum number of indistinguishable configurations. One is the trivial identity operation E. Each of the other kinds of symmetry operation has an associated symmetry element in the object. For example, our ammonia model has three reflection operations, each of which has an associated reflection plane as its symmetry element. It also has two rotation operations and these are associated with a common rotation axis as symmetry element. The axis is said to be three-fold in this case because the associated rotations are each one-third of a complete cycle. In general, rotation by iTt/n radians is said to occur about an -fold axis. Another kind of operation—one we have encountered before is inversion, and it has a point of inversion as its symmetry element. Finally, there is an operation known as improper rotation. In this operation, we first rotate the object by some fraction of a cycle about an axis, and then reflect it through a plane perpendicular to the rotation axis. The axis is the symmetry element and is called an improper axis. 

The multiplication table for the ammonia model is given in Table 13-7 in terms of the symmetry symbols just described. (A, B,C we taken to be [Pg.440]

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