Pyrazine crystal structure

The structure of lumazine has been studied more precisely by X-ray analysis (72AX(B)659). The crystal structure is built up of almost coplanar, hydrogen-bonded dimers of lumazine with the oxygens of the pyrimidine moiety in the keto form and the observed bond distances indicating the pyrazine ring electrons to be delocalized. 

In the crystal structure of the polymer phase (Fig. 17a), the polymer chains are aligned along the c-axis and the distance (3.71 A) between the centres of adjacent cyclobutane and pyrazine rings corresponds to half the c-axis repeat of the unit cell. For comparison between the monomer and polymer structures, an overlay plot of these structures is shown in Fig. 17b. It is clear that the solid-state reaction is associated with only very small atomic displacements at the site of the [2-1-2] photocyclization reaction (the displacement of the carbon atoms of the C=C double bonds of monomer molecules on forming the cyclobutane ring of the polymer is only ca. 0.8 A for one pair of carbon atoms and ca. 1.6 A for the other pair). Such small displacements are completely in accord with the assignment of this solid-state reaction as a topochemical transformation [124—127] (in which the crystal structure of the reactant monomer phase imposes geometric control on the pathway of the... 

In the X-ray analysis of a crystal structure the first step is the determination of the space group and the number of molecules in the unit cell. Occasionally it may be immediately apparent from such data that the molecule itself possesses certain elements of symmetry, and these may define or at least limit the possible molecular conformations. The elements most commonly found in aromatic molecules are centres of symmetry and twofold rotation axes. It might have been expected that the plane of symmetry would manifest itself in aromatic systems, but this is disappointingly rare. Indeed, amongst the structures reviewed here the only cases where a crystallographic symmetry plane coincides with that of a planar molecule occur in s-triazine and pyrazine (see Section V, A, 5). 

A combination semiempirical MO and X-ray study of pyrrolo[ 1,2-a]quinoxalines has been conducted [95JHC1317], and X-ray crystal structure reports of 1,2,3,5,6,8a-hexahydro-8,8a-diphenylimidazofl, 2-a]pyrazine [95AX(C)482], adducts of pyrazine-2,3-dicarboxylic acid and... 

The bonding in the cation LXX can be qualitatively explained in the same way as the bonding in the corresponding germanium species. The cationic pentamethylcyclopentadienyltin unit acts as a Lewis acid in its reactions with pyridine, pyrazine, and 2,2 -bipyridine, in which the adducts LXXXIa-d are formed [Eq. (41)] (205). X-Ray crystal structure data of adducts LXXIa and c... 

A lot of experimental data have been reported in CHEC-I. The x-ray crystal structure of pyra-zino[2,3-6]pyrazine and some special spectroscopic investigations were reported. More experimental... 

The X-ray crystal structures of pyrazine V.JV -dioxide (134) <02AX(E)1253>, the P-polymorph of phenazine (135) <02AX(C)181>, cobalt(III) complexes of pyrazine-2,6- and pyridine-2,6-dicarboxylic acids <02JIC458>, and bis-urea-substituted phenazines <02ZN(B)937> were reported. Fluorescent pyrido[l,2-a]quinoxalines 136 prepared as pH indicators were examined by X-ray crystallography <02JCS(P2)181>, as were macrocyclic quinoxaline-bridged porphyrinoids obtained from the condensation of dipyrrolylquinoxalines 137 and 1,8-diaminoanthracene... 

The pyrazine-bridged complex [OsRu(pyz)bipy4Cl2]2+ has been reported and its electronic spectrum and cyclic voltammetry have been measured two one-electron oxidations are observed.112 In recent work the X-ray crystal structure of [0s2(pyz)(NH3)1(l]Cl6-2H20 has been determined the Os —N (pyrazine) distance is 2.101(7) A.I14b... 

Fig. 45 X-ray crystal structure of the carcerand-like dimeric capsule 55 with pyrazine as a guest...

The crystal structure of 2-carbamoylpyrazine has been determined the pyrazine ring is completely planar, with mean C-N = 1.348 and C-C = 1.383 A, and made an angle about 5° to the amide group (C-0 = 1.244 and C-N = I.3I2 A) (1395). Investigations for 2-carbamoylpyrazine have been made of the heat of sublimation (1396), heat of fusion (1397), and solubilities in various solvents (1398) and the results correlated. The pA of 2-carbamoylpyrazine has been determined as — 0.5 (140). 

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