Pyrazine crystal phases

Crystal phases. The measured heat capacities for crystalline pyrazine in the range 20-40°C suggest that, in each of the phases involved, —50% of the molecules must be disordered.556... 

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... 

Polymers ofthe loop and chain type (A) can be obtained after crystallization by slow diffusion of the metallic salts and the quinoxaline or pyrazine derivatives in two different phases in the presence of organic spacers such as pyrazine (prz) or 4,4 -bipyridine (bpy) [see Eq. (8)]. 

Two crystal modifications and an amorphous phase were found for 24 by X-ray diffraction and differential thermogravimetric analysis [91KFZ(3)78]. The structure of a complex formed from 3-[p-methylphenyl)amino]-4-[(p-methylphenyl)imino]-4//-pyrido [1,2-a]pyrazine, sodium bis(trimethylsi-lyl)amide and (norbomadiene)Mo(CO)4 in THF was characterized by single-crystal X-ray diffraction [95JPR(337)38],... 

Rh2(bza)4(2-epyz)](2-epyz= 2-ethyl-pyrazine). tion (b, c), C02 isotherm curves with large leap Contact of C02 at ethyl lever on the channel during phase transition (d), and channel surface (a), topological change of inner space direction change in a single-crystal. 

The results of the fits of the filtered experimental spectra to the theoretical formula (1) are reported in Table 3 and Fig. 17. Phase shifts cp(k) and amplitude functions A(k) were extracted from the spectra of model compounds with known crystal structures Cu—O, Cu—C, and Cu—O from Na CufCjOJ 2 H O at 300 K and Cu—Cu from CUj(CH3COO)4 pyrazine at 30 K. These preliminary fits are also presented in Table 3... 

Structural parameters and interatomic distances derived from electron diffraction <77JST(42)121> and x-ray diffraction studies <76AX(B)3178> were given in CHEC-I. The molecular structure of pyrazine has been determined by combined analyses of data obtained by gas-phase electron diffraction (ED) and liquid-crystal NMR (LCNMR) <88JA2758>. The NMR spectrum gives structural information because the solute is partially oriented in the liquid-crystal solvent. The structural parameters determined from the ED, LCNMR data and in a joint analysis of both are listed in Table 2. There the C—C bonded distance is fixed since LCNMR data give no information on the absolute size of the molecule. Since pyrazine itself has no dipole moment, it should not show a microwave (pure rotation) spectrum. 

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