Mixed Crystals of Phenazine and N-oxyphenazine: Refinement of Crystal Structures

A. M. Glazer
1970 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  
PAGE 593 D a ta collection page 594 G row th a nd composition of R efinement and results 611 MIXED CRYSTALS 594 R eferences 622 Compositions (accurate to about 1% ) of mixed crystals of phenazine (P) and iV-oxyphenazine (NOP) were determined from their u.v. absorption spectra. Densities, habit and unit-cell dimensions were found at 20 °C for crystals containing respectively 0, 8, 52, 81 and 100 mole % N O P relative to P; and X-ray diffraction data (Fobs) were obtained for all these and also at
more » ... l these and also at -90 °C for NOP. All are isostructural (NOP being pseudocentrosymmetric) in P2Ja with two molecules per average unit cell. Atomic coordinates, bond lengths and anisotropic tem perature factors are listed, site-occupation factors for the oxygen atoms being used for the 8, 52 and 81 mole % compositions. A qualitative explanation, in terms of structure, is offered for the anisotropic therm al expansion of NOP. I ntroduction In 1961, Curti, Riganti & Locchi ( 1961) investigated the crystal structure of iV-oxyphenazine. The space group was found to be PZJc with two molecules in the unit cell. Since the molecules of iV-oxyphenazine are not centrosymmetric (figure it was decided that they must pack in the crystal in two possible orientations to produce an average centre of symmetry. Such arrange ments are fairly common in crystals, for example 1,4-bromochlorobenzene (Hendricks 1933), azulene (Robertson, Shearer, Sim & Watson 1962), anthrone (Srivastava 1964; Flack 1968). The crystal structure of iV-oxyphenazine was therefore refined by using a phenazine nucleus to which were attached oxygen atoms with half-weight, as in figure 1 b. The disordered structure was thus dealt with as if it were truly centrosymmetric. Refinement was carried out for the fractional coordinates and isotropic temperature factors. The final results ( = 0.17 approx.) seemed to indicate that the initial assumption of an average centre of symmetry was correct. Other evidence for this was provided by polarity tests and the N{z) test (Howells, Phillips & Rogers 1950). This was further tested by growing mixed crystals of iV-oxyphenazine and phenazine. The results of the structure analysis of phenazine (Herbstein & Schmidt 1955) showed that this centrosymmetric molecule (C12H 8N 2) packed in the crystal in the same space group as iV-oxyphenazine with two molecules in the unit cell. According to the theory of Kitaigorodskii (1957), which deals with the conditions for solubility in the solid state, complete miscibility between the two components is possible if the symmetry of the distribution of the molecules in the end-members is the same. Since Curti, Riganti and Locchi obtained a series of mixed crystals showing a gradual change in cell dimensions and intensities of reflexions, they considered this to be sufficient evidence for the existence of an average centre of symmetry. However, as will be shown in the following paper (part V) there is considerable evidence that there is a region of immiscibility in the mixed-crystal series. 64-2 A. M. GLAZER In the present paper, further refinement of the crystal structures of phenazine and N-oxyphenazine will be described, together with the structures of some of the mixed crystals. In later papers, the variation of physical properties with changing composition and the short-range-order diffuse scattering will be discussed. F i g u r e 1. (a) Formula for iV-oxyphenazine. (b) Formula assumed for purposes of calculation, showing numbering adopted in this paper. G row th and composition of mixed crystals Definite amounts of phenazine and iV-oxyphenazine were weighed out, mixed and dissolved together in methyl ethyl ketone, in small glass vessels made especially for this purpose. The solutions were then heated over a water-bath and excess methyl ethyl ketone was driven off, until some deposition of solid appeared on the walls of the glass vessels. These containers were sealed with ground-glass stoppers and allowed to cool in vacuum flasks. In this manner, small crops of needle or lath-like crystals could be grown in about 20 min. The crystals were removed from the liquor with a spatula, care being taken to avoid having much of the solution drying on their surfaces. This ensured that the outer portions of the crystals did not assume compositions differing from the interior. Preliminary X-ray photographs of the crystals, taken with the needle axis vertical, showed that there was no significant difference within any single batch and also that the crystals were all monoclinic, the b axis being the needle axis in each case. Ultraviolet absorption peaks were obtained at 250 nm for phenazine and 266 nm for A-oxyphenazine. Typical spectra are shown in figure 2. The molar extinction coefficients were measured: e lge phenazine 1.05 x 105 5.02 at 250 nm iV-oxyphenazine 0.86 x 105 4.94 at 266 nm By using these values and the relation D = eel, where D is the optical density, e the molar extinc tion coefficient, c the concentration/moll-1 and l the path length/cm, it was possible to determine the compositions of the mixed crystals to about 1 % accuracy. D ata collection The unit-cell dimensions, densities and habit were examined. The detailed results are given in table 1 and in part V of this paper. For convenience, a and c were taken so that the space group P21ja was assigned to A-oxyphenazine (rather than as given by Curti et al. 1961) IV. MIXED CRYSTALS OF PHENAZINE AND Y-OXYPHENAZINE 595 and to the mixed crystals, to agree with that already given by Herbstein & Schmidt (1955) for phenazine. Any attempts at cutting the crystals caused splitting, usually parallel to the length of the needles ([010]) and therefore it was found best to collect the necessary intensity data about the b axis only. To do this, the inclined-beam oscillation method of Milledge (1963) was used, with C uK a radiation. In this method a Weissenberg camera is employed, with the camera inclined at an 250 275 200 225 200 225 250 275 nm F i g u r e 2 . Typical u.v. spectra of solutions (in methyl ethyl ketone) of the mixed crystals, used in the determination of composition.
doi:10.1098/rsta.1970.0015 fatcat:woanp2uvivbdzevkcekt33vglm