Determination of the structure of 2(benzene-1,3,5-tricarboxylic acid)-1.5(pyrene)-2(methanol) and comparison with that of 2(benzene-1,3,5-tricarboxylic acid)-pyrene-2(ethanol).

The structure of the title methanol complex (P1;, Z = 2) has been determined and compared with that of the title ethanol complex (C2/c, Z = 8) using published data. Both complexes have layer structures, the (essentially planar) layers being constructed from rings of six TMA molecules, hydrogen bonded through four 'carboxyl dimers' and two 'interrupted dimers', where methanol (ethanol) is included in the R4(4)(12) (graph set) ring. The packing of the layers differs in the two complexes, leading to different three-dimensional structures. In the methanol complex, one pyrene molecule is located within the layer and the other, at a centre of symmetry, between the layers in one type of interlayer space, while the methyls of methanol protrude into the other type of interlayer space. In the ethanol complex, the superpositioning of the layers is such that two types of stack are formed; one of these is mixed, containing pyrene and one of the independent TMA molecules in alternating sequence, while the other stack contains only the second type of TMA. Spectroscopic study is needed to establish whether the partial mixed stack arrangement in the crystalline ethanol complex implies donor-acceptor interaction.

Correction of the space group of [Ni(bipy)(2)(ONO(2))(2)] x 2(pyrene).

The crystal structure of [Ni(bipy)(2)(ONO(2))(2)] x2(pyrene), poly[[[bis(nitrato-O)nickel(II)]bis(mu-4,4'-bipyridyl-N,N'] bis(pyrene)], was originally reported in space group Pn [Biradha et al. (1999). Chem. Commun. pp. 1327--1328]. Reasons are given for changing the space group to P2(1)/n. Consequently, incorrect descriptions of the title compound in the literature must be altered. In particular, the structure is not polar. It is further contended that description in terms of 'complementary, interpenetrating covalent and noncovalent two-dimensional networks' is misleading as the 'noncovalent network' (of pyrenes) has geometrical but not physical significance. The title compound is a typical host-guest inclusion complex.

How precise are measurements of unit-cell dimensions from single crystals?

The results of single-site and many-site measurements of cell dimensions from single crystals are compared for Bond and four-circle diffractometers using samples of corundum (essentially pure rhombohedral alpha-Al2O3, aluminum oxide) of high diffraction quality, where the effects of small changes in temperature and composition (Cr2O3, chromium oxide, in solid solution) can be taken into account. Similar comparisons are made for four-circle diffractometer measurements on ruby (alpha-Al2O3, with 0.46 wt % Cr in solid solution). The precisions are some parts in 10(5). There is partial support for the Taylor-Kennard [Acta Cryst. (1986), B42, 112-120] dictum that standard uncertainties (s.u.s) of cell parameters from routine four-circle diffractometer measurements are less than those for many-site measurements by factors of 5 for cell lengths and 2.5 for cell angles. For organic crystals, independent repetitions of adequate quality for comparison and analysis of routine four-circle diffractometer measurements are available only for alpha-oxalic acid dihydrate and anthracene. The experimental standard uncertainties given for these two crystals agree reasonably well with the sample s.u.s at room temperature, but appreciably less well at approximately 100 K, again giving partial support to the Taylor-Kennard dictum. The relation between specimen characteristics and attainable precision is emphasized; the precisions for routine measurements on good quality organic crystals are some parts in 10(4). Area-detector measurements of cell dimensions have also been appraised; currently published s.u.s from such measurements appear to be highly unreliable, and this is supported by a recent analysis of the operation of such diffractometers [Paciorek et al. (1999). Acta Cryst. A55, 543-557]. Formulation of a standard protocol for such measurements is badly needed. The dangers inherent in high degrees of replication are illustrated by recounting Kapteyn's Parable of the Chinese Emperor. Attention is drawn to the fact that there has been little improvement in claimed precisions over the past 40-60 years.

Determining the crystal structure of twinned 2-methylpyrazine.

Although systematic absences and symmetry relations among reflections pointed to space group I4(1)22 (one molecule in the asymmetric unit), a direct methods solution could only be obtained in I(-)4 (two molecules in the asymmetric unit). Refinement in I(-)4 was unsatisfactory until merohedral twinning was taken into account. The resulting molecular dimensions are in excellent agreement with analogous values in the literature. The molecular arrangement is described.

X-ray diffraction study of the crystal structure of the pi-molecular compound pyrene ... pyromellitic dianhydride at 19 K.

The crystal structure of the pyrene ... pyromellitic dianhydride (PMDA) pi-molecular compound [(C16H10:C10H2O6); PYRPMA] has been refined from intensities measured at 19 K using the low-temperature accessory designed by Samson, Goldish & Dick [J. Appl. Cryst. (1980), 13, 425-432] for a four-circle diffractometer. Earlier results for the ordered structure [Herbstein & Snyman (1969). Philos. Trans. R. Soc. London Ser. A, 264, 635-666] are confirmed and extended; at 19 K, a = 13.664 (3), b = 9.281 (2), c = 14.420 (3) A, beta = 91.80 (2) degrees, space group P2(1)/n, Z = 4, with two sets of pyrenes at independent centres of symmetry and the four PMDAs at general positions. The geometrical structures of the two components are in good agreement with quantum mechanical calculations. Analyses of thermal motion and packing show that one set of pyrenes is more tightly packed than the other; the principal interactions in the crystal are pi-pi* plane-to-plane interactions between pyrene and PMDA and > CH ... O = C <, between pyrene and PMDA, and between PMDAs.

X-ray diffraction study of the disorder-to-order transition at approximately 160 K in the pi-molecular compound pyrene ... pyromellitic dianhydride.

The crystal structure of pyrene ... pyromellitic dianhydride [(C16H10:C10H2O6); PYRPMA] has been studied over the range 300-19 K, using the low-temperature accessory designed by Samson, Goldish & Dick [J. Appl. Cryst. (1980), 13, 425-432] for a four-circle diffractometer. Earlier results for the disordered and ordered structures [Herbstein & Snyman (1969). Philos. Trans. R. Soc. London Ser. A, 264, 635-666] are confirmed and extended. At 295 K, a = 13.94 (1), b = 9.34 (1), c = 7.31 (1) A, beta = 93.65 (9) degrees, space group P2(1)/a, Z = 2, with pyrenes and pyromellitic dianhydrides (PMDAs) at crystallographic centres of symmetry. At 19 K, a = 13.664 (3), b = 9.281 (2), c = 14.420 (3) A, beta = 91.80 (2) degrees, space group P2(1)/n, Z = 4, with two sets of pyrenes at independent centres of symmetry and the four PMDAs at general positions. There are no discontinuities in cell dimensions with temperature (measurements at approximately 10 K intervals, down to 19 K) but db/dT and d beta/dT show discontinuities at approximately 167 K. Superlattice reflections appear below approximately 164 K (= Tc by X-ray diffraction), corresponding to the doubling of c and change of space group; the specific heat shows an anomalous increase over the range 120-155 K, giving a lambda-type peak. These results show that the transition is second order with regard to Ehrenfest's criteria. PYRPMA is a co-elastic crystal and quantitative analysis shows a linear dependence of the squares of spontaneous strain and of normalized superlattice intensity on temperature; hence, in terms of Landau theory, the transition is tricritical. However, the excess specific heat cannot be explained entirely on this basis. The physical nature of the transition is discussed. PYRPMA is so far unique among the pi-molecular compounds showing disorder-to-order transitions in the solid state in that there is a doubling of one of the axes; all indications are, however, that it resembles the other examples in the sense that subtle intermolecular packing interactions (here between pyrenes and PMDAs) are the driving force for the transition rather than electronic or charge-transfer interactions.

More space-group changes.

Revised structures are reported for 19 crystalline compounds, based on space groups of higher symmetry than originally reported. In four cases the Laue symmetry is changed, one from 1 to 2/m and three from 2/m to mmm; in the remaining fifteen a center of symmetry has been added. For eight of these latter compounds we have obtained F values and carried out least-squares refinements in the centrosymmetric space groups, with more satisfactory results than originally reported.