Arrays with Local Centers of Symmetry in Space Groups Pca2_1 and Pna2_1

Of the several hundred structures in the Cambridge Structural Database [version 4.6 (1992), Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, England] having space groups Pca2_1 or Pna2_1 and more than one molecule in the asymmetric unit (Z > 4), approximately three-quarters contain local centers of symmetry. These local centers, which are not crystallographic centers, occur predominantly near x = 1/8, y = ¼ in Pca2_1 or near x = 1/8, y = 0 in Pna2_1; this also holds for the limited number of examples we have examined of pseudo-centrosymmetric molecules with Z = 4. Local centers at these points create unusual correlations between corresponding atoms in the two molecules

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

The crystal structure of the pyrene···pyromellitic dianhydride (PMDA) π-molecular compound [(C _(16)H_(10):C_(10)-H_2O_6); 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) Å, β = 91.80 (2)°, space group P2_1/n = 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 π-π* plane-to-plane interactions between pyrene and PMDA and >CH···O=C<, between pyrene and PMDA, and between PMDAs

City of Cape Town Solar Water Heater By-law: Barriers to Implementation

The study of implementation has had tremendous importance for the study of policy. It opened up the black box of ‘after-a-formal-decision’ politics and demonstrated, among other things, that the political process continues all the way through to the final output of the policy process (Bardach 1977). It addressed the complexity of achieving policy goals, offered new insights into the importance of lower-level actors in policy, and attended to the effects that clients and extra-government groups had on the policy result (Schofield 2001). It became one of the most important sources for the development of new perspectives that tried to capture how policy processes cross the public-private divide, as evidenced by the new focus on governance (Rhodes 1997) or networks (Marin and Mayntz 1991). Implementation research has been particularly valuable in two somewhat contradictory ways

4-(Dimethyl­amino)benzaldehyde

The title compound, C9H11NO, crystallizes with two independent but essentially identical mol­ecules in the asymmetric unit, which are linked via a C—H⋯π inter­action. In both mol­ecules, the aldehyde and dimethyl­amine groups are essentially coplanar with the attached benzene ring. In the crystal structure, C—H⋯O hydrogen bonds link one type of independent mol­ecules into a chain along the a axis. In addition, the structure is stabilized by π–π stacking inter­actions involving the benzene rings [centroid-to-centroid distance = 3.697 (2) Å]

Trimesic acid dimethyl sulfoxide solvate: space group revision

The structure of the title solvate, C9H6O6·C2H6OS, was determined 30 years ago [Herbstein, Kapon & Wasserman (1978 ▶). Acta Cryst. B34, 1613–1617], with data collected at room temperature, and refined in the space group P21. The present redetermination, based on high-resolution diffraction data, shows that the actual space group is more likely to be P21/m. The crystal structure contains layers of trimesic acid molecules lying on mirror planes. A mirror plane also passes through the S and O atoms of the solvent molecule. The molecules in each layer are inter­connected through strong O—H⋯O hydrogen bonds, forming a two-dimensional supra­molecular network within each layer. The donor groups are the hydroxyls of the trimesic acid mol­ecules, while the acceptors are the carbonyl or the sulfoxide O atoms

X-ray diffraction study of the disorder-to-order transition at ~160 K in the π-molecular compound pyrene···pyromellitic dianhydride

The crystal structure of pyrene...pyromellitic dianhydride [(C_(16)H_(10):C_(10)H_2O_6); PYRPMA] has been studied over the range 300-19K, 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) Å, β = 93.65 (9)°, space group P2_l/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)Å, β = 91.80 (2)°, space group P2_l/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 ~10K intervals, down to 19 K) but db/dT and dβ /dT show discontinuities at ~167 K. Superlattice reflections appear below ~164K (= T_c 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 120-155K, giving a λ-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 π-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

X-ray diffraction study of the disorder-to-order transition at ~160 K in the π-molecular compound pyrene···pyromellitic dianhydride

The crystal structure of pyrene...pyromellitic dianhydride [(C_(16)H_(10):C_(10)H_2O_6); PYRPMA] has been studied over the range 300-19K, 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) Å, β = 93.65 (9)°, space group P2_l/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)Å, β = 91.80 (2)°, space group P2_l/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 ~10K intervals, down to 19 K) but db/dT and dβ /dT show discontinuities at ~167 K. Superlattice reflections appear below ~164K (= T_c 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 120-155K, giving a λ-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 π-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 Corrections: From Triclinic to Centred Monoclinic and to Rhombohedral; Also From P1 to P-1 and From Cc to C2/c

We present 14 examples of crystal structures that were originally described as triclinic, but are properly described as either C-centred monoclinic (ten examples) or rhombohedral (four examples). There is also one example each of changes from P1 to P1 and from Cc to C2/c

More Space-Group Corrections: From Triclinic to Centred Monoclinic and to Rhombohedral; Also From P1 to P-1 and From Cc to C2/c

We present 14 examples of crystal structures that were originally described as triclinic, but are properly described as either C-centred monoclinic (ten examples) or rhombohedral (four examples). There is also one example each of changes from P1 to P1 and from Cc to C2/c

The Bibliometric Properties of Article Readership Information

The NASA Astrophysics Data System (ADS), along with astronomy's journals and data centers (a collaboration dubbed URANIA), has developed a distributed on-line digital library which has become the dominant means by which astronomers search, access and read their technical literature. Digital libraries such as the NASA Astrophysics Data System permit the easy accumulation of a new type of bibliometric measure, the number of electronic accesses (``reads'') of individual articles. We explore various aspects of this new measure. We examine the obsolescence function as measured by actual reads, and show that it can be well fit by the sum of four exponentials with very different time constants. We compare the obsolescence function as measured by readership with the obsolescence function as measured by citations. We find that the citation function is proportional to the sum of two of the components of the readership function. This proves that the normative theory of citation is true in the mean. We further examine in detail the similarities and differences between the citation rate, the readership rate and the total citations for individual articles, and discuss some of the causes. Using the number of reads as a bibliometric measure for individuals, we introduce the read-cite diagram to provide a two-dimensional view of an individual's scientific productivity. We develop a simple model to account for an individual's reads and cites and use it to show that the position of a person in the read-cite diagram is a function of age, innate productivity, and work history. We show the age biases of both reads and cites, and develop two new bibliometric measures which have substantially less age bias than citationsComment: ADS bibcode: 2005JASIS..56..111K This is the second paper (the first is Worldwide Use and Impact of the NASA Astrophysics Data System Digital Library) from the original article The NASA Astrophysics Data System: Sociology, Bibliometrics, and Impact, which went on-line in the summer of 200