Neutron diffraction study of quinolinic acid recrystallized from D2O: evaluation of temperature and isotope effects in the structure

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0567740879008591.The structure of quinolinic acid recrystallized from D2 0 (2,3-pyridinedicarboxylic acid; C7H3D2N04 ) has been refined based on neutron diffraction data measured at four temperatures: 35, 80, 100 and 298 K. The principal temperature dependence in cell constants is observed for the b axis, which is perpendicular to the molecular planes. The refined thermal parameters have been extrapolated by a least-squares procedure, to yield values for T = 0 K which provide estimates of the combined effects of static disorder and zero-point motion. The D atom shifts toward the midpoint of the short intramolecular 0 - - - 0 hydrogen bond when the crystal is cooled, just as was found in an earlier study to occur for the H atom in the undeuterated material. At 100 and 298 K, the D atom is displaced significantly further from the bond midpoint than is the H atom at the same temperature. The magnitude of this isotope effect appears to be independent of temperature. The exchangeable protons in the crystal have not been completely replaced by D; refinement of the D scattering lengths indicates the presence of approximately 2-7% H attached to N(l) and 4-4% H in the short hydrogen bond.Research carried out at Brookhaven National Laboratory under contract with the US Department of Energy and supported by its Division of Basic Energy Sciences

A study of the charge density in putrescine diphosphate at 85 K

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0567740879005082.See article for abstract

A refinement of the crystal structure of quinolinic acid at 100 K with neutron diffraction data

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0567740878005117.The crystal structure of quinolinic acid (pyridine-2,3-dicarboxylic acid: C7H3N04 ) has been refined, based upon neutron diffraction data measured at 100 K. Crystal data: space group P2l/cm, a — 7-415 (5), b = 12-396 (9), c = 7-826 (6) A, P = 117-05 (4)°, Z = 4. The final unweighted R value based on F2 is 0-048 for all 1643 unique reflections, and bond distances have been determined with precision better than 0-003 A. The major temperature dependence in the cell constants is observed for the b axis, which is perpendicular to the molecular planes. Rigid-body analyses of the thermal parameters determined here at 100 K and those found in a prior study at 297 K indicate that the translational motion changes with temperature more along the b axis than in other directions. Significant differences between the structure at 100 and 297 K are observed in the intramolecular O • • • H • • • O hydrogen bond, where the H atom shifts towards the midpoint of the bond when the crystal is cooled. This temperature effect is discussed in the light of results of the rigid-body thermalmotion analyses

Neutron diffraction study at 37 K of sodium triaqua(ethylenediaminetetraacetato)samarate(III) pentahydrate, Na[Sm(C10H12N2O8)(H2O)3].5H2O

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0108270184009173.See article for abstract

C-H...O interactions and stacking of water molecules between pyrimidine bases in 5-nitro-1-([beta]-D-ribosyluronic acid)-uracil monohydrate [1-(5-nitro-2,4-dioxopyrimidinyl)-[beta]-D-ribofuranoic acid monohydrate]: a neutron diffraction study at 80 K

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0567740879006506.See article for abstract.Research carried out at Brookhaven National Laboratory under contract with the US Department of Energy, and supported by its Office of Basic Energy Sciences

Abundance of unknots in various models of polymer loops

A veritable zoo of different knots is seen in the ensemble of looped polymer chains, whether created computationally or observed in vitro. At short loop lengths, the spectrum of knots is dominated by the trivial knot (unknot). The fractional abundance of this topological state in the ensemble of all conformations of the loop of $N$ segments follows a decaying exponential form, $\sim \exp (-N/N_0)$, where $N_0$ marks the crossover from a mostly unknotted (ie topologically simple) to a mostly knotted (ie topologically complex) ensemble. In the present work we use computational simulation to look closer into the variation of $N_0$ for a variety of polymer models. Among models examined, $N_0$ is smallest (about 240) for the model with all segments of the same length, it is somewhat larger (305) for Gaussian distributed segments, and can be very large (up to many thousands) when the segment length distribution has a fat power law tail.Comment: 13 pages, 6 color figure