From Proton Disorder to Proton Migration: A Continuum in the Hydrogen Bond of a Proton Sponge in the Solid State

The crystal structure of the ionic complex of 1,8-bis(dimethylamino)naphthalene (DMAN) and 4,5-dichlorophthalic acid has been investigated using a multi-temperature X-ray diffraction approach. The two short intramolecular hydrogen bonds exhibit different behaviors, with the [O···H···O]- hydrogen bond appearing approximately centered throughout the experiment. The [N···H···N]+ hydrogen bond, on the other hand, exhibits a complicated temperature-dependent behavior, with disordered electron density being observed at lower temperatures, with apparent migration at higher temperatures. This continuum can be explained simply in terms of the changing local environment of the [N···H···N]+ hydrogen bond induced by lattice expansion. The related complex of DMAN with 3,4-furandicarboxylic acid shows no equivalent effects; this is consistent with the explanation proposed

Electron and Nuclear Positions in the Short Hydrogen Bond in Urotropine-<i>N</i>-oxide·Formic Acid

The crystal structure of urotropine-N-oxide·formic acid, as determined from multiple temperature single-crystal X-ray diffraction experiments in the range 123−295 K and from neutron diffraction at 123 K, is reported. There is a strong hydrogen bonding interaction between the OH of formic acid and the N-oxide of urotropine, with the oxygen−oxygen distance ranging from 2.4300(10) to 2.4469(10) Å. The electron density of the hydrogen atom associated with this interaction was located in the Fourier difference maps of the spherical atom refinement after all heavy atom positions were determined. The maximum of the electron density associated with the hydrogen bond is located approximately 1.16 Å from the formate segment, though the distribution of electron density is very broad. The electron density associated with the H atom is thus shown by these accurate X-ray diffraction experiments to be approximately centered at all temperatures studied. This was conclusively confirmed by single-crystal neutron diffraction data obtained at 123 K, from which statistically equivalent O−H distances of 1.221(7) and 1.211(7) Å were obtained

Structural Investigation of TS-1: Determination of the True Nonrandom Titanium Framework Substitution and Silicon Vacancy Distribution from Powder Neutron Diffraction Studies Using Isotopes

The first complete investigation of the structure of the selective oxidation catalyst TS-1 is presented. Constant wavelength powder neutron diffraction data collected on isotopically substituted titanium silicalite (TS-1) samples, with a Si:Ti molar ratio of 39:1, have been studied using a combination of single and multiple data set Rietveld analysis exploiting the scattering length contrast between the different titanium isotopes and silicon. This has allowed both the silicon vacancy and titanium site substitution distributions to be determined, which has not been possible previously. Both distributions are found to be nonrandom with the titanium preferentially substituting on 3 of the 12 crystallographically independent framework sitesT8, T10 and T3 (in order of decreasing titanium content)and silicon vacancies on 2 framework sitesT1 and T5. This study illustrates the power of isotopic substitution in powder neutron diffraction experiments to yield enhanced structural information in complex systems

On the Solid State Structure of 4-Iodobenzoic Acid

The solid-state structure of 4-iodobenzoic acid has been confirmed by variable temperature X-ray diffraction, variable temperature solid-state NMR and differential scanning calorimetry. 4-iodobenzoic acid crystallizes in the space group P21/n, and dimerizes in the solid state about a center of inversion. Using extensive X-ray crystallographic data collections, the placement of the carboxylate H atoms from the residual electron density in difference Fourier maps was determined. The position of the electron density associated with the proton is found to vary with temperature in that the population of the disordered sites changes with varying temperature. Determination of the crystal structure between the temperatures of 248 and 198 K was not possible due to a phase transition, an endothermic event occurring at 230.77 K. The phase transition is also indicated by a change in the relaxation time of the ring carbon atoms in the solid-state NMR data. Though the dominating force in the dimeric unit in the solid state is the presence of strong hydrogen bonds, there are also van der Waals forces present between the iodine atoms. In the layered structure, the iodine−iodine distance is within the van der Waals contact radii, an interaction which causes a deformation in the electron density of the iodine atoms

Electron and Nuclear Positions in the Short Hydrogen Bond in Urotropine-<i>N</i>-oxide·Formic Acid

The crystal structure of urotropine-N-oxide·formic acid, as determined from multiple temperature single-crystal X-ray diffraction experiments in the range 123−295 K and from neutron diffraction at 123 K, is reported. There is a strong hydrogen bonding interaction between the OH of formic acid and the N-oxide of urotropine, with the oxygen−oxygen distance ranging from 2.4300(10) to 2.4469(10) Å. The electron density of the hydrogen atom associated with this interaction was located in the Fourier difference maps of the spherical atom refinement after all heavy atom positions were determined. The maximum of the electron density associated with the hydrogen bond is located approximately 1.16 Å from the formate segment, though the distribution of electron density is very broad. The electron density associated with the H atom is thus shown by these accurate X-ray diffraction experiments to be approximately centered at all temperatures studied. This was conclusively confirmed by single-crystal neutron diffraction data obtained at 123 K, from which statistically equivalent O−H distances of 1.221(7) and 1.211(7) Å were obtained

An Analysis of the Thermal Motion in the Negative Thermal Expansion Material Sc₂(WO₄)₃ Using Isotopes in Neutron Diffraction

A full analysis of thermal motion has been carried out for the negative thermal expansion material Sc2(WO4)3 over the temperature range 50−823 K. By obtaining neutron diffraction data from isotopically pure samples of the compositions Sc2(184WO4)3, Sc2(186WO4)3 and natural Sc2(WO4)3, coupled with multidata set refinement methods, it is possible to extract anisotropic thermal parameters for individual atoms throughout the temperature range. Results indicate that the thermal motion of two Sc−O−W bridging oxygen atoms with the largest Sc−O−W angles is better represented by thermal toroids consistent with strong local motion of these units. The thermal behavior of the other oxygen atoms in the structures as a function of temperature is normal. Sc−O and W−O bond lengths corrected for the effect of correlated thermal motion show the expected increase with temperature

On the Solid State Structure of 4-Iodobenzoic Acid

The solid-state structure of 4-iodobenzoic acid has been confirmed by variable temperature X-ray diffraction, variable temperature solid-state NMR and differential scanning calorimetry. 4-iodobenzoic acid crystallizes in the space group P21/n, and dimerizes in the solid state about a center of inversion. Using extensive X-ray crystallographic data collections, the placement of the carboxylate H atoms from the residual electron density in difference Fourier maps was determined. The position of the electron density associated with the proton is found to vary with temperature in that the population of the disordered sites changes with varying temperature. Determination of the crystal structure between the temperatures of 248 and 198 K was not possible due to a phase transition, an endothermic event occurring at 230.77 K. The phase transition is also indicated by a change in the relaxation time of the ring carbon atoms in the solid-state NMR data. Though the dominating force in the dimeric unit in the solid state is the presence of strong hydrogen bonds, there are also van der Waals forces present between the iodine atoms. In the layered structure, the iodine−iodine distance is within the van der Waals contact radii, an interaction which causes a deformation in the electron density of the iodine atoms

Electron and Nuclear Positions in the Short Hydrogen Bond in Urotropine-<i>N</i>-oxide·Formic Acid

The crystal structure of urotropine-N-oxide·formic acid, as determined from multiple temperature single-crystal X-ray diffraction experiments in the range 123−295 K and from neutron diffraction at 123 K, is reported. There is a strong hydrogen bonding interaction between the OH of formic acid and the N-oxide of urotropine, with the oxygen−oxygen distance ranging from 2.4300(10) to 2.4469(10) Å. The electron density of the hydrogen atom associated with this interaction was located in the Fourier difference maps of the spherical atom refinement after all heavy atom positions were determined. The maximum of the electron density associated with the hydrogen bond is located approximately 1.16 Å from the formate segment, though the distribution of electron density is very broad. The electron density associated with the H atom is thus shown by these accurate X-ray diffraction experiments to be approximately centered at all temperatures studied. This was conclusively confirmed by single-crystal neutron diffraction data obtained at 123 K, from which statistically equivalent O−H distances of 1.221(7) and 1.211(7) Å were obtained

On the Solid State Structure of 4-Iodobenzoic Acid

The solid-state structure of 4-iodobenzoic acid has been confirmed by variable temperature X-ray diffraction, variable temperature solid-state NMR and differential scanning calorimetry. 4-iodobenzoic acid crystallizes in the space group P21/n, and dimerizes in the solid state about a center of inversion. Using extensive X-ray crystallographic data collections, the placement of the carboxylate H atoms from the residual electron density in difference Fourier maps was determined. The position of the electron density associated with the proton is found to vary with temperature in that the population of the disordered sites changes with varying temperature. Determination of the crystal structure between the temperatures of 248 and 198 K was not possible due to a phase transition, an endothermic event occurring at 230.77 K. The phase transition is also indicated by a change in the relaxation time of the ring carbon atoms in the solid-state NMR data. Though the dominating force in the dimeric unit in the solid state is the presence of strong hydrogen bonds, there are also van der Waals forces present between the iodine atoms. In the layered structure, the iodine−iodine distance is within the van der Waals contact radii, an interaction which causes a deformation in the electron density of the iodine atoms

Variable Temperature Powder Neutron Diffraction Study of SmNiO₃ through Its M−I Transition Using a Combination of Samarium and Nickel Isotopic Substitution

Neutron powder diffraction studies of 154Sm58NiO3, 154Sm60NiO3, and 154Sm62NiO3, at a range of temperatures through the M−I transition at approximately 128 °C, have been performed on the new general materials diffractometer (GEM) at ISIS, RAL. With combined data-set Rietveld analysis, using samples containing different nickel isotopes with contrasting scattering lengths, it has been found that extremely high quality structural parameters can be determined, even though total data collection times are more than an order of magnitude shorter than those previously used for this system. Rietveld analysis shows that the evolution of the structural parameters over the temperature range are smooth and that no symmetry change or abrupt structural transition occurs at the M−I transition. This is consistent with evolution of the high-temperature metallic material within the low-temperature insulating phase over the temperature range 108−128 °C. The key effects of thermal motion on the M−I transition have been extracted from the data and are discussed