RESUMO
Recent work on understanding why protons migrate with increasing temperature in short, strong hydrogen bonds is extended here to three more organic, crystalline systems. Inelastic neutron scattering and density functional theory based simulations are used to investigate structure, vibrations, and dynamics of these systems as functions of temperature. The mechanism determined in a previous work on urea phosphoric acid of low frequency vibrations stabilizing average crystal structures, in which the potential energy well of the hydrogen bond has its minimum shifted towards the center of the bond, is found to be valid here. The new feature of the N-H...O hydrogen bonds studied in this work is that the proton is transferred from the donor atom to the acceptor atom. Molecular dynamics simulations show that in an intermediate temperature regime, in which the proton is not completely transferred, the proton is bistable, jumping from one side of the hydrogen bond to the other. In the case of 3,5-pyridine dicarboxylic acid, which has been studied in most detail, specific phonons are identified, which influence the potential energy surface of the proton in the short, strong hydrogen bond.
RESUMO
The experimental charge density in the title complex has been re-examined. The original work, reported some 8 years ago [Smith et al. (1997). J. Am. Chem. Soc. 119, 5028-5034], was undertaken using a very early version of the XD software, which contained serious programming errors. A re-refinement, using the original data and a recent version of the XD software, shows that many of the unusual aspects of this earlier study are artefacts due to these programming errors. The topological properties of the newly obtained experimental density compare well with those calculated from a theoretical DFT (density-functional theory) UHF-SCF (unrestricted Hartree Fock-self-consistent field) density. This report corrects several erroneous conclusions regarding the charge density in the title complex--in particular, the highly unusual diffuse Laplacian distribution about the Ni atom, and the trifurcated bond path from the Ni atom to the alcohol oxygen donor atoms are no longer observed. An examination of a range of topological properties of the metal-ligand bonds leads to the conclusion that the Ni-N and Ni-O bonds have an intermediate character, with a significant shared interaction, but with a substantial ionic component. This new study also reveals a previously unrecognized intramolecular H...H interaction in the macrocyclic ligand.
RESUMO
The benefits of carrying out single crystal X-ray diffraction (SXRD) experiments at low temperatures have long been recognised by the scientific community, as clearly demonstrated by the massive increase in publications reporting the use of low temperature SXRD in the past 15 years. This tutorial review will summarise the advantages, many of them now often forgotten by its practitioners or never known by the newcomers to the field, of performing single crystal X-ray diffraction experiments at low temperatures. The instrumentation currently available to university laboratories, which has been greatly improved over the past 5 years, will also be briefly described and a few different examples covering a range of applications will be presented.
RESUMO
This communication presents the crystal structures of the high spin state at 300 K, the low spin state at 30 K and the metastable high spin state after irradiation at 30 K and an estimate of the critical LIESST temperature of [FeL2](BF4)2 which is shown to undergo a spin transition at 271 K.
RESUMO
Two subsets of data, corresponding to different crystalline modifications of the title compound, 5-oxatricyclo[5.1.0.0(1,3)]octane-4-one (C(7)H(8)O(2)), have been obtained from the same experiment. Both structures were successfully solved and refined. The packing of identical layers of molecules is different for monoclinic and orthorhombic forms.
