The C form of n-hexadecanoic acid

3 pages, 2 tables, 2 figures.In the crystal structure of the title compound, C16H32O2, the molecules are arranged into dimers through O-HO hydrogen bonds. These dimers are packed in bilayers with terminal methyl groups at both external faces, and these layers are parallel to the crystallographic (100) plane. All C-C bonds of the alkyl chain show an antiperiplanar (trans) conformation, with slight deviations from the ideal value in the C-C bonds close to the intermolecular hydrogen bonds. The similarity between the carboxyl C-O bond distances is consistent with the existence of cis-trans tautomerism.This research was supported through CICYT grant No. MAT2001±3352 and DGICYT grant No. BQU2002±1729.Peer reviewe

The C form of n-hexadecanoic acid

3 pages, 2 tables, 2 figures.In the crystal structure of the title compound, C16H32O2, the molecules are arranged into dimers through O-HO hydrogen bonds. These dimers are packed in bilayers with terminal methyl groups at both external faces, and these layers are parallel to the crystallographic (100) plane. All C-C bonds of the alkyl chain show an antiperiplanar (trans) conformation, with slight deviations from the ideal value in the C-C bonds close to the intermolecular hydrogen bonds. The similarity between the carboxyl C-O bond distances is consistent with the existence of cis-trans tautomerism.This research was supported through CICYT grant No. MAT2001±3352 and DGICYT grant No. BQU2002±1729.Peer reviewe

Competing Intermolecular Interactions in the High-Temperature Solid Phases of Even Saturated Carboxylic Acids (C10H19O2H to C20H39O2H)

Structural knowledge of the high-temperature phases of saturated carboxylic acids (CnH2n-1O2H) from C6H11O2H to C23H45O2H is now complete. Crystal structures of the high-temperature phases of even acids from decanoic (C10H19O2H) to eicosanoic (C20H39O2H) are reported. The crystal structures of the six compounds were determined from powder X-ray diffraction data following direct space methods and refined by the Rietveld method combined with force field geometry optimization. The combination proved to be a valuable approach to obtain structures that are chemically sensible and in close agreement with the powder pattern. At the end of the process solid-state DFT calculations were applied to improve the overall accuracy of the system but in this case DFT did not render better structures. The high-temperature solid phases of even carboxylic acids are all P2(1)/c with Z=4, the molecules are united into dimers via strong hydrogen bonds. Two major types of interactions govern the crystal packing of carboxylic acids, hydrogen bonds and van der Waals interactions. A survey of the intermolecular interactions has revealed that hydrogen bonds are the dominant interaction for acids with less than 23 carbon atoms in the alkyl chain while van der Waals interactions dominate the packing for acids with more than 23 carbon atoms

Polymorphism of even saturated carboxylic acids from n-decanoic to n-eicosanoic acid

The nature and the mechanism of the magnetic hysteresis for the thermal spin crossover exhibited by an iron (II) compound is investigated by means of variable-temperature powder and single-crystal x-ray diffraction. The unit cell temperature dependence clearly evidences the amplitude of the strong structural rearrangement that accompanies the spin crossover - corresponding to a variation of 8.6% for one of the unit cell parameters - as well as the structural hysteresis width. In this regard, the present x-ray study reveals significant differences in the spin crossover features according to the nature of the sample - powder or single crystal - that should be taken into account in the analysis of physical properties. Concerning the interplay between structural and magnetic transitions, quenching effects show that the structural transition and the spin crossover are indissociable. Furthermore, investigations of the mechanism itself of the thermal spin crossover confirm the presence of spin-like domains in the conversion region, either in the cooling or in the warming loops. The non-dependence with temperature of these domains inside the hysteresis loop demonstrates the stability of the microscopic and macroscopic structures in the corresponding thermodynamic conditions. This result is of interest in the context of the potential use of hysteresis loops to obtain high-temperature photo-conversion

Polymorphism of even saturated carboxylic acids from n-decanoic to n-eicosanoic acid

The nature and the mechanism of the magnetic hysteresis for the thermal spin crossover exhibited by an iron (II) compound is investigated by means of variable-temperature powder and single-crystal x-ray diffraction. The unit cell temperature dependence clearly evidences the amplitude of the strong structural rearrangement that accompanies the spin crossover - corresponding to a variation of 8.6% for one of the unit cell parameters - as well as the structural hysteresis width. In this regard, the present x-ray study reveals significant differences in the spin crossover features according to the nature of the sample - powder or single crystal - that should be taken into account in the analysis of physical properties. Concerning the interplay between structural and magnetic transitions, quenching effects show that the structural transition and the spin crossover are indissociable. Furthermore, investigations of the mechanism itself of the thermal spin crossover confirm the presence of spin-like domains in the conversion region, either in the cooling or in the warming loops. The non-dependence with temperature of these domains inside the hysteresis loop demonstrates the stability of the microscopic and macroscopic structures in the corresponding thermodynamic conditions. This result is of interest in the context of the potential use of hysteresis loops to obtain high-temperature photo-conversion