Crystallography in Structural Chemistry

This anniversary article has three functions: It marks Volume 25 of our journal; it honors 2014, the International Year of Crystallography; and it celebrates the centennial from the birth of a great crystallographer, Aleksandr I. Kitaigorodskii.     

Molecular structure of p-diisocyanobenzene from gas-phase electron diffraction and theoretical calculations and effects of intermolecular interactions in the crystal on the benzene ring geometry

In this study, the molecular structure of p-diisocyanobenzene has been determined by gas-phase electron diffraction and quantum chemical calculations. The electron diffraction intensities from a previous study by Colapietro et al. (J Mol Struct 125:19–32, 1984) have been reanalyzed using geometrical constraints and initial values of vibrational amplitudes from computations. The equilibrium structure of the molecule has D _2h symmetry, whereas the average geometry in the gaseous phase is best described by a non-planar model of C _2v symmetry. The lowering of symmetry is due to large-amplitude motion of the substituents out of the plane of the benzene ring. The non-planar model has an internal ring angle at the ipso position, ∠_aC2–C1–C6 = 120.6 ± 0.2°, about 1° smaller than that from the previous study, but consistent with the quantum chemical calculations. The mean length of the ring C–C bonds and the length of the triple bond are accurately determined as 〈r _g(C–C)〉 = 1.398 ± 0.003 Å and r _g(N≡C) = 1.177 ± 0.002 Å, respectively. Comparison with the gaseous isoelectronic molecules p-diethynylbenzene and p-dicyanobenzene shows that the differences in the mean lengths of the ring C–C bonds and in the lengths of the triple bonds determined by electron diffraction are equal or closely similar to the corresponding differences from quantum chemical calculations. The present experimental value of the ipso angle in free p-diisocyanobenzene is slightly, but significantly smaller than that obtained by X-ray crystallography. The difference is confirmed by computational modeling of the crystal structure and appears to be due to –N≡C···H–C intermolecular interactions in the crystal.     

Comprehensive study of the structure of aluminum trihalides from electron diffraction and computation

The molecular geometries of the complete series of aluminum trihalides, AlX₃ (X = F, Cl, Br, and I), are discussed based on new gas-phase electron diffraction (GED) experiments (AlCl₃ and Al₂Cl₆) and on the reanalysis of our previous GED data (for the rest of the series) with an improved and analogous treatment of the temperature effect on the average bond lengths. This is the first time that all structure analyses are based on GED experiments of homogeneous vapors; either as pure monomers or dimers. The structures of other small oligomers as well as those of the crystals of aluminum trihalides are also discussed.     

Ulf Lagerkvist and his Nobel histories

Upon retirement from his professorship in medical biochemistry at Gothenburg University, Ulf Lagerkvist (1926?C2010) wrote several books about great discoveries in chemistry and biology and examined both their awarded and missing Nobel Prizes. He combined a deep interest in historical development, intimate knowledge in his subject matters, and a natural gift of an engaging writing style. His latest, posthumous book was about the missing Nobel Prize for Dmitrii Mendeleev??s Periodic Table of the Elements.     

Quasilinear molecule par excellence, SrCl2: structure from high-temperature gas-phase electron diffraction and quantum-chemical calculations--computed structures of SrCl2.argon complexes

The molecular geometry of strontium dichloride has been determined by high-temperature electron diffraction (ED) and computational techniques. The computation at the MP2 level of theory yields a shallow bending potential with a barrier of about 0.1 kcal mol(-1) at the linear configuration. The experimentally determined thermal average Sr--Cl bond length, r(g), is 2.625+/-0.010 A and the bond angle, angle-spherical(a), is 142.4+/-4.0 degrees . There is excellent agreement between the equilibrium bond lengths estimated from the experimental data, 2.607+/-0.013 A, and computed at different levels of theory and basis sets, 2.605+/-0.006 A. Based on anharmonic analyses of the symmetric and asymmetric stretching as well as the bending motions of the molecule, we estimated the thermal average structure from the computation for the temperature of the ED experiment. In order to emulate the effect of the matrix environment on the measured vibrational frequencies, a series of complexes with argon atoms, SrCl(2)Ar(n) (n=1-7), with different geometrical arrangements were calculated. The complexes with six or seven argon atoms approximate the interaction best and the computed frequencies of these molecules are closer to the experimental ones than those computed for the free SrCl(2) molecule.     

Adsorption of poly(ethylene oxide) at the air/water interface: a dynamic and static surface tension study

The adsorption of polyethylene oxide (PEO) homologues in a wide range of molecular weight (from M(PEO)=200 to 10(6)) at the air/aqueous solution interface was investigated by dynamic and static surface tension measurements. An approximate estimate for the lower limit of PEO concentration was given at which reliable equilibrium surface tension can be determined from static surface tension measurements. It was shown that the observed jump in the earlier published sigma-lg(c(PEO)) curves is attributable to the nonequilibrium surface tension values at low PEO concentrations. The adsorption behavior of short chain PEO molecules (M(PEO)1000) is similar to that of the ordinary surfactants. The estimated standard free energy of PEO adsorption, DeltaG(0), increases linearly with the PEO molecular weight until M(PEO)=1000. In this molecular weight range, DeltaG(0) was found to be approximately the fifth of the hydrophobic driving force related to the adsorption of a surfactant with the same number of methylene groups. In the case of the longer chain PEOs the driving force of adsorption is so high that the adsorption isotherm is near saturation in the experimentally available polymer concentration range. Above a critical molecular weight the PEO adsorption reveals universal features, e.g., the surface tension and the surface density of segments do not depend on the polymer molecular weight.     

ChemInform Abstract: Molecular Structure,Bonding, and Jahn—Teller Effect in Gold Chlorides: Quantum Chemical Study of AuCl3, Au2Cl6, AuCl‐4, AuCl,and Au2Cl2 and Electron Diffraction Study of Au2Cl6.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

ChemInform Abstract: Intriguing Gold Trifluoride — Molecular Structure of Monomers and Dimers: An Electron Diffraction and Quantum Chemical Study.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

ChemInform Abstract: Gas‐Phase Structures of Iron Trihalides: A Computational Study of all Iron Trihalides and an Electron Diffraction Study of Iron Trichloride.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.