Femtosecond diffraction with chirped electron pulses

Here, we report on the possible achievement, in ultrafast electron diffraction and imaging, of temporal resolution of tens of femtoseconds through the use of chirped electron packets in combination with energy filtering. Space–charge forces in multi-electron packets accelerate leading electrons and retard trailing ones, thus inducing correlations of momentum and time. By resolving the diffraction images with an energy analyzer, well-defined temporal slices of the long electron packet can be selected. Numerical simulations show that conventional electron sources are sufficient to reach the 30-fs domain of resolution without electron packet compression. They also reveal the influence of packet shape, electron density and photoemission bandwidth on the achievable time resolution.     

Anomeric effect in N-azidomethylpyrrolidine: gas-phase electron diffraction and theoretical study

Gas-phase electron-diffraction data and high-level quantum chemical calculations have been used to study the conformational behaviour of N-azidomethylpyrrolidine. The two most stable conformers with a relative abundance of about 80% at 298 K possess gauche orientation of the azidomethyl group around the C-N(pyr) bond (C-N(azido)gauche with respect to the endocyclic N(pyr)-C bond). This orientation is a strong manifestation of an anomeric effect. The influence of the anomeric effect is also reflected in shortening of the C-N(pyr) bond and lengthening of the C-N(azido) bond as compared to such bonds in other compounds.     

Molecular structure of 1,2,3-triethyldiaziridine: Gas-phase electron diffraction and theoretical calculations

For the first time, the equilibrium molecular structure and conformational composition (6 to 8 conformers) of 1,2,3-triethyl-diaziridine in the gas phase were determined by gas-phase electron diffraction. Using 1D and 2D ¹H and ¹³C NMR spectro-scopy, it was shown that in a CDCl₃ solution under normal conditions on the NMR time scale, the molecule exists only as one conformer. The enthalpy of formation Δ_fH ⁰₂₉₈ of the studied molecule in the gas phase was calculated by the method of atomization reactions and is equal to 92.2 ± 1.7 kJ mol⁻¹.     

Structural dynamics of surfaces by ultrafast electron crystallography: experimental and multiple scattering theory

Recent studies in ultrafast electron crystallography (UEC) using a reflection diffraction geometry have enabled the investigation of a wide range of phenomena on the femtosecond and picosecond time scales. In all these studies, the analysis of the diffraction patterns and their temporal change after excitation was performed within the kinematical scattering theory. In this contribution, we address the question, to what extent dynamical scattering effects have to be included in order to obtain quantitative information about structural dynamics. We discuss different scattering regimes and provide diffraction maps that describe all essential features of scatterings and observables. The effects are quantified by dynamical scattering simulations and examined by direct comparison to the results of ultrafast electron diffraction experiments on an in situ prepared Ni(100) surface, for which structural dynamics can be well described by a two-temperature model. We also report calculations for graphite surfaces. The theoretical framework provided here allows for further UEC studies of surfaces especially at larger penetration depths and for those of heavy-atom materials.     

High-precision X-ray diffraction data,experimental and theoretical study of 2H-MoS2

The crystal structure of MoS₂ was for the first time refined based on precise three-dimensional X-ray diffraction data collected from the synthesized single crystal. The results of the refinement using the Hansen—Coppens multipole model based on X-ray diffraction data and quantum chemical calculations for the molybdenum disulfide monolayer and single crystal were analyzed in terms of Bader’s quantum theory of Atoms in Molecules (QTAIM). An analysis of the bond critical points (bcp) for S...S attests to a weak van der Waals interaction having the energy of ～1.25 kcal mol^?1 per S...S contact and the binding energy of the layers of ～7.5 kcal mol^?1.     

First critical steps for understanding complex advanced materials: Synergy between X-ray diffraction and electron microscopy

Attaining novel structures in compounds displaying remarkable properties is the goal of numerous solid-state chemists and physicists. Several approaches for predicting new structures exist but, whatever they are, one of the essential steps is to understand their architecture. It needs to collect the maximum information for deciphering the building code. In this paper, we give a personal account and focus on the powerful synergy of transmission electron microscopy and X-ray diffraction techniques for achieving it. We have chosen the example of the misfit-layered cobaltites, which appear more and more complex as we attain each new level in the hierarchy of their structures. We have limited our illustration to the first period of the misfit cobaltites “research story”, from the discovery of highly complex oxides to the understanding of the architecture building, without reporting on the numerous further papers devoted to iso-structural compounds and to their remarkable properties of thermoelectric power.     

Chiolite,a case study for combining NMR crystallography,diffraction and structural simulation

Chiolite has been selected as a test case for developing a general approach to solve inorganic structures from powders by combining NMR, modeling, and X-ray diffraction. The different steps of the strategy are successfully performed, building the candidate integrant units using NMR, simulating candidate crystal structures using the computational AASBU method, and checking the consistency of the candidate structures against the diffraction data analyzed with FOX computer program.     

The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione aluminium: gas-phase electron diffraction,quantum chemical calculations and X-ray crystallography

Structural Chemistry - The molecular structure of tris-2,2,6,6-tetramethyl-heptane-3,5-dione aluminium, or Al(thd)3, has been determined by quantum chemical (DFT) calculations, X-ray...     

X-ray diffraction and scanning electron microscopy of galvannealed coatings on steel

The formation of Fe–Zn intermetallic compounds, as relevant in the commercial product galvannealed steel sheet, was investigated by scanning electron microscopy and different methods of X-ray diffraction. A scanning electron microscope with high resolution was applied to investigate the layers of the galvannealed coating and its topography. Grazing incidence X-ray diffraction (GID) was preferred over conventional Bragg–Brentano geometry for analysing thin crystalline layers because of its lower incidence angle α and its lower depth of information. Furthermore, in situ experiments at an environmental scanning electron microscope (ESEM) with an internal heating plate and at an X-ray diffractometer equipped with a high-temperature chamber were carried out. Thus, it was possible to investigate the phase evolution during heat treatment by X-ray diffraction and to display the growth of the ζ crystals in the ESEM.     

Structure and conformational properties of 1,3,3-trimethyl-1,3-azasilinane: gas electron diffraction, dynamic NMR, and theoretical study

Structure and the conformational properties of 1,3,3-trimethyl-1,3-azasilinane have been studied. According to gas electron diffraction (GED), the molecule exists in a slightly distorted chair conformation with the N-Me group in equatorial position. High-level quantum chemical calculations excellently reproduce the experimental geometry. Employing variable temperature (1)H and (13)C NMR spectroscopy down to 103 K, the conformational equilibrium could be frozen and the barrier to ring inversion determined.     

Molecular structures of benzoic acid and 2-hydroxybenzoic acid, obtained by gas-phase electron diffraction and theoretical calculations

The structures of benzoic acid (C6H5COOH) and 2-hydroxybenzoic acid (C6H4OHCOOH) have been determined in the gas phase by electron diffraction using results from quantum chemical calculations to inform restraints used on the structural parameters. Theoretical methods (HF and MP2/6-311+G(d,p)) predict two conformers for benzoic acid, one which is 25.0 kJ mol(-1) (MP2) lower in energy than the other. In the low-energy form, the carboxyl group is coplanar with the phenyl ring and the O-H group eclipses the C=O bond. Theoretical calculations (HF and MP2/6-311+G(d,p)) carried out for 2-hydroxybenzoic acid gave evidence for seven stable conformers but one low-energy form (11.7 kJ mol(-1) lower in energy (MP2)) which again has the carboxyl group coplanar with the phenyl ring, the O-H of the carboxyl group eclipsing the C=O bond and the C=O of the carboxyl group oriented toward the O-H group of the phenyl ring. The effects of internal hydrogen bonding in 2-hydroxybenzoic acid can be clearly observed by comparison of pertinent structural parameters between the two compounds. These differences for 2-hydroxybenzoic acid include a shorter exocyclic C-C bond, a lengthening of the ring C-C bond between the substituents, and a shortening of the carboxylic single C-O bond.     

Amyloidosis of Alzheimer's Abeta peptides: solid-state nuclear magnetic resonance, electron paramagnetic resonance, transmission electron microscopy, scanning transmission electron microscopy and atomic force microscopy studies

Aggregation cascade for Alzheimer's amyloid-beta peptides, its relevance to neurotoxicity in the course of Alzheimer's disease and experimental methods useful for these studies are discussed. Details of the solid-phase peptide synthesis and sample preparation procedures for Alzheimer's beta-amyloid fibrils are given. Recent progress in obtaining structural constraints on Abeta-fibrils from solid-state NMR and scanning transmission electron microscopy (STEM) data is discussed. Polymorphism of amyloid fibrils and oligomers of the 'Arctic' mutant of Abeta(1-40) was studied by (1)H,(13)C solid-state NMR, transmission electron microscopy (TEM) and atomic force microscopy (AFM), and a real-time aggregation of different polymorphs of the peptide was observed with the aid of in situ AFM. Recent results on binding of Cu(II) ions and Al-citrate and Al-ATP complexes to amyloid fibrils, as studied by electron paramagnetic resonance (EPR) and solid-state (27)Al NMR techniques, are also presented.     

Dissociative electron attachment to gas phase valine: a combined experimental and theoretical study

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.     

Molecular structure and conformation of chloronitromethane as determined by gas-phase electron diffraction and theoretical calculations

The molecular structure of chloronitromethane was studied in the gas phase at a nozzle-tip temperature of 373 K. The experimental data were interpreted using a dynamic model where the molecules are undergoing torsional motion governed by a potential function: V = V2/2x(1 - cos 2tau) + V4/2x(1 - cos 4tau) with V2 = 0.81(30) and V4 = 0.12(40) kcal/mol (tau is the dihedral angle between the C-Cl and N-O bond). The conformer with a zero degree dihedral angle is the most stable conformer. Comparison with results from HF/MP2/B3LYP 6-311G(d,p) calculations were made. The important geometrical parameter values (for the eclipsed form) obtained from least-squares refinements are the following: r(C-H) = 1.061(18)A, r(C-N) = 1.509 (5)A, r(N-O) = 1.223(1)A, r(C-Cl) = 1.742(2)A, angleClCN = 115.2(7) degrees, angleO4NC = 118.9(10) degrees, angleO5NC = 114.9(16) degrees, and angleClCH 115(4) degrees.     

Molecular structures of 3-hydroxybenzoic acid and 4-hydroxybenzoic acid, obtained by gas-phase electron diffraction and theoretical calculations

The structures of 3-hydroxybenzoic acid and 4-hydroxybenzoic acid have been determined by gas-phase electron diffraction using results from quantum chemical calculations to inform the choice of restraints applied to some of the structural parameters. The results from the study presented here demonstrate that resonance hybrids are not as helpful in rationalizing the structures of 2-, 3-, and 4-hydroxybenzoic acids as are models based upon electrostatic effects.