Progress in detection of and correction for low-energy contamination.

Contamination with low-energy radiation leads to an increased number of weighted residuals being larger in absolute terms than three standard uncertainties. For a Gaussian distribution, these rare events occur only in 0.27% of all cases, which is a small number for small- to medium-sized data sets.

On the accuracy and precision of X-ray and neutron diffraction results as a function of resolution and the electron density model.

X-ray diffraction is the main source of three-dimensional structural information. In total, more than 1.5 million crystal structures have been refined and deposited in structural databanks (PDB, CSD and ICSD) to date.

Diferrocenyl Thioketone: Reactions with (Bisphosphane)Pt(0) Complexes-Electrochemical and Computational Studies.

Diferrocenyl thioketone reacts smoothly with (bisphosphane)Pt(0) complexes in toluene solution at room temperature yielding 1:1 adducts identified as ferrocenyl (Fc) functionalized platinathiiranes. Their structures were unambiguously confirmed by means of spectroscopic methods as well as by X-ray diffraction analysis. A unique, ferrocene-rich platinathiirane, bearing three Fc-units, was prepared starting with [bis(diphenylphosphino)ferrocene] Pt(0(-norbornene).

[3]rotaxanes composed of two dibenzo-24-crown-8 ether wheels and an azamacrocyclic complex.

The azamacrocyclic complex was used as a platform for the construction of [3]rotaxanes containing two DB24C8 macrocycles per molecule. The complex unit incorporates two electron deficient π-bond systems and two N-H hydrogen bond donating groups which facilitated the formation of a 1 : 2 interlocked structure. Synthesis and properties of such compounds are presented.

Regioselective Functionalization of the [ closo-1-CBH] Anion through Iodonium Zwitterions.

Reactions of [ closo-1-CBH-1-R] (2, R = H, COOH, CH) with PhI(OAc) lead to mixtures of regioisomers [ closo-1-CBH-1-R-6-IPh] (5[6]) and [ closo-1-CBH-1-R-10-IPh] (5[10]) in ratios of ∼3:1 to 1:1, of which the former isomer undergoes selective reactions with nucleophiles (MeCN, pyridine, MeC(═NH)NH, CN). The products and the unreacted 10-isomers 5[10] are separated achieving kinetic resolution of the isomeric iodonium zwitterions. Pure 5[10] is reacted with nucleophiles (pyridine, 4-CHOPyridine, MeNCHS, PhCO, CN, N, I, MeC(═NH)NH, and MeCN), giving substitution products.

Magnetostructural Investigation of Orthogonal 1-Aryl-3-Phenyl-1,4-Dihydrobenzo[e][1,2,4]triazin-4-yl Derivatives.

3-Phenyl-1,4-dihydrobenzo[e][1,2,4]triazinyl radicals with the N(1) position substituted with naphth-2-yl (1 b), naphth-1-yl (1 c), pyren-1-yl (1 d), anthracen-9-yl (1 e), 2-trifluoromethylphenyl (1 f), 3-trifluoromethylphenyl (1 g), and 2-iodophenyl (1 h) were characterized by using single-crystal X-ray diffraction, variable-temperature magnetic susceptibility, and DFT computational methods. The substituent at N(1) is essentially orthogonal to the heterocycle plane in 1 f and 1 h, and with a high torsion angle in 1 c and 1 d. Radicals 1 c and 1 h form unusual infinite chains with crisscrossing hetero-co-facial π-π interactions, whereas radical 1 d forms analogous homo-co-facial arrangements.

A theoretical-electron-density databank using a model of real and virtual spherical atoms.

A database describing the electron density of common chemical groups using combinations of real and virtual spherical atoms is proposed, as an alternative to the multipolar atom modelling of the molecular charge density. Theoretical structure factors were computed from periodic density functional theory calculations on 38 crystal structures of small molecules and the charge density was subsequently refined using a density model based on real spherical atoms and additional dummy charges on the covalent bonds and on electron lone-pair sites. The electron-density parameters of real and dummy atoms present in a similar chemical environment were averaged on all the molecules studied to build a database of transferable spherical atoms.

Synthesis and characterization of 12-pyridinium derivatives of the [closo-1-CB11H12]- anion.

Diazotization of [closo-1-CB11H10-1-R-12-NH2](-)[NMe4](+) (4[NMe4]) in neat 4-methoxypyridine leads to 12-(4-methoxypyridinium) zwitterions [closo-1-CB11H10-1-R-12-(4-MeOC5H4N)] (2) in ∼50% yield. Demethylation of 2 with LiCl in dimethylformamide provides access to 12-pyridones 5[NMe4], which can be O-alkylated with alkyl triflates giving 12-(4-alkoxypyridinium) zwitterions, such as 1. This three-step process is more efficient than direct diazotization of amine 4[NMe4] in neat higher 4-alkoxypyridine.

A Comparative Study of Transferable Aspherical Pseudoatom Databank and Classical Force Fields for Predicting Electrostatic Interactions in Molecular Dimers.

Accurate and fast evaluation of electrostatic interactions in molecular systems is one of the most challenging tasks in the rapidly advancing field of macromolecular chemistry and drug design. Electrostatic interactions are of crucial importance in biological systems. They are well represented by quantum mechanical methods; however, such calculations are computationally expensive.

Statistical analysis of multipole-model-derived structural parameters and charge-density properties from high-resolution X-ray diffraction experiments.

A comprehensive analysis of various properties derived from multiple high-resolution X-ray diffraction experiments is reported. A total of 13 charge-density-quality data sets of α-oxalic acid dihydrate (C2H2O4·2H2O) were subject to Hansen-Coppens-based modelling of electron density. The obtained parameters and properties were then statistically analysed yielding a clear picture of their variability across the different measurements.

An improved experimental databank of transferable multipolar atom models--ELMAM2. Construction details and applications.

ELMAM2 is a generalized and improved library of experimentally derived multipolar atom types. The previously published ELMAM database is restricted mostly to protein atoms. The current database is extended to common functional groups encountered in organic molecules and is based on optimized local axes systems taking into account the local pseudosymmetry of the molecular fragment.

Verification of structural and electrostatic properties obtained by the use of different pseudoatom databases.

The existing pseudoatom databases (ELMAM, Invariom, UBDB and ELMAM2) enable structure refinement to be performed with the use of aspherical scattering factors computed from the transferable aspherical atom model (TAAM) as an alternative to independent atom model refinement. In addition, electrostatic properties can be estimated with the help of the databases. The quality of the structural and electrostatic properties obtained from the individual databases was tested.

Structural analysis and multipole modelling of quercetin monohydrate--a quantitative and comparative study.

The multipolar atom model, constructed by transferring the charge-density parameters from an experimental or theoretical database, is considered to be an easy replacement of the widely used independent atom model. The present study on a new crystal structure of quercetin monohydrate [2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one monohydrate], a plant flavonoid, determined by X-ray diffraction, demonstrates that the transferred multipolar atom model approach greatly improves several factors: the accuracy of atomic positions and the magnitudes of atomic displacement parameters, the residual electron densities and the crystallographic figures of merit. The charge-density features, topological analysis and electrostatic interaction energies obtained from the multipole models based on experimental database transfer and periodic quantum mechanical calculations are found to compare well.

X-ray diffraction, FT-IR, and (13)C CP/MAS NMR structural studies of solvated and desolvated C-methylcalix[4]resorcinarene.

Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and (13)C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P2(1)/n space group with three CH(3)CN and two H(2)O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form).

Determination of experimental charge density in model nickel macrocycle: [3,11-bis(methoxycarbonyl)-1,5,9,13-tetraazacyclohexadeca-1,3,9,11-tetraenato-(2-)-kappa(4)N]nickel(II).

Experimental and theoretical atomic charges and d-orbital populations were obtained for [3,11-bis(methoxycarbonyl)-1,5,9,13-tetraazacyclohexadeca-1,3,9,11-tetraenato(2-)-kappa(4)N]nickel(II) monocrystal (16Ni) using the Hansen and Coppens formalism. Several models of this structure were tested as a function of quality of the fit, convergence of the refinement, value of residual peaks and holes, and Hirshfeld's rigid-bond test. The models with Ni-atomic scattering factors applied for the metal center are significantly better than those with the Ni-ionic scattering factors.

Neutral nickel(II) and copper(II) tetraazamacrocyclic complexes as molecular rods attached to gold electrodes.

New dithiolated derivatives of neutral Cu(II) and Ni(II) tetraazamacrocyclic complexes have been synthesized and characterized by spectroscopic and diffractional methods. These rod-shaped molecules were assembled in monocomponent and mixed monolayers on gold electrodes. In the mixed monolayers, the active molecules were embedded in a hexanethiol matrix.

Fine-tuning of properties of bismacrocyclic dinuclear cyclidene receptors by N-methylation.

N-Methylated bismacrocyclic Cu and Ni complexes were synthesised and structurally characterised in the solid state. Their properties in solution were analysed by using NMR and ESR spectroscopies and electrochemical methods. Face-to-face biscyclidenes linked through polymethylene chains form rectangular boxlike cations.

Intermetallic interactions in face-to-face homo- and heterodinuclear bismacrocyclic complexes of copper(II) and nickel(II).

New face-to-face heterodinuclear complexes containing copper(II) and nickel(II) in identical tetraazamacrocyclic environments have been synthesized and characterized using ESI mass-spectrometry, X-ray diffraction, spectroscopic methods, and elemental analysis. These new bismacrocyclic systems were compared with the respective mono- and bismacrocyclic and [2]catenane homonuclear complexes. Interactions between the metal centers were monitored by magnetic and electrochemical measurements.