Rainer Glaser

Push–pull substitution versus intrinsic or packing related N–N gauche preferences in azines. Synthesis, crystal structures and packing of asymmetrical acetophenone azines

The asymmetrical E,E-configured para-disubstituted 4-methoxyacetophenone azines with 4′-bromo-(1), 4′cyano-(2) and 4′-nitro-(3) substituents have been synthesized and their crystal structures have been determined. The synergetic reinforcement of ‘push’ and ‘pull’ effects does not suffice to overcome the intrinsic or packing induced N–N gauche preference, and azines 1–3 all assume distinctly gauche N–N conformations (dihedral angle of 115–140°). The crystal packing is characterized by offset T-shaped and parallel displaced face-to-face arene-arene contacts between pairs of azines with parallel or anti-parallel D→A orientations. This crystal architecture results in a net dipole of crystals of 1. The structural data are analysed in comparison with the symmetrical E,E-configured para-disubstituted acetophenone azines 4–7. With regard to this best possible reference data set, the structural parameters of 1–3 do not show any significant manifestation of special electronic interactions over the N-perturbed extended π-system to be associated with the asymmetry of the azines. While there exists no structural evidence of conjugation in these solid state structures, our results do not rule out asymmetry effects on the electronic structure of the ground or the excited states.

THE CARRIERS OF THE INTERSTELLAR UNIDENTIFIED INFRARED EMISSION FEATURES: CONSTRAINTS FROM THE INTERSTELLAR C-H STRETCHING FEATURES AT 3.2-3.5 μm

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have recently been ascribed to mixed aromatic/aliphatic organic nanoparticles. More recently, an upper limit of <9% was placed on the aliphatic fraction (i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers based on the observed intensities of the 3.4 μm and 3.3 μm emission features by attributing them to aliphatic and aromatic C-H stretching modes, respectively, and assuming A 3.4/A 3.3 ≈ 0.68 derived from a small set of aliphatic and aromatic compounds, where A 3.4 and A 3.3 are, respectively, the band strengths of the 3.4 μm aliphatic and 3.3 μm aromatic C-H bonds. To improve the estimate of the aliphatic fraction of the UIE carriers, here we analyze 35 UIE sources exhibiting both the 3.3 μm and 3.4 μm C-H features and determine I 3.4/I 3.3, the ratio of the power emitted from the 3.4 μm feature to that from the 3.3 μm feature. We derive the median ratio to be I 3.4/I 3.3 ≈ 0.12. We employ density functional theory to compute A 3.4/A 3.3 for a range of methyl-substituted PAHs. The resulting A 3.4/A 3.3 ratio well exceeds ~1.4, with an average ratio of A 3.4/A 3.3 ≈ 1.76. By attributing the 3.4 μm feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity and superhydrogenation), we derive the fraction of C atoms in aliphatic form from I 3.4/I 3.3 ≈ 0.12 and A 3.4/A 3.3 ≈ 1.76 to be ~2%. We therefore conclude that the UIE emitters are predominantly aromatic.

Asymmetrization Effects on Structures and Populations of the Ground State of Dipolar Donor)Acceptor-Substituted Molecular Organic NLO Materials

Asymmetric donor–acceptor‐substituted π‐conjugated systems with low dipole moments and structural components that favor parallel alignment of neighboring molecules are potential molecular organic materials with nonlinear optical properties for which near prefect dipole parallel alignment is possible. The asymmetrical 4‐methoxysubstituted acetophenone azines with the substituents fluorine (1), chlorine (2), bromine (3), cyano (4), and nitro (5) in the 4′‐position have been studied in this context, and for 2 and 3 the dipole parallel alignment has indeed been accomplished in crystals of the pure material. In the present study, the effects of asymmetrization on the structure and the electronic structures of the ground states of 1–5 have been explored at the RHF/6‐31G* level. The properties of the optimized structures of the azines 1–5, a comparative analysis of asymmetrical and symmetrical azines, and natural population analyses, all show no significant evidence for conjugation over the azine bridge. The concept of azine spacers as “conjugation stoppers” is useful and the dipole moments of asymmetrical azines are relatively small. The analysis suggests that the charge transfer between the donor and acceptor contributes to the dipole moment much less than is generally assumed. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1130–1140, 1998

Thermochemistry of the initial steps of methylaluminoxane formation. Aluminoxanes and cycloaluminoxanes by methane elimination from dimethylaluminum hydroxide and its dimeric aggregates.

Results are presented of ab initio studies at levels MP2(full)/6-31G* and MP2(full)/6-311G** of the hydrolysis of trimethylaluminum (TMA, 1) to dimethylaluminumhydroxide (DMAH, 2) and of the intramolecular 1,2-elimination of CH(4) from 2 itself to form methylaluminumoxide 3, from its dimeric aggregate 4 to form hydroxytrimethyldialuminoxane 5 and dimethylcyclodialuminoxane 6, and from its TMA aggregate 7 to form 8 and/or 9, the cyclic and open isomers of tetramethyldialuminoxane, respectively. Each methane elimination creates one new Lewis acid site, and dimethylether is used as a model oxygen-donor molecule to assess the most important effects of product stabilization by Lewis donor coordination. It is found that the irreversible formation of aggregate 4 (ΔG(298) = -29.2 kcal/mol) is about three times more exergonic than the reversible formation of aggregate 7 (ΔG(298) = -9.9 kcal/mol), that the reaction free enthalpies for the formations of 5 (ΔG(298) = -9.0 kcal/mol) and 6 (ΔG(298) = -18.8 kcal/mol) both are predicted to be quite clearly exergonic, and that there is a significant thermodynamic preference (ΔG(298) = -7.2 kcal/mol) for the formation of 6 over ring-opening of 5 to hydroxytrimethyldialuminoxane 10. The mechanism for oligomerization is discussed based on the bonding properties of dimeric aggregates and involves the homologation of HO-free aluminoxane with DMAH (i.e., 9 to 13), and any initially formed hydroxydialuminoxane 10 is easily capped to trialuminoxane 13. Our studies are consistent with and provide support for Sinns proposal for the formation of oligoaluminoxanes, and in addition, the results point to the crucial role played by the kinetic stability of 5 and the possibility to form cyclodialuminoxane 6. Dialuminoxanes 9 and 10 are reversed-polarity heterocumulenes, and intramolecular O→Al dative bonding competes successfully with Al complexation by Lewis donors. Intramolecular O→Al dative bonding is impeded in cyclodialuminoxane 6, and the dicoordinate oxygen in 6 is a strong Lewis donor. Ethylene polymerization catalysts contain highly oxophilic transition metals, and our studies suggest that these transition metal catalysts should discriminate strongly in favor of cycloaluminoxane-O donors even if these are present only in small concentrations in the methylaluminoxane (MAO) cocatalyst.

Chemistry Is in the News: Taxonomy of authentic news media‐based learning activities1

A brief history is given of approaches that aim at achieving a connectedness of the content of organic chemistry courses to real world issues. Recently, such approaches have relied more and more on online media resources, the tools of the Internet and the World Wide Web. We propose a six‐level taxonomy of ‘authentic news media‐based learning activities’ to provide a conceptual framework for the description and discussion of such approaches. The Chemistry Is in the News project was designed to allow students to draw explicit connections between the course content and real world issues in ways that engage the students in a full range of cognitive skills. The activities consisted in the study, creation, and peer review of news portfolios by student collaborative groups. A news portfolio consists of an authentic news article taken from the popular press with interpretive comments and questions.

Electronic Excitations in Homopolyatomic Bismuth Cations: Spectroscopic Measurements in Molten Salts and an ab initio CI-Singles Study

The electronic excitations of the low-valence bismuth cluster cations Bi5(3+), Bi8(2+), and Bi9(5+) have been studied with experimental and theoretical techniques. The UV-visible spectra of the bismuth ions were measured in acidic chloroaluminate melts (mixture of 1-methyl-3-benzyl imidazolium chloride and AlCl3). The spectra of the Bi5(3+) and Bi8(2+) ions agree fairly well with previous reports, but also revealed additional low-energy absorptions. Ab initio methods were employed to assign the experimentally observed electronic transitions of these homopolyatomic bismuth cations. Structures were optimized at the RHF, MP2, and B3LYP levels of theory by using split-valence LANL2DZ basis sets that were augmented with one and two sets of pure d functions. The computed structures agree well with the results of neutron diffraction analyses of melts. Electronically excited states of the three clusters were treated by using the CI-Singles theory. The results of these calculations were used to explain the observed UV-visible spectra. The observed electronic excitations in the UV-visible range are all found to result from transitions involving the molecular orbitals formed by 6p-atomic-orbital overlap. This leads to the necessity of using basis sets that include d-type functions, which allow for an adequate description of the bonding that results from such p-orbital overlap. Spin-orbit coupling becomes increasingly important with increasing atomic number and its consideration is necessary when describing the electronic transitions in clusters of heavy atoms. The calculations show that singlet-triplet transitions, which are made accessible by strong spin-orbit coupling, are responsible for some of the observed absorptions.

Ab initio calculations on phosphorous compounds II: effects of distributions on ligand apicophilicity in phosphoranes

Geometry optimizations at the HF/3‐21G(*) and HF/6‐31G* levels of ab initio theory have been carried out for various isomers of model disubstituted phosphoranes PH3XY(X, YOH, CH3, NH2, and SH). Reasonable agreement was obtained between the optimized geometries and available crystal structure data for analogous compounds. The isomers were further characterized by frequency calculations. The MP2/6‐31G*//6‐31G* + ZPE energy data reveal that the interactions between the ligands are relatively small (0–4 kcal mol−1) for the most stable conformations of the isomers. Hence, for these conformations the apicophilicities (based upon monosubstituted phosphoranes) are approximately additive. The less stable PH3XY conformations are in general transition states or higher‐order saddle points, and their interligand interactions are larger in magnitude (up to 10 kcal mol−1); the results with these conformations suggest that apicophilicities may not be as additive for some highly substituted phosphoranes.

A MNDO-study of solvent free and solvated dimeric lithium ion pairs of acetaldoxime. Models for dimeric aggregates of lithiated oxime ethers

Abstract Cyclic and stack-type structures of solvent-free and solvated dimeric lithium ion pairs of isomeric carbanions of acetaldoxime were investigated as models for lithiated oxime ethers. The adequacy of MNDO is evaluated critically with reference to ab initio results. Severe overestimation of the pyramidalization at the carbanionic center and insufficient carbon lone-pair—lithium core repulsion are found to cause the following artifacts: excessive charge localization, the breakdown of the pseudo-π conjugation within the ligands, and orbital orientation towards the cation; standard MNDO calculations yield entirely unacceptable results. A simple method is proposed to counteract some of these deficiencies and results thus obtained are in significantly better agreement with ab initio data. Solvated cyclic ( C i symmetric) dimers were found to be the most important aggregated species; they are preferred over bis -solvated monomeric ion pairs at the MNDO level. The magnitudes of the relative energies are such that a definitive conclusion regarding the predominance of monomeric or dimeric aggregates cannot be made. Mechanistic implications with regard to the experimentally observed syn -axial entry of electrophiles into configurationally fixed lithiated oxime ethers are discussed.

On the reaction mechanism of tirapazamine reduction chemistry: unimolecular N-OH homolysis, stepwise dehydration, or triazene ring-opening.

The initial steps of the activation of tirapazamine (TPZ, 1, 3-amino-1,2,4-benzotriazine 1,4-N,N-dioxide) under hypoxic conditions consist of the one-electron reduction of 1 to radical anion 2 and the protonation of 2 at O(N4) or O(N1) to form neutral radicals 3 and 4, respectively. There are some questions, however, as to whether radicals 3 and/or 4 will then undergo N-OH homolyses 3 → 5 + ·OH and 4 → 6 + ·OH or, alternatively, whether 3 and/or 4 may react by dehydration and form aminyl radicals via 3 → 11 + H(2)O and 4 → 12 + H(2)O or phenyl radicals via 3 → 17 + H(2)O. These outcomes might depend on the chemistry after the homolysis of 3 and/or 4, that is, dehydration may be the result of a two-step sequence that involves N-OH homolysis and formation of ·OH aggregates of 5 and 6 followed by H-abstraction within the ·OH aggregates to form hydrates of aminyls 11 and 12 or of phenyl 17. We studied these processes with configuration interaction theory, perturbation theory, and density functional theory. All stationary structures of OH aggregates of 5 and 6, of H(2)O aggregates of 11, 12, and 17, and of the transition state structures for H-abstraction were located and characterized by vibrational analysis and with methods of electron and spin-density analysis. The doublet radical 17 is a normal spin-polarized radical, whereas the doublet radicals 11 and 12 feature quartet instabilities. The computed reaction energies and activation barriers allow for dehydration in principle, but the productivity of all of these channels should be low for kinetic and dynamic reasons. With a view to plausible scenarios for the generation of latent aryl radical species without dehydration, we scanned the potential energy surfaces of 2-4 as a function of the (O)N1-Y (Y = C5a, N2) and (O)N4-Z (Z = C4a, C3) bond lengths. The elongation of any one of these bonds by 0.5 Å requires less than 25 kcal/mol, and this finding strongly suggests the possibility of bimolecular reactions of the spin-trap molecules with 2-4 concomitant with triazene ring-opening.

Stabilities and Spin Distributions of Benzannulated Benzyl Radicals

The effects have been studied of mono- and dibenzannulation of a benzyl radical with hybrid density functional theory (B3LYP) and quadratic configuration interaction theory (QCISD). Bond dissociation energies and enthalpies are reported that were determined at the common level QCISD/6-311G**//B3LYP/6-31G* for the benzylic C-H bonds of toluene 1H, the monobenzannulated polycyclic aromatic hydrocarbons (PAH) 1- and 2-methylnaphthalene 2H and 3H, the dibenzannulated PAHs 9-methylanthracene 4H and 9-methylphenanthrene 5H, and the model hydrocarbons 1-phenylpropene 6H and propene 7H. The conformational preferences and the symmetries of 1H-7H and of their corresponding radicals 1-7 have been determined. The analysis of the electron and spin density distributions of radicals 1-7 at the QCI level are reported, and these high-level data are discussed in comparison to results obtained with density functional theory and with an awareness of a general perception shaped by Hückel molecular orbital theory. The results show in a compelling fashion that electron and spin delocalization onto an annulated arene is not the decisive principle for stabilization of the benzyl radicals formed by homolysis of the methylated PAHs C10H7-CH3 and C14H9-CH3, and instead, the analysis of QCI spin density distributions suggests that spin delocalization onto annulated arenes is avoided as much as possible while spin polarization does occur to a significant extent.