Sterically Active Electron Pairs in Lead Sulfide? An Investigation of the Electronic and Vibrational Properties of PbS in the Transition Region Between the Rock Salt and the α‐GeTe‐Type Modifications

Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree–Fock and density functional theory to globally search for possible thermodynamically stable and metastable structures. The perhaps most fascinating observation was that besides the experimentally known modification exhibiting the rock salt structure a second minimum exists close‐by on the landscape showing the low‐temperature α‐GeTe‐type structure. In the present study, we investigate the possible reasons for the existence of this metastable modification; in particular we address the question, whether the α‐GeTe‐type modification might be stabilized (and conversely the rock salt modification destabilized) by steric effects of the non‐bonding electron pair.     

What Controls the Magnetic Interaction in bis‐μ‐Alkoxo MnIII Dimers? A Combined Experimental and Theoretical Exploration

The synthesis and magnetic characterisation of a series of bis‐μ‐alkoxide bridged Mn^III dinuclear complexes of general formula [Mn^III₂(μ‐OR)₂(biphen)₂(ROH)_x(L)_y] (where R=Me, Et; H₂biphen=2,2′‐biphenol and L=terminally bonded N‐donor ligand) is described, doubling the literature basis set for this type of complex. Building on these findings we have categorised all known μ‐OR bridged Mn^III dinuclear complexes into one of three classifications with respect to their molecular structures. We have then employed DFT and MO calculations to assess all potential magneto‐structural correlations for this class of compound in order to identify the structural requirements for constructing ferromagnetic family members. Our analysis indicates that the most influential parameter which governs the exchange interaction in this class of compounds is the relative orientation of the JT axes of the Mn^III atoms. A perpendicular orientation of the JT axes leads to a large ferromagnetic contribution to the exchange. These results also suggest that a large ferromagnetic interaction and a large anisotropy are unlikely to co‐exist in such structural types.     

Multicenter Homogeneous Dendritic Catalysts: The Higher the Generation,the Better the Reactivity and Selectivity? – A Comparative Study of the Catalytic Efficiency of Dendrimeric [1,1′‐Binaphthalene]‐2,2′‐diol‐Derived Catalysts

The G0 and G1 generations of optically active, multicenter 1,1′‐binaphthalene‐based dendritic ligands 4 and 5 constructed on a rigid oligo(arylene) framework were prepared by divergent synthesis. Their corresponding aluminum complexes 1 and 2 , respectively, were shown to possess slightly better reactivity and enantioselectivity than those of a monomeric 1,1′‐binaphthalene catalyst 3 in the Diels–Alder reaction between cyclopentadiene and 3‐[(E)‐but‐2‐enoyl]‐oxazolidin‐2‐one.     

How Can Rotaxanes Be Modified by Varying Functional Groups at the Axle?—A Combined Theoretical and Experimental Analysis of Thermochemistry and Electronic Effects

We present theoretically as well as experimentally determined thermochemical data of the non-covalent interactions in different axle-substituted pseudorotaxanes. The overall interaction energy lies in the region of 35 kJ mol(-1), independent of the substitution pattern at the axle. Because rearrangement energies of 7 and 3 kJ mol(-1) are required for wheel and axle, respectively, the sum of the net interactions of individual non-covalent bonds must exceed 10 kJ mol(-1) to achieve a successful host-guest interaction. The geometrical analysis shows three hydrogen bonds, and the close inspection of the individual dipole moments as well as the individual hydrogen bonds reveals trends according to the different functional groups at the axle. The individual trends for the different hydrogen bonds almost lead to a cancellation of the substitution effects. From solvent-effect considerations it can be predicted that the pseudorotaxane is stable in CHCl(3) and CH(2)Cl(2), whereas it would dethread in water. Comparing experimentally and theoretically calculated Gibbs free enthalpies, we find reasonable agreement if an exchange reaction of one solvent molecule instead of the direct formation reaction is considered.     

Synthesis and Antimalarial‐Activity Evaluation of Tetraoxane?Triazine Hybrids and Spiro[piperidine‐4,3′‐tetraoxanes]

A series of tetraoxane? triazine hybrids and spiro[piperidine‐4,3′‐tetraoxanes] have been synthesized, and all the compounds were screened for in vitro antimalarial activity against chloroquine‐sensitive (D6) and chloroquine‐resistant (W2) strains of Plasmodium falciparum. Most of the spiro[piperidine‐4,3′‐tetraoxanes] exhibited moderate to good antimalarial activities, and two compounds have shown good antimalarial activity with IC₅₀ values in the range of 0.30 to 0.70 μM against both the strains with high selectivity index and no cytotoxicity towards mammalian kidney cell line.     

Differentiation between [1,2,4]triazolo[1,5‐a] pyrimidine and [1,2,4]triazolo[4,3‐a]‐ pyrimidine regioisomers by 1H?15N HMBC experiments

The condensation of malonoaldehyde derivatives with either a 3‐amino‐[1,2,4]‐triazole or a 3,5‐diamino‐[1,2,4]‐triazole precursor was studied. In agreement with previous reports, two different bicycles, namely, bearing the regioisomeric [1,2,4]triazolo[1,5‐a]pyrimidine ( 1 ) or[1,2,4] triazolo [4,3‐a]pyrimidine ( 2 ) structural surrogates, could be obtained. We found that, depending on the triazole precursor, only one regioisomer resulted, either of the 1 or 2 series. We also observed that these two structural surrogates could be unambiguously differentiated by indirectly measuring their ¹⁵N chemical shifts by ¹H? ¹⁵N HMBC experiments. The occasional conversion of [1,2,4]triazolo[4,3‐a]pyrimidines to the [1,2,4]triazolo[1,5‐a]pyrimidine counterparts could be unequivocally determined by ¹⁵N NMR data. Copyright © 2010 John Wiley & Sons, Ltd.     

Protonation of a Biologically Relevant CuII μ‐Thiolate Complex: Ligand Dissociation or Formation of a Protonated CuI Disulfide Species?

The proton‐induced electron‐transfer reaction of a Cu^II μ‐thiolate complex to a Cu^I‐containing species has been investigated, both experimentally and computationally. The Cu^II μ‐thiolate complex [Cu^II₂( L^MeS )₂]²⁺ is isolated with the new pyridyl‐containing ligand L^MeSSL^Me , which can form both Cu^II thiolate and Cu^I disulfide complexes, depending on the solvent. Both the Cu^II and the Cu^I complexes show reactivity upon addition of protons. The multivalent tetranuclear complex [Cu^I₂Cu^II₂( LS )₂(CH₃CN)₆]⁴⁺ crystallizes after addition of two equivalents of strong acid to a solution containing the μ‐thiolate complex [Cu^II₂( LS )₂]²⁺ and is further analyzed in solution. This study shows that, upon addition of protons to the Cu^II thiolate compound, the ligand dissociates from the copper centers, in contrast to an earlier report describing redox isomerization to a Cu^I disulfide species that is protonated at the pyridyl moieties. Computational studies of the protonated Cu^II μ‐thiolate and Cu^I disulfide species with LSSL show that already upon addition of two equivalents of protons, ligand dissociation forming [Cu^I(CH₃CN)₄]⁺ and protonated ligand is energetically favored over conversion to a protonated Cu^I disulfide complex.     

Electron ionization mass spectra of 8‐aryl‐3,4‐dioxo‐2H,8H‐6,7‐dihydroimidazo[2,1‐c][1,2,4]triazines. Do they exhibit tautomerism in the gas phase?

The electron ionization mass spectra of the title compounds (1: a R = H, b 2-CH(3), c 4-CH(3), d 2,3-diMe, e 2-OCH(3), f 4-OCH(3), g 2-Cl, h 3-Cl, i 4-Cl, j 3,4-diCl) were recorded at 70 eV to determine the effects of substituents and the possible keto-enol tautomerism. The compounds showed several common fragment ions but also fragment ions which divided them into three classes, namely 1a-1d (parent compound and Me-substituted derivatives), 1e and 1f (MeO-substituted derivatives), and 1g-1j (Cl-substituted derivatives). The presence of the HOCN(+.) ion as well as the exponential dependence of its total ion current in the case of p- and also 3-Cl-substituted compounds (1a, c, f, h-j) on the Hammett sigma constants and the loss of CHO or one or two HOCN moieties can be somewhat easier explained by the presence of the enol form but as a whole the results support the predominance of the keto form, in parallel to the situation in solution.     

Which One is Bulkier: The 3,5‐Dimethylphenyl or the 2,6‐Dimethylphenyl Group? Development of Size‐Complementary Molecular and Macromolecular [2]Rotaxanes

We developed novel size‐complementary molecular and macromolecular rotaxanes using a 2,6‐dimethylphenyl terminal group as the axle‐end‐cap group in dibenzo‐24‐crown‐8‐ether (DB24C8)‐based rotaxanes, where the 2,6‐dimethylphenyl group was found to be less bulky than the 3,5‐dimethylphenyl group. A series of molecular and macromolecular [2]rotaxanes that bear a 2,6‐dimethylphenyl group as the axle‐end‐cap were synthesized using unsubstituted and fluorine‐substituted DB24C8. Base‐induced decomposition into their constituent components confirmed the occurrence of deslipping, which supports the size‐complementarity of these rotaxanes. The deslipping rate was independent of the axle length but dependent on the DB24C8 substituents. A kinetic study indicated the rate‐determining step was that in which the wheel is getting over the end‐cap group, and deslipping proceeded via a hopping‐over mechanism. Finally, the present deslipping behavior was applied to a stimulus‐degradable polymer as an example for the versatile utility of this concept in the context of stimulus‐responsive materials.     

Theoretical Investigation on the Structural and Spectroscopic Properties of “Cross” [Anthraquinone-Diamine/Diimine-Pt] Complexes

Russian Journal of Physical Chemistry A - The Frontier molecular orbitals, UV–Vis absorption spectra, electron excitation properties, and triplet excited states of 1 and 2 were investigated...     

Synthesis of 1H,7H,12bH‐Pyrano[3′,4′: 5,6]pyrano[3,4‐c][1]benzopyran‐1‐one via Domino Knoevenagel/Hetero‐Diels?Alder Reaction with Theoretical Investigations

The CuI‐catalyzed intramolecular oxa‐Diels? Alder reaction of 2‐(prop‐2‐yn‐1‐yloxy)benzaldehydes as unactivated terminal alkynes with 4‐hydroxy‐6‐methyl‐2H‐pyran‐2‐one is described. The reaction proceeds with remarkable chemoselectivity to yield pyranones 3 (Scheme 1). A theoretical investigation of the reaction in terms of HOMO? LUMO interactions in the gas phase is also reported. The reaction could be regarded as an inverse‐electron‐demand Diels? Alder cycloaddition. The theoretical results are in high agreement with the experimental evidences.     

Catalyst‐Switchable Regiocontrol in the Direct Arylation of Remote C?H Groups in Pyrazolo[1,5‐a]pyrimidines

The regiodivergent palladium‐catalyzed C H arylation of pyrazolo[1,5‐a]pyrimidine has been achieved, wherein the switch in regioselectivity between positions C3 and C7 is under complete catalyst control. A phosphine‐containing palladium catalyst promotes the direct arylation at the most acidic position (C7), whereas a phosphine‐free catalyst targets the most electron‐rich position (C3).     

Intra‐ and Intermolecular Dispersion Interactions in [n]Cycloparaphenylenes: Do They Influence Their Structural and Electronic Properties?

Cycloparaphenylenes (CPPs) are nanosized structures with unique isolated and bulk properties, and are synthetic targets for the template‐driven bottom‐up synthesis of carbon nanotubes. Thus, a systematic understanding of the supramolecular order at the nanoscale is of utmost relevance for molecular engineering. In this study, it is found that intramolecular noncovalent (dispersion) interactions must be taken into account for obtaining accurate estimates of the structural and optoelectronic properties of [n]CPP compounds, and their influence as the number of repeat units increases from n=4 to n=12 is also analyzed, both in the gas phase and in solution. The supramolecular self‐assembly, for which both intra‐ and intermolecular noncovalent interactions are relevant, of [6]CPP is also investigated by calculating the binding energies of dimers taken along several crystal directions. These are also used to estimate the cohesive energy of the crystal, which is compared to the value obtained by means of dispersion‐corrected DFT calculations using periodic boundary conditions. The reasonable agreement between both computational strategies points towards a first estimate of the [6]CPP cohesive energy of around 50 kcal mol^?1.     

Microwave Accelerated Polymerization of 2‐Phenyl‐2‐oxazoline: Microwave or Temperature Effects?

Summary: Investigations regarding the cationic ring‐opening polymerization of 2‐phenyl‐2‐oxazoline under microwave irradiation and conventional heating are reported. This study was inspired by contradictory reports of the (non‐)existence of non‐thermal microwave effects that might accelerate the cationic ring‐opening of 2‐oxazolines. The polymerization of 2‐phenyl‐2‐oxazoline was investigated under pressure in acetonitrile and under reflux (or at the boiling point of butyronitrile in a closed vessel) in butyronitrile utilizing a single‐mode microwave reactor and automated synthesis robots with conventional heating.