Quadricyclanes. Part II: Electronic structure and chemical reactivity

The electronic structure of quadricyclane and 3-methylidenequadricyclane obtained by photoelectron spectroscopy, is used as a basis for the discussion of cycloadditions to these systems. The electronic structure of 3-heteroquadricyclanes, arrived at by theoretical calculations, agrees well with that expected from the above measured systems. A surprising outcome is that the orbital most responsible for the observed 2,4-cycloadditions to these heterosystems in not the HOMO but the third highest orbital which lies well below the former. This strongly suggests that these 2,4-cycloadditions proceed not in a concerted fashion but presumably involve as rate-determining step the formation of a resonance-stabilized zwitterionic intermediate. The nature of this intermeiate is discussed and the feasability of its formation investigated on the basis of thermochemical considerations.     

Crystalline Inclusion Compounds Derived from Derivatives of Mandelic Acid. Host Synthesis, Inclusion Formation, and X-Ray Structures of a Free Host Compound and of DMSO Inclusion Complexes in Both Racemic and Optically Resolved Forms

New chiral host compounds based on mandelic acid derivatives having methyl (6a, b and 8a, b) or bromo substituents (7a, b) attached to the phenyl ring of mandelic acid and involving additional aromatic groups were synthesized. The inclusion properties of both the racemic and the optically resolved host species are reported, including solvent co-crystallization as well as chiroselective and vapour sorptive inclusion. The structures of the free racemic host compound 6b and of the DMSO inclusion compounds of optically resolved and racemic 8 (8a and 8b, respectively) have been determined by X-ray analysis. Enantiomeric pairs of molecules in 6b form centro-symmetric dimers by mutual hydrogen bonding of one hydroxyl group while the other is involved in O-–H ... interactions. The guest molecules in the DMSO complexes of 8a and 8b are bound via hydrogen bonds to two host molecules related by translation along crystallographic axes. Parallels to previous hosts of this type are drawn.     

Studies on Radical Cations I. Molecular and electronic structure of [3]radialene cation produced by γ-irradiation in condensed phase

γ-Irradiation of tris (methylidene)-cyclopropane (‘[3]radialene’) 1 in a rigid electron scavenging matrix (butylchoride/i-pentane, 1:1) at 77 K leads to formation of its molecular cation 1 ⁺. Slight softening of the matrix by a temperature increase of 3–5 K results in formation of a newly absorbing species, tentatively assigned arising from structrral relaxation of 1 ⁺ by π-bond rotation:      

Synthesis and properties of fluorine-containing heterocyclic compounds. V. Trifluoromethyl-1,8- and 1,10-phenanthrolines

The condensation of ethyl trifluoroacetoacetate with various heterocyclic amines yielded 2-(trifluoromethyl)phenanthrolin-4-ones. The structures of these compounds were established from a study of their infrared and ultraviolet spectra. Substituted phenanthrolines were prepared by nucleophilic attack on a chlorophenanthroline. The oxidation of 5-methoxy-2-trifluoromethyl-1,10-phenanthroline yielded an o-quinone which was treated with typical quinone reagents. A molecular orbital calculation was carried out in an attempt to explain the ease of reduction of the 1,10-phenanthroline system and the colors of the reduced compounds.     

Sequential energy and electron transfer in aggregates of tetrakis[oligo(p-phenylene vinylene)] porphyrins and C60 in water

Chiral aggregation of oligo(p-phenylene vinylene)-functionalized Zn and free-base porphyrins is observed in water. The formation of mixed assemblies containing both porphyrins results in sequential energy transfer from OPV via zinc porphyrin to free-base porphyrin. Furthermore, the incorporation of C60 as electron acceptor yields a charge separated state by ultimate electron transfer.     

Über die Stärke der Si—Si-bindung in Abhängigkeit vom Substituenten

The strength of the Si?Si-bond in disilanes varies with the substituents of silicon. It is shown, that the increase of the Si?Si stretching force constant inX ₃SiSiX ₃ parallels an increase of the SiH-stretching force constant in the correspondingX ₃SiH-compound. The coupling constantJ(²⁹SiH) alters in a similar manner. Exceptions are found in same disilanes with strong d π p π-interactions from the substituents.     

‘Lone pair’ Electronic Structure,Conformation and Oxidation Behaviour of Diaziridines

Photoelectron spectroscopic investigations indicate that (i) monocyclic diaziridines prefer a trans-conformation of the nitrogen-lone pairs, and (ii) the interaction between these is comparable to that exhibited by open-chain analogues, i.e. alkyl-substituted hydrazines. The chemical experience concerning the particular case of ? NH? NH? →? N?N? oxidation of diaziridines can therefore not be accounted for by a prevailing unfavorable ‘lone pair’ interaction. Thermochemical as well as entropy arguments may serve as an explanation. A rather unsatisfactory agreement between predictions from various theoretical procedures and experimental results for the ionization potentials of ‘lone pair’ MO electrons is noted.     

Zur Kenntnis schichtförmiger Si-Verbindungen des Typs (SiX) n

Zusammenfassung Die Reaktion zwischen CaSi₂ und JCl liefert in einer Festkörperreaktion unter Aufspaltung des Schichtgitters ein stöchiometrisch und strukturell definiertes (SiCl) _n . Eigenschaften und Hydrolyse der Verbindung werden besprochen. Solvolyse mit Alkoholen führt zu entsprechenden Alkoxyderivaten, die Umsetzung mit Lithiummethyl zur methylierten Verbindung.Mit 2 Abbildungen4. Mitt.:E. Hengge undH. Grupe, Chem. Ber.97, 1783 (1964).     

Solution and X-Ray Crystal Structures of the Di- and Tetra-allyl Ether of tert- utylcalix[4]arene

The di- and tetra-allyl ethers of tert-butylcalix[4]arene 1 and 2 have been prepared by alkylation of tert-butylcalix[4]arene with allyl bromide and K₂CO₃ using different reaction times. Solution ¹H NMR measurement of the di-allyl ether 1 and X-ray crystal structures of the complexes of 1 with chloroform (1a) or methanol (1b) indicate the cone conformation of 1 in which intramolecular hydrogen bonding can be maximized. The crystalline state conformers 1a and 1b are distorted in different grades depending on the solvent. While methanol is incorporated in the macrocycle, chloroform molecules do not occupy the cage. The solution ¹H NMR spectra of tetra-allyl ether 2 show the co-existence of the cone and partial cone conformation. The partial cone conformer of 2 was investigated by X-ray crystallography. In this compound hydrogen bonding is not existent. The conformer distribution is likely affected by steric and template effects.     

An improved clean-up strategy for simultaneous analysis of polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF), and polychlorinated biphenyls (PCB) in fatty food samples

The study and extension of a simple automated clean-up method for polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) to a broad range of polychlorinated biphenyls (PCB) is described. The isolation of seven PCDD, ten PCDF, and three coplanar PCB (cPCB) is extended to eight monoortho substituted PCB and seven so-called "marker PCB" (Aroclor 1260) for fatty food samples. This enables quantification of 35 compounds - including all congeners with a WHO toxic equivalent factor (TEF)--in a single extraction and single purification step. The chromatographic behaviour of mono-ortho PCB and marker PCB on a variety of adsorbents, including basic alumina, has been studied. Partitioning of analytes through multi-column sequences is described and correlated with their structural and electronic properties, by use of molecular modelling calculations. The fractionation process available with the Power-Prep automated clean-up system enables rapid independent analysis of the different groups of compounds. Gas chromatography with high resolution mass spectrometry (GC-HRMS) is used for the PCDD/F and cPCB fraction and quadrupole ion-storage tandem in time mass spectrometry (GC-QISTMS) for analysis of the remaining PCB. A comparison study was performed on quality-control samples and real fatty food samples to evaluate the robustness of the new strategy compared with a reference method. On the basis of this simultaneous clean-up, a rapid simplified strategy for PCDD/F and selected PCB analysis determination is proposed for fatty food samples.