Theoretical reference values for the AE6 and BH6 test sets from explicitly correlated coupled-cluster theory

We propose a new computational protocol to obtain highly accurate theoretical reference data. This protocol employs the explicitly correlated coupled-cluster method with iterative single and double excitations as well as perturbative triple excitations, CCSD(T)(F12), using quadruple-z\zeta basis sets. Higher excitations are accounted for by conventional CCSDT(Q) calculations using double-z\zeta basis sets, while core/core-valence correlation effects are estimated by conventional CCSD(T) calculations using quadruple-z\zeta basis sets. Finally, scalar-relativistic effects are accounted for by conventional CCSD(T) calculations using triple-z\zeta basis sets. In the present article, this protocol is applied to the popular test sets AE6 and BH6. An error analysis shows that the new reference values obtained by our computational protocol have an uncertainty of less than 1 kcal/mol (chemical accuracy). Furthermore, concerning the atomization energies, a cancellation of the basis set incompleteness error in the CCSD(T)(F12) perturbative triples contribution with the corresponding error in the contribution from higher excitations is observed. This error cancellation is diminished by the CCSD(T*)(F12) method. Thus, we recommend the use of the CCSD(T*)(F12) method only for small- and medium-sized basis sets, while the CCSD(T)(F12) approach is preferred for high-accuracy calculations in large basis sets.     

The electronic spectrum of the C(s)-C11H3 radical

The electronic gas-phase absorption spectrum of the bent carbon-chain radical, HC(4)CHC(6)H with C(s) symmetry, is recorded in the 595 nm region by cavity ring-down spectroscopy through an expanding hydrogen plasma. An unambiguous spectroscopic identification becomes possible from a systematic deuterium labeling experiment. A comparison of the results with recently reported spectra of the nonlinear HC(4)CHC(4)H and HC(4)C(C(2)H)C(4)H radicals with C(2v) symmetry provides a more comprehensive understanding of the molecular behavior of π-conjugated bent carbon-chain systems upon electronic excitation. We find that the electronic excitation in the bent carbon-chain HC(4)CHC(2n)H (n = 1-4) series exhibits a similar trend as in the linear HC(2n+1)H (n = 3-6) series, shifting optical absorptions towards longer wavelengths for increasing overall bent chain lengths. The π-conjugation in bent HC(4)CHC(2n)H (n = 1-4) chains is found to be generally smaller than in the linear HC(2n+1)H (n = 3-6) case for equivalent numbers of C-atoms. The addition of an electron-donating group to the bent chain causes a slight decrease of the effective conjugation.     

(Thio)ureido anion receptors based on a 1,3-alternate oxacalix[2]arene[2]pyrimidine scaffold

In pursuit of highly preorganized macrocyclic host molecules for the complexation of anions, a series of oxacalix[2]arene[2]pyrimidine-based bis(thio)ureido receptors were synthesized and fully characterized. The pincer-like 1,3-alternate conformation of the oxacalix[4]arene scaffold, essential for an efficient host-guest interaction, was visualized by single-crystal X-ray analysis and supported by variable-temperature NMR studies. The anion binding properties of the receptors were evaluated via (1)H NMR titration experiments, showing intermolecular interactions with H(2)PO(4)(-), AcO(-), BzO(-), and Cl(-) ions. The host molecule bearing 4-nitrophenyl substituents on the bisurea binding pocket showed association constants in the range of 200-400 M(-1) in the strongly competitive solvent mixture of DMSO/0.5% H(2)O.     

Synthesis of functionalized dioxa-aza[7]helicenes using palladium catalyzed arylations

Despite the recent reports on transition-metal catalyzed cycloisomerization strategies toward helicenes, the amount of palladium catalyzed routes remains rather scarce. Within this letter the successful preparation and characterization of novel dioxa-aza[7]helicenes using palladium mediated coupling reactions is presented.     

Synthesis and properties of methoxyphenyl-substituted derivatives of indolo[3,2-b]carbazole

The synthesis and full characterization of new derivatives of indolo[3,2-b]carbazole with differently substituted phenyl groups at nitrogen atoms is reported. Comparative study on their thermal, optical electrochemical, and photoelectrical properties is presented. The synthesized compounds are electrochemically stable. Their highest occupied molecular orbital energy values range from -5.14 to -5.07 eV. The electron photoemission spectra of the films of synthesized materials revealed the ionization potentials of 5.31-5.47 eV. Hole drift mobility of the amorphous film of 5,11-bis(3-methoxyphenyl)-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole exceed 10(-3) cm(2)/V·s at high electric fields, as it was established by xerographic time-of-flight technique. In contrast to diphenylamino substituted derivatives of carbazole, no effect of the position of methoxy groups on the photoelectrical properties was observed for the synthesized methoxyphenyl-substituted derivatives of indolo[3,2-b]carbazole. The indolo[3,2-b]carbazole core has a larger resonance structure that includes 3 phenyl rings, and thus the energy gap of the HOMO and LUMO π orbitals is lower as compared to that of carbazoles. With a larger energy difference between the phenyl substituents and the core moiety, the indolo[3,2-b]carbazole derivatives studied all have a weaker coupling between the phenyl group and a much weaker dependence of the molecular properties on the position of substituents on the phenyl groups as compared to those observed in substituted carbazoles.     

Synthesis of novel dendrimers containing pyrimidine units

A novel convergent approach to dendritic macromolecules is described in which 4,6-dichloro-2-(4-methoxyphenyl)-pyrimidine is used as the building block. The nucleophilic aromatic substitution reaction at this AB₂-monomer was used as the key step in the propagation of the dendrons. Different core reagents were used to form the dendrimers, including a 5,15-bis(pyrimidyl)porphyrin core. Fourth-generation dendrons and third-generation dendrimers could be synthesized. The presented dendrimers are promising candidates to be used in applications where a more rigid structure and a larger resistance towards the applied conditions is required.     

Influence of the coordination number of nitrogen on the structure of 6aλ4-thia-1,3,4,6-tetraazapentalenic systems. Crystal structure analyses of 3-(2-pyridylimino)-3H-[1,2,4]thiadiazolo[4,3-a]pyridine and its 1-methylated salt

Structural data were obtained by X-ray crystallography for the title compounds which show that they are essentially planar and exhibit an approximately linear N2-S1-N8 arrangement. In compound 3 the separation between the sulfur atom and the pyridine nitrogen atom (2.61 Å) is larger than the Huggins constant energy distance (2.58 Å), suggesting that there is little or no bonding between them. The methylated salt 4 , on the contrary, has a closer S…N(pyridine) distance (2.19 Å) with an estimated bond dissociation energy of 6 kcal/mole.     

Polyketide Synthase-Mediated O-Methyloxime Formation in the Biosynthesis of the Oximidine Anticancer Agents

Bacterial trans-acyltransferase polyketide synthases (trans-AT PKSs) are modular megaenzymes that employ unusual catalytic domains to assemble diverse bioactive natural products. One such PKS is responsible for the biosynthesis of the oximidine anticancer agents, oxime-substituted benzolactone enamides that inhibit vacuolar H⁺-ATPases. Here, we describe the identification of the oximidine gene cluster in Pseudomonas baetica and the characterization of four novel oximidine variants, including a structurally simpler intermediate that retains potent anticancer activity. Using a combination of in vivo, in vitro and computational approaches, we experimentally elucidate the oximidine biosynthetic pathway and reveal an unprecedented mechanism for O-methyloxime formation. We show that this process involves a specialized monooxygenase and methyltransferase domain and provide insight into their activity, mechanism and specificity. Our findings expand the catalytic capabilities of trans-AT PKSs and identify potential strategies for the production of novel oximidine analogues.     

A journey through quality control

The Euroanalysis VII conference in Vienna included a two-day session: Quality Assurance in Analytical Chemistry. The contributions comprised 15 lectures devoted to: intra-laboratory quality measures, inter-laboratory control, formal aspects and accreditation and implementation. The paper presents an overview of the main items developed by the contributors.A survey on the session on Quality Assurance in Analytical Chemistry of Euroanalysis VIIThe authors thank the organizers of EUROANALYSIS VII and in particular Dr. B. Griepink and Dr. E. Maier of the Community Bureau of Reference (BCR) of the CEC for their support and considerable contributions.     

Effects of Ultraviolet-B Exposure on the Resistance to Listeria monocytogenes in the Rat

A rat infection model using the bacterial pathogen Listeria monocytogenes was employed to analyze the im-munosuppressive activity of UVB radiation. Rats were exposed to suberythemal doses of UVB radiation for 5 or 7 consecutive days, using Kromayer or FS40 lamps respectively. Subsequently, the rats were infected subcuta-neously or intravenously with Listeria . Exposure to UVB resulted in an increased number of bacteria in the spleen 4 days after infection. Listeria -specific lymphocyte proliferation assays as well as delayed-type hypersensitivity reactions demonstrated that T cell-mediated immunity to Listeria was impaired by UVB as measured 4 and 8 days after infection. In addition, UVB exposure decreased phagocytotic activity of peripheral blood macrophages. This study demonstrated that suberythemal doses of UVB radiation caused a delay in the clearance of Listeria bacteria from the spleen of the rats and that this was probably caused by impaired nonspecific phagocytosis of Listeria by macrophages in addition to an impaired activity of Listeria -specific T cells.