Ab initio study of interaction-induced NMR shielding constants in mixed rare gas dimers

The interaction-induced contribution to the NMR shielding constants in homonuclear A2 and heteronuclear AB (A,B=He,Ne,Ar) dimers is obtained ab initio by employing a coupled cluster singles and doubles with perturbative treatment of triples wave function model and extended correlation-consistent basis sets. The second virial coefficients entering the expansion of the property with the density are then computed in a fully quantum mechanical approach, for temperatures ranging from the limit of dissociation of the dimer to well above standard conditions. The results can be used to describe the density and temperature dependence of the shielding constants in binary mixtures of helium, neon, and argon. The predicted effects should be observable for the interaction of 21Ne with other rare gases.     

Toward the total synthesis of spirastrellolide A. Part 3: intelligence gathering and preparation of a ring-expanded analogue

Different methods for the formation of the C.25-C.26 bond of spirastrellolide A () are evaluated that might qualify for the end game of the projected total synthesis, with emphasis on metathetic ways to forge the macrocyclic frame.     

Stereochemistry of rHint1 hydrolase assisted cleavage of P-N bond in nucleoside 5'-O-phosphoramidothioates

The Hint-1 hydrolase assisted cleavage of the P-N bond in adenosine-5'-O-[N-(tryptophanylamide)]phosphoramidothioate proceeds with retention of configuration at the phosphorus atom which is consistent with the formation of a covalent enzyme-substrate complex.     

TLC of alkaloids on cyanopropyl bonded stationary phases. Part II. Connection with RP18 and silica plates

Some standards of the alkaloids and synthetic or natural mixtures are separated by two-dimensional thin-layer chromatography (TLC) on different adsorbent layers. Normal- and reversed-phase systems are used to obtain significant differences in the separation selectivity. Optimization of the one-dimensional TLC separation of the alkaloids' standards is performed on cyanopropyl-silica, RP18W, and silica layers in various eluents containing (besides diluent and modifier) silanol blockers, such as diethyl amine or ammonia. The most selective systems are used for the separation of the alkaloids' mixtures by two-dimensional TLC with an adsorbent gradient method. The mixtures of alkaloids or plant extracts (Chelidonium majus, Fumaria officinalis, or Glaucium flavum) are chromatographed in system I; the plates are connected with the plate pre-coated with various adsorbent, and partly separated fractions are transferred to the second layer and developed in system II. CN-silica-RP18W and CN-silica-silica are used as the connected layers. The alkaloids are identified by R(F) values of standards, and the components of plant extracts are identified in both systems, and by the comparison of UV spectra obtained in diode array detector densitometry.     

The application of gradient reversed-phase high-performance liquid chromatography to the pK(a) and log k(w) determination of polyprotic analytes

The purpose of this work was to propose a theoretical model of the combined pH/organic modifier gradient in reversed-phase high-performance liquid chromatography (RP HPLC) with special emphasis on its applicability to polyprotic analytes. The model was developed and approximated to be useful for a data set comprising organic modifier gradients obtained at different pH changes and different gradient durations. It was evaluated regarding its ability to describe experimental data. The chromatographic pK(a) and lipophilicity parameter, log k(w), were obtained by fitting to the proposed model and comparing to the literature values.     

Synthesis and structural characterisation of alkylaluminium and alkylgallium pinacolates

The influence of the steric repulsion of ligands on the structure of alkylaluminium and alkylgallium 2,3-dimethylbutane-2,3-diolates (pinacolates) has been studied. Reaction of Me₃Al and pinacol afforded the compound Me₅Al₃[OC(CH₃)₂C(CH₃)₂O]₂ (1) possessing two diolate ligands. The treatment of pinacol with the more sterically hindered ^tBu₃Al resulted in the formation of aluminium tripinacolate ^tBu₃Al₃[OC(CH₃)₂C(CH₃)₂O]₃ (2) as the only product. In contrast, the reaction of the less reactive ^tBu₃Ga with pinacol stopped at the stage of the compound ^tBu₃Ga₂[OC(CH₃)₃C(CH₃)₂O] [OC(CH₃)₃C(CH₃)₂OH] (3), which is an intermediate product of the reaction of 1,2-diols with group 13 trialkyls. Compounds 1, 2 and 3 were crystallographically characterised.     

Two-Dimensional Thin-Layer Chromatography of Structural Analogs. Part II. Method for Quantitative Analysis of Selected Coumarins in Plant Material

JPC – Journal of Planar Chromatography – Modern TLC - Coumarins are interesting group of natural compounds, because of their biological and pharmacological activity, and are widely...     

Is the Hoveyda–Grubbs Complex a Vinylogous Fischer‐Type Carbene? Aromaticity‐Controlled Activity of Ruthenium Metathesis Catalysts

Three naphthalene-based analogues (4 a-c) of the Hoveyda-Grubbs metathesis catalyst exhibited immense differences in reactivity. Systematic structural and spectroscopic studies revealed that the ruthenafurane ring present in all 2-isopropoxyarylidene chelates possesses some aromatic character, which inhibits catalyst activity. This aromatic stabilization within the chelate ring may be controlled by variation of the polycyclic core topology as was demonstrated for tetraline and phenanthrene derivatives (4 d, e). General conclusions about a new mode of ligand-structure tuning in catalytic systems are presented.     

The time of graph bootstrap percolation

Graph bootstrap percolation, introduced by Bollobás in 1968, is a cellular automaton defined as follows. Given a “small” graph H and a “large” graph , in consecutive steps we obtain from G_t by adding to it all new edges e such that contains a new copy of H. We say that G percolates if for some , we have G_t = K_n. For H = K_r, the question about the size of the smallest percolating graphs was independently answered by Alon, Frankl and Kalai in the 1980's. Recently, Balogh, Bollobás and Morris considered graph bootstrap percolation for and studied the critical probability , for the event that the graph percolates with high probability. In this paper, using the same setup, we determine, up to a logarithmic factor, the critical probability for percolation by time t for all © 2016 Wiley Periodicals, Inc. Random Struct. Alg., 51, 143–168, 2017     

Stability of minimizers of set optimization problems

We study the asymptotic behavior of sequences of minimization problems in set optimization. More precisely, considering a sequence of set optimization problems \((P_n)\) converging in some sense to a set optimization problem (P) we investigate the upper and lower convergences of the sets of minimizers of the problems \((P_n)\) to the set of minimizers of the problem (P).