Mechanism of nucleophilic substitutions at phenacyl bromides with pyridines. A computational study of intermediate and transition state

DFT computations have been performed on nucleophilic substitutions of phenacyl bromides with pyridines to investigate the mechanism of the reaction. In contrast with earlier suppositions, tetrahedral intermediate is not formed by the addition of pyridine on the C?O group of phenacyl bromide, because the total energy of the reacting species increases continuously, when the distance between the N and C(?O) atoms of reactants is shorter than 2.7 Å. At a greater distance, however, a bridged complex of the reactants is observed, in which the N atom of pyridine is slightly closer to the C atom of the C?O, than to the C atom of the CH₂Br group of phenacyl bromide, the distances are 2.87 and 3.05 Å, respectively. The attractive forces between the oppositely polarized N and C(?O) atoms in the complex decrease the free energy of activation of the S_N2 attack of pyridine at the CH₂Br group. The calculated structural parameters of the S_N2 transition states (TS) indicate, that earlier TSs are formed when the pyridine nucleophile bears electron‐donating (e‐d) groups, while electron‐withdrawing (e‐w) groups on phenacyl bromide substrate increase the tightness of the TS. Free energies of activation computed for the S_N2 substitution agree well with the data calculated from the results of kinetic experiments and correlate with the σ_Py substituent constants, derived for pyridines, and with the Hammett σ constants, when the substituents (4‐MeO‐4‐NO₂) are varied on the pyridine or on the phenacyl bromide reactants. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the oxidation of tetrathionate by iodine in a slightly acidic medium

The iodine-tetrathionate reaction has been reinvestigated spectrophotometrically at T = 25.0 +/- 0.1 degrees C and at an ionic strength of 0.5 M adjusted by sodium acetate as a buffer component in both the absence and presence of the iodide ion in the pH range of 4.25-5.55. The reaction was found to be independent of pH within the range studied, and it was clearly demonstrated that the reaction proceeds via an intermediate S4O6I- formed in a pre-equilibrium. Iodide dependence of the kinetic curves strongly suggests that the iodide ion has to be involved in this equilibrium. Further reactions of the intermediate, including its hydrolysis and reaction with iodide, leads to the strict stoichiometry characterized by S4O62- + 7I2 + 10H2O --> 4SO42- + 14I- + 20H+. A seven-step kinetic model with three fitted kinetic parameters is suggested and discussed. A rate equation is also derived from which a sound explanation of the iodide dependence of the apparent rate coefficient is presented. Furthermore, it has also been pointed out that formation of the triiodide ion alone is not sufficient to take the retardation effect of the iodide ion into account quantitatively.     

Adiabatic Jacobi corrections for H2+-like systems

The Coulomb three-body problem in Jacobi coordinates was solved by treating the distance of the particles having equal charge as a parameter. This method allows computation of electronic energies with finite nuclear masses while maintaining the notion of a potential energy curve. The rotationless ground-state electronic and the so-called adiabatic Jacobi correction (AJC) energies are presented for H2+, D2+, and HD+ at fixed internuclear separations. The AJCs are defined as the difference between the results obtained from calculations using proper finite and infinite nuclear masses. Except at the united atom limit, the AJCs are smaller than the traditional first-order diagonal Born-Oppenheimer corrections. Expectation values of proton-electron, p-e, and deuteron-electron, d-e, distances for HD+ have been computed as a function of internuclear separation. Similarly to the fully nonadiabatic approach, the present method is able to follow the symmetry breaking in HD+. Exact and approximate analytical and numerical results are given for counterfactual systems as well. In these cases changes are allowed for the values of the electron rest mass or the elementary charge, as well as for the mass or charge of the unique particle (electron).     

Chain Intersecting Families

Let be a family of subsets of an n-element set. is called (p,q)-chain intersecting if it does not contain chains and with . The maximum size of these families is determined in this paper. Similarly to the p = q = 1 special case (intersecting families) this depends on the notion of r-complementing-chain-pair-free families, where r = p + q − 1. A family is called r-complementing-chain-pair-free if there is no chain of length r such that the complement of every set in also belongs to . The maximum size of such families is also determined here and optimal constructions are characterized. The first author is a member of the Egerváry Research Group (EGRES). Research is supported by OTKA grants T 037547 and TS 049788, by European MCRTN Adonet, Contract Grant No. 504438 and by the Egerváry Research Group of the Hungarian Academy of Sciences. The work of the second author was partially supported by the Hungarian National Foundation for Scientific Research (OTKA), grant numbers T037846 and NK62321.     

Biological systems as nanoreactors: anomalous small-angle scattering study of the CdS nanoparticle formation in multilamellar vesicles

The formation of cadmium sulfide (CdS) particles in the gaps between the layers of the multilamellar vesicles is described, introducing a new pathway in the preparation of nanometer-scale particles. The in situ structural characterization of both the CdS particles and the vesicles as a reaction medium was performed in the early and final states of the process by using anomalous small-angle X-ray scattering (ASAXS) and freeze-fracture methods. The ASAXS method provides the separation of the scattering of nanoparticles present in a small amount, whereby the monitoring of their formation and growth in the whole time range of manufacturing has become possible.     

Protolimonoids and norlimonoids from the stem bark of Toona ciliata var. pubescens

Six new tirucallane protolimonoids, toonapubesins A-F (1-6), one new rearranged tirucallane protolimonoid, toonapubesin G (7), and two new 21,22,23-trinorapotirucallane limonoids, toonapubesic acids A (8) and B (9), possessing an unprecedented carbon skeleton, along with five known tirucallane protolimonoids (10-14) and one known apotirucallane limonoid (15), were isolated from the stem bark of Toona ciliata var. pubescens. Their structures and relative configurations were determined by detailed spectroscopic analysis and by chemical methods. The proposed structures of 8 and 11 were confirmed by X-ray diffraction analysis of their respective derivatives (8a and 11a). The absolute configuration of 8 was determined by a novel solid-state TDDFT ECD approach on its derivative 8a while the absolute configuration of 10 was determined by the modified Mosher's method. In addition, the structures of dyvariabilin H (10c) proposed by Sticher et al. and cneorin-NP(36) (11b) by Mondon et al. were corrected as 10 and 11, respectively. Toonapubesin G (7) showed promising inhibitory activity against CDC25B with an IC(50) value of 2.1 μM, while compound 8a showed significant cell protecting activity against H(2)O(2)-induced SH-SY5Y cell damage with 11.5% increase in cell viability.     

Rotating full- and reduced-dimensional quantum chemical models of molecules

A flexible protocol, applicable to semirigid as well as floppy polyatomic systems, is developed for the variational solution of the rotational-vibrational Schro?dinger equation. The kinetic energy operator is expressed in terms of curvilinear coordinates, describing the internal motion, and rotational coordinates, characterizing the orientation of the frame fixed to the nonrigid body. Although the analytic form of the kinetic energy operator might be very complex, it does not need to be known a priori within this scheme as it is constructed automatically and numerically whenever needed. The internal coordinates can be chosen to best represent the system of interest and the body-fixed frame is not restricted to an embedding defined with respect to a single reference geometry. The features of the technique mentioned make it especially well suited to treat large-amplitude nuclear motions. Reduced-dimensional rovibrational models can be defined straightforwardly by introducing constraints on the generalized coordinates. In order to demonstrate the flexibility of the protocol and the associated computer code, the inversion-tunneling of the ammonia ((14)NH(3)) molecule is studied using one, two, three, four, and six active vibrational degrees of freedom, within both vibrational and rovibrational variational computations. For example, the one-dimensional inversion-tunneling model of ammonia is considered also for nonzero rotational angular momenta. It turns out to be difficult to significantly improve upon this simple model. Rotational-vibrational energy levels are presented for rotational angular momentum quantum numbers J = 0, 1, 2, 3, and 4.     

Integration of ground aerogel particles as chromatographic stationary phase into microchip

C16 modified and ground monolithic silica aerogel particles in submicrometer size, as a new type of stationary phase was prepared and integrated in polydimethylsiloxane (PDMS) microchip. The aerogel particles were packed into the microfluidic channel using a simple procedure, which does not require any special frit or fabrication step to retain the particles. The subnanoliter volume of samples can be transported through the porous, short length of packing with low pressure (< 3 bar). Food dyes as test components could be separated using low pressure within 6s. A 50-fold preconcentration could be achieved by retaining 100 nL volume of sample on the packing and elution with methanol.     

Influence of the solvation process on solute adsorption in reversed phase liquid chromatography

The adsorption isotherms of phenol were acquired by frontal analysis on six different reversed phase adsorbents from five different organic solvent solutions. The end-capped octadecyl columns only differed in the bonding density of the C(18) ligands. The inverse method was used to confirm the estimated isotherm parameters derived from the frontal experiments. The effect of the bonding density of the end-capped octadecyl bonded phase on the adsorption properties of phenol from different mobile phase compositions was investigated. The adsorption behavior of phenol has changed from Langmuir type to BET type with the change of the organic modifier and the bonding density of the adsorbent.