Structural and magnetic properties of homoleptic iron(III) complexes containing N-(8-quinolyl)-salicylaldimine [Fe(Qsal)2]+X− {X = I or (Qsal)FeCl3}

The structural properties of two homoleptic iron(III) complexes containing N-(8-quinolyl)-salicylaldimine (qsalH) are reported on the basis of two single-crystal X-ray diffraction experiments. The complexes are shown to share the general formula [Fe(qsal)₂]⁺X^? {X = I (1) or (qsal)FeCl₃ (2)} and include one new complex 2 and one previously reported complex 1 for which structural details were until now unknown. Variable-temperature magnetic susceptibility measurements on both samples indicate the nature of the spin state of the metal ion, which is low-spin for 1 and high-spin for 2. Mössbauer spectra are recorded for 1 at 293 and 5.6 K and indicate unusual temperature-dependent behaviour, which corroborates an earlier report.     

The Synthesis of Graft,Block, and Star Polymers Using Enolate-Initiated Anionic Polymerization

Abstract

Initiation of the polymerization of methacrylate monomers by enolate ions affords broad opportunities for the design of several types of macromolecules. Alone, or in combination with other techniques, enolate initiation provides routes to the preparation of well-Characterized graft copolymers, block copolymers (of both AB and BAB types), star polymers, and H-polymers. The general mechanism of polymerization initiated by lithiated esters and silyl ketene acetals is discussed, and examples are presented of the synthesis of each of the kinds of polymer molecules mentioned above.     

Semiclassical Limit of Focusing NLS for a Family of Square Barrier Initial Data

The small dispersion limit of the focusing nonlinear Schrödinger equation (NLS) exhibits a rich structure of sharply separated regions exhibiting disparate rapid oscillations at microscopic scales. The non‐self‐adjoint scattering problem and ill‐posed limiting Whitham equations associated to focusing NLS make rigorous asymptotic results difficult. Previous studies have focused on special classes of analytic initial data for which the limiting elliptic Whitham equations are wellposed. In this paper we consider another exactly solvable family of initial data,the family of square barriers,ψ 0(x) = qχ[?L,L] for real amplitudes q. Using Riemann‐Hilbert techniques, we obtain rigorous pointwise asymptotics for the semiclassical limit of focusing NLS globally in space and up to an O(1) maximal time. In particular, we show that the discontinuities in our initial data regularize by the immediate generation of genus‐one oscillations emitted into the support of the initial data. To the best of our knowledge, this is the first case in which the genus structure of the semiclassical asymptotics for focusing NLS have been calculated for nonanalytic initial data. © 2013 Wiley Periodicals, Inc.     

Complex phosphorus thermochemistry. Volume-based thermodynamics and the estimation of standard enthalpies of formation of gas phase ions: Delta(f)H(o) (PCl4+, g) and Delta(f)H(o) (PCl6-, g)

Energy-resolved collision-induced dissociation in a flowing afterglow-guided ion beam tandem mass spectrometer has recently enabled the accurate determination of the standard enthalpy of formation of the gaseous phosphorus pentachloride cation, Delta(f)H(o) ([PCl4(+)], g), found to be 414 +/- 17 kJ mol(-1) (giving a value of 378 +/- 18 kJ mol(-1) at 0 K). Such experimental values for the standard enthalpy of formation of gas phase complex are now being incorporated into the NIST standard reference data program. Such results, can, inter alia, provide a benchmark by which to test earlier computationally based methods which were made to estimate such quantities in the absence of any experimental data. The establishment of this value experimentally also affords us with the opportunity to explore the likely success of newer, simpler approaches. Previous large-scale direct minimization computations to estimate this (and other) standard enthalpies of formation match very well these new experimental results. This paper raises the question as to whether the much simpler volume-based thermodynamics (VBT) approach could yield equally satisfactory results and so circumvent, completely, the need for detailed modeling of the lattices involved. The conclusion is that the VBT approach portrays the extremely complex thermodynamics quite adequately. Thus for the purposes of obtaining basic thermodynamic data, complex modeling of the underlying structures involved may no longer be necessary. At least this should be the case for highly symmetrical ions, like PCl4(+), where detailed packing with counterions is possibly less important than in other cases and where covalent interactions (less easily modeled) with neighboring ions is unlikely to be strongly featured. Other gaseous complex ion enthalpies of formation are also predicted here.     

Total synthesis of +/--hyphodermins A and D

An efficient formal synthesis of (+/-)-hyphodermins A and D, metabolites of Hyphoderma radula, has been completed in 12 and 11 steps, respectively. The tricyclic carbon skeleton of enone 6 was rapidly assembled from diester 11 via an alpha brominationn-elimination sequence followed by anhydride formation. Regioselective reduction of the lactone group of enone 6 with LiAlH(t-BuO) 3 gave lactol 15. Lactol 15 was converted in two steps to (+/-)-hyphodermin D, without the need for complex protection-deprotection strategies. Lactol 15 was converted in three steps to (+/-)-hyphodermin A, via the key step of epoxidation of an enone in the presence of a THP lactol. A combination of NMR and ab initio studies suggests that the structures of hyphodermin C and D should be interchanged.     

Molecular modeling and mutagenesis reveals a tetradentate binding site for Zn2+ in GABA(A) alphabeta receptors and provides a structural basis for the modulating effect of the gamma subunit

Gamma-aminobutyric acid type A receptors (GABA(A)-R) containing alpha1beta2gamma2 subunits are weakly inhibited by Zn2+, whereas receptors containing only the alpha1beta2 subunits are strongly inhibited. We built homology models of the ion pores of alpha1beta2 and alpha1beta2gamma2 GABA(A)-R using coordinates of the nicotinic acetylcholine receptor as a template. Threading the GABA(A)-R beta2 sequence onto this template placed the 17' histidine and the 20' glutamate residues at adjacent locations in the mouth of the pore, such that a nearly ideal tetradentate site for Zn2+ was formed from two histidine and two glutamate residues between adjacent beta subunits in the alpha1beta2 GABA(A)-R. Following optimization with CHARMM, the distance between the alpha-carbons of the adjacent histidine residues was approximately 9.2 A, close to the ideal distance for a Zn2+ binding site. Loss of inhibition by Zn2+ in alpha1beta2gamma2 GABA(A)-R can be explained by the geometry of these residues in the arrangement alpha1beta2gamma2alpha1beta2, in which the nearest C-alpha-C-alpha distance between the histidine residues is 15.5 A, too far apart for an energetically optimal Zn2+ binding site. We then mutated the gamma subunit at the 17' and/or 20' positions. Zn2+ inhibition was not restored in alpha1beta2gamma2 (I282H) receptors. A novel finding is that the modeling shows the native 20' lysine in gamma2 can compete with Zn2+ for binding to the inserted 17' histidine. Sensitivity to Zn2+ was restored in the double mutant receptor, alpha1beta2gamma2 (I282H; K285E), in which the competition with lysine was removed and a more favorable Zn2+ binding site was formed.     

Solvent effects on oxygen-17 chemical shifts in methyl formate: linear solvation shift relationships

A multiple linear regression analysis has been carried out using the Kamlet-Abboud-Taft solvatochromic parameters in order to quantify the solvent effects on the (17)O chemical shifts of methyl formate (MF). The influence of the solvents upon the carbonyl oxygen chemical shifts is smaller for MF than for N-methylformamide (NMF). The influence (in parts per million) of the solvent polarity-polarizability reduces from -21.9pi* in amides to -9.6pi* in MF. The influence of the solvent hydrogen-bond-donor acidities reduces from -42.0alpha in formamides to -16.9alpha in MF. The solvent effects upon the dicoordinated oxygen chemical shifts of MF are smaller in magnitude and opposite in direction, i.e., 4.8pi* and 2.6alpha, than those for the carbonyl oxygen. (17)O hydration shifts have been calculated for the NMF + (H(2)O)(6) and MF + (H(2)O)(5) complexes by the ab initio GIAO method at the 6-311 + G** level. The hydration shifts calculated for the carbonyl oxygens of NMF and MF and for the dicoordinated oxygen of MF, -102.4, -64.7, and 17.6 ppm, respectively, show the same trend as the corresponding empirical hydration shifts, -101.7, -42.0, and 14.2 ppm.