Microwave‐Assisted Heterogeneous Cross‐Coupling Reactions Catalyzed by Nickel‐in‐Charcoal (Ni/C)

A study involving the relatively rare combination of heterogeneous catalysis conducted under microwave conditions is presented. Carbon–carbon bond formation, including Negishi and Suzuki couplings, can be quickly effected with aryl chloride partners by using a base metal (nickel) adsorbed in the pores of activated charcoal. Aminations were also studied, along with cross‐couplings of vinyl alanes with benzylic chlorides as a means to stereodefined allylated aromatics. Reaction times for all these processes are typically reduced from several hours to minutes in a microwave reactor.     

UV Raman demonstrates that alpha-helical polyalanine peptides melt to polyproline II conformations

We examined the 204-nm UV Raman spectra of the peptide XAO, which was previously found by Shi et al.'s NMR study to occur in aqueous solution in a polyproline II (PPII) conformation. The UV Raman spectra of XAO are essentially identical to the spectra of small peptides such as ala(5) and to the large 21-residue predominantly Ala peptide, AP. We conclude that the non-alpha-helical conformations of these peptides are dominantly PPII. Thus, AP, which is highly alpha-helical at room temperature, melts to a PPII conformation. There is no indication of any population of intermediate disordered conformations. We continued our development of methods to relate the Ramachandran Psi-angle to the amide III band frequency. We describe a new method to estimate the Ramachandran Psi-angular distributions from amide III band line shapes measured in 204-nm UV Raman spectra. We used this method to compare the Psi-distributions in XAO, ala(5), the non-alpha-helical state of AP, and acid-denatured apomyoglobin. In addition, we estimated the Psi-angle distributions of peptide bonds which occur in non-alpha-helix and non-beta-sheet conformations in a small library of proteins.     

One and two hydrogen atom migrations in the retro-diels-alder fragmentation of norbornene and bicyclo [2.2.2] octene derivatives under electron impact

Norbornene derivatives with one or two carbonyl-containing substituents at positions 5 and 6 (anhydrides, esters, amides, cyclic ketones) undergo an electron impact induced rerro-Diels-Alder fragmentation accompanied by the migration of one hydrogen atom (RDA + H) giving rise to the [dienophile + H]⁺ ions. Bicyclo[2.2.2]octene derivatives substituted at C(5) and C(6) by a ring with two carbonyl groups (anhydrides, imides, diketones) undergo an RDA fragmentation accompanied by the transfer of two hydrogen atoms (RDA + 2H). Bicyclo[2.2.2]octene 5,6-diesters undergo both the RDA + H and RDA + 2H fragmentations, and the relative abundance of the resulting [dienophile + H]⁺ and [dienophile + 2H]⁺˙ ions is strongly affected by configuration.     

TDDFT Studies of Electronic Structure and First Hyperpolarizability of Tetra-nuclear Cubane-like Transition Metal Clusters

A series of tri-nuclear transition metal clusters with incomplete cubane-like configurations have been studied by TDDFT method. The calculations show that they have enormously large second-order polarizabilities () and are potential nonlinear optical materials for infrared double frequency conversion1. In this paper some tetra-nuclear transition metal clusters with cubane-like configurations, MCu3X4 (PPh3)3 (M=W, Mo; X= S, O, Cl, Se, Br), were studied by TDDFT method for a reference…     

双圈图的零度与其匹配数之间的关系

Let G be a graph with n(G) vertices and m(G) be its matching number.The nullity of G,denoted by η(G),is the multiplicity of the eigenvalue zero of adjacency matrix of G.It is well known that if G is a tree,then η(G) = n(G)-2m(G).Guo et al.[Jiming GUO,Weigen YAN,Yeongnan YEH.On the nullity and the matching number of unicyclic graphs.Linear Alg.Appl.,2009,431:1293 1301]proved that if G is a unicyclic graph,then η(G)equals n(G)-2m(G)-1,n(G)-2m(G),or n(G)-2m(G) +2.In this paper,we prove that if G is a bicyclic graph,then η(G) equals n(G)-2m(G),n(G)-2m(G)±1,n(G)-2m(G)±2or n(G)-2m(G) + 4.We also give a characterization of these six types of bicyclic graphs corresponding to each nullity.     

A phenomenological theory for polarization flop in spiral multiferroic TbMnO<Subscript>3</Subscript>

A phenomenological Landau theory has been used to explain magnetic field-driven polarization flop in TbMnO ₃. The Néel wall-like magnetic structure in spiral multiferroics induces a space-dependent internal magnetic field which exerts a torque on spins to rotate bc-spiral to ab-spiral. The external magnetic field is argued to be competing with easy axis anisotropy and the system stabilizes when anisotropy is minimum. With the help of Landau free energy with DM magnetoelectric coupling and a general ansatz for magnetization, the phenomenon of polarization flop has been explained. Relation between T_flop and critical magnetic field has been established and found to be in good agreement with the experiment. This could be an indication that anisotropy of the system is temperature- and magnetic field-dependent.     

Car-Parrinello molecular dynamics simulations and EPR property calculations on aqueous ubisemiquinone radical anion

Car-Parrinello molecular dynamics (CPMD) simulations have been performed on ubisemiquinone radical anion in aqueous solution. The different types of hydrogen bonding formed between the semiquinone and the solvent were studied in terms of frequency and directionality, in comparison with the parent benzosemiquinone radical anion. The EPR parameters (g-tensors and hyperfine coupling constants) were obtained from quantum chemical property calculations performed on snapshots along the MD trajectory, and the contributions of different solvation effects to the EPR parameters have been evaluated. The influence of the anion’s conformational behaviour was examined, including the orientation-dependent effects of hyperconjugation on side-chain hyperfine coupling.     

Dependence of glycine CH2 stretching frequencies on conformation, ionization state, and hydrogen bonding

We experimentally and theoretically examined the conformation, pH, and temperature dependence of the CH2 stretching frequencies of glycine (gly) in solution and in the crystalline state. To separate the effects of the amine and carboxyl groups on the CH2 stretching frequencies we examined the Raman spectra of 2,2,2-d3-ethylamine (CD3-CH2-NH2) and 3,3,3-d3-propionic acid (CD3-CH2-COOH) in D2O. The symmetric (nusCH2) and asymmetric (nuasCH2) stretching frequencies show a significant dependence on gly conformation. We quantified the relation between the frequency splitting (Delta = nuasCH2-nusCH2) and the xi angle which determines the gly conformational geometry. This relation allows us to determine the conformation of gly directly from the Raman spectral frequencies. We observe a large dependence of the nusCH2 and nuasCH2 frequencies on the ionization state of the amine group, which we demonstrate theoretically results from a negative hyperconjugation between the nitrogen lone pair and the C-H antibonding orbitals. The magnitude of this effect is maximized for C-H bonds trans to the nitrogen lone pair. In contrast, a small dependence of the CH2 stretching frequencies on the carboxyl group ionization state arises from delocalization of electron density from carboxyl oxygen to C-H bonding orbitals. According to our experimental observations and theoretical calculations the temperature dependence of the nusCH2 and nuasCH2 of gly is due to the change in the hydrogen-bonding strength of the amine/carboxyl groups to water.     

Computable shuffle sums of ordinals

The main result is that for sets , the following are equivalent:

Geometric and chelation influences on the electronic structure and optical properties of tetra(carboxylic acid)phenyleneethynylene dyes

A quantum-chemical study on the consequences of geometric modification and chelation on the electronic structure and optical properties of a tetra(carboxylic acid)phenyleneethynylene dye, of interest for chemical sensing applications, is presented. Rotation within the central biphenylene and complexation with divalent metal ions--in particular Cu2+--lead to notable changes in the absorption and emission profiles. Calculations at both the density functional theory (DFT) and Hartree-Fock (HF) levels are used to evaluate geometric potential energy surfaces for rotation within the central biphenylene unit; HF coupled with configuration interaction singles (HF-CIS) is used to investigate the first excited state of the dye. Time-dependent DFT (TDDFT) calculations are employed to assess changes in optical absorption and fluorescence as a function of geometry and chelation.