Effect of the geometric phase on the dynamics of the hydrogen-exchange reaction

A recent puzzle in nonadiabatic quantum dynamics is that geometric phase (GP) effects are present in the state-to-state opacity functions of the hydrogen-exchange reaction, but cancel out in the state-to-state integral cross sections (ICSs). Here the authors explain this result by using topology to separate the scattering amplitudes into contributions from Feynman paths that loop in opposite senses around the conical intersection. The clockwise-looping paths pass over one transition state (1-TS) and scatter into positive deflection angles; the counterclockwise-looping paths pass over two transition states (2-TS) and scatter into negative deflection angles. The interference between the 1-TS and 2-TS paths thus integrates to a very small value, which cancels the GP effects in the ICS. Quasiclassical trajectory (QCT) calculations reproduce the scattering of the 1-TS and 2-TS paths into positive and negative deflection angles and show that the 2-TS paths describe a direct insertion mechanism. The inserting atom follows a highly constrained "S-bend" path, which allows it to avoid both the other atoms and the conical intersection and forces the product diatom to scatter into high rotational states. By contrast, the quantum 2-TS paths scatter into a mainly statistical distribution of rotational states, so that the quantum 2-TS total ICS is roughly twice the QCT ICS at 2.3 eV total energy. This suggests that the S-bend constraint is relaxed by tunneling in the quantum system. These findings on H+H(2) suggest that similar cancellations or reductions in GP effects are likely in many other reactions.     

Gentle immobilization of nonionic polymersomes on solid substrates

Vesicles from Pluronic L121 (PEO5-PPO68-PEO5) triblock copolymers were first stabilized by a permanent interpenetrating polymer network and then gently immobilized onto a glass or mica surface. Fluorescence-labeled micrometer-sized vesicles were visualized with confocal laser scanning microscopy, and smaller sized capsules, around 100 nm, were probed by liquid atomic force microscopy. The immobilized vesicles were weakly attached to a negatively charged surface via negatively charged polyelectrolytes in combination with Mg2+ ions and can be reversibly detached from the surface by slightly elevated temperatures. To illustrate that the immobilized vesicles remain responsive to external stimuli, we show that it is possible to transform their shape from spherical to cylindrical by introducing a second Pluronic, namely, P123 (PEO20-PPO70-PEO20). The detailed transition process has been recorded in real time by confocal laser scanning microscopy. Electron microscopy studies confirmed that a similar morphology change also occurs in the bulk.     

Tris(dimethylamido)bis(dimethylamine)titanium(IV) chloridobis(dimethylamine)[tris(pentafluorophenyl)boron–amido][tris(pentafluorophenyl)boron–nitrido]titanate(IV) toluene solvate

The title ionic solid, [Ti(C₂H₆N)₃(C₂H₇N)₂][Ti(C₁₈BF₁₅N)(C₁₈H₂BF₁₅N)Cl(C₂H₇N)₂]·C₇H₈, (I), comprises a cation with three dimethylamide ligands in the equatorial plane and two dimethylamine ligands positioned axially in a trigonal–bipyramidal geometry about the central Ti^IV atom. The anion has a highly distorted octahedral structure. The two dimethylamine ligands are coordinated mutually trans. The chloride is trans to the tris(pentafluorophenyl)boron–amide, while the sixth coordination site is occupied by an ortho‐F atom of the tris(pentafluorophenyl)boron–amide group in a trans disposition with respect to the tris(pentafluorophenyl)boron–nitride ligand. The most significant feature of the anion is the presence of an unprecedented terminal Ti[triple‐bond]N moiety [1.665 (2) Å], stabilized by coordination to B(C₆F₅)₃, with a Ti[triple‐bond]N—B angle of 169.50 (19)°.     

Generalizations of the projection method with applications to SOR theory for hermitian positive semidefinite linear systems

Summary Ann×n complex matrixB is calledparacontracting if B₂1 and 0x[N(I-B)]Bx₂<x₂. We show that a productB=B _k B _k–1 ...B ₁ ofk paracontracting matrices is semiconvergent and give upper bounds on the subdominant eigenvalue ofB in terms of the subdominant singular values of theB _i 's and in terms of the angles between certain subspaces. Our results here extend earlier results due to Halperin and due to Smith, Solomon and Wagner. We also determine necessary and sufficient conditions forn numbers in the interval [0, 1] to form the spectrum of a product of two orthogonal projections and hence characterize the subdominant eigenvalue of such a product. In the final part of the paper we apply the upper bounds mentioned earlier to provide an estimate on the subdominant eigenvalue of the SOR iteration matrix associated with ann×n hermitian positive semidefinite matrixA none of whose diagonal entries vanish.The work of this author was supported in part by NSF Research Grant No. MCS-8400879     

New seven-coordinate thiourea and N,N,N′,N′-tetramethylthiourea complexes of molybdenum(II) and tungsten(II)

The compounds [MI₂(CO)₃(NCMe)₂] (M = Mo or W) react with one equivalent of thiourea (tu) in MeOH or N,N,N′,N′-tetramethylthiourea (tmtu) in CH₂Cl₂ at room temperature to initially afford the monoacetonitrile compounds [MI₂(CO)₃(NCMe)L] (L = tu or tmtu) which rapidly transform to the isolated iodide bridged dimers, [M(μ-I)I(CO)₃L]₂ with loss of acetonitrile. Reaction of [WI₂(CO)₃(NCMe)₂] with two equivalents of tu or tmtu gave the expected mononuclear seven-coordinate compounds [WI₂(CO)₃L₂]. However, reaction of [MoI₂(CO)₃(NCMe)₂] with two equivalents of tu or tmtu rapidly affords the iodide-bridged dimers [Mo(μ-I)I(CO)₂L₂]₂ with loss of carbon monoxide from [MoI₂(CO)₃L₂]. The low temperature (−70°C) ¹³C NMR spectrum of [Mo(μ-I)I(CO)₂ {SC(NMe₂)₂}₂]₂ suggests the complex is based on two capped octahedra with a carbonyl ligand capping each octahedral face.     

A temperature-dependent order-disorder and crystallographic phase transition in a 0D Fe(II) spin crossover compound and its non-spin crossover Co(II) isomorph

The new dipyridylamino/triazine ligand DDE (N(2),N(2),N(4),N(4)-tetraethyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine) has been incorporated into the mononuclear Fe(II) SCO compounds cis-[Fe(II)(NCSe)(2)(DDE)(2)] (1), cis-[Fe(II)(NCBH(3))(2)(DDE)(2)] (2), and cis-[Fe(II)(NCS)(2)(DDE)(2)] (3). Magnetic susceptibility measurements reveal that each of 1, 2 and 3 undergoes a complete, continuous spin transition with a T(?) of ～260 K, ～300 K and ～205 K, respectively. An analogue and isomorph of 1, cis-[Co(II)(NCSe)(2)(DDE)(2)] (4), remains high spin down to low temperatures. Variable temperature single crystal data reveal that 1 and 4 undergo a crystallographic phase transition (from orthorhombic Pbcn at high temperatures to monoclinic P2/c at low temperatures) accompanied by an order-disorder transition of ethyl moieties of the DDE ligand. In the Pbcn phase, the structures of 1 and 4 contain one crystallographically unique M(II) centre, while in the P2/c phase, 1 and 4 contain two crystallographically unique M(II) centres. Variable temperature powder X-ray diffraction experiments reveal that the crystallographic phase transition occurs at ～250 K for 1. The occurrence of the concomitant order-disorder and crystallographic phase transitions undergone by 1 and 4 is not directly apparent in their magnetic susceptibility measurements, and this is likely due to the local environment of the M(II) centres remaining largely undisturbed as the transitions occur. The compound 2 is isostructural to 1 and 4 at low temperatures.     

Relationship of stereochemical and skeletal diversity of small molecules to cellular measurement space

Systematic and quantitative measurements of the roles of stereochemistry and skeleton-dependent conformational restriction were made using multidimensional screening. We first used diversity-oriented synthesis to synthesize the same number (122) of [10.4.0] bicyclic products (B) and their corresponding monocyclic precursors (M). We measured the ability of these compounds to modulate a broad swath of biology using 40 parallel cell-based assays. We analyzed the results using statistical methods that revealed illuminating relationships between stereochemistry, ring number, and assay outcomes. Conformational restriction by ring-closing metathesis increased the specificity of responses among active compounds and was the dominant factor in global activity patterns. Hierarchical clustering also revealed that stereochemistry was a second dominant factor; whereas the stereochemistry of macrocyclic appendages was a determinant for bicyclic compounds, the stereochemistry of the carbohydrates was a determinant for the monocyclic compounds of global activity patterns. These studies illustrate a quantitative method for measuring stereochemical and skeletal diversity of small molecules and their cellular consequences.     

Pressure effects on Friedel-Crafts alkylation reactions in supercritical difluoromethane.

Dielectrometry is used as a novel technique for following the rate of a Friedel-Crafts alkylation reaction in supercritical (sc) difluoromethane. Although the process was not optimized, good product yields were obtained and reaction rates were found to be larger than in CO2 at comparable conditions. The reaction order is determined using the initial rate method and the reaction is shown to be first-order with respect to both anisole and t-butyl chloride. The reaction rate constant is unaffected by pressures above 120 bar but close to the critical pressure the value decreases with increasing pressure. It is also shown that the product distribution for the alkylation of anisole shows significant pressure dependence with substitution at the ortho-position being favored at lower pressures, which is ascribed to hydrogen bonding. This pressure dependency is not observed in sc CO2 or using toluene as a substrate, which supports the idea that hydrogen bonding may be important in the reaction mechanism. The effect of the different reagents and temperature on the rate of the alkylation reaction was also determined.     

Structure and dynamics of the solvated electron in alcohols from resonance Raman spectroscopy

Resonance Raman (RR) spectroscopy is used to probe the structure and excited-state dynamics of the solvated electron in the primary liquid alcohols methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), and n-butanol (n-BuOH). The strong resonance enhancements (>or=10(4) relative to pure solvent) of the libration, CO stretch, COH bend, CH3 bend, CH2 bend, and OH stretch reveal significant Franck-Condon coupling of the intermolecular and intramolecular vibrational modes of the solvent to the electronic excitation of the solvated electron. All enhanced bands are fully accounted for by a model of the solvated electron that is comprised of several nearby solvent molecules that are only perturbed by the presence of the electron; no new molecular species are required to explain our data. The 340 cm(-1) downshift observed for the OH stretch frequency of e-(MeOH), relative to pure solvent, strongly suggests that the methanol molecules in the first solvent shell have the hydroxyl group directed linearly toward the excess electron density. The smaller downshifts observed for e-(EtOH), e-(n-PrOH), and e-(n-BuOH) are explained in terms of a OH group that is bent 28-40 degrees from linear. The Raman cross sections and absorption spectra are modeled, lending quantitative support for the inhomogeneous origin of the broad absorption spectra, the necessity of OH local motion in all enhanced Raman modes of the alcohols, and the dominant librational response of the solvent upon photoexcitation of the electron.     

The application of vinylogous iminium salt derivatives to the synthesis of Ningalin B hexamethyl ether

A vinylogous iminium salt derivative has been used to prepare a 2,3,4-trisubstituted pyrrole synthon in a regioselective, efficient and convenient manner. This pyrrole synthon was subsequently converted to the multidrug-resistant (MDR) reversal agent, Ningalin B hexamethyl ether, via alkylation, hydrolysis and oxidative lactonization steps. This methodology provides an alternative pathway to this important class of pyrrole containing marine natural products.