Unraveling the electronic structure of transition metal dimers using resonant four‐wave mixing

The potential of two‐color resonant and degenerate four‐wave mixing spectroscopy for investigations of the complex spectra of transition metal dimers is explored. Two‐color resonant and degenerate four‐wave mixing spectroscopy scans of the well‐known A‐X and B‐X transitions in Cu₂ are reported and compared with previous experimental data obtained from standard single‐resonance techniques. The selectivity of the method is shown to enable the measurement of isotopologue pure spectra without the need for isotopically enriched metal targets. Specific subsets of the rovibronic structure are separated in a congested spectral region of overlapping transitions. The sensitivity of the method compares satisfactorily with linear spectroscopic methods such as laser‐induced fluorescence and cavity ring‐down. A new laser vaporization source for the production of transition metal dimers and clusters has been constructed. The new design aims for a high number density and maximum possible shot‐to‐shot stability. The possibilities of further applications of non‐linear four‐wave mixing spectroscopy to Cu₂ and other transition metal dimers are discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

High-throughput characterization of sediment organic matter by pyrolysis–gas chromatography/mass spectrometry and multivariate curve resolution: A promising analytical tool in (paleo)limnology

Molecular-level chemical information about organic matter (OM) in sediments helps to establish the sources of OM and the prevalent degradation/diagenetic processes, both essential for understanding the cycling of carbon (C) and of the elements associated with OM (toxic trace metals and nutrients) in lake ecosystems. Ideally, analytical methods for characterizing OM should allow high sample throughput, consume small amounts of sample and yield relevant chemical information, which are essential for multidisciplinary, high-temporal resolution and/or large spatial scale investigations. We have developed a high-throughput analytical method based on pyrolysis–gas chromatography/mass spectrometry and automated data processing to characterize sedimentary OM in sediments. Our method consumes 200 μg of freeze-dried and ground sediment sample. Pyrolysis was performed at 450 °C, which was found to avoid degradation of specific biomarkers (e.g., lignin compounds, fresh carbohydrates/cellulose) compared to 650 °C, which is in the range of temperatures commonly applied for environmental samples. The optimization was conducted using the top ten sediment samples of an annually resolved sediment record (containing 16–18% and 1.3–1.9% of total carbon and nitrogen, respectively). Several hundred pyrolytic compound peaks were detected of which over 200 were identified, which represent different classes of organic compounds (i.e., n-alkanes, n-alkenes, 2-ketones, carboxylic acids, carbohydrates, proteins, other N compounds, (methoxy)phenols, (poly)aromatics, chlorophyll and steroids/hopanoids). Technical reproducibility measured as relative standard deviation of the identified peaks in triplicate analyses was 5.5 ± 4.3%, with 90% of the RSD values within 10% and 98% within 15%. Finally, a multivariate calibration model was calculated between the pyrolytic degradation compounds and the sediment depth (i.e., sediment age), which is a function of degradation processes and changes in OM source type. This allowed validation of the Py–GC/MS dataset against fundamental processes involved in OM cycling in aquatic ecosystems.     

Improving simulated annealing through derandomization

We propose and study a version of simulated annealing (SA) on continuous state spaces based on \((t,s)_R\)-sequences. The parameter \(R\in \bar{\mathbb {N}}\) regulates the degree of randomness of the input sequence, with the case \(R=0\) corresponding to IID uniform random numbers and the limiting case \(R=\infty \) to (t, s)-sequences. Our main result, obtained for rectangular domains, shows that the resulting optimization method, which we refer to as QMC-SA, converges almost surely to the global optimum of the objective function \(\varphi \) for any \(R\in \mathbb {N}\). When \(\varphi \) is univariate, we are in addition able to show that the completely deterministic version of QMC-SA is convergent. A key property of these results is that they do not require objective-dependent conditions on the cooling schedule. As a corollary of our theoretical analysis, we provide a new almost sure convergence result for SA which shares this property under minimal assumptions on \(\varphi \). We further explain how our results in fact apply to a broader class of optimization methods including for example threshold accepting, for which to our knowledge no convergence results currently exist. We finally illustrate the superiority of QMC-SA over SA algorithms in a numerical study.     

Negishi cross-coupling reactions catalyzed by an aminophosphine-based nickel system: a reliable and general applicable reaction protocol for the high-yielding synthesis of biaryls

Treatment of NMP solutions of NiCl(2) with 1,1',1'-(phosphanetriyl)tripiperidine (≈2.05 equiv), dissolved in THF, in air at 25 °C forms a highly active catalytic system for the cross-coupling of a large variety of electronically activated, non-activated, deactivated, and ortho-substituted, heterocyclic, and functionalized aryl bromides and aryl chlorides with diarylzinc reagents. Very high levels of conversion and yields were obtained within 2 h at 60 °C in the presence of only 0.1 mol% of catalyst (based on nickel) and thus at catalyst loadings far lower than typically reported for nickel-catalyzed versions of the Negishi reaction. Various aryl halides-which may contain trifluoromethyl groups, fluorides, or other functional groups such as acetals, ketones, ethers, esters, lactones, amides, imines, anilines, alkenes, pyridines, quinolines, and pyrimidines-were successfully converted into the corresponding biaryls. Electronic and steric variations are tolerated in both reaction partners. Experimental observations indicate that a molecular (Ni(I)/Ni(III)) mechanism is operative.     

Theoretical and experimental studies on the carbon-nanotube surface oxidation by nitric acid: interplay between functionalization and vacancy enlargement

The nitric acid oxidation of multiwalled carbon nanotubes leading to surface carboxylic groups has been investigated both experimentally and theoretically. The experimental results show that such a reaction involves the initial rapid formation of carbonyl groups, which are then transformed into phenol or carboxylic groups. At room temperature, this reaction takes place on the most reactive carbon atoms. At higher temperatures a different mechanism would operate, as evidenced by the difference in activation energies. Experimental data can be partially related to first-principles calculations, showing a multistep functionalization mechanism. The theoretical aspects of the present article have led us to propose the most efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies.     

Shedding light on a dark state: the energetically lowest quintet state of C2

In this work we present a deperturbation study of the d?(3)Π(g), v=6 state of C(2) by double-resonant four-wave mixing spectroscopy. Accurate line positions of perturbed transitions are unambiguously assigned by intermediate level labeling. In addition, extra lines are accessible by taking advantage of the sensitivity and high dynamic range of the technique. These weak spectral features originate from nearby-lying dark states that gain transition strength through the perturbation process. The deperturbation analysis of the complex spectral region in the (6,5) and (6,4) bands of the Swan system (d(3)Π(g)-a?(3)Π(u)) unveils the presence of the energetically lowest high-spin state of C(2) in the vicinity of the d?(3)Π(g), v=6 state. The term energy curves of the three spin components of the d state cross the five terms of the 1?(5)Π(g) state at rotational quantum numbers N ≤ 11. The spectral complexity for transitions to the v = 6 level of d?(3)Π(g) state is further enhanced by an additional perturbation at N = 19 and 21 owing to the b?(3)Σ(g)(-), v=19 state. The spectroscopic characterization of both dark states is accessible by the measurement of 122 "window" levels. A global fit of the positions to a conventional Hamiltonian for a linear diatomic molecule yields accurate molecular constants for the quintet and triplet perturber states for the first time. In addition, parameters for the spin-orbit and L-uncoupling interaction between the electronic levels are determined. The detailed deperturbation study unravels major issues of the so-called high-pressure bands of C(2). The anomalous nonthermal emission initially observed by Fowler in 1910 [Mon. Not. R. Astron. Soc. 70, 484 (1910)] and later observed in numerous experimental environments are rationalized by taking into account "gateway" states, i.e., rotational levels of the d?(3)Π(g), v=6 state that exhibit significant (5)Π(g) character through which all population flows from one electronic state to the other.     

Conformational transitions of glycine induced by vibrational excitation of the O-H stretch

Vibrational energy flow and conformational transitions following excitation of the OH stretching mode of the most stable conformer of glycine are studied by classical trajectories. "On the fly" simulations with the PM3 semiempirical electronic structure method for the potential surface are used. Initial conditions are selected to correspond to the ν=1 excitation of the OH stretch. The main findings are: (1) An an equilibrium-like ratio is established between the populations of the 3 lowest-lying conformers after about 10 picoseconds. (2) There is a high probability throughout the 150 ps of the simulations for finding the molecule in geometries far from the equilibrium structures of the lowest-energy conformers. (3) Energy from the initial excited OH (ν=1) stretch flows preferentially to 5 other vibrational modes, including the bending motion of the H atom. (4) RRK theory yields conformational transition rates that deviate substantially from the classical trajectory results. Possible implication of these results for vibrational energy flow and conformational transitions in small biological molecules are discussed.     

A short note on groups in separably closed valued fields

In this note we show that groups with definable generics in a separably closed valued field K of finite imperfection degree can be embedded into groups definable in the algebraic closure of K.