Nonparametric identification of linearizations and uncertainty using Gaussian process models – application to robust wheel slip control

Gaussian process prior models offer a nonparametric approach to modelling unknown nonlinear systems from experimental data. These are flexible models which automatically adapt their model complexity to the available data, and which give not only mean predictions but also the variance of these predictions. A further advantage is the analytical derivation of derivatives of the model with respect to inputs, with their variance, providing a direct estimate of the locally linearized model with its corresponding parameter variance. We show how this can be used to tune a controller based on the linearized models, taking into account their uncertainty. The approach is applied to a simulated wheel slip control task illustrating controller development based on a nonparametric model of the unknown friction nonlinearity. Local stability and robustness of the controllers are tuned based on the uncertainty of the nonlinear models’ derivatives

EMC and power quality standards for 20-kHz power distribution

The Space Station Power Distribution System has been baselined as a sinusoidal single phase, 440 VRMS system. This system has certain unique characteristics directly affecting its application. In particular, existing systematic description and control documents were modified to reflect the high operating frequency. This paper will discuss amendments made on Mil STD 704 (Electrical Power Characteristics), and Mil STD 461-B (Electromagnetic Emission and Susceptibility Requirements for the Control of Electromagnetic Interference). In some cases these amendments reflect changes of several orders of magnitude. Implications and impacts of these changes are discussed

COCO: A Platform for Comparing Continuous Optimizers in a Black-Box Setting

We introduce COCO, an open source platform for Comparing Continuous Optimizers in a black-box setting. COCO aims at automatizing the tedious and repetitive task of benchmarking numerical optimization algorithms to the greatest possible extent. The platform and the underlying methodology allow to benchmark in the same framework deterministic and stochastic solvers for both single and multiobjective optimization. We present the rationales behind the (decade-long) development of the platform as a general proposition for guidelines towards better benchmarking. We detail underlying fundamental concepts of COCO such as the definition of a problem as a function instance, the underlying idea of instances, the use of target values, and runtime defined by the number of function calls as the central performance measure. Finally, we give a quick overview of the basic code structure and the currently available test suites.Comment: Optimization Methods and Software, Taylor & Francis, In press, pp.1-3

How the Lewis Base F– Catalyzes the 1,3-Dipolar Cycloaddition between Carbon Dioxide and Nitrilimines

Bio-organic Synthesi

How solvation influences the S(N)2 versus E2 competition

We have quantum chemically investigated how solvation influences the competition between the S(N)2 and E2 pathways of the model F- + C2H5Cl reaction. The system is solvated in a stepwise manner by going from the gas phase, then via microsolvation of one to three explicit solvent molecules, then last to bulk solvation using relativistic density functional theory at (COSMO)-ZORA-OLYP/QZ4P. We explain how and why the mechanistic pathway of the system shifts from E2 in the gas phase to S(N)2 upon strong solvation of the Lewis base (i.e., nucleophile/protophile). The E2 pathway is preferred under weak solvation of the system by dichloromethane, whereas a switch in reactivity from E2 to S(N)2 is observed under strong solvation by water. Our activation strain and Kohn-Sham molecular orbital analyses reveal that solvation of the Lewis base has a significant impact on the strength of the Lewis base. We show how strong solvation furnishes a weaker Lewis base that is unable to overcome the high characteristic distortivity associated with the E2 pathway, and thus the S(N)2 pathway becomes viable.Bio-organic Synthesi

Rational tuning of the reactivity of three-membered heterocycle ring-openings via SN2 reactions

The development of small molecule covalent inhibitors and probes continuously pushes the rapidly evolving field of chemical biology forward. A key element in these molecular tool compounds is the "electrophilic trap" that allows for a covalent linkage with the target enzyme. The reactivity of this entity needs to be well balanced to effectively trap the desired enzyme, while not being attacked by off-target nucleophiles. We here investigate the intrinsic reactivity of substrates containing a class of widely used electrophilic traps, the three-membered heterocycles with an N- (aziridine), P- (phosphirane), O- (epoxide) and S-atom (thiirane) as heteroatom. Using quantum chemical approaches, we studied the conformational flexibility and nucleophilic ring-opening reaction of a series of model substrates, in which these electrophilic traps are mounted on a cyclohexene scaffold (C6H10Y with Y = NH, PH, O, S). It is revealed that the activation energy of the ring-opening reaction does not necessarily follow the trend that is expected from C-Y leaving-group bond strength, but steeply decreases from NH, to PH, to O, to S. We illustrate that the HOMONu-LUMOSubstrate interaction is an all-important factor for the observed reactivity. In addition, we show that the activation energy of aziridines and phosphiranes can be tuned far below that of the corresponding epoxides and thiiranes by the addition of proper electron-withdrawing ring substituents. Our results provide mechanistic insights to rationally tune the reactivity of this class of popular electrophilic traps and can guide the experimental design of covalent inhibitors and probes for enzymatic activity.NWONWO-Rekentijd grant 17569 and 11116Bio-organic Synthesi

Turnover of dimethylsulfoniopropionate and dimethylsulfide in the marine environment:A mesocosm experiment

The production of dimethylsulfoniopropionate (DMSP) by marine phytoplankton and the fate of the produced DMSP and dimethylsulfide (DMS) were studied in 4 pelagic mesocosms during an algal bloom over a period of 1 mo. Bacterial numbers, concentrations of particulate and dissolved DMSP, DMS, and chlorophyll a were monitored, as well as the turnover rates of DMS and DMSP. Of the total amount of DMSP produced, only a fraction could be detected as DMS in the water column. DMS production in the water column did not necessarily correlate with algal senescence, but also occurred during the maximum of the algal bloom. The flux of DMS to the atmosphere played a minor role as a sink for DMS. Evidence is presented that shows bacterial consumption to be a major sink for DMS, under conditions of both high and low DMS water concentrations. DMSP was degraded either via cleavage or via demethylation; the results indicate a predominant role for the latter route

A unified framework for understanding nucleophilicity and protophilicity in the SN2/E2 competition

The concepts of nucleophilicity and protophilicity are fundamental and ubiquitous in chemistry. A case in point is bimolecular nucleophilic substitution (S(N)2) and base-induced elimination (E2). A Lewis base acting as a strong nucleophile is needed for S(N)2 reactions, whereas a Lewis base acting as a strong protophile (i.e., base) is required for E2 reactions. A complicating factor is, however, the fact that a good nucleophile is often a strong protophile. Nevertheless, a sound, physical model that explains, in a transparent manner, when an electron-rich Lewis base acts as a protophile or a nucleophile, which is not just phenomenological, is currently lacking in the literature. To address this fundamental question, the potential energy surfaces of the S(N)2 and E2 reactions of X-+C2H5Y model systems with X, Y = F, Cl, Br, I, and At, are explored by using relativistic density functional theory at ZORA-OLYP/TZ2P. These explorations have yielded a consistent overview of reactivity trends over a wide range in reactivity and pathways. Activation strain analyses of these reactions reveal the factors that determine the shape of the potential energy surfaces and hence govern the propensity of the Lewis base to act as a nucleophile or protophile. The concepts of "characteristic distortivity" and "transition state acidity" of a reaction are introduced, which have the potential to enable chemists to better understand and design reactions for synthesis.Bio-organic Synthesi

C(spn)−X (n=1–3) bond activation by palladium

We have studied the palladium-mediated activation of C(sp(n))-X bonds (n = 1-3 and X = H, CH3, Cl) in archetypal model substrates H3C-CH2-X, H2C=CH-X and HC equivalent to C-X by catalysts PdLn with L-n = no ligand, Cl-, and (PH3)(2), using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp(3))-X, to C(sp(2))-X, to C(sp)-X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp(3), to sp(2), to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(sp(n))-X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst-substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp(3))-X to C(sp)-X.Bio-organic Synthesi

Scaling Analysis of Fluctuating Strength Function

We propose a new method to analyze fluctuations in the strength function phenomena in highly excited nuclei. Extending the method of multifractal analysis to the cases where the strength fluctuations do not obey power scaling laws, we introduce a new measure of fluctuation, called the local scaling dimension, which characterizes scaling behavior of the strength fluctuation as a function of energy bin width subdividing the strength function. We discuss properties of the new measure by applying it to a model system which simulates the doorway damping mechanism of giant resonances. It is found that the local scaling dimension characterizes well fluctuations and their energy scales of fine structures in the strength function associated with the damped collective motions.Comment: 22 pages with 9 figures; submitted to Phys. Rev.