139,704 research outputs found
Mechanistic insight into the Photoredox-Nickel-HAT triple catalyzed arylation and alkylation of α-amino Csp3-H bonds
No full textWe report here a comprehensive computational analysis of the mechanisms of the photoredox-nickel-HAT (HAT: Hydrogen atom transfer) catalyzed arylation and alkylation of α-amino Csp3-H bonds developed by MacMillan and co-workers. Different alternatives for the three catalytic cycles were tested to identify unambiguously the operative reaction mechanism. Our analysis indicated that the IrIII photoredox catalyst, upon irradiation with visible light, can be either reduced or oxidized by the HAT and nickel catalysts, respectively, indicating that both reductive and oxidative quenching catalytic cycles can be operative, although the reductive cycle is favored. Our analysis of the HAT cycle indicated that activation of a α-amino Csp3-H bond of the substrate is facile and selective relative to activation of a β-amino Csp3-H bond. Finally, our analysis of the nickel cycle indicated that both arylation and alkylation of α-amino Csp3-H bonds occurs via the sequence of nickel oxidation states NiI-NiII-NiI-NiIII and of elementary steps: Radical addition-SET-oxidative addition-reductive elimination
Pair natural orbital and canonical coupled cluster reaction enthalpies involving light to heavy alkali and alkaline earth metals: The importance of sub-valence correlation
No full textIn this work, we tested canonical and domain based pair natural orbital coupled cluster methods (CCSD(T) and DLPNO-CCSD(T), respectively) for a set of 32 ligand exchange and association/dissociation reaction enthalpies involving ionic complexes of Li, Be, Na, Mg, Ca, Sr, Ba and Pb(ii). Two strategies were investigated: in the former, only valence electrons were included in the correlation treatment, giving rise to the computationally very efficient FC (frozen core) approach; in the latter, all non-ECP electrons were included in the correlation treatment, giving rise to the AE (all electron) approach. Apart from reactions involving Li and Be, the FC approach resulted in non-homogeneous performance. The FC approach leads to very small errors (<2 kcal mol-1) for some reactions of Na, Mg, Ca, Sr, Ba and Pb, while for a few reactions of Ca and Ba deviations up to 40 kcal mol-1 have been obtained. Large errors are both due to artificial mixing of the core (sub-valence) orbitals of metals and the valence orbitals of oxygen and halogens in the molecular orbitals treated as core, and due to neglecting core-core and core-valence correlation effects. These large errors are reduced to a few kcal mol-1 if the AE approach is used or the sub-valence orbitals of metals are included in the correlation treatment. On the technical side, the CCSD(T) and DLPNO-CCSD(T) results differ by a fraction of kcal mol-1, indicating the latter method as the perfect choice when the CPU efficiency is essential. For completely black-box applications, as requested in catalysis or thermochemical calculations, we recommend the DLPNO-CCSD(T) method with all electrons that are not covered by effective core potentials included in the correlation treatment and correlation-consistent polarized core valence basis sets of cc-pwCVQZ(-PP) quality
Treating Subvalence Correlation Effects in Domain Based Pair Natural Orbital Coupled Cluster Calculations: An Out-of-the-Box Approach
No full textThe validity of the main approximations used in canonical and domain based pair natural orbital coupled cluster methods (CCSD(T) and DLPNO-CCSD(T), respectively) in standard chemical applications is discussed. In particular, we investigate the dependence of the results on the number of electrons included in the correlation treatment in frozen-core (FC) calculations and on the main threshold governing the accuracy of DLPNO all-electron (AE) calculations. Initially, scalar relativistic orbital energies for the ground state of the atoms from Li to Rn in the periodic table are calculated. An energy criterion is used for determining the orbitals that can be excluded from the correlation treatment in FC coupled cluster calculations without significant loss of accuracy. The heterolytic dissociation energy (HDE) of a series of metal compounds (LiF, NaF, AlF3, CaF2, CuF, GaF3, YF3, AgF, InF3, HfF4, and AuF) is calculated at the canonical CCSD(T) level, and the dependence of the results on the number of correlated electrons is investigated. Although for many of the studied reactions subvalence correlation effects contribute significantly to the HDE, the use of an energy criterion permits a conservative definition of the size of the core, allowing FC calculations to be performed in a black-box fashion while retaining chemical accuracy. A comparison of the CCSD and the DLPNO-CCSD methods in describing the core-core, core-valence, and valence-valence components of the correlation energy is given. It is found that more conservative thresholds must be used for electron pairs containing at least one core electron in order to achieve high accuracy in AE DLPNO-CCSD calculations relative to FC calculations. With the new settings, the DLPNO-CCSD method reproduces canonical CCSD results in both AE and FC calculations with the same accuracy
Thermal compensation system in advanced and third generation gravitational wave interferometric detectors
No full textOn September 14th 2015 the first gravitational wave signal has been detected by the Advanced LIGO interferometers, opening the era of the gravitational astronomy and giving new opportunities to investigate the universe. The Advanced LIGO and Advanced Virgo interferometers are now back in a commissioning phase in order to improve their sensitivity for the next observing run, which will start in the first months of 2019.In the high-frequencies region of their sensitivity band, the detectors are shot-noise limited: the sensitivity in this frequency window could be improved increasing the laser input power, but this increases also the optical aberrations due to the thermal effects. The optical power absorptions in the substrate and coatings of the optics induce both an increase of the optical path length in the substrates of the mirrors (thermal lensing) and a thermal expansion of the optic itself along the optical axis (thermo-elastic deformation). Both these aberrations reduce the sensitivity of the detector, limiting its performances. In order to face and minimize them, an adaptive Thermal Compensation System is required in order to guarantee the proper operation of the interferometer.An overview of the present Thermal Compensation System system installed on Advanced Virgo, with also a focus on the possible improvements of the actual actuators for the next generation of detectors, is presented here
A new capacitive read-out for EXPLORER and NAUTILUS
No full textWe describe the present status of a new read-out for the EXPLORER and NAUTILUS gravitational wave detectors. The read-out is based on a double-gap capacitive transducer and a double-SQUID amplifier. The transducer has been tested at liquid helium temperature and a Q of 1.0.106 has been measured with a biasing field of 20 MV/m. The double- SQUID amplifier has been tested down to 2 K with a high-Q resonant input load, showing very good stability and energy resolution, of about 70℠. With the new read-out, NAUTILUS, cooled to 100 mK, could reach a peak sensitivity of 3 . 10 -22Hz-1/2 and a bandwidth, at the level of 10 -21Hz-1/2, of about 35 Hz. © 2006 IOP Publishing Ltd
Accurate experimental and theoretical enthalpies of association of TiCl4with typical Lewis bases used in heterogeneous Ziegler-Natta catalysis
No full textAdducts of TiCl4with Lewis bases used as internal or external donors in heterogeneous Ziegler-Natta (ZN) catalysis represent a fundamental interaction contributing to the final composition of MgCl2supported ZN-catalysts. This study presents the accurate experimental evaluation, from titration calorimetry, of the formation enthalpy of TiCl4adducts with 15 Lewis bases of industrial interest. In addition, we report the accurate energies of association of TiCl4with the same Lewis bases from calculations at the DLPNO-CCSD(T) level of theory. These accurate experimental and theoretical association values are compared with selected methods based on density functional theory (DFT) in combination with popular continuum solvation models. Calculations suggest that the PBE-D3, and M06 functionals in combination with a triple-ζ plus polarization quality basis set provide the best performance when the basis set superposition error (BSSE) is not removed from the association energies. Cleaning the association energies with the BSSE with the counterpoise protocol suggests B3LYP-D3, TPSS-D3 and M06L as the best performing functionals. The introduction of solvent effects with the PCM and SMD continuum solvation models allows the DFT-based association enthalpies to be compared with the experimental values obtained from titration calorimetry. Both solvation models in combination with the PBE-D3, PBE0-D3, B3LYP-D3, TPSS-D3, M06L, and M06 functionals provide association enthalpies close to the experimental values with MUEs in the range of 10-15 kJ mol-1
Stroboscopic torsion pendulum
No full textWe report on a simple, inexpensive readout for torsion pendulums, suitable for robust applications like teaching-lab equipment or monitoring of large ampl- itude oscillations. A short light pulse is recorded every time a reflective band on the pendulum inertial member passes in front of the sensor, an infrared LED pair (emitter–receiver). Simple algebraic manipulations on the time series of these pulses arrival times allow one to extract the resonant frequency as well as the decay time of the pendulum. This readout is insensitive to the amplitude of oscillation and is therefore suitable for monitoring torsional oscillations of large amplitude, where traditonal readouts like optical levers or auto- collimators encounter dynamic range limitations
Spatially-localized time dependent solutions including turbulence and their interactions in 2D Kolmogorov flow
No full textIn 2D Kolmogorov flow in small aspect ratio domains, spatially-localized solutions such as kink, traveling or time-dependent kink-antikink pars coexist. However, the conservation of the flow rate in the y direction strongly restrict combination of localized solutions and their positioning. We find that by adding a homogeneous flow U y their positioning is controlled and each of localized solutions including a spatially-localized chaos is isolated. Numerical results suggest that these isolated solutions can be elements constructing a whole flow
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