1,126 research outputs found
Introduction of a weighting scheme for the X-ray restrained wavefunction approach: advantages and drawbacks
No full textIn a quite recent study [Genoni et al. (2017). IUCrJ, 4, 136-146], it was observed that the X-ray restrained wavefunction (XRW) approach allows a more efficient and larger capture of electron correlation effects on the electron density if high-angle reflections are not considered in the calculations. This is due to the occurrence of two concomitant effects when one uses theoretical X-ray diffraction data corresponding to a single-molecule electron density in a large unit cell: (i) the high-angle reflections are generally much more numerous than the low- and medium-angle ones, and (ii) they are already very well described at unrestrained level. Nevertheless, since high-angle data also contain important information that should not be disregarded, it is not advisable to neglect them completely. For this reason, based on the results of the previous investigation, this work introduces a weighting scheme for XRW calculations to up-weight the contribution of low- and medium-angle reflections, and, at the same time, to reasonably down-weight the importance of the high-angle data. The proposed strategy was tested through XRW computations with both theoretical and experimental structure-factor amplitudes. The tests have shown that the new weighting scheme works optimally if it is applied with theoretically generated X-ray diffraction data, while it is not advantageous when traditional experimental X-ray diffraction data (even of very high resolution) are employed. This also led to the conclusion that the use of a specific external parameter λJ for each resolution range might not be a suitable strategy to adopt in XRW calculations exploiting experimental X-ray data as restraints
New Catalytic Methods for Carbon Nitrogen Double Bond Transformations
No full textOver the last years, the use of trichlorosilane as a reducing agent has attracted much attention; the employment of a metal-free methodology could address the cost and waste remediation issues associated with main group hydrides, as well as avoid the expense and potentially toxic nature of metal catalysts. To promote the reaction, the trichlorosilane needs to be activated by coordination with a Lewis base: in particular, the use of chiral Lewis bases offers the potential to control the absolute stereochemistry of the process.1
Recently we decided to extend this methodology to the enantioselective reduction of fluorinated ketoimines, due to the great interest that organofluorine chemistry has received in many fields, such as material and pharmaceutical sciences.2 In spite of the great activity that fluorine attracted lately, it continues to challenge the organic chemistry community, since the presence of fluorine functional groups profoundly modifies the physicochemical and biological properties. In particular, the stereocontrol at carbon center featuring a fluorinated motif is an highly challenging task. The use of trichlorosilane combines the advantages of an environmentally friendly technique and the avoidance of the problems linked to the stereoselective insertion of a fluorinated group, while retaining high levels of enantioselectivity.
During our studies we’ve synthesized a set of fluorinated aromatic ketimines, both aromatic and aliphatic. Their trichlorosilane mediated reduction, after a proper tuning of reaction and workup conditions, allowed us to isolate the corresponding amines with high chemical yield and very good enantioselectivity, up to 90% e.e. Some variously substitued aromatic substrates were also examined, showing a good tolerance for electrowithdrawing and electrodonating substituents on the aromatic ring.
References:
1. a) Guizzetti S., Benaglia M. Eur. J. Org. Chem. 2010, 5529–5541, b) Jones S., Warner C. J. A. Org. Biomol. Chem. 2012,
10, 2189–2200
2. Nie J., Guo H., Cahard D, Ma J. Chem. Rev. 2011, 111, 455–52
lamaGOET : an interface for quantum crystallography
No full textIn quantum crystallography, theoretical calculations and crystallographic refinements are closely intertwined. This means that the employed software must be able to perform both quantum-mechanical calculations and crystallographic least-squares refinements. So far, the program Tonto is the only one able to do that. The lamaGOET interface described herein deals with this issue since it interfaces dedicated quantum-chemical software (the widely used Gaussian package and the specialized ELMOdb program) with the refinement capabilities of Tonto. Three different flavours of quantum-crystallographic refinements of the dipetide glycyl-L-threonine dihydrate are presented to showcase the capabilities of lamaGOET: Hirshfeld atom refinement (HAR), HAR-ELMO, namely HAR coupled with extremely localized molecular orbitals, and X-ray constrained wavefunction fitting
Quantum filtering of a thermal master equation with a purified reservoir
No full textWe consider a system subject to a quantum optical master equation at finite temperature and study a class of conditional dynamics obtained by monitoring its totally or partially purified environment. More specifically, drawing from the notion that the thermal state of the environment may be regarded as the local state of a lossy and noisy two-mode squeezed state, we consider conditional dynamics (“unravellings”) resulting from the homodyne detection of the two modes of such a state. Thus, we identify a class of unravellings parametrized by the loss rate suffered by the environmental two-mode state, which interpolate between direct detection of the environmental mode alone (occurring for total loss, whereby no correlation between the two environmental modes is left) and full access to the purification of the bath (occurring when no loss is acting and the two-mode state of the environment is pure). We hence show that, while direct detection of the bath is not able to reach the maximal steady-state squeezing allowed by general-dyne unravellings, such optimal values can be obtained when a fully purified bath is accessible. More generally we show that, within our framework, any degree of access to the bath purification improves the performance of filtering protocols in terms of achievable squeezing and entanglement
Introduction of a weighting scheme for the X-ray restrained wavefunction approach: advantages and drawbacks
No full textIn a quite recent study [Genoni et al. (2017). IUCrJ, 4, 136-146], it was observed that the X-ray restrained wavefunction (XRW) approach allows a more efficient and larger capture of electron correlation effects on the electron density if high-angle reflections are not considered in the calculations. This is due to the occurrence of two concomitant effects when one uses theoretical X-ray diffraction data corresponding to a single-molecule electron density in a large unit cell: (i) the high-angle reflections are generally much more numerous than the low- and medium-angle ones, and (ii) they are already very well described at unrestrained level. Nevertheless, since high-angle data also contain important information that should not be disregarded, it is not advisable to neglect them completely. For this reason, based on the results of the previous investigation, this work introduces a weighting scheme for XRW calculations to up-weight the contribution of low- and medium-angle reflections, and, at the same time, to reasonably down-weight the importance of the high-angle data. The proposed strategy was tested through XRW computations with both theoretical and experimental structure-factor amplitudes. The tests have shown that the new weighting scheme works optimally if it is applied with theoretically generated X-ray diffraction data, while it is not advantageous when traditional experimental X-ray diffraction data (even of very high resolution) are employed. This also led to the conclusion that the use of a specific external parameter lambda(J) for each resolution range might not be a suitable strategy to adopt in XRW calculations exploiting experimental X-ray data as restraints
Information/disturbance trade-off in single and sequential measurements on a qudit signal
No full textWe address the trade-off between information gain and state disturbance in measurement performed on qudit systems and devise a class of optimal measurement schemes that saturate the ultimate bound imposed by quantum mechanics to estimation and transmission fidelities. The schemes are minimal, i.e. they involve a single additional probe qudit, and optimal, i.e. they provide the maximum amount of information compatible with a given level of disturbance. The performances of optimal single-user schemes in extracting information by sequential measurements in a N-user transmission line are also investigated, and the optimality is analyzed by explicit evaluation of fidelities. We found that the estimation fidelity does not depend on the number of users, neither for single-measure inference nor for collective one, whereas the transmission fidelity decreases with N. The resulting trade-off is no longer optimal and degrades with increasing N. We found that optimality can be restored by an effective preparation of the probe states and present explicitly calculations for the 2-user case
Optimal feedback control of linear quantum systems in the presence of thermal noise
No full textWe study the possibility of taking bosonic systems subject to quadratic Hamiltonians and a noisy thermal environment to nonclassical stationary states by feedback loops based on weak measurements and conditioned linear driving. We derive general analytical upper bounds for the single-mode squeezing and multimode entanglement at steady state, depending only on the Hamiltonian parameters and on the number of thermal excitations of the bath. Our findings show that, rather surprisingly, larger number of thermal excitations in the bath allow for larger steady-state squeezing and entanglement if the efficiency of the optimal continuous measurements conditioning the feedback loop is high enough. We also consider the performance of feedback strategies based on homodyne detection and show that, at variance with the optimal measurements, it degrades with increasing temperature
DENPOL : A new program to determine electron densities of polypeptides using extremely localized molecular orbitals
No full textA new method to compute high-quality electron densities of polypeptides is proposed. The method is based on the transferability properties of extremely localized molecular orbitals, which can be used to describe with great accuracy the different functional groups of a molecule. It is therefore possible to generate a database of such orbitals, each of them associated with specific amino acids or with the peptide bond. A new program, DENPOL, has been written in order to build up the electron density of a generic polypeptide using this database. Due to both the large number of orbitals required to describe a polypeptide and the non-orthogonal nature of these orbitals, a Divide & Conquer strategy has been used to assemble the final electron density. The application of this approach is particularly efficient thanks to the extreme localization of the orbitals. The comparison with the corresponding electron densities generated by the Hartree–Fock method, shows the accuracy of the proposed approach and indicates that the electron densities generated by DENPOL are very close to those generated by an ab initio approach
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