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Register a new userOn a complementary scale of crystal-field strength
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A new measure of the crystal-field strength, complementary to the conventional one, is defined. It is based on the rotational invariants |B_{k0}|_{av} or |sum_{k}B_{k0}|_{av}, k=2,4,6, of the crystal-(ligand)-field (CF) Hamiltonian H_{CF} parametrizations, i.e. on the axial CF parameters modules averaged over all reference frame orientations. They turn out to be equal to |H_{CF}^{(k)}|_{av} and |H_{CF}|_{av}, respectively. While the traditional measure is established on the parametrization modules or on the second moment of the CF energy levels, the introduced scale employs rather the first moment of the energy modules and has better resolving power. The new scale is able to differentiate the strength of various iso-modular parametrizations according to the classes of rotationally equivalent parametrizations. Using both the compatible CF strength measures one may draw more accurate conclusions about the Stark levels arrays and particularly their total splitting magnitudes.
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To contribute to the understanding of noncovalent binding of halogenated molecules with a biological activity, electrostatic potential (ESP) maps of more than 2,500 compounds were thoroughly analysed. A peculiar region of positive ESP, called $sigma$-hole, is a concept of central importance for halogen bonding. We aim at simplifying the view on $sigma$-holes and provide general trends in organic drug-like molecules. The results are in fair agreement with crystallographic surveys of small molecules as well as of biomolecular complexes and attempt to improve the intuition of chemists when dealing with halogenated compounds.
New charge transfer crystals of $pi$-conjugated, aromatic molecules (phenanthrene and picene) as donors were obtained by physical vapor transport. The melting behavior, optimization of crystal growth and the crystal structure is reported for charge transfer salts with (fluorinated) tetracyanoquinodimethane (TCNQ-F$_x$, x=0, 2, 4), which was used as acceptor material. The crystal structures were determined by single-crystal X-ray diffraction. Growth conditions for different vapor pressures in closed ampules were applied and the effect of these starting conditions for crystal size and quality is reported. The process of charge transfer was investigated by geometrical analysis of the crystal structure and by infrared spectroscopy on single crystals. With these three different acceptor strengths and the two sets of donor materials, it is possible to investigate the distribution of the charge transfer systematically. This helps to understand the charge transfer process in this class of materials with $pi$-conjugated donor molecules.
We consider the problem of photon creation from vacuum inside an ideal cavity with vibrating walls in the resonance case, taking into account the interaction between the resonant field mode and a detector modeled by a quantum harmonic oscillator. The frequency of wall vibrations is taken to be twice the cavity normal frequency, modified due to the coupling with the detector. The dynamical equations are solved with the aid of the multiple scales method. Analytical expressions are obtained for the photon mean numbers and their variances for the field and detector modes, which are supposed to be initially in the vacuum quantum states. We analyze different regimes of excitation, depending on the ratio of the modulation depth of the time-dependent cavity eigenfrequency to the coupling strength between the cavity mode and detector. We show that statistical properties of the detector quantum state (variances of the photon numbers, photon distribution function, and the degree of quadrature squeezing) can be quite different from that of the field mode. Besides, the mean number of quanta in the detector mode increases with some time delay, compared with the field mode.
Fe-gluconate, Fe(C_6H_11O_7_2xH_2O is a well-known material widely used for iron supplementation. On the other hand, it is used in food industry as a coloring agent, in cosmetic industry for skin and nail conditioning and metallurgy. Despite of wide range of applications its physical properties were not studied extensively. In this study, Fe-gluconate with three different amount of water viz. x=2 (fully hydrated, 0 < x < 2 (intermediate) and x=0 (dry) was investigated by means of X-ray diffraction (XRD) and Mossbauer spectroscopic (MS) methods. The former in the temperature range of 20-300 K, and the latter at 295 K. Based on the XRD measurements crystallographic structures were determined: monoclinic (space group I2) for the hydrated sample and triclinic (space group P1) for the dry sample. The partially hydrated sample was two-phased. Unit cells parameters for both structures show strong, very complex and non-monotonic temperature dependences. Mossbauer spectroscopic measurements gave evidence that iron in all samples exist in form of Fe(II) and Fe(III) ions. The amount of the latter equals to ca.30% in the hydrated sample and to ca.20% in the dry one.
Nuclear Magnetic Resonance (NMR) spectroscopy is particularly well-suited to determine the structure of molecules and materials in powdered form. Structure determination usually proceeds by finding the best match between experimentally observed NMR chemical shifts and those of candidate structures. Chemical shifts for the candidate configurations have traditionally been computed by electronic-structure methods, and more recently predicted by machine learning. However, the reliability of the determination depends on the errors in the predicted shifts. Here we propose a Bayesian framework for determining the confidence in the identification of the experimental crystal structure, based on knowledge of the typical error in the electronic structure methods. We also extend the recently-developed ShiftML machine-learning model, including the evaluation of the uncertainty of its predictions. We demonstrate the approach on the determination of the structures of six organic molecular crystals. We critically assess the reliability of the structure determinations, facilitated by the introduction of a visualization of the of similarity between candidate configurations in terms of their chemical shifts and their structures. We also show that the commonly used values for the errors in calculated $^{13}$C shifts are underestimated, and that more accurate, self-consistently determined uncertainties make it possible to use $^{13}$C shifts to improve the accuracy of structure determinations.
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