Spin relaxation: under the sun, anything new?

Spin relaxation has been at the core of many studies since the early days of NMR, and the undelying theory worked out by its founding fathers. However, this theory has been recently questioned (Bengs and Levitt (2020)) in the light of Linblad theory of quantum Markovian master equations. In this article, we review the conventional approach of quantum mechanical theory of NMR relaxation and show that under the usual assumptions, it is equivalent to the Linblad formulation. We also comment on the debate over semi-classical versus quantum versions of spectral density functions involved in relaxation.

On Concept of Hybrid in Colloid Sciences

The concepts hybrid and hybridization are common in many scientific fields, as in the taxonomic parts of botany and zoology, in modern genetic, and in the quantum–mechanical theory of atomic–molecular orbitals, which are of foremost relevance in most aspects of modern chemistry. Years later, scientists applied the concept hybrid to colloids, if the particles’ domains are endowed with functionalities differing each from the other in nature and/or composition. For such denomination to be fully valid, the domains belonging to a given hybrid must be recognizable each from another in terms of some intrinsic features. Thus, the concept applies to particles where a given domain has its own physical state, functionality, or composition. Literature examples in this regard are many. Different domains that are present in hybrid colloids self-organize, self-sustain, and self-help, according to the constraints dictated by kinetic and/or thermodynamic stability rules. Covalent, or non-covalent, bonds ensure the formation of such entities, retaining the properties of a given family, in addition to those of the other, and, sometimes, new ones. The real meaning of this behavior is the same as in zoology; mules are pertinent examples, since they retain some features of their own parents (i.e., horses and donkeys) but also exhibit completely new ones, such as the loss of fertility. In colloid sciences, the concept hybrid refers to composites with cores of a given chemical type and surfaces covered by moieties differing in nature, or physical state. This is the result of a mimicry resembling the ones met in a lot of biological systems and foods, too. Many combinations may occur. Silica nanoparticles on which polymers/biopolymers are surface-bound (irrespective of whether binding is covalent or not) are pertinent examples. Here, efforts are made to render clear the concept, which is at the basis of many applications in the biomedical field, and not only. After a historical background and on some features of the species taking part to the formation of hybrids, we report on selected cases met in modern formulations of mixed, and sometimes multifunctional, colloid entities.

Исследование с помощью молекулярной динамики образования димеров на поверхности (001) GaAs при низких температурах

The simulation of dimers formation during the low-temperature reconstruction of GaAs (001) surface terminated with Ga or As atoms was performed by the molecular dynamics method using the analytical Bond-Order Potential based on quantum mechanical theory incorporating both σ- and π- bonds between atoms. A decrease in values of potential energy of the atoms during formation of isolated surface dimer have been determined. It has been found that potential energy of an atom in As-dimer is several tenths of an eV lower than in Ga-dimer. Kinetics of the initial stages of Ga-dimers formation in the temperature range of 25 - 40 K was studied. It was found that the characteristic thermal activation energy of single isolated Ga-dimers formation is ~ 29 meV, which is lower than the same value for As-dimers (~ 38 meV). Time constants characterizing the average rate of transformation of one dimer into a chain of two dimers at temperature range of 28 - 37 K were estimated. Inverse values of these parameters for paired Ga- and As-dimers are in the ranges of 10^11 – 10^12 s^-1 and 10^9 – 10^10 s^-1, respectively, while corresponding parameters for the formation of single dimers are in the ranges of 4·10^6 – 10^8 s^-1 and 1.4·10^6 – 7.4·10^7 s^-1.

Abstract-algebraic interpretations of the energy–time uncertainty relation prove quantization of time

In modern physics, the general theory of relativity (GTR) successfully predicts the vital structure of gravity. The GTR confirmed that gravity is the distortion of four-dimensional spacetime [Formula: see text] by massive bodies. Such a prediction of the GTR is one of the vital successes towards the development goals of modern physics. Though, the central foundation for all the calculations of the GR is the hypothesis of continuum spacetime. In this paper, the author introduces a theorem of the quantum mechanical theory of time (QMT) to test whether the metric time is discrete or continuous. To prove the theorem, the author applied the set theory of cardinal numbers on the energy–time uncertainty relationship. The proof of the theorem confirmed that the metric time is discrete, and has an intrinsic quantum nature. The result implied that the continuum spacetime assumption of the GTR is found fundamentally erroneous. Therefore, spacetime is discrete and needs to be analyzed by the principles of quantum mechanics (QM).

A Hidden Side of the Conformational Mobility of the Quercetin Molecule Caused by the Rotations of the O3H, O5H and O7H Hydroxyl Groups: In Silico Scrupulous Study

In this study at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of quantum-mechanical theory it was explored conformational variety of the isolated quercetin molecule due to the mirror-symmetrical hindered turnings of the O3H, O5H and O7H hydroxyl groups, belonging to the A and C rings, around the exocyclic C–O bonds. These dipole active conformational transformations proceed through the 72 transition states (TSs; C1 point symmetry) with non-orthogonal orientation of the hydroxyl groups relatively the plane of the A or C rings of the molecule (HO7C7C8/HO7C7C6 = ±(89.9–93.3), HO5C5C10 = ±(108.9–114.4) and HO3C3C4 = ±(113.6–118.8 degrees) (here and below signs ‘±’ corresponds to the enantiomers)) with Gibbs free energy barrier of activation ΔΔGTS in the range 3.51–16.17 kcal·mol−1 under the standard conditions (T = 298.1 K and pressure 1 atm): ΔΔGTSO7H (3.51–4.27) < ΔΔGTSO3H (9.04–11.26) < ΔΔGTSO5H (12.34–16.17 kcal mol−1). Conformational dynamics of the O3H and O5H groups is partially controlled by the intramolecular specific interactions O3H…O4, C2′/C6′H…O3, O3H…C2′/C6′, O5H…O4 and O4…O5, which are flexible and cooperative. Dipole-active interconversions of the enantiomers of the non-planar conformers of the quercetin molecule (C1 point symmetry) is realized via the 24 TSs with C1 point symmetry (HO3C3C2C1 = ±(11.0–19.1), HC2′/C6′C1′C2 = ±(0.6–2.9) and C3C2C1′C2′/C3C2C1′C6′ = ±(1.7–9.1) degree; ΔΔGTS = 1.65–5.59 kcal·mol−1), which are stabilized by the participation of the intramolecular C2′/C6′H…O1 and O3H…HC2′/C6′ H-bonds. Investigated conformational rearrangements are rather quick processes, since the time, which is necessary to acquire thermal equilibrium does not exceed 6.5 ns.