Ring-expanded bicyclic β-lactams: a structure-chiroptical properties relationship investigation by experiment and calculations

In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic β-lactams to the sign of the 220 nm band observed in their electronic circular dichroism (ECD) spectra is examined for ring-expanded cephalosporin analogues. To this end, a series of model compounds with a seven-membered ring condensed with the β-lactam unit was synthesized. A key step of their synthesis was either the ring-closing metathesis (RCM) or the free radical cyclization leading to the seven-membered ring with an S, O, or C atom at the 6 position in the bicyclic skeleton. To investigate the scope and limitations of the simple, empirically established helicity rule, a combination of ECD spectroscopy, variable-temperature ECD measurements, X-ray analysis, and time-dependent density functional theory (TD-DFT) calculations was used. A comparison of the experimental ECD spectra with the spectra simulated by TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 240-215 nm spectral range. The results suggest that the helicity rule does not apply to the investigated compounds because of the planarity of their amide chromophore. Thus, these compounds do not constitute an exception to the rule that was established for bi- and polycyclic β-lactams with the nonplanar amide chromophore only.     

Red- and blue-shifted hydrogen bonds: the Bent rule from quantum theory of atoms in molecules perspective

MP2/6-311++G(3pd,3df) calculations were performed on complexes of acetylene and fluoroform acting as the proton donating systems and different Lewis bases being the proton acceptors since these complexes are linked through C-H···Y hydrogen bonds. Quantum Theory of Atoms in Molecules (QTAIM) is applied to explain the nature of these interactions. The characteristics of bond critical points are presented for these complexes. The inter-relations between energetic and geometrical parameters as well as the parameters derived from the Natural Bond Orbital (NBO) theory are analyzed here. Red- and blue-shifted hydrogen bonds are detected for the complexes investigated and the differences between those interactions are analyzed from the QTAIM perspective. It is shown that such differences are in agreement with the Bent rule. The position of the bond critical point of the proton donating bond is connected with the nature of hydrogen bonding, that is, if it is blue- or red-shifted.     

Effects of static magnetic field on electrolyte solutions under kinetic condition

Electrolyte solutions were exposed for different time to weak static magnetic field (MF) generated from a stack of magnets (B = 15 mT) at the flow rate of 1.4 mL/s. The conductivity was measured as a function of time following the application of MF. It was found that the changes in conductivity depend on the kind of electrolyte and the magnetic exposure time and are related to the thermodynamic function of hydration.     

Nonadiabatic molecular dynamics study of the cis-trans photoisomerization of azobenzene excited to the S1 state

Ab initio nonadiabatic dynamics simulations of cis-to-trans isomerization of azobenzene upon S(1) (n-π*) excitation are carried out employing the fewest-switches surface hopping method. Azobenzene photoisomerization occurs purely as a rotational motion of the central CNNC moiety. Two nonequivalent rotational pathways corresponding to clockwise or counterclockwise rotation are available. The course of the rotational motion is strongly dependent on the initial conditions. The internal conversion occurs via an S(0)/S(1) crossing seam located near the midpoint of both of these rotational pathways. Based on statistical analysis, it is shown that the occurrence of one or other pathway can be completely controlled by selecting adequate initial conditions.     

Enantioselective synthesis of all-carbon quaternary stereogenic centers in acyclic systems

Most of the methods for the creation of all-carbon quaternary stereogenic centers in acyclic systems were developed in the last decade showing the contemporary interest of this field of research. Initial strategies, where chiral entities were linked to carbon skeleton, to enantioselective catalysis and finally to strategies where several carbon-carbon bonds were created in a single-pot operation, reflect the constant strive and creativity in the design and execution of synthetic sequences. This feature article summarizes these sequences and is divided into sections on substitution and additions reactions, alkylation, aldol, Mannich and rearrangements reactions. It is safe to predict that this field of chemistry will continue to grow exponentially in the coming decades and the ready availability of a wide range of these chiral entities will provide an excellent opportunity to further enrich mainstream synthetic methodologies.     

FT NIR Raman studies of alginic acid-benzimidazole polymer composite

New bio-inspired polymer composites of alginic acid and benzimidazole were created and characterized by FT NIR Raman spectroscopy. The obtained films with 1:0.5, 1:1 and 1:1.5 molar ratio are homogeneous, with good mechanical properties. Raman spectra recorded at room temperature revealed that the obtained films are a new compound with a different molecular structure and physical properties compared with pure substrates: alginic acid and benzimidazole. Raman band related to vibration of COOH entity at 1740 cm(-1) of alginic acid disappears in the alginic acid:benzimidazole composites, in which new Raman band related to COO(-) was found. Additionally, characteristic lines observed in polymer composites which may be associated with vibrations of NH groups, can be attributed to the linking of proton to deprotonated N atom in benzimidazole group. Possibility of such proton exchange is a promising property which might facilitate the application of obtained composites to anhydrous proton conducting electrolytes in fuel cells.     

Surface‐Plasmon‐Coupled Emission of Rhodamine 110 in a Silica Nanolayer

The first observation of strong directional surface‐plasmon‐coupled emission (SPCE) of Rhodamine 110 in silica nanofilms deposited on silver nanolayers is reported. The preparation of the material is described in detail. The intensity of SPCE exceeds 10 times that of free space fluorescence and total linear light polarization in the SPCE ring is observed. A new experimental setup and an original data collection method is presented. Our material completely preserves its fluorescence properties for at least eight months.     

Single dibenzoterrylene molecules in naphthalene and 2,3-dimethylnaphthalene crystals: vibronic spectra

Fluorescence excitation spectra of single dibenzoterrylene (DBT) molecules embedded in naphthalene (N) and 2,3-dimethylnaphthalene (2,3-DMN) crystals were studied at 5 K. The frequencies characterizing the vibronic structure of single DBT molecules in an N crystal agree with the theoretical prediction for the isolated DBT molecule. The 'dipolar' disorder encountered in 2,3-DMN crystals leads to a broad distribution of frequencies of the (0,0) lines of single DBT molecules. Moreover, the observed vibronic frequencies and intensities in the spectrum of DBT in 2,3-DMN crystals are slightly different to those in an N crystal. We conclude that the structure of DBT molecules in a 2,3-DMN crystal is disturbed in comparison with isolated DBT and the main change concerns its central tetracene moiety.     

The effect of lipid oxidation on the water permeability of phospholipids bilayers

The effect of lipid oxidation on water permeability of phosphatidylcholine membranes was investigated by means of both scattering stopped flow experiments and atomistic molecular dynamics simulations. Formation of water pores followed by a significant enhancement of water permeability was observed. The molecules of oxidized phospholipids facilitate pore formation and subsequently stabilize water in the membrane interior. A wide range of oxidation ratios, from 15 to 100 mol%, was considered. The degree of oxidation was found to strongly influence the time needed for the opening of a pore. In simulations, the oxidation ratio of 75 mol% was found to be a threshold for spontaneous pore formation in the tens of nanosecond timescale, whereas 15 mol% of oxidation led to significant water permeation in the timescale of seconds. Once a pore was formed, the water permeability was found to be virtually independent of the oxidation ratio.     

Cold heteromolecular dipolar collisions

Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.