Physchem, Volume 2, Issue 3 (September 2022) – 7 articles

The structure of two biochars and of their activated carbons was investigated by Electron Paramagnetic Resonance, Solid State Nuclear Magnetic Resonance, and Fourier Transform Infrared spectroscopies, together with X-ray diffraction and nitrogen adsorption/desorption isotherm measurements. The biochars were obtained from wood and Posidonia Oceanica by slow pyrolysis up to 600 °C, whereas the activated carbons were prepared from the biochars by impregnation with KOH, heating up to 800 °C. Two different KOH:biochar mass ratios were tested in the case of Posidonia, namely 4:1 and 2:1, while only the 4:1 ratio was used for wood. When the larger ratio was used, activation significantly increased the microporosity of the starting biochar, also creating bottle-neck pores not accessible to water molecules, and induced the formation of larger condensed aromatic networks arranged in interconnected conducting domains. In the case of Posidonia, activation using the 2:1 ratio mainly created mesopores and induced an increase in organic radical content by almost four orders of magnitude. This huge increase was related to the presence of minerals in the starting biochar. Full article

The report elucidates for the first time a significant effect of a strong magnetic field combined with thermal treatment on the texture of thin liquid-crystalline films in a smectic state. The metastable texture generated in the magnetic field was arrested via the crystallization of mesogens while they cooled to room temperature. The effect was demonstrated on a series of wedge-shaped amphiphilic mesogens based on 1,2,3-tris-(dodecyloxy)benzene (TDOB): asymmetric 2,3,4-tris-(dodecyloxy)benzenesulfonic acid (TDOBSH) and its sodium (TDOBSNa) and pyridine (TDOBSPyr) salts. The thermotropic properties and the structure of the liquid crystal phases of the synthesized compounds were studied using differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. It was shown that, depending on the type of counterion, the synthesized mesogens formed different supramolecular structures. The largest effect of the intense magnetic field was observed for the pyridine salt for which an ordered primitive cubic phase texture was generated upon the normal application of the magnetic field with respect to the film. In contrast, for the corresponding acid, an improvement of the gyroid cubic phase orientation was detected with the magnetic field oriented along the film. A highly ordered columnar phase of the sodium salt was affected only minimally by the magnetic field. Full article

This study presents the method of fixed-photon-energy double-imaging photoelectron photoion coincidence (i²PEPICO) utilized to investigate the dissociation of state-selected ions. Vacuum ultraviolet (VUV) synchrotron radiation at one fixed photon energy of hν = 21.2 eV, the He(I) atomic resonance energy, is employed as a light source to ionize molecules. Various dynamic information including time-of-flight (TOF) mass spectra, mass-selected photoelectron spectra (PES), and electron and ion kinetic energy correlation diagrams corresponding to each mass are obtained efficiently with the multiplexed capabilities of i²PEPICO, thereby revealing the detailed dissociation mechanisms of ions. As representative examples, dissociation of state-selected O₂⁺ ions prepared in the b⁴∑_g⁻ and B²∑_g⁻ electronic states and CH₃F⁺ ions in the X²E, A²A₁, and B²E states were selected and investigated. Full article

The zero-point energies (ZPEs) of paramagnetic molecules, free and compressed in a C₅₉N paramagnetic cage, were computed. The excess of energy acquired by molecules under compression depended on the deuterium and tritium isotopes which ranged from 6–8 kcal/mol for H₂⁺ to 1.0–1.5 kcal/mol for HO^• and HO₂. The differences in the ZPEs of compressed isotopic molecules resulted in large deuterium and tritium isotope effects which differed for singlet and triplet spin states. The hyperfine coupling (HFC) constants for protons and ¹⁷O nuclei decreased under compression, confirming the leakage of the unpaired π-electron from the central oxygen atom of guest molecules into the system of π-electrons of the cage, and its distribution over 60 atoms of the C₅₉N. The latter seems to be the reason why the nitrogen-14 HFCs for C₅₉N remain almost unchanged upon encapsulation of guest molecules. The singlet-triplet splitting is shown to depend on the Coulomb interaction, which controls the sign of the exchange potential. The importance of compression effects on the functioning of enzymes as molecular compressing devices is discussed. Full article

Like most photosynthetic organisms, cyanobacteria are vulnerable to fluctuations in light intensity, which can damage their photosynthetic machinery. To protect against this, they use a photoprotective mechanism called non-photochemical quenching (NPQ), where excess absorbed photo-energy is dissipated as heat. In cyanobacteria, light activation of Orange Carotenoid Protein (OCP) is the critical first step in the NPQ response. OCP is also the only known photosensitive protein, which uses carotenoid for its activation. We summarize the current knowledge on the light induced reactions of OCP; the different mechanisms of activation that have been proposed; photocycle kinetics and characteristics; and the reported structural intermediates. We discuss the possible interpretations of reported experimental results, and we formulate important open questions and directions for future work, to reveal the molecular and structural basis of photosensing by OCP. Full article

The determination of the NMR spin–spin relaxation rate of water in (purely) aqueous particulate dispersions has been shown to be a convenient and facile experimental approach to probing the composition of near particle surface structures. Here, a systematic study has been undertaken of both non-aqueous and mixed aqueous–non-aqueous solvent particulate dispersions to explore the universality of the solvent relaxation technique. As in the aqueous case, a linear relationship between the surface area present and the solvent relaxation rate is observed, confirming the rapid exchange of the solvent molecules between the surface and the bulk and thereby illustrating the viability of the experimental methodology to study such systems. Crucially, the surface enhancement effect was considerably weaker in non-aqueous systems compared with aqueous dispersions and reflects a potential limitation of the wider deployment of this experimental methodology. Full article

The molecular mechanisms of three intramolecular rearrangements (I, the rearrangement of allyloxycycloheptatriene to yield tricyclic ketones; II, the cycloaddition of a nitrone-alkene to render two tricyclic isoxazolidines; and III, the decomposition of N-carbamoyl-L-proline in tetrahydro-1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione plus water, or tetrahydro-1H,3H-pyrrolo[1,2-c]oxazole-1,3-dione plus ammonia) have been studied by means of the bonding evolution theory (BET). The thermal rearrangement I is composed by a sigmatropic rearrangement coupled to an intramolecular Diels–Alder reaction. The sigmatropic reaction comprises four steps: (1) rupture of an O-C single chemical bond, (2) transformation of a C-O single to double bond, (3) creation of pseudo-radical centers on carbon atoms coupled with a double C-C bond evolving to single and the other C-C double bond migration, and (4) formation of the new C-C single bond. For the Diels–Alder reaction, the process can be described as an initial formation of up to four monosynaptic V(C) basins in two successive steps, coupled with the loss of the double bond character of the three initial double bonds, followed by the consecutive formation of two new C-C bonds, with the new double C-C bond formation sensed in between the formation of the first and the second C-C bonds. For reaction II, the bond forming process is described by the depopulation of N-C and C-C double bonds with the creation of a V(N) and two V(C) monosynaptic basins, followed by an O-C and C-C bond-forming processes via the creation of V(O,C) and V(C,C) disynaptic basins. Finally, for the thermal decomposition III, the reaction mechanism for the water elimination takes place in four events which can be summarized as follows: (1) the depopulation of V(N) with the formation of C-N, (2) the rupture of the C-O bond with transfer of its population to V(O), (3) the restoration of an N nitrogen lone pair via H-N bond cleavage, and (4) the formation of O-H illustrating the water molecule release. For the case of deamination, the events (1) and (2) correspond to the breaking and forming process of H-O and H-N bonds, respectively, while last events deal with the C-O bond formation and the elimination of the NH₃ molecule. Full article