Pressure versus Temperature Effects on Intramolecular Electron Transfer in Mixed‐Valence Complexes

Mixed‐valence trinuclear carboxylates, [M₃O(O₂CR)₆L₃] (M=metal, L=terminal ligand), have small differences in potential energy between the configurations M^IIM^IIIM^III?? M^IIIM^IIM^III??M^IIIM^IIIM^II, which means that small external changes can have large structural effects, owing to the differences in coordination geometry between M²⁺ and M³⁺ sites (e.g., about 0.2 Å for Fe? O bond lengths). It is well‐established that the electron transfer (ET) between the metal sites in these mixed‐valence molecules is strongly dependent on temperature and on the specific crystal environment; however, herein, for the first time, we examine the effect of pressure on the electron transfer. Based on single‐crystal X‐ray diffraction data that were measured at 15, 90, 100, 110, 130, 160, and 298 K on three different crystals, we first unexpectedly found that our batch of Fe₃O (O₂CC(CH₃)₃)₆(C₅H₅N)₃ ( 1 ) exhibited a different temperature dependence of the ET process than previous studies of compound 1 have shown. We observed a phase transition at around 130 K that was related to complete valence trapping and Hirshfeld surface analysis revealed that this phase transition was governed by a subtle competition between C? H???π and π???π intermolecular interactions. Subsequent high‐pressure single‐crystal X‐ray diffraction at pressures of 0.15, 0.35, 0.45, 0.74, and 0.96 GPa revealed that it was not possible to trigger the phase transition (i.e., valence trapping) by a reduction of the unit‐cell volume, owing to this external pressure. We conclude that modulation of the ET process requires anisotropic changes in the intermolecular interactions, which occur when various directional chemical bonds are affected differently by changes in temperature, but not by the application of pressure.     

Membranes producing nitrogen-enriched combustion air in diesel engines: Assessment via dimensionless numbers

The challenge of reducing nitrogen oxides (NO_X) emissions in diesel engine exhaust gas can be met by the use of nitrogen-enriched air (NEA) for combustion. In this work, the separation performance of membranes producing NEA is simulated and experimentally validated. Retentate nitrogen concentration and stage-cut depend on the membrane properties oxygen–nitrogen selectivity, oxygen permeance and membrane area as well as on the operating parameters, feed pressure, permeate pressure and feed flow rate. This complex dependence is presented using correlations of dimensionless numbers for retentate oxygen mole fractions between 16 and 21 mol%. It is herewith possible to calculate membrane area and compression power loss for any given membrane at any engine load point. The correlations also serve to identify the most suitable membrane and operating parameters when having to meet design criteria, e.g. limited construction space or maximum power loss. In a design example, both membrane area and power demand are calculated for three state-of-the-art membranes at a number of characteristic load points of a medium-duty diesel truck engine.     

New egg-shaped fullerenes: non-isolated pentagon structures of Tm3N@C(s)(51 365)-C84 and Gd3N@C(s)(51 365)-C84

Although there are 51 568 non-IPR and 24 IPR structures for C84, the egg-shaped endohedral fullerenes Tm3N@C(s)(51 365)-C84 and Gd3N@C(s)(51 365)-C84 utilize the same non-IPR cage structure as found initially for Tb3N@C(s)(51 365)-C84.     

Gd3N@C2n (n = 40, 42, and 44): remarkably low HOMO-LUMO gap and unusual electrochemical reversibility of Gd3N@C88

High-performance liquid chromatography was used to isolate two new trimetallic nitride endohedral fullerenes, Gd3N@C2n (n = 42 and 44), and they were characterized by MALDI-TOF mass spectrometry, UV-vis-NIR, and cyclic voltammetry. It was found that their electronic HOMO-LUMO gaps depend pronouncedly on the size of the cage, from a large band gap for Gd3N@C80 (2.02 V) to a small band gap for Gd3N@C88 (1.49 V). The electrochemical properties also change dramatically with the size of the cage, going from irreversible for the C80 cage to reversible for Gd3N@C88. The latter is the largest trimetallic cluster inside C88 isolated and characterized to date. Gd3N@C88 has one of the lowest electrochemical energy gaps for a nonderivatized metallofullerene.     

Lipophilic components from the Ecuadorian plant Schistocarpha eupatorioides

Phytochemical investigation of secondary metabolites of the Ecuadorian plant Schistocarpha eupatorioides (Fenzl) Kuntze (Asteraceae) afforded three phytyl fatty acid esters along with a mixture of unidentified polyprenols, the very well known sterols beta-sitosterol and stigmasterol, and their corresponding fatty acid esters and glucosyl derivatives. The structures of the compounds were elucidated on the basis of various spectroscopic means. In addition, a volatile fraction was separated the composition of which, comprising sesquiterpene hydrocarbons as the main fraction, was determined by GC-MS.     

Photoreactivity of biologically active compounds. XIX: Excited states and free radicals from the antimalarial drug primaquine

The formation and reactivity of excited states and free radicals from primaquine, a drug used in the treatment of malaria, was studied in order to evaluate the primary photochemical reaction mechanisms. The excited primaquine triplet was not detected, but is likely to be formed with a short lifetime (<50 ns) and with a triplet energy <250 kJ/mol as the drug is an efficient quencher of the fenbufen triplet and the biphenyl triplet, and forms ${}^1{\text{O}}_2$ by laser flash photolysis $({}^{\text{PQ}}{\Phi }_{\mathrm{\Delta }}=0.025)$. Primaquine (PQ) exists as the monocation $({\text{PQH}}^{+})$ in aqueous solution at physiological pH. ${\text{PQH}}^{+}$ photoionises by a biphotonic process and also forms the monoprotonated cation radical $({\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}})$ by one electron oxidation by ${\text{HO}}^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ (k_q = 6.6 × 10⁹ M^?1 s^?1) and ${\text{Br}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ (k_q = 4.7 × 10⁹ M^?1 s^?1) at physiological pH, detected as a long-lived transient decaying essentially by a second order process (k₂ = 7.4 × 10⁸ M^?1 s^?1). ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ is scavenged by O₂, although at a limited rate (k_q = 1.0 × 10⁶ M^?1 s^?1). The reduction potential (E°) of ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}/{\text{PQH}}^{+}$ is < +1015 mV, as measured versus tryptophan (${\text{TRP}}^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}/\text{TRPH}$). Primaquine also forms ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ at pH 2.4, by one electron oxidation by ${\text{Br}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ and proton loss (k_q = 2.7 × 10⁹ M^?1 s^?1). The non-protonated cation radical (${\text{PQ}}^{+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$) is formed during one electron oxidation with ${\text{Br}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ at alkaline conditions (k_q = 4.2 × 10⁹ M^?1 s^?1 at pH 10.8). The estimated pK_a-value of ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}/{\text{PQ}}^{+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ is pK_a ～ 7–8. Primaquine is not a scavenger of ${\text{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ at physiological pH. Thus self-sensitization by ${\text{O}}_2^{\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ is eliminated as a degradation pathway in the photochemical reactions. Impurities in the raw material and photochemical degradation products initiate photosensitized degradation of primaquine in deuterium oxide, prevented by addition of the ${}^1{\text{O}}_2$ quencher sodium azide. Photosensitized degradation by formation of ${}^1{\text{O}}_2$ is thus important for the initial photochemical decomposition of primaquine, which also proceeds by free radical reactions. Formation of ${\text{PQH}}^{2+\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ is expected to play an essential part in the photochemical degradation process in a neutral, aqueous medium.     

A Carbon Nano‐Onion–Ferrocene Donor–Acceptor System: Synthesis,Characterization and Properties

Don't cry! The attachment of ferrocene moieties on the surface of carbon nano‐onions influences the electrochemical properties of these moieties and the photophysical properties of the carbon nano‐onions (see figure). Quantum chemical calculations confirm that the spectral properties of carbon nano‐onions depend on their size and the degree of functionalisation.