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Synthesis of μ <sub/>2</sub> -Oxo-Bridged Iron(III) Tetraphenylporphyrin-Spacer-Nitroxide Dimers and their Structural and Dynamics Characterization by using EPR and MD Simulations
摘要： Iron(III) porphyrins have the propensity to form m2-oxo-dimers, the structures of which resemble two wheels on an axle. Whereas their crystal structure is known, their solution structure and internal dynamics is not. In the present work, the structure and dynamics of such dimers were studied by means of electron paramagnetic resonance (EPR) spectroscopy and quantum chemistry based molecular dynamics (MD) simulations by using the semiempirical tight-binding method (GFN-xTB). To enable EPR investigation of the dimers, a nitroxide was attached to each of the tetraphenylporphyrin cores through a linear and a bent linker. The inter-nitroxide distance distributions within the dimers were determined by continuous-wave (cw)-EPR and pulsed electron–electron double resonance (PELDOR or DEER) experiments and, with the help of MD, interpreted in terms of the rotation of the porphyrin planes with respect to each other around the Fe–O–Fe axis. It was found that such rotation is restricted to the four registers defined by the phenyl substituents. Within the registers, the rotation angle swings between 308 and 608 in the proximal and between 1258 and 1458 in the distal register. With EPR, all four angles were found to be equally populated, whereas the 308 and 1458 angles are strongly favored to the expense of the 608 and 1258 angles in the MD simulation. In either case, the internal dynamics of these dimers thus resemble the motion of a step motor.
关键词： DEER spectroscopy,pulsed dipolar spectroscopy,nitroxide,complex formation,metal centers
更新于2025-09-19 17:15:36
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Complex Formation of Cobalt(II) Octakis(3,5-di-tert-butylphenoxy)phthalocyanine with 2′-(Pyridin-4-yl)-5′-(Pyridin-2-yl)-1′-(Pyridin-2-ylmethyl)-2′,4′-Dihydro-1′H-Pyrrolo[ 3′,4′:1,2][C60-Ih][5,6]fullerene
摘要： The formation of the donor–acceptor complex of cobalt(II) octakis(3,5-di-tert-butylphenoxy)phthalocyanine with 2'-(pyridin-4-yl)-5'-(pyridin-2-yl)-1'-(pyridin-2-ylmethyl)-2',4'-dihydro-1'H-pyrrolo[3',4':1,2][C60-Ih][5,6]fullerene (Py3C60) in toluene was studied by chemical kinetics (thermodynamics) and spectroscopy methods in comparison with a similar system based on 4-picoline (Pic). In both cases, the reaction occurred in one step giving donor–acceptor complexes of 1 : 1 stoichiometry, which were characterized by key spectral (UV-vis, IR, 1H NMR) parameters and thermodynamic equilibrium parameters; in the case of reaction with pyridyl-substituted fullerene, full kinetic description was obtained. The stability constants for (Pic)CoPc(3,5-tBuPhO)8 and (Py3C60)CoPc(3,5-tBuPhO)8 are (1.01 ± 0.16) × 104 L/mol (logK = 4.00, Δ G = –22.84 kJ/mol) and (1.84 ± 0.16) × 105 L/mol (logK = 5.26, Δ G = –30.04 kJ/mol), respectively. The obtained results are relevant for the development of new efficient components for photovoltaic cells based on phthalocyanine–fullerene systems.
关键词： 4-picoline,thermodynamics,phthalocyanine–fullerene complex,kinetics,donor–acceptor complex formation,pyridyl-fullerene,spectral properties,cobalt(II) phthalocyanine
更新于2025-09-09 09:28:46
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Encyclopedia of Spectroscopy and Spectrometry || Ion Collision, Theory ☆
摘要： Collisions between an ion and neutral species result in a number of possible outcomes depending upon the chemical and physical properties of the two reactants, their relative velocities, and the impact parameter of their trajectories. These include elastic and inelastic scattering of the colliding particles, charge transfer (including dissociative charge transfer), atom abstraction, complex formation and dissociation of the colliding ion. Each of these reactions may be characterized in terms of their energy-dependent rate coefficients, cross sections and reaction kinetics. This article outlines a theoretical framework for discussing these processes that emphasizes simple models and classical mechanics. The discussion of collision processes has been divided into two categories: low-energy and high-energy thermal or quasi-thermal collisions. Experiments under conditions – swarms, drift tubes, chemical ionization and ion cyclotron resonance – are strongly influenced by long-range forces and often involve ‘capture collisions’ in which atom exchange and extensive energy exchange are common characteristics. High-energy collisions are typically impulsive, involve short-range intermolecular forces and are direct, fast processes.
关键词： Low-energy Collisions,Elastic and Inelastic Scattering,Reaction Kinetics,Theory,Complex Formation,Rate Coefficients,Charge Transfer,Atom Abstraction,High-energy Collisions,Ion Collision,Dissociation,Cross Sections
更新于2025-09-04 15:30:14
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Equilibria in a ZnCl2–H2O–NaOH System, According to Data from Potentiometric Titration, and Selecting Conditions for the Hydrochemical Synthesis of ZnS and ZnSe Films
摘要： Equilibrium processes that occur in the ZnCl2–H2O–NaOH system are studied via the potentiometric titration of zinc chloride in the 0.00006–1.01 mol/L range of concentrations. Based on mathematical simulations, a number of polynuclear structures and soluble phases are identified in the system. Values of the instability constants of complex forms and the constants of the dynamic equilibria of weakly soluble zinc compounds are calculated, along with their stoichiometric compositions and areas of sustainable existence. Refined boundary conditions are determined, along with areas of the formation of ZnSe and ZnS when they are precipitated using sodium selenosulfate and thiourea, respectively. ZnSe and ZnS layers around 1000 and 200 nm thick are obtained via hydrochemical deposition on glass-ceramic substrates. Based on electron microscopy data, it is concluded that the layers consist of spherical aggregates with mean sizes of around 350–450 nm for ZnSe and 50–200 nm for ZnS, generated from primary particles with sizes of around 20–60 and 20–30 nm, respectively.
关键词： equilibrium processes,zinc selenide,complex formation,zinc sulfide,zinc chloride,potentiometric titration,thin films
更新于2025-09-04 15:30:14
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A Spectroscopic Study of a Cyclodextrin-Based Polymer and the “Molecular Accordion” Effect
摘要： The formation of host-guest complexes was studied for two hosts; β-cyclodextrin (β-CD) and a cross-linked polymer containing an equimolar ratio of β-CD and hexamethylene diisocyanate (HDI), denoted as HDI-1. The thermodynamics of host-guest binding was studied with 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) using steady-state fluorescence spectroscopy in aqueous solution at variable temperature and ambient pH. The association of 1,8-ANS with β-CD and HDI-1, showed a fluorescence enhancement of ~4 and 12 units, respectively. Greater fluorescence enhancement for the polymer/dye system indicates the presence of multiple binding sites (inclusion vs. interstitial). By contrast, the β-CD/dye system adopt trends that indicate the formation of well-defined inclusion complexes. HDI-1 has inclusion sites (β-CD) and interstitial domains (HDI) that afford dual binding with variable binding affinity. Simplified binding models employed herein address the role of inclusion binding without an explicit account for higher order or secondary binding equilibria. The approximate 1:1 binding constant (K1:1) for CD/1,8-ANS is about two-fold greater over the HDI-1/1,8-ANS system. HDI-1 displays cooperative effects among the polymer subunits, according to changes in relative fluorescence intensity due to structural transitions and binding site loci. The relative fluorescence intensities of the HDI-1/1,8-ANS system relate to a reversible temperature-driven structural transition (globular ? extended) between 5 oC and 60 °C of the polymer, in contrast to the β-CD/1,8-ANS complex. The temperature- and guest-driven structural transition, described as the “molecular accordion” effect is supported by new insight provided by complementary fluorescence and 1H NMR spectral results in aqueous solution.
关键词： Complex formation,Host-Guest,Fluorescence,Responsive polymers,Cyclodextrin
更新于2025-09-04 15:30:14