71 research outputs found
On the strong difference in reactivity of acyclic and cyclic diazodiketones with thioketones: experimental results and quantum-chemical interpretation
No full textThe 1,3-dipolar cycloaddition of acyclic 2-diazo-1,3-dicarbonyl compounds (DDC) and thioketones preferably occurs with Z,E-conformers and leads to the formation of transient thiocarbonyl ylides in two stages. The thermodynamically favorable further transformation of C=S ylides bearing at least one acyl group is identified as the 1,5-electrocyclization into 1,3-oxathioles. However, in the case of diazomalonates, the dominating process is 1,3-cyclization into thiiranes followed by their spontaneous desulfurization yielding the corresponding alkenes. Finally, carbocyclic diazodiketones are much less reactive under similar conditions due to the locked cyclic structure and are unfavorable for the 1,3-dipolar cycloaddition due to the Z,Z-conformation of the diazo molecule. This structure results in high, positive values of the Gibbs free energy change for the first stage of the cycloaddition process
ULTRAFAST PHOTOCHEMISTRY OF POLYATOMIC MOLECULES CONTAINING LABILE HALOGEN ATOMS IN SOLUTION
No full textTime-resolved relaxation dynamics of near-infrared excited electronic states in transition metal complexes
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Previous issue date: 6Sub-100 fs time-resolved, broadband transient absorption spectroscopy was employed to investigate ultrafast radiationless relaxation dynamics of near-infrared, metal-centered (MC), electronic excited states of several d5 and d9 transition metal complexes (e.g., CuCl4 2−, CuBr4 2−, IrBr6 2−, IrCl6 2−, etc.) in acetonitrile solution. The results yield insights into the topology of the involved potential energy surfaces, Jann-Teller distortions, and the dynamics through conical intersections connecting the first excited and ground electronic states (energy gap, less than 8000 cm−1 ). Furthermore, it was found that the addition of water to the solutions efficiently quenches the MC excited states via energy transfer
TIME-RESOLVED RELAXATION DYNAMICS OF NEAR-INFRARED EXCITED ELECTRONIC STATES IN TRANSITION METAL COMPLEXES.
No full textSub-100 fs time-resolved, broadband transient absorption spectroscopy was employed to investigate ultrafast radiationless relaxation dynamics of near-infrared, metal-centered (MC), electronic excited states of several d and d transition metal complexes (e.g., \chem{CuCl_4}, \chem{CuBr_4}, \chem{IrBr_6}, \chem{IrCl_6}, etc.) in acetonitrile solution. The results yield insights into the topology of the involved potential energy surfaces, Jann-Teller distortions, and the dynamics through conical intersections connecting the first excited and ground electronic states (energy gap, less than 8000 \wn). Furthermore, it was found that the addition of water to the solutions efficiently quenches the MC excited states via energy transfer
Методы оптической спектроскопии с временным разрешением
No full textIn this tutorial we outline the fundamentals of time-resolved spectroscopy techniques and the applications of the modern experimental methods for studying ultrafast photochemical and photophysical processes . This tutorial is intended for students, master students and post-graduate students of the Institute of Chemistry of St. Petersburg State University and other universities, as well as teachers who are studying at advanced training courses.В данном учебном пособии излагаются основы методов спектроскопии с временным разрешением и возможности применения современных экспериментальных методов для изучения быстрых фотохимических и фотофизических процессов. Пособие предназначено да студентов, магистров и аспирантов института химии СПбГУ и других ВУЗов, а также преподавателей, обучающихся на курсах повышения квалификации
Reductive and Coordinative Effects of Hydrazine in Structural Transformations of Copper Hydroxide Nanoparticles
No full textShape-specific copper oxide nanostructures have attracted increasing attention due to their widespread applications in energy conversion, sensing, and catalysis. Advancing our understanding of structure, composition, and surface chemistry transformations in shaped copper oxide nanomaterials during changes in copper oxidation state is instrumental from both applications and preparative nanochemistry standpoints. Here, we report the study of structural and compositional evolution of amorphous copper (II) hydroxide nanoparticles under hydrazine reduction conditions that resulted in the formation of crystalline Cu2O and composite Cu2O-N2H4 branched particles. The structure of the latter was influenced by the solvent medium. We showed that hydrazine, while being a common reducing agent in nanochemistry, can not only reduce the metal ions but also coordinate to them as a bidentate ligand and thereby integrate within the lattice of a particle. In addition to shape and composition transformation of individual particles, concurrent interparticle attachment and ensemble shape evolution were induced by depleting surface stabilization of individual nanoparticles. Not only does this study provide a facile synthetic method for several copper (I) oxide structures, it also demonstrates the complex behavior of a reducing agent with multidentate coordinating ability in nanoparticle synthesis
Acid‐catalyzed decomposition and stability of diazofuranones: Experimental and mechanistic study
No full textRoaming isomerization of photoexcited halogenated alkanes in the gas and liquid phases
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Previous issue date: 6Made available in DSpace on 2018-01-29T23:02:12Z (GMT). No. of bitstreams: 6
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Previous issue date: 6Recent experimental and computational gas-phase studies have brought to light a new type of unimolecular decomposition called a “roaming mechanism. It has only been observed in the gas phase, and whether it also occurs in solution is an intriguing question. Using ultrafast transient absorption spectroscopy, we report direct isomerization of CHBr, BBr, and PBr geminal tribromides in solution within the first 100 fs after S-excitation. The gas-phase conditions do not affect the earliest course of similar isomerization of CHBr. High-level ab initio simulations on XBr (X = B, P, and CH) suggest that isomerization is governed by an energetically and dynamically accessible S/S conical intersection and can be best described as a roaming-mediated pathway. Following the initial relaxation from the Franck-Condon point, “wandering” of the central atoms and migration of Br atom starts on a planar region of the S surface, and in the vicinity of the conical intersection (~40 fs) the XBr and Br fragments become separated to 3 AA. After passage through the conical intersection, the partially dissociated bromine atom slips off the XBr bisector plane, and forms the Br–Br bond of the BrXBr–Br isomer (~60 fs). We give examples of similar roaming isomerization in several other di- and polyhalogenated alkanes
Solvation and complexation of europium(III) ions in triflate and chloride aqueous-organic solutions by TRLF spectroscopy
No full textM.: The effect of the charge-separating interface on exciton dynamics in photocatalytic colloidal heteronanocrystals
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