89 research outputs found
Switching on molecular iodine elimination through isomerization: The F2C–I–I isomer of difluorodiiodomethane
No full textWe report a UMP2/LANL2MB computational investigation of the ground-state pathways involved in the 350-nm photoinduced formation of molecular iodine from difluorodiiodomethane. Formation of I-2 from the conventional CF2I2 isomer is highly endothermic proceeding through a cyclic-skewed transition state at 62.4 kcal mol (1) above CF2I2. A minimum corresponding to iso- difluorodiiodomethane (F2C-I-I) is calculated at 24.8 kcal mol (1) above CF2I2. The formation of CF2 + I-2 from F2C-I-I is endothermic by only 18.8 kcal mol (1). The results suggest that solvent- induced geminate recombination of CF2I and I photofragments leads to hot F2C-I-I, which dissociates to form I-2 on a picosecond time scale. (c) 2008 Elsevier B. V. All rights reserved
Structure of the Photochemical Reaction Path Populated via Promotion of CF2I2into Its First Excited State
Get PDFThe photochemical reaction path following the promotion of CF2I2 into its lowest-lying excited electronic singlet state has been modeled using ab initio multiconfigurational quantum chemical calculations. It is found that a conical intersection drives the electronically excited CF2I2* species either to the CF2I + I radical pair or back to the starting CF2I2 structure. The structures of the computed relaxation pathways explain the photoproduct selectivity previously observed in the gas phase. Furthermore, the results provide the basis for explaining the condensed-phase photochemistry of CF2I2
Comparative Study of Uracil Excited-State Photophysics in Water and Acetonitrile via RMS-CASPT2-Driven Quantum-Classical Trajectories
No full textWe present a nonadiabatic molecular dynamics study of the ultrafast processes occurring in uracil upon UV light absorption, leading to electronic excitation and subsequent nonradiative decay. Previous studies have indicated that the mechanistic details of this process are drastically different depending on whether the process takes place in the gas phase, acetonitrile, or water. However, such results have been produced using quantum chemical methods that did not incorporate both static and dynamic electron correlation. In order to assess the previously proposed mechanisms, we simulate the photodynamics of uracil in the three environments mentioned above using quantum-classical trajectories and, for solvated uracil, hybrid quantum mechanics/molecular mechanics (QM/MM) models driven by the rotated multistate complete active space second-order perturbation (RMS-CASPT2) method. To do so, we exploit the gradient recently made available in OpenMolcas and compare the results to those obtained using the complete active space self-consistent field (CASSCF) method only accounting for static electron correlation. We show that RMS-CASPT2 produces, in general, a mechanistic picture different from the one obtained at the CASSCF level but confirms the hypothesis advanced on the basis of previous ROKS and TDDFT studies thus highlighting the importance of incorporating dynamic electron correlation in the investigation of ultrafast electronic deactivation processes
Matrix isolation and computational studies of the CF2I radical
No full textWe report experimental and computational studies of the spectroscopy and photochemistry of the CF2I radical, which was generated using pulsed discharge or photolytic techniques and trapped in Ar or Ne matrices held at ∼5 K. Ground state calculations were performed using UM062X and UB3LYP density functionals and the UMP2, UCCSD, and CASPT2 ab initio methods, all with high-quality basis sets. The photochemical reaction path from the lowest-lying electronically excited state was mapped using the CASPT2//CASSCF methodology. A conical intersection was found to drive the excited state species to the photoproduct/reactant wells on the ground state potential energy surface. © 2010 Elsevier B.V. All rights reserved
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Get PDFExcited-state trajectories computed at the complete active space second-order perturbation theory (CASPT2) reveal how vibrational excitation controls the molecular approach to the intersection space that drives the photodissociation of a prototypical halogenated methyl radical, namely CF2I. Translating the Franck-Condon structure along the ground-state CASPT2 vibrational modes in this system followed by propagating the displaced structures in the first excited doublet state simulates specific vibrational excitations and vibrationally mediated dynamics, respectively. Three distinct situations are encountered: the trajectories (i) converge to an energetically flat segment of the intersection space, (ii) locate a segment of the intersection space, and (iii) access a region where the intersection space degeneracy is lifted to form a ridge of avoided crossings. The computational protocol documented herein can be used as a tool to design control strategies based on selective excitation of vibrational modes, including adaptive feedback schemes using coherent light sources
Roaming isomerization of photoexcited halogenated alkanes in the gas and liquid phases
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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
Ultrafast dynamics of dibromocycloalkanes
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Previous issue date: 6Carbocyclic geminal dibromides are important intermediates in synthesis of complex heterocyclic molecules and natural products.a Ultrafast time-resolved techniques can help to obtain information about the electronic structure of the intermediates which appears in the chemistry and photochemistry of these molecules. Also, relaxation dynamics and reactivity of these intermediates in different solvents can be characterized. This information can be used for the design of different compounds with desired properties. In the current work, 1,1-dibromocycloalkanes with 3-, 4-, and 5-member rings were synthesized using previously described procedures.b c d For all three samples, ultrafast time-resolved absorption experiments with UV-excitation were performed in acetonitrile and methylcyclohexane solvents. It was shown that excitation of dibromocycloalkane solutions with 250 nm short (40 fs) pulses forms excited state absorption (ESA) of parent molecules in the spectral range from 360 to 760 nm. Within 800 fs, ESA decays with beginning of formation product bands. Next within 50 ps, these broad bands reach their maximum intensity and form well-defined broad peaks centered at 550, 600 and 400 nm for 3-, 4-, and 5-member rings respectively. These product species are long-lived and begin to decay at 1 ns. Obtained results suggest the formation of isomer products (CH2)n − C − Br − Br (n=1-3), similar to the isomeric species (HBrCBr-Br) observed in isomerization of bromoform.e
До питання про участь В. В. Тарновського- молодшого у земському ліберальному русі на півночі Лівобережної України (60–80-ті рр. ХІХ ст.)
No full textThe article is devoted to the actual and unresolved scientific problem of Ukrainian historical science – the question of the probable participation of the bright representative of the Ukrainian national aristocracy, a well-known public figure, philanthropist and patron, V. V. Tarnovsky-Jr. in the zemsky liberal movement in the north of the Left Bank of Ukraine (60–80 r.). On the basis of little-known and not well-researched sources discovered by the author, which are first introduced into scientific circulation; with the help of scientific methods of research – analysis and synthesis, deduction and induction, typology, classification and generalization, anthropological and retrospective method, content analysis, historical and dissemination and historical and genetic methods, thoroughly considered historical facts that showed the presence of contacts and contacts V. V. Tarnovsky-Jr. with the opposition fronts. As a result, the author comes to the reasonable conclusion that the younger Tarnovsky was not a full member of the specified liberation movement, there was no evidence of his active political activity in this field; his involvement in zemsky liberalism was formal and symbolic. Mutual interest of Frontender and younger Tarnovsky was caused by political calculations. However, the undeniable positive of these contacts should be considered the beginning of the extremely active and large-scale charitable work of V. Tarnovsky-Jr. in favor of the zemsky institutions of the region and the public of the region.Стаття присвячена актуальній та не розв’язаній до цього часу науковій проблемі української історичної науки – питанню вірогідної участі яскравого представника української національної аристократії, відомого громадського діяча, благодійника та мецената – В. В. Тарновського-молодшого у земському ліберальному русі на півночі Лівобережної України (60–80-ті рр. ХІХ ст.). На основі виявлених автором маловідомих та малодосліджених джерел, які вперше вводяться у науковий обіг, всебічно розглянуті історичні факти, які свідчили про наявність контактів та зв’язків В. Тарновського-молодшого з опозиційною фрондою. У результаті автор приходить до обґрунтованого висновку про те, що молодший Тарновський не був повноправним членом вказаного визвольного руху, жодних підтверджень його активної політичної діяльності на цій ниві не виявлено, його причетність до земського лібералізму була формальною і символічною
Watching ultrafast barrierless excited-state isomerization of pseudocyanine in real time
Get PDFThe photoinduced excited-state processes in 1,1'-diethyl-2,2'-cyanine iodine are investigated using femtosecond time-resolved pump-probe spectroscopy. Using a broad range of probe wavelengths, the relaxation of the initially prepared excited-state wavepacket can be followed down to the sink region. The data directly visualize the directed downhill motion along the torsional reaction coordinate and suggest a barrierless excited-state isomerization in the short chain cyanine dye. Additionally, ultrafast ground-state hole and excited-state hole replica broadening is observed. While the narrow excited-state wavepacket broadens during pump-probe overlap, the ground-state hole burning dynamics takes place on a significantly longer time-scale. The experiment reported can be considered as a direct monitoring of the shape and the position of the photoprepared wavepacket on the excited-state potential energy surface
Reactivity of iso-diiodomethane and iso-iodoform, isomers of CH2I2 and CHI3, toward the double bond of a variety of cycloalkenes
Get PDFThe metastable CH2I-I and CHI2-I isomers formed by UV photolysis of CH2I2, and CHI3 transfer methylene and iodomethylene groups, respectively, to a variety of cycloalkenes, leading to their cyclopropanation. More than a 100-fold increase of the reaction rate with increasing solvent polarity suggests a dipolar transition state. The fastest second-order rates observed were in CH3CN. However, CH2Cl2 will be the more appropriate reaction medium because the isomer thermal stability is greater in CH2Cl2 than in the more polar CH3CN
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