The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both ...experimentally and theoretically. Lifetime measurements at the origin of the first ππ* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is ∼50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH2 and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the NPheH bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial ππ* surface, most importantly the one involving a transfer of electronic excitation to an nπ* surface, induced by distortions of the backbone peptide bond.
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A conformation-selective photophysics study in phenylalanine model peptides, combining pump-probe gas phase experiments and excited state calculations, highlights for the first time the quenching ...properties of a primary amide group (through its nπ* excited state) along with the effect of vibrational energy that facilitates access to the conical intersection area.
The C 1s, N 1s, and O 1s ionization energies were calculated for the three stable nitroxide free radicals, viz., tempo and its two analogues, and compared to their most recent high-resolution core ...photoelectron spectra. We compare the performance of unrestricted and restricted open shell based ΔHF, ΔMP2, and ΔB3LYP methods. A mixed basis set is employed in all cases, which consists of the core–valence correlation-consistent triple-ζ basis set (cc-pCVTZ or cc-pwCVTZ) on the atom whose core–electron binding energy is calculated, model core potentials on the remaining first row atoms, and the cc-pVDZ basis set on the hydrogen atoms. The best overall performance for the three free radicals is offered by the restricted open shell based ΔB3LYP method. Surprisingly, both the restricted open and unrestricted second-order perturbation theories perform relatively poorly and typically do not warrant additional computational effort over the reference ΔHF methods. This is particularly true of the ΔZAPT method, which exhibits a number of grave failures that render it unsuitable for calculating the core-ionization spectra.
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The photochemical reactivity of diphenyldiazomethane 1 and phenyl 1- and 2-adamantyl diazomethanes 2 and 3, respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination ...of N2 upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of 1 and 2, detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet car-1 in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet car-2 does not undergo ISC but reacts in the intermolecular insertion reactions into C–H bonds. Car-3 has an α-C–H bond next to the carbene center and reacts rapidly in the intramolecular C–H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from 3 is highly indicative of triplet car-3’s intermediate formation. The TA spectra from the S1–S3 states of 1–3 were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.
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A combination of multireference CASSCF/CASPT2 and time-dependent DFT (TD-B3P86) theoretical treatments was employed to test their predictions against recently proposed assignments of the vacuum-UV ...spectra of fluorene and its three heteroanaloguesdibenzofuran, carbazole, and dibenzothiopheneup to the ionization threshold. For the low-lying transitions, the theoretically based assignments are generally not problematic because of the well-resolved bands, although, even in this region, the two methods yield some opposing predictions. Further on toward the vacuum region, the assignments prove increasingly challenging because of predicted crowding of transitions, many of which exhibit significant intensity. Some of the transitions in this region and beyondtoward the ionization thresholdsare thus necessarily assigned only tentatively. Overall, the two methods are frequently found to complement each other well, and equivalent transitions usually appear as bracketed from the high- (CASPT2) and low- (TD-B3P86) energy sides.
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Two BODIPY phenolic esters 1 and 2 were synthesized and their acid-catalyzed hydrolytic and esterase enzymatic stability was demonstrated. Their photophysical properties were investigated in solvents ...of different polarity. Their spectral properties are the typical of BODIPY dyes with narrow absorption and emission spectra in the visible spectral region and large molar absorption coefficients (≈500 nm, ε = 40000-50000 M−1cm−1). The derivative with methyl substituted BODIPY core 2 shows quantum yields of fluorescence one magnitude higher (Φf = 0.30–0.55) compared to the BODIPY without methyl groups. Solvents affect very weakly spectral properties, with more pronounced effect of the solvent polarizability than the solvent polarity. Both BODIPY dyes show pH responsive fluorescence in aqueous solution (pKa = 2.72 ± 0.08 and 4.18 ± 0.05), with quenching of fluorescence by protonation due to PET from the BODIPY to phenol, rendering them applicable for intracellular pH measurements. The feasibility of the PET in the protonated form of 1 was corroborated by electrochemical measurements and calculation of ΔETGo for PET, and (time-dependent) density functional theory computations. This less frequently encountered quenching mechanism where the BODIPY is an electron donor provides new opportunities in the rational design of new generations of fluorescent sensors. The applicability for intracellular measurements was demonstrated by confocal fluorescence microscopy on H460 human cancer cell line.
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•Two BODIPY phenolic esters 1 and 2 were synthesized and their photophysical properties were investigated.•The BODIPY with methyl substituents 1 shows quantum yields of fluorescence one magnitude higher (Φf = 0.30–0.55) compared to 2.•Both BODIPY dyes show pH responsive fluorescence in aqueous solution (pKa = 2.72 ± 0.08 and 4.18 ± 0.05).•In acidic media, quenching of fluorescence takes place due to PET from the BODIPY to phenol.•The applicability was demonstrated by confocal fluorescence microscopy on H460 human cancer cell line.
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Ab initio multiconfigurational CASSCF and CASPT2 methods were employed in studying the reaction mechanisms and kinetics of the gas-phase ozone additions to ethene, fluoroethene, and chloroethene up ...to the formation of the primary addition products (primary ozonides). Reactants, transition-state structures, and products were optimized, and harmonic vibrational frequencies were calculated at the CASSCF/cc-pVTZ level. For kinetic calculations, the electron energies of all the stationary points were further refined by utilizing the CASPT2 method with the optimized CASSCF/cc-pVTZ wave functions taken as the zeroth order. The rate constants and Arrhenius kinetic parameters were finally calculated in terms of the conventional transition-state theory. The favorable conformations of the ozone approach to the two asymmetrically substituted haloalkenes are at first governed by the electrostatic repulsion in the transition-state structures and later by the gradually predominating anomeric effect. The bond formation in the primary haloozonides was analyzed by monitoring the changes in the occupation numbers of the active orbitals in the course of the optimizations. For all the reactions thus studied, close agreement is found with the experimental kinetics, which makes the future use of the same approach very promising.
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9.
A multifaceted approach to hydrogen storage CHURCHARD, Andrew J; BANACH, Ewa; JARON, Tomasz ...
Physical chemistry chemical physics : PCCP,
01/2011, Volume:
13, Issue:
38
Journal Article
Peer reviewed
The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the ...problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.
The reactions of α-hydroxyalkyl radicals in aqueous medium are of interest because they exhibit a rich variety of fundamentally important competing mechanisms, such as proton-coupled electron ...transfer (PCET), hydrogen atom transfer, free radical substitutions, abstractions and additions, etc. We present a theoretical study of the mechanism and kinetics of the aqueous reactions of α-hydroxyisopropyl (2-propanol) radical with four halogenated organic substrates: iodoacetate (IAc), iodoacetamide (IAm), 5-bromouracil (5-BrU), and carbon tetrachloride (CCl4). The reactions are studied using density functional theory (DFT) (M06-2X), and the solvent is modeled as a polarizable continuum, either without the explicit solvent molecules or with one added water molecule. For an additional refinement, the double hybrid DFT B2PLYP energies were calculated at the M06-2X stationary points. Within this framework, for each substrate, we determine the most favorable radical-induced decomposition pathway among the several found and compare the thermochemical predictions against the experimental kinetics. The following dominant decomposition mechanisms are inferred: PCET for IAc, PCET-H2O and the I-atom abstraction for IAm, the ortho-addition to the double bond for 5-BrU, and the Cl-atom abstraction for CCl4. These pathways are invariably characterized by the negative apparent activation energies. Whereas for 5-BrU and CCl4 the transition state theory rate constants are in good agreement with the experiment, the rate constants for IAc and IAmthe two substrates reacting preferably via the PCETare difficult to predict correctly. Consequently, the corresponding reaction barriers necessitate lowering by 1–3 kcal mol–1 to bring them in accord with experiment. The B2PLYP method provides a worthwhile improvement over the M06-2X energetics although the largest errors remain for the two PCET processes.
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