# PubMed Journal Database | The journal of physical chemistry. A

19:20 EST 8th December 2013 | BioPortfolio

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## Showing PubMed Articles 1–25 of 3,200+ from The journal of physical chemistry. A

832282

Electronic Communication in Linear Oligo(azobenzene) Radical Anions.

The radical anions of five bis(azobenzene) and one tris(azobenzene) compounds were studied by optical and electron paramagnetic resonance (EPR) spectroscopies in polar aprotic solvents. The radicals with planar or almost-planar bridges are charge-delocalized mixed-valence species. Localization of charge occurs only on radicals with highly twisted biphenyl bridges. The electronic coupling between the azobenzene charge-bearing units, calculated as half the energy of the intervalence band for the charge-deloca...

832207

Direct Probing of Solvent Accessibility and Mobility at the Binding Interface of Polymerase (Dpo4)-DNA Complex.

Water plays essential structural and dynamical roles in protein-DNA recognition through contributing to enthalpic or entropic stabilization of binding complex and by mediating intermolecular interactions and fluctuations for biological function. These interfacial water molecules are confined by the binding partners in nanospace, but in many cases they are highly mobile and exchange with outside bulk solution. Here, we report our studies of the interfacial water dynamics in the binary and ternary complexes o...

832110

Pariser-Parr-Pople Model Based Investigation of Ground and Low-lying Excited States of Long Acenes.

Several years back Angliker }}\emph{\normalsize{et al}}{\normalsize{. {[}Chem. Phys. Lett. }}\textbf{\normalsize{1982}}{\normalsize{, }}\emph{\normalsize{87}}\textbf{\normalsize{,}}{\normalsize{{} 208{]} predicted nonacene to be the first linear acene with the triplet state $1^{3}B_{2u}$ as the ground state, instead of the singlet $1^{1}A_{g}$ state. However, contrary to that prediction, in a recent experimental work Tönshoff and Bettinger {[}}}\textit{\emph{\normalsize{Angew. Chem. Int. Ed}}}\emph{\normal...

832066

Description of Aromaticity with the Help of Vibrational Spectroscopy: Anthracene and Phenanthrene.

A new approach is presented to determine π-delocalization and the degree of aromaticity utilizing measured vibrational frequencies. For this purpose, a perturbation approach is used to derive vibrational force constants from experimental frequencies and calculated normal mode vectors. The latter are used to determine the local counterparts of the vibrational modes. Next, relative bond strength orders (RBSO) are obtained from the local stretching force constants, which provide reliable descriptors of CC and...

832038

Theoretical Methodologies for Calculation of Judd-Ofelt Intensity Parameters of Poly-Europium Systems.

In this study, we propose three different methods for calculation of Judd-Ofelt intensity parameters of poly-europium systems. The first, named Centroid Method, proposes the calculation considering the center of the dimeric system as the half distances between the two europium centers. The second method, called Overlapped Polyhedra Method, proposes the calculation considering the overlapping of both europium polyhedra, and the last one, Individual Polyhedron Method, proposes the use of theoretical mean valu...

827181

Electrophilic Aromatic Substitution: Enthalpies of Hydrogenation of the Ring Determine Reactivities of C6H5X. The Direction of the C6H5-X Bond Dipole Determines Orientation of the Substitution.

There are still some secrets left to this well-studied reaction. Previously unreported relationships discovered are as follows. The ordering of reactivities of C6H5X is the same as that of enthalpies of hydrogenation of the ring to the correspondingly substituted cyclohexane. The orientation of substitution (meta or ortho/para) is controlled by the dipole direction of the ipso-C-X bond, like an ON/OFF switch. The difference between the halogens and other deactivating groups is that the bond between the atom...

827152

Moderately Strong Phenols Dissociate by Forming an Ion-Pair Kinetic Intermediate.

We show computational evidence that ground-state moderately-strong hydroxyarenes (Ar-OH, pKa ~ 0) dissociate by forming an ion-pair intermediate which lives for 3-5 picoseconds. The concentration of this intermediate is approximately three times smaller than that of the un-ionized acid at pH ~ 0.6 and is characterized by average C-O bond lengths (1.30 Å) that are intermediate between those of un-ionized (1.29 Å) and fully dissociated (1.34 Å) species. During the lifetime of the ion-pair intermediate the...

827122

Molecular Structural Transformations Induced by Spatial Confinement in Barium Fluoride Cells.

Confinement of peptides and surfactants in narrow spaces of the order of 50 microns between BaF2 plates is found to induce new structural transformations. Optical microscopy, transmission electron microscopy and vibrational spectroscopic studies have been used to uncover the evidence for these structural transformations. These studies indicate that, using spatial confinement, small micelles can be converted to larger aggregates and peptide monomers can be converted to beta sheets even in monomer promoting 1...

827089

Classical Trajectory Study of Energy Transfer in Collisions of Highly Excited Allyl Radical with Argon.

Predicting the results of collisions of polyatomic molecules with a bath of atoms is a research area that has attracted substantial interest both in experimental and theoretical chemistry. Energy transfer, which is the consequence of such collisions, plays an important role in gas-phase kinetics and relaxation of excited molecules. We present a study of energy transfer in single collisions of highly vibrationally excited allyl in argon. We evolve a total of 52,000 classical trajectories on a potential energ...

826966

Photochemistry of Adsorbed Nitrate Formed From Nitric Acid Adsorption on α-Fe2O3 Particle Surfaces: The Role of Adsorbed Water in the Formation of Nitrogen-Reduced Surface Species.

The surface photochemistry of nitric acid (HNO3) adsorbed on hematite (α-Fe2O3) particle surfaces under various environmental conditions has been investigated. Surface product formation and photochemistry of adsorbed nitrate studied using X-ray photoelectron spectroscopy (XPS). Adsorbed nitrate resulted from gas phase nitric acid adsorption showed a N1s peak at 407.4 eV. Upon irradiation with light (λ > 300 nm), the coverage of nitrate decreases, as indicated by the decrease in the peak at 407.4 eV, and t...

826911

Atmospheric Chemistry of CF3CF2CHO: Absorption Cross Sections in the UV and IR Regions, Photolysis at 308 nm, and Gas-Phase Reaction with OH Radicals (T = 263-358 K).

The relative importance in the atmosphere of UV photolysis of perfluoropropionaldehyde, CF3CF2CHO, and reaction with hydroxyl (OH) radicals has been investigated in this work. Firstly, the forbidden n-π* transition of the carbonyl chromophore was characterized between 230 and 380 nm as a function of temperature (269 - 298 K) and UV absorption cross sections, σλ, were determined in those ranges. In addition, IR absorption cross sections were determined between 4000 and 500 cm-1. Pulsed laser photolysis (P...

822313

Fourth-Order Perturbative Model for Photoinduced Internal Conversion Processes.

Essential to the functionality of numerous biological and synthetic molecular systems is the ability to rapidly convert electronic excitation energy into heat. Such internal conversion (IC) transitions often cannot be described by traditional second-order kinetic theories because of time-coincident electronic and nuclear relaxation processes. Here, we present a perturbative fourth-order phenomenological model for photoinduced IC that incorporates effects associated with finite laser bandwidths and non-equil...

822245

Looking at Radiation Damage on Prebiotic Building Blocks.

A number of complex organic molecules have been detected in the interstellar medium, as well as in meteorites or comets. Among them, some exobiologic-relevant molecules have attracted particular interest. In the hypothesis of an exogen transport of prebiotic building blocks at the origin of life, the survival of such species and particularly their resistance to the solar UV radiation or cosmic rays is a key issue. For that purpose, we have performed a theoretical approach of the charge transfer dynamics ind...

822224

Measurements of the Sensitivity of Aerosol Hygroscopicity and the Kappa Parameter to the O/C Ratio.

We report measurements of the subsaturated hygroscopic growth of aerosol particles composed of single organic components of varying oxygen-to-carbon ratio up to relative humidities approaching saturation using the techniques of aerosol optical tweezers and an electrodynamic balance. The variation in the hygroscopicity parameter ĸ between compounds of even the same O/C ratio is found to be significant with, for example, a range in ĸ values from 0.12 to 0.38 for compounds with an O/C of 1. The measurements...

822174

Theoretical Prediction of XRgCO(+) ions (X = F, Cl and Rg = Ar, Kr, Xe).

In this work we have predicted novel rare gas containing cationic molecules, XRgCO+ (X = F, Cl and Rg = Ar, Kr, Xe) using ab initio quantum chemical methods. Detail structural, stability, vibrational frequency and charge distribution values are reported using density functional theory, second order Møller-Plesset perturbation theory, and coupled-cluster theory based methods. These ions are found to be metastable in nature and exhibit a linear geometry with C∞v symmetry in their minima energy structures,...

822113

Reduction of Earth Alkaline Metal Salts in THF Solution Studied by Picosecond Pulse Radiolysis.

Picosecond pulse radiolysis of Tetrahydrofuran (THF) solutions containing earth alkaline metal salt, MII(ClO4)2, at different concentrations are performed using two different supercontinua as probe pulse, one covering the visible and another the near infrared (NIR) down to the visible. Two types of line scan detectors are used to record the absorption spectra in the range from 400 to 1500 nm. Because of the strong overlap between the spectra of the absorbing species in the present wavelength range, global m...

822059

Comparison of Unimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels.

Quantum mechanical molecular modeling is used [M06-2X/6-311++G(2df,p)] to compare activation energies and rate constants for unimolecular decomposition pathways of saturated and unsaturated carboxylic acids that are important in the production of biofuels and that are models for plant and algae-derived intermediates. Dehydration and decarboxylation reactions are considered. The barrier heights to decarboxylation and dehydration are similar in magnitude for saturated acids (~71 kcal mol(-1)), with an approxi...

822019

Catastrophic Growth of Gas Hydrates in the Presence of Kinetic Hydrate Inhibitors.

The effect of the concentration of kinetic hydrate inhibitors, polyvinylpyrrolidone (PVP) and polyvinylcaprolactum (PVCap), on the onset and growth of synthetic natural gas hydrate is investigated by measuring hydrate onset time and gas consumption rate. Although hydrate onset time is extended by increasing the concentration from 0.5 to 3.0 wt% for both PVP and PVCap, the growth rate of hydrates shows different tendency depends on the type of kinetic hydrate inhibitor and its concentration. For PVCap soluti...

821968

CIMS sulfuric acid detection efficiency enhanced by amines due to higher dipole moments - a computational study.

Quantum chemical calculations have been performed on negatively charged nitric acid - sulfuric acid - dimethylamine clusters. The cluster energies were combined with a kinetic model to study the chemical ionization of sulfuric acid molecules and sulfuric acid - dimethylamine clusters with nitrate ions. Both the sulfuric acid monomer and the H2SO4·(CH3)2NH cluster get ionized, but the cluster has a much higher dipole moment, and thus a higher collision rate with charger ions. Clustering of sulfuric acid wit...

818572

Chain-Reaction Mechanism in Hydrogen/Fluorine Combustion.

Vibrationally excited species have been considered to play significant roles in H2/F2 reaction systems. In the present study, in order to obtain further understanding of the chain-reaction mechanism in the combustion of mixtures containing H2 and F2, burning velocities for H2/F2/O2/N2 flames were measured and compared to that obtained from kinetic simulations using a detailed kinetic model, which involves the vibrationally excited species, HF(v) and H2(v), and the chain-branching reactions, HF(v > 2) + F2 =...

813629

Experimental and Theoretical Charge Density Analysis of a Bromoethyl Sulfonium Salt.

Bromoethyl sulfonium trifluoromethanesulfonate is a salt complex in which a sulphur atom makes three covalent bonds. This molecule has been proved to act as an efficient annulation reagent which results in the formation of synthetically challenging and pharmaceutically important 4-, 5-, 6- and 7-membered heterocycles in excellent yields. The charge density of the molecule was determined from both experimentally and theoretically derived diffraction data. The stereochemistry and electron density topology of...

808921

Conformational Control in the Population of the Triplet State and Photoreactivity of Nitronaphthalene Derivatives.

Nitronaphthalene derivatives (NNDs) are among the most abundant volatile nitro-polycyclic aromatic hydrocarbons found in the Earth's atmosphere. Investigations of the atmospheric degradation processes show that photolysis is the major degradation pathway under ambient conditions. In this contribution, we present photochemical measurements and quantum-chemical calculations of three major NNDs. It is shown that the magnitude of the photodegradation and triplet quantum yields in 1 nitronaphthalene (1NN), 2-met...

808869

Fe L-edge X-ray Absorption Spectra of Fe(II) Polypyridyl Spin Crossover Complexes from Time Dependent Density Functional Theory.

L-edge near-edge X-ray ne structure spectroscopy (NEXAFS) has become a pow- erful tool to study the electronic structure and dynamics of metallo-organic and bio- logical compounds in solution. Here we present a series of density functional theory calculations of Fe L-edge NEXAFS for spin crossover (SCO) complexes within the time- dependent framework. Several key factors that control the L-edge excitations have been carefully examined using Fe(II) polypyridyl complex [Fe(tren(py)3)]2+ (where tren(py)3 = tris...

808836

On the Anisotropy of van der Waals Atomic Radii of O, S, Se, F, Cl, Br, and I.

The Cambridge Structural Database (CSD) was used to obtain flattening factors to describe the overall anisotropy of non-bonding van der Waals (vdW) contacts between several main group elements. The method for obtaining the flattening factors is based on a novel minimization process. Results show that the vdW contact distances are significantly dependent on the environment and the orientations of the surrounding covalently bonded atoms: head-on vdW contacts are generally shorter than sideways contacts in ove...

808811

Effects of Iodine on the Relaxation Dynamics of a Photoexcited I(-)(H2O)4 Cluster.

The Born-Oppenheimer molecular dynamics are used to examine the relaxation dynamics of the charge-transfer-to-solvent (CTTS) photoexcited electron in I(-)(H2O)4. The dynamics are initiated from the C1' cluster configuration, which contains a dangling water molecule. The iodine atom is found to exert a repulsive force on the photoexcited electron at the beginning but an attractive force at later times of the simulation. This dual repulsion-and-attraction role of the iodine atom is found to be dependent on th...

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