SciCombinator

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Journal: The journal of physical chemistry. A

28

Laser-ablated beryllium atom has been codeposited at 4K with hydrogen sulfide in excess noble gas matrices. Four noble-gas compounds NgBeS (Ng = Ne, Ar, Kr, Xe) and BeS2 molecule are identified on the basis of the S-34 isotopic substitution, DFT and CCSD(T) theoretical predictions, and comparison of noble-gas substitution. The agreement between the experimental and calculated vibrational frequencies supports the identification of these molecules. The dissociation energies are calculated at 1.6, 12.6, 10.7 and 13.4 kcal/mol for NeBeS, ArBeS, KrBeS and XeBeS, respectively, at the CCSD(T) level. The BeS Lewis acid molecule favors strong chemical binding between the Be and Ng atoms.

Concepts: Water, Molecule, Chemistry, Atom, Chemical bond, Chemical compound, Helium, Noble gas

28

Trinitrobenzene (TNB) and trinitrotoluene (TNT) react in N,N-dimethylformamide (DMF) to form multiple species in solution. Despite structural similarities, electronic spectra show that the reactivity is different for TNB and TNT. In addition to reaction with the DMF solvent, residual water in nominally dry DMF generates sufficient hydroxide for reaction with TNB and TNT. Multiple sigma adducts are formed and observed to be fluorescent, which has not been previously reported. Both TNB and TNT show the capacity to form sigma adducts with hydroxide and DMF, while methyl hydrogens of TNT can be deprotonated by hydroxide.

Concepts: Oxygen, Toluene, Tetrahydrofuran, Multiplication, Trinitrotoluene

28

The helical structure is experimentally determined by circular dichroism (CD) spectra. The sign and shape of the CD spectra are different between B-DNA with a right-handed double-helical structure and Z-DNA with a left-handed double-helical structure. In particular, the sign at around 295 nm in CD spectra is positive for B-DNA, which is opposite to that of Z-DNA. However, it is difficult to determine the helical structure from the UV absorption spectra. Three important factors that affect the CD spectra of DNA are 1) the conformation of dG monomer, 2) the hydrogen-bonding interaction between two helices and 3) the stacking interaction between nucleic acid bases. We calculated the CD spectra of 1) the dG monomer at different conformations, 2) the composite of dG and dC monomers, 3) two dimer models that simulate separately the hydrogen-bonding interaction and the stacking interaction and 4) the tetramer model that includes both hydrogen-bonding and stacking interactions simultaneously. The helical structure of DNA can be clarified by a comparison of the experimental and SAC-CI theoretical CD spectra of DNA and that the sign at around 295 nm of the CD spectra of Z-DNA reflects from the strong stacking interaction characteristic of its helical structure.

Concepts: DNA, Protein, Nucleic acid, Monomer, Double helix, Circular dichroism, Alpha helix, Helices

28

Although 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP) is only weakly fluorescent in solution, two of its crystal polymorphs in which molecules are packed as stacked pairs and in nearly coplanar conformation exhibit bright excited-state intramolecular proton transfer (ESIPT) luminescence of different colors (blue-green and yellow). In order to clarify the enhanced and polymorph-dependent luminescence of HPIP in the solid state, the potential energy surfaces (PESs) of HPIP in the ground (S(0)) and excited (S(1)) states were analyzed computationally by means of ab initio quantum chemical calculations. The calculations reproduced the experimental photophysical properties of HPIP in solution, indicating that the coplanar keto form in the first excited (S(1)) state smoothly approaches the S(0)/S(1) conical intersection (CI) coupled with the twisting motion of the central C-C bond. The S(1)-S(0) energy gap of the keto form became sufficiently small at the torsion angle of 60°, and the corresponding CI point was found at 90°. Since a minor role of the proximity effect was indicated experimentally and theoretically, the observed emission enhancement of the HPIP crystals was ascribed to the following two factors: (1) suppression of efficient radiationless decay via the CI by fixing the torsion angle at the nearly coplanar conformation of the molecules in the crystals and (2) inhibition of excimer formation resulting from the lower excited level of the S(1)-keto state compared to the S(0)-S(1) excitation energy in the enol form. However, the fluorescence color difference between the two crystal polymorphs having slightly different torsion angles was not successfully reproduced, even at the MS-CASPT2 level of theory.

Concepts: Fluorescence, Electron, Chemistry, Atom, Chemical bond, Computational chemistry, Angle, Keto-enol tautomerism

28

Reactions between dilute methane and non-energetic hydroxyl radicals were carried out at 3.5 K. The temperature was kept low in order to characterize the step-wise reaction and prevent parasitic side reactions. The hydroxyl radicals originate from discharged H2O/He mixtures. The reactions were monitored in-situ using a Fourier transform infrared spectrometer. The formation of CH3 radicals was confirmed simultaneously with the formation of water ice. Subsequent recombination reactions lead to the formation of ethane (C2H6). Production of ethane and water ice occur preferentially to the formation of methanol.

Concepts: Spectroscopy, Alkane, Fourier transform, Infrared spectroscopy, Fourier transform spectroscopy, Methane, Fourier analysis, Ethane

28

Strong electric fields open new routes for the control of radiation-less decay in molecules with conical intersections. Here, we present quantum chemical and quantum dynamical simulations which demonstrate that the radiation-less decay and related photoisomerization of pyridinylidene-phenoxide can be effectively manipulated with strong electric fields by shifting the conical intersection. Moreover, we show the effects of the electric field on the orientation of the molecules and on the photoexcitation and discuss the conditions for which the field induced coupling between rotational and vibronic states can be neglected.

Concepts: Electron, Electromagnetism, Electric charge, Electromagnetic field, Quantum mechanics, Chemistry, Quantum chemistry, Conical intersection

28

Kinetics of humic acid (HA) adsorption onto soil minerals (kaolinite and hematite) has been investigated under various conditions. The influence of ionic strength, pH, and solution cations on the rate of adsorption has been studied. The rate and the amount of adsorbed humic acid onto soil minerals increased with increasing ionic strength, decreasing pH, and in the presence of Ca(2+) as background electrolyte. The adsorption equilibrium data showed that adsorption behavior of humic acid could be described more reasonably by Freundlich adsorption isotherm than Langmiur adsorption isotherm. Pseudo first order and pseudo second order kinetic models were used to evaluate the kinetic data and the rate constants. The results explained that humic acid adsorption on hematite and kaolinite was more conforming with pseudo second order kinetics.

Concepts: Soil, Adsorption, Rate equation, Colloidal chemistry, Freundlich equation, Humus, Humic acid, Weathering

28

A series of ruthenium-polypyridyl complexes were studied using UV/vis absorption and luminescence spectroscopy as well as luminescence lifetime determination by time-correlated single photon counting (TCSPC). The complexes were characterized with regard to the variation in the electronic band gap as a result of the sequential variation of the auxiliary ligand 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), and 2,2'-biquinoline (biq) ligands while the main ligand remained constant for three different main ligand types. Luminescence yields were calculated and correlated with structural and electronic variation. It was found that both the absorption and emission characteristics could be tailored through the systematic variation of the reduction potential of the individual auxiliary ligand. This was shown to be the case regardless of the functional group at the end of the main ligand. Stokes shift and Raman spectroscopy were employed as a means to gauge the effect of ligand change on the conjugation and vibrational characteristics of the complexes. Luminescence yield and lifetimes were also shown to be well-defined with regards to systematic structure variations. The well-defined trends established elucidate the effect which variation of auxiliary ligands has on the electronic and excited state characteristics of the ruthenium-polypyridyl systems. These well-defined relationships can potentially be extended to optimize luminescence yield and lifetimes and therefore suitability of such compounds for the application in for example photodynamic therapy.

Concepts: Spectroscopy, Photon, Ligand, Raman spectroscopy, Absorption spectroscopy, Stokes shift, Potential, Phenanthroline

28

The spin and orbital magnetic moments, as well as the magnetic anisotropy energy (MAE), of small 4d transition metal ™ clusters are systematically studied by using the spin-orbit coupling (SOC) implementation of the density-functional theory (DFT). The effects of spin-orbit interactions on geometrical structures and spin moments are too weak to alternate relative stabilities of different low-lying isomers. Remarkable orbital contributions to cluster magnetic moments are identified in Ru, Rh, and Pd clusters, in contrast to immediate quenching of the atomic orbital moment at the dimer size in other elemental clusters. More interestingly, there is always collinearity between total spin and orbital moments (antiferromagnetic or ferromagnetic coupling depends on the constituent atoms whose 4d subshell is less or more than half-filled). The clusters preserve the validity of Hund’s rules for the sign of orbital moment. The calculations on MAEs reveal the complicated changes of the easy axes in different structures. The perturbation theory and the first-principles calculations are compared to emphasize how MAEs evolve with cluster size. Finally, large orbital moments combined with strong spin-orbit coupling are proposed to account for large MAEs in Ru, Rh, and Pd clusters.

Concepts: Quantum mechanics, Angular momentum, Fundamental physics concepts, Atom, Magnetism, Celestial mechanics, Magnetic anisotropy, Electron shell

28

Photochromic Schiff bases 5-diethylamino-2-[(4-diethylamino-benzylidene)-hydrazonomethyl]-phenol (DDBHP) and N,N'-bis(4-N,N-diethylaminosalisalidene) hydrazine (DEASH) with both the proton and charge transfer moieties have been synthesized, and their photophysical properties such as excited state intramolecular charge transfer (ICT) and proton transfer (ESIPT) processes have been reported on the basis of steady-state and time-resolved spectral measurement in various solvents. The ground-state six-membered intramolecular hydrogen bonding network at the proton transfer site accelerates the ESIPT process for these compounds. Both the compounds show large Stokes-shifted emission bands for proton transfer and charge transfer processes. The hydrogen bonding solvents play a crucial role in these photophysical processes. Excited-state dipole moment of DDBHP and DEASH calculated by the solvatochromic method supports the polar character of the charge transfer excited state. Introduction of -NEt(2) groups to the reported salicylaldehyde azine (SAA) Schiff base results an increase in fluorescence lifetime from femtosecond to picosecond time scale for the proton transfer process.

Concepts: Electron, Oxygen, Hydrogen, Atom, Hydrogen bond, Imine, Schiff base, Hugo Schiff