The universality of quantum halo states enables a comparison of systems from different fields of physics, as demonstrated in two- and three-body clusters. In the present work, we studied weakly bound helium tetramers in order to test whether some of these four-body realistic systems qualify as halos. Their ground-state binding energies and structural properties were thoroughly estimated using the diffusion Monte Carlo method with pure estimators. Helium tetramer properties proved to be less sensitive on the potential model than previously evaluated trimer properties. We predict the existence of realistic four-body halo 4He23He2, whereas 4He4 and 4He33He are close to the border and thus can be used as prototypes of quasi-halo systems. Our results could be tested by the experimental determination of the tetramers’ structural properties using a setup similar to the one developed for the study of helium trimers.
In this article, the extrapolation procedures of π−π* electronic transition energy on π-conjugated oligomers are reexamined. Different models, including the simplest coupled oscillator, the free electron, the Hückel approach, the molecular exciton model, and some specific fitting-functions, are compared using the transition energies derived from theoretical calculations on three thiophene-based oligomer series. Specifically, oligomers of up to 30 repeating units have been considered to include the saturation effects as a function of chain length. The coupled
oscillator model of W. Kuhn and the fitting-function of Hirayama are the models that present the better suit on the transition energy interpolation as a function of chain length. Using only the first four oligomers of the series (n = 2 up to 8) yields an estimation of the transition energy on the
polymer limit with an average error of ∼1.5%. The vertical and adiabatic ionization potential present a better fit with the Hückel model approach. Finally, implications of the environmental
polarity on the electronic properties, molecular geometry, charge distribution, and aromaticity are shortly discussed.
Solutions of 30 mg L-1 of the herbicide atrazine have been degraded by environmentally friendly electrochemical advanced oxidation processes (EAOPs) such as anodic oxidation (AO), electro-Fenton (EF), and photoelectro-Fenton (PEF) using a small open and cylindrical cell with a boron-doped diamond (BDD)
anode. AO has been carried out either with a stainless steel cathode or an O2 diffusion cathode able to generate H2O2. Hydroxyl radicals (•OH) formed at the BDD surface in all EAOPs and in the bulk from Fenton’s reaction between added Fe2+ and electrogenerated H2O2 in EF and PEF are the main oxidants. All treatments yielded almost overall mineralization, although the rate for total organic carbon (TOC) removal is limited by the oxidation of persistent byproducts with •OH at the BDD surface. In AO, TOC abatement is enhanced by
parallel electrochemical reduction of organics at the stainless steel cathode, while in PEF, it also increases from additional photolysis of intermediates by UVA light under the synergistic action of •OH in the bulk. The effect of current and pH on the degradative behavior of EAOPs has been examined to determine their optimum values. Atrazine decay always follows a pseudo-first-order reaction, being more rapidly destroyed from •OH in the bulk than at the BDD surface. Aromatic intermediates such as desethylatrazine, desethyldesisopropylatrazine, and cyanuric acid and short linear carboxylic acids such as formic, oxalic, and oxamic have been identified and quantified by reversed-phase and ion-exclusion HPLC, respectively. Released inorganic ions such as Cl-, NO3-, and NH4 + have been followed by ionic chromatography.
A theoretical study of the water bend-to-libration energy transfer in liquid H2O has been performed by means
of nonequilibrium classical molecular dynamics computer simulations. Attention has been focused on the
time scale and mechanism of the decay of the fundamental H2O bend vibration and the related issue of the
decay of water librational (hindered rotational) excitations, including the important role of that for the excited
molecule itself. The time scales found are 270 fs for the decay of the average energy of an H2O molecule
excited to the ν ) 1 state of the bending oscillator and less than 100 fs for excess rotational (librational)
kinetic energy, both consistent with recent ultrafast infrared experimental results. The energy flow to the
excited molecule rotation and through the first several solvent shells around the excited water molecule is
discussed in some detail.
The interaction of polyaniline with alkylsulfonate dopants have been investigated at the atomic level using quantum mechanical methods. Calculations have been performed on complexes formed by dopant molecules with an alkyl group ranging from methyl to nonyl and model oligoanilines of different sizes. The stabilization provided by the formation of the alkylsulfonate···oligoaniline complexes (70−90 kcal/mol) is significantly higher than that found for conventional hydrogen bonds (5−12 kcal/mol) but lower than that obtained for methylsulfate···alkylammonium and methylsulfate···Na+ systems (120−135 kcal/mol). On the other hand, the influence of size of the alkyl group contained in the dopant on the interaction is practically negligible, whereas, in opposition, the number of aniline units used to represent polyaniline significantly affects the energetics of the interaction. Specifically, the interaction energy of an alkyl-dopant molecule and an infinite polyaniline chain has been predicted to be around −65 kcal/mol. The overall results allow the conclusion that the interaction between alkylsulfonate dopants and polyaniline is a very local phenomenon.
Casanovas Salas, Jordi; Bertran, O.; Armelin, E.; Torras, J.; Estrany, F.; Aleman, C. Journal of physical chemistry A Vol. 112, num. 42, p. 10650-10656 DOI: 10.1021/jp805719s Data de publicació: 2008-10 Article en revista
Formation of intra- and intermolecular hydrogen bonds in 2-thiophen-3-ylmalonic acid, the precursor of a polythiophene derivative bearing two carboxylic acid groups in the side chain, have been examined by Fourier transform infrared (FTIR) spectroscopy and ab initio quantum mechanical calculations. Interactions found in the FTIR spectra recorded for the melted and solid states are in good agreement with results provided by MP2/6-31+G(d,p) calculations on monomers and dimers, respectively. Specifically, inter- and intramolecular hydrogen bonds were detected in the solid and melted states, respectively. Calculations on dimers stabilized by intermolecular hydrogen bonds exclusively and by both intra- and intermolecular interactions indicated that the former structures are significantly more stable than the latter ones, which is fully consistent with experimental observations. On the other hand, intramolecular interactions in isolated monomers are favored in the melted state, which is dominated by a thermally driven entropic process.
We report a quantum mechanical study on the electrostatic interactions found in poly(L-lysine)•alkyl sulfate complexes, which are materials with interesting technological properties able to adopt self-assembled supramolecular structures. For this purpose, 19 reduced complexes were considered in the gas phase, chloroform solution, and aqueous solution. Calculations in the gas phase were carried out up to the MP2/6-311++G(d,p) level, while the influence of the bulk solvent was investigated at the HF/6-311++G(d,p) level using the polarizable continuum model. The electrostatic interaction characteristics of poly(L-lysine)•alkyl sulfate complexes have been compared with those of alkyltrimethylammonium•poly(a,L-glutamate) complexes, a related family of materials previously investigated.