Direct dynamics study of ultrafast vibrational energy relaxation in ice Ih

Car-Parrinello molecular dynamics (CPMD) and a previously developed wave packet model are used to study ultrafast relaxation in water clusters. Water clusters of 15 water molecules are used to represent ice Ih. The relaxation is studied by exciting a symmetric or an asymmetric stretch mode of the central water molecule. The CPMD results suggest that relaxation occurs within 100 fs. This is in agreement with experimental work by Woutersen and Bakker and the earlier wave packet calculations. The CPMD results further indicate that the excitation energy is transferred both intramolecularly and intermolecularly on roughly the same time scale. The intramolecular energy transfer occurs predominantly between the symmetric and asymmetric modes while the bend mode is largely left unexcited on the short time scale studied here.     

A CNDO/2 calculation on stilbene

The equilibrium conformations of trans- and cis-stilbene and the rotational barrier between the two isomers were calculated using the CNDO/2 method. The results for the equilibrium angles and distances are in good agreement with experimental data. However, according to the calculations the phenyl rings would have a nearly free rotation which is only limited by steric hindrance. This result obviously deviates from the current view. The results are discussed in relation to previous theoretical and experimental evidence.

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Zusammenfassung Die Gleichgewichtskonformation von trans- und cis-Stilben und die Rotationsbarriere zwischen zwei Isomeren wurde nach der CNDO/2-Methode berechnet. Die Ergebnisse in bezug auf die Geometrie stimmen mit experimentellen Daten gut überein, die Rotationsbarriere für die Drehung der Phenylringe dagegen ist praktisch Null.

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Résumé La méthode CNDO/2 a été employée pour calculer les conformations d'équilibre du trans- et du cis-stilbène ainsi que la barrière de rotation entre ces deux isomères. Angles d'équilibre et distances calculés sont en bon accord avec les données expérimentales. Cependant les calculs indiquent que les cycles phényles devraient posséder une rotation libre soumise seulement à l'empêchement stérique. Ce résultat s'écarte de l'opinion courante. Les résultats sont discutés en relation avec les données théoriques et expérimentales antérieures.

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This work was supported by grant No 2741-4 from The Swedish Natural Science Research Council and grant No 69-927/U 697 from The Swedish Board for Technical Development.     

A multiconfigurational scf study of the transformation of methylene peroxide to dioxirane Intermediates in the ozonolysis of ethylene

The transformation of methylene peroxide to dioxirane, molecules of importance for understanding the mechanism of ozonolysis of ethylene, has been studied with ab initio MC SCF calculations. The results, an exothermicity of 106 kJ/mol and reaction barrier of 90 kJ/mol, support the Criegee mechanism for liquid-phase ozonolysis and the idea that dioxirane is important for gas-phase ozonolysis of ethylene.     

A new method for approximate estimates of the dispersion interaction between two molecules

A new method to evaluate the dispersion interaction between two weakly interacting closed shell molecules is presented. The method is based on a second-order sum-over-states perturbation method, where the nominator is approximated by products of one electron integrals.     

Convergent iterative schemes for time parallelization

Parallel methods are usually not applied to the time domain because of the inherit sequentialness of time evolution. But for many evolutionary problems, computer simulation can benefit substantially from time parallelization methods. In this paper, we present several such algorithms that actually exploit the sequential nature of time evolution through a predictor-corrector procedure. This sequentialness ensures convergence of a parallel predictor-corrector scheme within a fixed number of iterations. The performance of these novel algorithms, which are derived from the classical alternating Schwarz method, are illustrated through several numerical examples using the reservoir simulator Athena.

     

Spatial distribution of stabilizer-derived nitroxide radicals during thermal degradation of poly(acrylonitrile-butadiene-styrene) copolymers: a unified picture from pulsed ELDOR and ESR imaging

Double Electron-Electron Resonance (DEER) provides information on the spatial distribution of radicals on the length scale of a few nanometres, while Electron Spin Resonance Imaging (ESRI) provides information on a length scale of millimetres with a resolution of about 100 micrometres. Despite the gap between these length scales, results from the two techniques are found to complement and support each other in the characterization of the identity and distribution of nitroxide radicals derived from the Hindered Amine Stabilizer (HAS) Tinuvin 770 in poly(acrylonitrile-butadiene-styrene) (ABS) copolymers. DEER measurements demonstrate that there is no significant formation of biradicals from the bifunctional HAS, and provide the distributions of local radical concentrations. These distributions are poorly resolved for model-free analysis of the DEER data by the Tikhonov regularization; the resolution was significantly improved by utilizing information obtained by ESRI. DEER data can be fitted with only one adjustable parameter, namely the average radical concentration, if 1D and 2D spectral--spatial ESRI results on both the spatial distribution of nitroxides and their distribution between the acrylonitrile--styrene-rich (SAN) and butadiene-rich (B) microphases are considered.