Pooria Farahani

Mechanisms for the breakdown of halomethanes through reactions with ground-state cyano radicals.

One route to break down halomethanes is through reactions with radical species. The capability of the artificial force-induced reaction algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes (CX3 Y; X=H, F; Y=Cl, Br) and ground-state ((2) Σ(+) ) cyano radicals (CN). For CH3 Cl+CN, 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (10(8) s(-1)  M(-1) at 298 K) is obtained for hydrogen abstraction. For CH3 Br, the rate constants for hydrogen and halogen abstraction are similar (10(9) s(-1)  M(-1) ), whereas replacing hydrogen with fluorine eliminates the hydrogen-abstraction route and decreases the rate constants for halogen abstraction by 2-3 orders of magnitude. The detailed mapping of stationary points allows accurate calculations of product distributions, and the encouraging rate constants should motivate future studies with other radicals.

A two-scale approach to electron correlation in multiconfigurational perturbation theory.

We present a new approach for the calculation of dynamic electron correlation effects in large molecular systems using multiconfigurational second‐order perturbation theory (CASPT2). The method is restricted to cases where partitioning of the molecular system into an active site and an environment is meaningful. Only dynamic correlation effects derived from orbitals extending over the active site are included at the CASPT2 level of theory, whereas the correlation effects of the environment are retrieved at lower computational costs. For sufficiently large systems, the small errors introduced by this approximation are contrasted by the substantial savings in both storage and computational demands compared to the full CASPT2 calculation. Provided that static correlation effects are correctly taken into account for the whole system, the proposed scheme represent a hierarchical approach to the electron correlation problem, where two molecular scales are treated each by means of the most suitable level of theory.

A combined theoretical and experimental study on the mechanism of spiro-adamantyl-1,2-dioxetanone decomposition

1,2-Dioxetanones have been considered as model compounds for bioluminescence processes. Theunimolecular decomposition of these prototypes leads mainly to the formation of triplet excited stateswher ...

Advances in computational photochemistry and chemiluminescence of biological and nanotechnological molecules

Recent advances (2014–2015) in computational photochemistry and chemiluminescence derive from the development of theory and from the application of state-of-the-art and new methodology to challenging electronic-structure problems. Method developments have mainly focused, first, on the improvement of approximate and cheap methods to provide a better description of non-adiabatic processes, second, on the modification of accurate methods in order to decrease the computation time and, finally, on dynamics approaches able to provide information that can be directly compared with experimental data, such as yields and lifetimes. Applications of the ab initio quantum-chemistry methods have given rise to relevant findings in distinct fields of the excited-state chemistry. We briefly summarise, in this chapter, the achievements on photochemical mechanisms and chemically-induced excited-state phenomena of interest in biology and nanotechnology.

Ab initio quantum mechanical calculation of the reaction probability for the Cl- + PH2Cl -> ClPH2 + Cl- reaction

Abstract The SN2 substitution reactions at phosphorus play a key role in organic and biological processes. Quantum molecular dynamics simulations have been performed to study the prototype reaction Cl - + PH 2 Cl → ClPH 2 + Cl - , using one and two-dimensional models. A potential energy surface, showing an energy well for a transition complex, was generated using ab initio electronic structure calculations. The one-dimensional model is essentially reflection free, whereas the more realistic two-dimensional model displays involved resonance structures in the reaction probability. The reaction rate is almost two orders of magnitude smaller for the two-dimensional compared to the one-dimensional model. Energetic errors in the potential energy surface is estimated to affect the rate by only a factor of two. This shows that for these types of reactions it is more important to increase the dimensionality of the modeling than to increase the accuracy of the electronic structure calculation.

Ab initio quantum mechanical calculation of the reaction probability for the reaction

Abstract The SN2 substitution reactions at phosphorus play a key role in organic and biological processes. Quantum molecular dynamics simulations have been performed to study the prototype reaction Cl - + PH 2 Cl → ClPH 2 + Cl - , using one and two-dimensional models. A potential energy surface, showing an energy well for a transition complex, was generated using ab initio electronic structure calculations. The one-dimensional model is essentially reflection free, whereas the more realistic two-dimensional model displays involved resonance structures in the reaction probability. The reaction rate is almost two orders of magnitude smaller for the two-dimensional compared to the one-dimensional model. Energetic errors in the potential energy surface is estimated to affect the rate by only a factor of two. This shows that for these types of reactions it is more important to increase the dimensionality of the modeling than to increase the accuracy of the electronic structure calculation.

Ab initio quantum mechanical calculation of the reaction probability for the Cl-+PH2Cl→ClPH2+Cl-Cl-+PH2Cl→ClPH2+Cl- reaction

Abstract The SN2 substitution reactions at phosphorus play a key role in organic and biological processes. Quantum molecular dynamics simulations have been performed to study the prototype reaction Cl - + PH 2 Cl → ClPH 2 + Cl - , using one and two-dimensional models. A potential energy surface, showing an energy well for a transition complex, was generated using ab initio electronic structure calculations. The one-dimensional model is essentially reflection free, whereas the more realistic two-dimensional model displays involved resonance structures in the reaction probability. The reaction rate is almost two orders of magnitude smaller for the two-dimensional compared to the one-dimensional model. Energetic errors in the potential energy surface is estimated to affect the rate by only a factor of two. This shows that for these types of reactions it is more important to increase the dimensionality of the modeling than to increase the accuracy of the electronic structure calculation.