67,560 research outputs found
Poster P15: Excited states of molecular solutes with Quantum Monte Carlo: vertical transition and geometry optimization
No full textRecently, we have developed a novel Polarizable Continuum Model (PCM), which includes both surface and volume polarization of the dielectric medium (pure SVPE scheme), designed for the Quantum Monte Carlo (QMC) treatment of the solute. In particular, the treatment of volume polarization, due to quantum mechanical penetration of the solute charge density in the solvent domain, is based on quantum Monte Carlo techniques. The method allows to accurately solve Poisson's equation of the solvation model coupled with the Schrödinger equation for the solute [1,2,3]. The present model has been now extended to treat the effects of solvation in solute vertical electronic transitions and to the search of the solute equilibrium geometry in the excited states. For the first case, here we show the results of our study
performed on fast n → pi* and pi → pi* vertical transitions of s-trans- acrolein in water [4]. To perform calculations in a non-equilibrium solvation regime for the solute excited state, we have added a correction to the global dielectric polarization charge density, obtained self consistently with the solute ground-state wave function by assuming a linear-response scheme. The calculated solvatochromic shifts are properly described. For the second case, we start from recent
advances made to carry out the ground- and excited-state geometry optimization within QMC [5]. For the present purpose, we have extended the calculation of the forces to include solvent e_ects through our QMC implementation of PCM [6]. We show results, performed at the variational Monte Carlo level, on the excited-state geometry optimization of some small organic molecules in water solution and we make a comparison with the more widely used TDDFT and CASPT2 methods.
[1] C. Amovilli, C. Filippi, F. M. Floris, J. Phys. Chem. B (2006) 110 26225.
[2] C. Amovilli, C. Filippi, F. M. Floris, J. Chem. Phys. (2008) 129 244106.
[3] F. M. Floris, C. Filippi, C. Amovilli, J. Chem. Phys. (2012) 137 075102.
[4] F. M. Floris, C. Filippi, C. Amovilli, J. Chem. Phys. (2014) 140 034109.
[5] R. Guareschi, C. Filippi, J. Chem. Theor. Comput. (2013) 9 5513.
[6] R. Guareschi, F. M. Floris, C. Amovilli, C. Filippi, in preparation (2014)
Substituent effects on the stereochemistry of gas-phase intracomplex nucleophilic substitutions
No full textThe mechanism and stereochemistry of the intracomplex solvolysis of proton-bound complexes [Y center dot center dot center dot H center dot center dot center dot M](+) between M = (CH3OH)-O-18 and Y = 1-arylethanol [(S)-1-(para-tolyl)ethanol (1(S)), (S)-1-(para-chlorophenyl)ethanol (2(S)), (S)-1-(meta-alpha,alpha,alpha-trifluoromethylphenyl)ethanol (3(S)), (S)-1-(Para-alpha,alpha,alpha-trifluoromethylphenyl)-ethanol (4(S)), (R)-1-(pentafluorophenyl)ethanol (5(R)), (R)-alpha-(trifluoromethyl)benzyl alcohol (6(R)), and (R)-1-phenylethanol (7(R))] have been investigated in the gas phase (CH3F; 720 Torr) in the 25-140 degrees C temperature range. Gas-phase solvolysis of [Y center dot center dot center dot H center dot center dot center dot M](+) (Y = 2(S), 3(S), 4(S), and 7(R)) leads to extensive racemization above a characteristic temperature t(#) (e.g. at t(#) > 60 degrees C for 7(R)), whereas below that temperature the reaction displays a preferential retention of configuration. Predominant retention of configuration is instead observed in the intracomplex solvolysis of [Y center dot center dot center dot H center dot center dot center dot M](+) (Y = 1(S), 4(S), 5(R), and 6(R)) with the temperature range investigated (25 <= T <= 120 degrees C). These results indicate that the intracomplex solvolysis proceeds through the intermediacy of the relevant benzylic cations (Bz(+)), which is electrostatically coordinated to a H2O and a (CH3OH)-O-18 molecule (a pure S(N)1 mechanism). The obtained gas-phase mechanism is discussed in the light of related solution data. It is concluded that the stereochemistry of unimolecular solvolytic reactions is determined by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature and the dynamics of the solvent cage as well
Enantioselective supramolecular devices in the gas phase. Resorcin[4] arene as a model system
No full textFraschetti C, Letzel M, Filippi A, Speranza M, Mattay J. Enantioselective supramolecular devices in the gas phase. Resorcin[4] arene as a model system. Beilstein Journal of Organic Chemistry. 2012;8:539-550.This review describes the state-of-art in the field of the gas-phase reactivity of diastereomeric complexes formed between a chiral artificial receptor and a biologically active molecule. The presented experimental approach is a ligand-displacement reaction carried out in a nano ESI-FT-ICR instrument, supported by a thermodynamic MS-study and molecular-mechanics and molecular-dynamics (MM/MD) computational techniques. The noncovalent ion-molecule complexes are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects
fMRI of Memory
No full textNumerous fMRI studies have investigated the network of brain regions critical for memory. While neuropsychological techniques can delineate brain regions that are necessary for intact memory function, neuroimaging techniques can be used to investigate which regions are recruited during healthy memory formation, storage, and retrieval. For example, fMRI studies have shown that lateral prefrontal cortex (PFC) supports some components of working memory function. However, working memory is not localized to a single brain region but is likely a property of the functional interaction between the PFC and posterior brain regions. The medial temporal lobe (MTL) and its connections with neocortical, prefrontal, and limbic structures are implicated in episodic memory. Semantic memory is mediated by a network of neocortical structures, including lateral and anterior temporal lobes, and inferior frontal cortex, possibly to a greater extent in the left hemisphere. Memory for semantic information benefits from the MTL for only a limited time, and can be acquired, albeit slowly and with difficulty, without it. To date, most of the emphasis has been on exploring the unique aspects of these different types of memory. Some evidence, however, of functional overlap in general retrieval processes does exist
Sustainable building equipment: an excursus through main environmental performance rating systems. Part II: Indoor Environmental Quality & Quality of Service
No full textSustainable building equipment: an excursus through main environmental performance rating systems. Part I: Resource Consumption & Environmental Loadings
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Diffusion Monte Carlo method with lattice regularization
No full textWe introduce an efficient lattice regularization scheme for quantum Monte Carlo calculations of realistic electronic systems. The kinetic term is discretized by a finite difference Laplacian with two mesh sizes, a and a', chosen so that the electrons can diffuse in a configuration space which is in practice indistinguishable from the continuum, and the different length scales in the system can be efficiently taken in account. The regularized Hamiltonian goes to the continuous limit for a -> 0 and allows the inclusion of nonlocal potentials in a consistent variational scheme, substantially improving the accuracy upon previous nonvariational approaches
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