Kaline Coutinho

Bio: Kaline Coutinho is an academic researcher from University of São Paulo. The author has contributed to research in topics: Solvation & Solvent effects. The author has an hindex of 31, co-authored 128 publications receiving 3167 citations. Previous affiliations of Kaline Coutinho include Universidade de Mogi das Cruzes & Andrés Bello National University.

Solvent effects in emission spectroscopy: A Monte Carlo quantum mechanics study of the n←π* shift of formaldehyde in water

Abstract: Supermolecular calculations that treat both the solute and the solvent quantum-mechanically are performed to analyze the solvatochromism of the first emission transition of formaldehyde in water. The liquid structures are generated by NVT Metropolis Monte Carlo simulation assuming a fully relaxed excited state. The autocorrelation function is calculated to obtain an efficient ensemble average. A detailed analysis of the hydrogen bonds and their contribution to the solvation shift is presented. On average, 0.7 hydrogen bonds are formed in the excited state, about three times less than in the ground state. Quantum-mechanical calculations using the intermediate neglect of differential overlap with singly excited configuration interaction (INDO/CIS) are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The third solvation shell extends up to 10 A from the center of mass of formaldehyde, showing the very long-range effects on the solvation shift of this polar molecule. The largest cluster includes one formaldehyde and 142 water molecules. INDO/CIS calculations are performed on this cluster with a properly antisymmetric reference ground state wave function involving all valence electrons. The estimated limit value for the solvatochromic shift of the n-π* emission transition of fully relaxed formaldehyde in water, compared to the gas phase, is ≈1650 cm−1. The total Stokes shift of formaldehyde in water is calculated as ≈550 cm−1.

An efficient statistically converged average configuration for solvent effects

Abstract: Using statistically uncorrelated solute–solvent configurations generated by Monte Carlo simulation a simpler and efficient implementation of the averaged solvent electrostatic potential is made. An average configuration alone is used such that one single quantum mechanical calculation reproduces the converged statistical average obtained from the entire simulation. Applications are presented for solvent effects in a variety of properties of acetone and aminopurine in water. In all cases, excellent agreement is obtained using the average configuration and the average from the full statistical distribution.

Solvent Effects from a Sequential Monte Carlo - Quantum Mechanical Approach

Abstract: An approach based on the sequential use of Monte Carlo simulation and Quantum Mechanics is suggested for the treatment of solvent effects with special attention to solvatochromic shifts. The basic idea is to treat the solute, the solvent and its interaction by quantum mechanics. This is a totally discrete model that avoids the use of a dielectric continuum. Statistical analysis is used to obtain uncorrelated structures. The radial distribution function is used to determine the solvation shells. Quantum mechanical calculations are then performed in supermolecular structures and the spectral shifts are obtained using ensemble average. Attention is also given to the case of specific hydrogen bond between the solute and solvent.

A Monte Carlo-quantum mechanics study of the solvatochromic shifts of the lowest transition of benzene

Abstract: We examine the spectroscopic red shifts that occur when benzene is dissolved in (liquid) benzene, in cyclohexane, in carbon tetrachloride, and in water. For this we develop a mixed classical/quantum model in which uncorrelated structures are obtained from Monte Carlo simulation, and these structures are then used for quantum chemical calculations including the chromophore and all solvent molecules within the first radial distribution maxima. We discuss the effects of different sampling techniques and the inclusion of more, or less, solvent molecules in the quantum chemical supermolecule calculation. We obtain shifts of −306 cm−1, −268 cm−1, −456 cm−1, and −122 cm−1, in excellent agreement with the experimentally observed shifts of −332 cm−1, −308 cm−1, −458 cm−1, and −143 cm−1, respectively. We note that the larger shift observed in carbon tetrachloride that is not expected on the basis of larger dielectric constant results from small contributions of the charge transfer type from solvent to solute.

Solvent effects on the UV-visible absorption spectrum of benzophenone in water: A combined Monte Carlo quantum mechanics study including solute polarization

Abstract: The entire ultraviolet-visible absorption spectrum of benzophenone in water is studied and compared with the same spectrum in gas phase. Five transitions are considered, and the corresponding solvatochromic shifts are obtained and compared to experiment. Using a sequential procedure of Monte Carlo simulations and quantum mechanical calculations, liquid configurations were generated and an averaged spectrum of the solution was calculated. The solute polarization was included by an iterative procedure where the atomic charges of the solute were obtained as an average with the solvent distribution. The calculated average dipole moment of benzophenone in water, with MP26-31++G(d,p), converges to the value of 5.84+/-0.05 D, 88% larger than the gas-phase value of 3.11 D. Using 100 statistically uncorrelated configurations and solvation shells with 235 explicit water molecules selected by a minimum-distance distribution of solvent shells, instead of the usual radial distribution, the excitation energies were obtained from solute-solvent all-valence-electron INDO/CIS calculations. The shift of the weak n-pi(*) transition is obtained as 2045+/-40 cm(-1) and the strong and broad pi-pi(*) shift as -1790+/-30 cm(-1). These results are in good agreement with the experimental values of 2200 and -1600 cm(-1), respectively. Standard procedure used by common force fields to generate atomic charges to describe the electrostatic moments of the solute, with HF6-31G(d), gives a dipole moment of 3.64 D. Using these standard charges in the simulation, the average shifts are calculated as 1395+/-35 and -1220+/-25 cm(-1), both about 600 cm(-1) smaller in magnitude than those obtained with the average converged fully polarized solute. The influence of the solute polarization in the solute-solvent interaction and, in particular, in solute-solvent hydrogen bonds is analyzed.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Quantum mechanical continuum solvation models.

Abstract: 6.2.2. Definition of Effective Properties 3064 6.3. Response Properties to Magnetic Fields 3066 6.3.1. Nuclear Shielding 3066 6.3.2. Indirect Spin−Spin Coupling 3067 6.3.3. EPR Parameters 3068 6.4. Properties of Chiral Systems 3069 6.4.1. Electronic Circular Dichroism (ECD) 3069 6.4.2. Optical Rotation (OR) 3069 6.4.3. VCD and VROA 3070 7. Continuum and Discrete Models 3071 7.1. Continuum Methods within MD and MC Simulations 3072

New developments in the polarizable continuum model for quantum mechanical and classical calculations on molecules in solution

Abstract: The polarizable continuum model (PCM), used for the calculation of molecular energies, structures, and properties in liquid solution has been deeply revised, in order to extend its range of applications and to improve its accuracy. The main changes effect the definition of solute cavities, of solvation charges and of the PCM operator added to the molecular Hamiltonian, as well as the calculation of energy gradients, to be used in geometry optimizations. The procedure can be equally applied to quantum mechanical and to classical calculations; as shown also with a number of numerical tests, this PCM formulation is very efficient and reliable. It can also be applied to very large solutes, since all the bottlenecks have been eliminated to obtain a procedure whose time and memory requirements scale linearly with solute size. The present procedure can be used to compute solvent effects at a number of different levels of theory on almost all the chemical systems which can be studied in vacuo.

Time-dependent density functional theory for molecules in liquid solutions

Abstract: A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular polarizabilities and of electronic transition energies. The solute-solvent operator has been derived from a PCM approach depending on solute electrostatic potential: Recently, it has been shown that such an approach also provides an excellent treatment of the solute electronic charge lying far from the nuclei, being particularly reliable for this kind of applications. The method has been tested for formaldehyde in water and in diethyl-ether, and then applied to the calculation of solvent effects on the n→π* transition of diazabenzenes in different solvents. The computed transition energies are in fairly...

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg