16 research outputs found
Solving the CH riddle: the fundamental role of spin to explain metastable anionic methane
When atoms or molecules exist in the form of stable negative ions, they play
a crucial role in the gas phase chemistry. Determining the existence of such an
ion, its internal energy and its stability are necessary prerequisites to
analyze the role of this ion in a particular medium. Experimental evidence of
the existence of a negative methane ion CH has been provided over a
period of 50 years. However, quantum chemistry had not been able to explain its
existence, and a detailed recent study has shown that the experimentally
observed species cannot be described by the attachement of an electron in the
ground state of CH. Here we describe CH as being a metastable
species in its lowest quartet spin state and we find that this species is a
CH-:H exciplex with three open shells, lying 5.8 eV above the methane
singlet ground state but slightly below the dissociation fragments. The
formation of charged exciplexes is a novel mechanism to explain small molecular
anions with implications in a plethora of basic and applied research fields
Coupling a polarizable water model to the hydrated ionâwater interaction potential: A test on the Cr3+ hydration
A strategy to build interaction potentials for describing ionic hydration of highly charged monoatomic cations by computer simulations, including the polarizable character of the solvent, is proposed. The method is based on the hydrated ion concept that has been previously tested for the case of Cr3+ aqueous solutions [J. Phys. Chem. 100, 11748 (1996)]. In the present work, the interaction potential of [Cr(H2O6)]3+ with water has been adapted to a water model that accounts for the polarizable character of the solvent by means of a mobile charge harmonic oscillator representation (MCHO model) [J. Chem. Phys. 93, 6448 (1990)]. Monte Carlo simulations of the Cr3+ hexahydrate plus 512 water molecules have been performed to study the energetics and structure of the ionic solution. The results show a significant improvement in the estimate of the hydration enthalpy [ LlHhydr(Cr3+)=-1109.6:±70 kcal/mol] that now matches the experimental value within the uncertainty of this magnitude. The use of the polarizable water model lowers by ïżœ140 kcal/mol the statistical estimation of the [Cr(H2O6)]3+ hydration enthalpy compared to the nonpolarizable model. (-573 kcal/mol for the polarizable model vs -714 kcal/mol for the nonpolarizable one.) This improvement reflects a more accurate treatment of the many-body nonadditive effects.DirecciĂłn General de Investigaciones CientĂfica y TĂ©cnica PB95-0549DGAPA-UNAM ES-112896CONACyT L004-
Liquid methanol Monte Carlo simulations with a refined potential which includes polarizability, nonadditivity, and intramolecular relaxation
Monte Carlo simulations of liquid methanol were performed using a refined ab initio derived potential which includes polarizability, nonadditivity, and intramolecular relaxation. The results present good agreement between the energetic and structural properties predicted by the model and those predicted by ab initio calculations of methanol clusters and experimental values of gas and condensed phases. The molecular level picture of methanol shows the existence of both rings and linear polymers in the methanol liquid phase
Ab Initio Study of the Hydrolysis Reactions of Neutral and Anionic MgâPyrophosphate Complexes in the Gas Phase
A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence
Monte Carlo and molecular dynamics simulations were done with three recent water models TIP4P/2005 (Transferable Intermolecular Potential with 4 Points/2005), TIP4P/Ice (Transferable Intermolecular Potential with 4 Points/ Ice) and TIP4Q (Transferable Intermolecular Potential with 4 charges) combined with two models for methane: an all-atom one OPLS-AA (Optimal Parametrization for the Liquid State) and a united-atom one (UA); a correction for the CâO interaction was applied to the latter and used in a third set of simulations. The models were validated by comparison to experimental values of the free energy of hydration at 280, 300, 330 and 370 K, all under a pressure of 1 bar, and to the experimental radial distribution functions at 277, 283 and 291 K, under a pressure of 145 bar. Regardless of the combination rules used for ÏC,O, good agreement was found, except when the correction to the UA model was applied. Thus, further simulations of the sI hydrate were performed with the united-atom model to compare the thermal expansivity to the experiment. A final set of simulations was done with the UA methane model and the three water models, to study the sI hydrate-liquid water-gas coexistence at 80, 230 and 400 bar. The melting temperatures were compared to the experimental values. The results show the need to perform simulations with various different models to attain a reliable and robust molecular image of the systems of interest
An application of flexible constraints in Monte Carlo simulations of the isobaric-isothermal ensemble of liquid water and ice Ih with the polarizable and flexible mobile charge densities in harmonic oscillators model
The method of flexible constraints was implemented in a Monte Carlo code to perform numerical simulations of liquid water and ice Ih in the constant number of molecules, volume, and temperature and constant pressure, instead of volume ensembles, using the polarizable and flexible mobile charge densities in harmonic oscillators (MCDHO) model. The structural and energetic results for the liquid at T=298âK and Ï=997âkgâmâ3 were in good agreement with those obtained from molecular dynamics. The density obtained at P=1âatm with flexible constraints, Ï=1008âkgâmâ3, was slightly lower than with the classical sampling of the intramolecular vibrations, Ï=1010âkgâmâ3. The comparison of the structures and energies found for water hexamers and for ice Ih with six standard empirical models to those obtained with MCDHO, show this latter to perform better in describing water far from ambient conditions: the MCDHO minimum lattice energy, density, and lattice constants were in good agreement with experiment. The average â HOH of the water molecule in ice was predicted to be slightly larger than in the liquid, yet 1.2% smaller than the experimental value
Flexible constraints: An adiabatic treatment of quantum degrees of freedom, with application to the flexible and polarizable mobile charge densities in harmonic oscillators model for water
In classical molecular simulations chemical bonds and bond angles have been modeled either as rigid constraints, or as nearly harmonic oscillators. However, neither model is a good description of a chemical bond, which is a quantum oscillator that in most cases occupies the ground state only. A quantum oscillator in the ground state can be represented more faithfully by a flexible constraint. This means that the constraint length adapts itself, in time, to the environment, such that the rotational and potential forces on the constraint cancel out. An accurate algorithm for flexible constraints is presented in this work and applied to study liquid water with the flexible and the polarizable âmobile charge densities in harmonic oscillatorsâ model. The iterations for the flexible constraints are done simultaneously with those for the electronic polarization, resulting in negligible additional computational costs. A comparison with fully flexible and rigidly constrained simulations shows little effect on structure and energetics of the liquid, while the dynamics is somewhat faster with flexible constraints
A Four-Site Molecular Model for Simulations of Liquid Methanol and WaterâMethanol Mixtures: MeOH-4P
In
this work, we present a new four-site potential for methanol,
MeOH-4P, fitted to reproduce the dielectric constant Δ, the
surface tension Îł<sub><i>s</i></sub>, and the liquid
density Ï of the pure liquid at <i>T</i> = 298.15
K and <i>p</i> = 1 bar. The partial charges on each site
were taken from the OPLS/2016 model with the only difference of putting
the negative charge on the fourth site (<i>M</i>) instead
of on the O atom, as done in four-site water models. The original
Lennard-Jones (LJ) parameters of OPLS/2016 for the methyl moiety (Me)
were modified for the fitting of Ï and Îł<sub><i>s</i></sub>, whereas the parameters of the TIP4P-FB water model were used
for the O atom without change. Taking into account the energetic cost
of the enhanced dipole relative to the isolated molecule, the results
from simulations with this model showed good agreement with experiments
for Ï, α<sub><i>p</i></sub>, Îș<sub><i>T</i></sub>, <i>C</i><sub><i>p</i></sub>, and Î<i>H</i><sub><i>v</i>â<i>l</i></sub>. Also, the temperature dependence of Îł<sub><i>s</i></sub> and Δ is satisfactory in the interval
between 260 and 360 K, and the critical point description is similar
to that of OPLS/2016. It is shown that orientational correlations,
described by the Kirkwood factor <i>G</i><sub><i>k</i></sub>, play a prominent role in the appropriate description of dielectric
constants in existing models; unfortunately, the enhancement of the
dipole moment produced a low diffusion coefficient <i>D</i><sub>MeOH</sub>; thus, a compromise was required between a good reproduction
of Δ and an acceptable <i>D</i><sub>MeOH</sub>. The
use of a fourth site resulted in a significant improvement for waterâmethanol
mixtures described with TIP4P-FB and MeOH-4P, respectively, but required
the modification of the LJ geometric combination rule to allow a good
description of the methanol molar-fraction dependence of Ï,
Δ, and methanol (water) diffusion coefficients <i>D</i><sub>MeOH</sub> (<i>D</i><sub>H<sub>2</sub>O</sub>) and
excess volume of mixing Î<i>V</i><sub>mix</sub> in
the entire range of composition. The resulting free energy of hydration
Î<i>G</i><sub>hyd</sub> shows excellent agreement
with experiments in the interval between 280 and 360 K