Ab initio and analytic intermolecular potentials for Ar-CF4

An advanced bath model to simulate association followed by ensuing dissociation dynamics of benzene + benzene system: a comparative study of gas and condensed phase results

The role of the environment (N₂ molecules) on the association followed by the ensuing dissociation reaction of benzene + benzene system is studied here with the help of a new code setup. Chemical dynamics simulations are performed to investigate this reaction in vacuum as well as in a bath of 1000 N₂ molecules, equilibrated at 300 K. Bath densities of 20 and 324 kg m^-3 are considered with a few results from the latter density. The simulations are performed at three different excitation temperatures of benzene, namely, 1000, 1500, and 2000 K, with an impact parameter range of 0-12 Å for both vacuum and bath models. Higher association probabilities and hence, higher temperature dependent association rate constants are obtained in the condensed phase. In the condensed phase, when a trajectory takes a longer time for the monomers to associate, the associated complex is formed with a longer lifetime and provides a lower rate of ensuing dissociation. Higher association rate and lower dissociation rate in condensed phase dynamics are due to the energy transfer process. Hence, the energy transfer phenomenon plays a decisive role in the association/dissociation dynamics, which is completely ignored in the same reaction when studied in vacuum.

Chemical Dynamics Simulations of Intermolecular Energy Transfer: Azulene + N2 Collisions

Chemical dynamics simulations were performed to investigate collisional energy transfer from highly vibrationally excited azulene (Az*) in a N2 bath. The intermolecular potential between Az and N2, used for the simulations, was determined from MP2/6-31+G* ab initio calculations. Az* is prepared with an 87.5 kcal/mol excitation energy by using quantum microcanonical sampling, including its 95.7 kcal/mol zero-point energy. The average energy of Az* versus time, obtained from the simulations, shows different rates of Az* deactivation depending on the N2 bath density. Using the N2 bath density and Lennard-Jones collision number, the average energy transfer per collision ⟨ΔEc⟩ was obtained for Az* as it is collisionally relaxed. By comparing ⟨ΔEc⟩ versus the bath density, the single collision limiting density was found for energy transfer. The resulting ⟨ΔEc⟩, for an 87.5 kcal/mol excitation energy, is 0.30 ± 0.01 and 0.32 ± 0.01 kcal/mol for harmonic and anharmonic Az potentials, respectively. For comparison, the experimental value is 0.57 ± 0.11 kcal/mol. During Az* relaxation there is no appreciable energy transfer to Az translation and rotation, and the energy transfer is to the N2 bath.

Intermolecular potential for binding of protonated peptide ions with perfluorinated hydrocarbon surfaces

An analytic potential energy function was developed to model both short-range and long-range interactions between protonated peptide ions and perfluorinated hydrocarbon chains. The potential function is defined as a sum of two-body potentials of the Buckingham form. The parameters of the two-body potentials were obtained by fits to intermolecular potential energy curves (IPECs) calculated for CF4, which represents the F and C atoms of a perfluoroalkane chain, interacting with small molecules chosen as representatives of the main functional groups and atoms present in protonated peptide ions: specifically, CH4, NH3, NH4(+), and HCOOH. The IPECs were calculated at the MP2/aug-cc-pVTZ level of theory, with basis set superposition error (BSSE) corrections. Good fits were obtained for an energy range extending up to about 400 kcal/mol. It is shown that the pair potentials derived from the NH3/CF4 and HCOOH/CF4 fits reproduce acceptably well the intermolecular interactions in HCONH2/CF4, which indicates that the parameters obtained for the amine and carbonyl atoms may be transferable to the corresponding atoms of the amide group. The derived potential energy function may be used in chemical dynamics simulations of collisions of peptide-H(+) ions with perfluorinated hydrocarbon surfaces.

Potential energy surfaces for small alcohol dimers I: Methanol and ethanol

Potential energy landscapes for homogeneous dimers of methanol and ethanol were calculated using counterpoise (CP) corrected energies at the MP26-311+G(2df,2pd) level. The landscapes were sampled at approximately 15 dimer separation distances for different relative monomer geometries, or routes, given in terms of a relative monomer yaw, pitch, and roll and the spherical angles between the monomer centers (taken as the C atom attached to the O). The 19 different routes studied for methanol and the 22 routes examined for ethanol include 607 CP corrected energies. Both landscapes can be adequately represented by site-site, pairwise-additive models, suitable for use in molecular simulations. A modified Morse potential is used for the individual pair interactions either with or without point charges to represent the monomer charge distribution. A slightly better representation of the methanol landscape is obtained using point charges, while the potential energy landscape of ethanol is slightly better without point charges. This latter representation may be computationally advantageous for molecular simulations because it avoids difficulties associated with long-range effects of point-charge-type models.

Theoretical Study of the Ar−, Kr−, and Xe−CH4, −CF4 Intermolecular Potential-Energy Surfaces

We present a theoretical study of the intermolecular potentials for the Ar, Kr, and Xe-CH4, -CF4 systems. The potential-energy surfaces of these systems have been calculated utilizing second-order Möller-Plesset perturbation theory and coupled-cluster theory in combination with correlation-consistent basis sets (aug-cc-pvnz; n = d, t, q). The calculations show that the stabilizing interactions between the rare gases and the molecules are slightly larger for CF4 than for CH4. Moreover, the rare-gas-CX4 (X = H, F) potentials are more attractive for Xe than for Kr and Ar. Our highest quality ab initio data (focal-point-CCSD(T) extrapolated to the complete basis set limit) have been used to develop pairwise analytical potentials for rare-gas-hydrocarbon (-fluorocarbon) systems. These potentials can be applied in classical-trajectory studies of rare gases interacting with hydrocarbon surfaces.