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Ab initio studies of fluorocarbons: Barriers to internal rotation, hydrogen-bonding, and intermolecular potentials
Abstract
A systematic ab initio study of some environmentally related fluorocarbons is presented. Molecular orbital theory (MO) and density functional theory (DFT) are used to examine barriers to internal rotations, hydrogen bonding, and intermolecular potentials in these compounds. The structures and vibrational frequencies for eclipsed and staggered conformers of CF2HCH3, CFCFH2, C2F 5H, and C2F6 are calculated by using the B3LYR HF, and MP2 methods. A reassignment of the experimental IR spectra of CF 3CFH2, and C2F6 is suggested. Electron correlation is included by means of Møller-Plesset perturbation methods up to the fourth order. Vibrational frequencies are used to account for zero-point energy and thermal contributions to electronic energies. The dependence of the rotational barriers with temperature is studied at 20 K intervals from 0 to 300 K. Ab initio calculations are also used to provide information on the mixed dimers and cyclic trimers of the methanol-trifluoromethanol system along with the monomers and corresponding dimers and trimers. A systematic study of the dimer and cyclic trimer of trifluoromethanol is also conducted at the Hartree-Fock level. The presence of a secondary hydrogen-bonding interaction in the trifluoromethanol dimer is discussed. An extensive and systematic evaluation of cooperative effects in the various systems is provided. Finally, the intermolecular potential energy surface of the CF4 -CF4 system is explored by means of ab initio molecular orbital calculations at the HF and MP2 level of theory. A number of progressively larger basis sets (up to 6-311+G(3d)) are used in order to evaluate the basis set effect. Basis set superposition error (BSSE) is very large for all basis sets considered. The HF energy component of the total interaction energy is somewhat affected by the choice of the basis set. The dispersion energy component of the interaction energy is greatly changed with the choice of the basis set. The Lennard-Jones and the Buckingham potential energy functions are fitted to the ab initio MP2/6-31+G(3d) interaction energies for nine different orientations and a total of 130 configurations.
Subject Area
Physical chemistry|Chemistry
Recommended Citation
Parra, Ruben Dario, "Ab initio studies of fluorocarbons: Barriers to internal rotation, hydrogen-bonding, and intermolecular potentials" (1999). ETD collection for University of Nebraska-Lincoln. AAI9942144.
https://digitalcommons.unl.edu/dissertations/AAI9942144