THEORETICAL STUDIES OF LIQUID HYDROGEN FLUORIDE AND THE SOLVATION OF CARBONIUM IONS
Abstract
Monte Carlo simulations for the planar and perpendicular allyl cation in liquid hydrogen fluoride have been carried out in the NPT ensemble at 1 atm and 0(DEGREES)C. The intermolecular potential functions for the solute-solvent interactions have been developed from ab initio molecular orbital calculations with the split valence 4-31G basis set. The two conformers are represented with different charge distributions, but with the same Lennard-Jones parameters. The solvent-solvent interactions are described by a function of the TIPS form including one Lennard-Jones term and three charged sites for each HF monomer. Detailed structural and thermodynamic results on the solvation of localized and delocalized carbonium ions were obtained. In particular, the planar cation interacts with the liquid primarily through hydrogen bonds, while strong coordination of two HF monomers to the formally charged carbon in the perpendicular conformer is striking. Insight into solvent effects on the relative energies of isomeric carbonium ions were also obtained. Consistent with traditional electrostatic concepts, the more localized, perpendicular conformer is found to be better solvated, thereby reducing the rotational barrier by ca. 21 (+OR-) 6 kcal/mol. This result is also in qualitative accord with recent analyses of NMR data for substituted allyl cations under stable ion conditions. As a prerequisite for the dilute solution simulations, it was necessary to carry out extensive investigations of pure liquid HF and of carbonium ion-FH interactions. Information was also obtained on other topics including hydrogen bonding to carbonium ions, three-body effects for ion-molecule complexes, and the influence of one and two HF molecules on the rotational barrier for the allyl cation.
Degree
Ph.D.
Subject Area
Organic chemistry
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