INVESTIGATIONS OF STRUCTURE IN ELECTROLYTIC SOLUTIONS IN APROTIC SOLVENTS VIA INFRARED SPECTROSCOPY AND MONTE CARLO SIMULATIONS

DANNI LOUIS HARRIS, Purdue University

Abstract

Structure of electrolytic solutions local to anions in aprotic solvents is investigated via the digitized infrared spectra in the region corresponding to excitation of the fundamentals of the asymmetric stretching modes of the anions. The nature of the second coordination sphere ion pair is investigated in solutions of salts of the Co(CO)(,4)('-) and NO(,3)('-) anions both with and without high formation constant cation binding ligands. The latter species is reported to be a nonconducting ion pair in which the anion occupies a position opposite an interstice between solvent molecules of the first coordination sphere of the cation. This species is found in addition to contact and long range ion pairs in several aprotic solvents. Triple ions are present in small amounts in many systems studied. Spectroscopic assignments of the abovementioned species are corroborated by conductivity studies, though it is shown that conductance-concentration equations to date at best provide semi-quantitative estimates of populations of associated ionic species. Cryrogenic spectroscopic studies of ligand doped solutions of NaCo(CO)(,4) in THF and THP provide thermodynamic information about the interconversion of the three types of ion pairs found in these solvents. The latter results are compared with the models of these three species and are found to be consistent. The abovementioned experimental evidence is juxtaposed with the results of Hartree Fock calculations of LiNO(,3)/CH(,3)CN clusters, including three-body effects, to comprehend the solvent stabilization of the S.C.I.P. structure found in LiNO(,3)/CD(,3)CN solutions. It is found that three-body terms in the total energy are important factors in the stabilization of second coordination sphere ion pairs relative to contact ion pairs in this system. A pair potential is developed for interactions in Li('+)/NO(,3)('-)/CH(,3)CN systems from restricted Hartree Fock calculations on dimers using the 6-31G and 6-31G* basis sets. The (1-6-12) pair potentials are utilized in a Monte Carlo simulation utilizing preferential sampling techniques to elucidate solvent mediation of Li('+) ... NO(,3)('-) interactions in a second coordination sphere ion pair in 128 acetonitriles. These results confirm the basic hypotheses concerning the nature of this species arising from the experimental data.

Degree

Ph.D.

Subject Area

Chemistry

COinS