Ab Initio Studies of Hydrogen Bonding and Solvent Effects in Some Molten Salts

• Main Authors: Chih-Ku Liu, 劉芝谷
• Other Authors: Shao-Pin Wang
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/96964091801042832402

碩士 === 國立成功大學 === 化學系碩博士班 === 91 === The H-bonding in four molten salts, (EMI+BF4－), (EMI+PF6－), (BMI+BF4－), (BMI+PF6－), were first examined by two criteria including the commonly used F…H distance and the orbital interaction energies, □E(2). It is found the C(2)-H1 is the predominant H-donating site, which is consistent with related studies reported in literature. Our results, however, can be employed to account for the recent NMR studies on (EMI+BF4－), in which the relaxation times measured for both □-carbons also revealed discontinuity as phase change occurred. Further investigation performed on effects of alkyl groups (R and R’) on H-bonding has also been made. It is also found that the ring hydrogens, H6, attached to nitrogen is more favored than C(2)-H1 in formation of hydrogen bonding and hence would raise the glass transition temperature. Due to H-bonging between the fluorides and C(2)-H1, the alkyl’s hydrogen, the neighboring □- and/or the □-hydrogens on the alkyl chains would participate H-bonding depending on the length of the substituted R and R’ groups on the two ring nitrogen atoms, respectively. Therefore, the H-bonding network is more complicated than those found experimentally, which were in general carried out focused on H1. The relative capabilities in H-bonding formation of BF4－ and PF6－ anions cannot be distinctly determined in the current study, most likely resulted from the multiply-sited H-bonding network. As both R and R’ contain at least two carbons, however, PF6－ exhibits higher capability than BF4－ in formation of H-bonding with the same RR’I+ through analysis of the magnitudes of □E(2). Effects of benzene, a non-polar solvent, on (EMI+AlCl4－) has been thoroughly examined. The unexpected experimental observations can be ascribed to the orbital interactions between AlCl4－and benzene, which in return weaken the interaction between the counter ions. Analysis of solvation energies indicated that solvents with higher dielectric constants would increase the conductivity of the four above-stated salts as expected. The two protic solvents, ethanol and methanol, could lead to the same conclusion, which provide a research subject in due course.