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Theoretical Study of Ion Pair M+XO4- (M=Li, Na, K and X=F, Cl, Br, I) Structures:

2009년 8월 12일 15시 01분 14초
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목 <발표Ⅰ>
저자 및
Mohammad Harun, 최철호
경북대학교 화학과, Korea
We have investigated the structures and energetics of M+XO4- salt ion pairs (M=Li, Na, K and X=F, Cl, Br, I) using density functional theory (DFT) and ab initio calculations. The calculated all M+XO4- species have a bidentate and tridentate ground state structure and there have a general trend observed in their stability. The calculations showed that the (1) Molecular structure of alkaline metal perfluorate, M+FO4- [M=Li, Na, K] favors the tridentate structure with bidentate isomer higher in energy (2) M+ClO4-, M+BrO4- and M+IO4- ion pairs while M=Li and Na, favors the bidentate structure with tridentate association higher in energy (3) K+ClO4-, K+BrO4- molecule exists as bidentate and tridentate forms having almost similar energies and K+IO4- has only bidentate isomer. As a general trend for M+XO4- (M=Li, Na, K and X=Cl, Br, I) ion pairs we conclude, as the cation size decreases and anion size increases the bidentate structure favors with tridentate association higher in energy. The most interesting dual minima structure exists in NaClO4 and NaFO4 molecule cases. Both have four possible isomeric forms, two are bidentate and two are tridentate like and each bidentate and tridentate structure posses the dual minima (covalent and ionic type) which is a new invention. Bidenate and tridentate are almost isoenergetic, bidentate structure favours only 1 kcal/mol over tridentate one but the energy differences between this two respective dual minima of NaClO4 structures are about 35 kcal/mol by MP2/aug-cc-pVTZ and 41 kcal/mol by CCSD(T) which is quite high. Surprisingly DFT calculations with various functionals were seriously failed to predict the structures from Möller-Plesset second-order perturbation (MP2) theory and the most reliable coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)].