Ion pairs of lithium salts based on three organic anions, namely
Tf-, TFSI-, and TFSM-, have
been studied
by ab initio quantum-chemical methods. These calculations allowed
us to draw the conformational energy
diagram for TFSM- and TFSI-.
Geometries and binding energies have been evaluated at the
restricted
Hartree−Fock (RHF) level using standard basis set (3-21+G* and
6-31+G*). For the reference
Tf-−Li+
,
electron correlation effects have been determined at the MP2 level.
Wave function analyses have been
performed by the natural bond orbital (NBO) method. Bidentate
structures involving two oxygen atoms (one
of each SO2 group in the
TFSI-,Li+ and
TFSM-,Li+ systems) are preferred.
The affinities of these anions
for the lithium cation are strong and lie in the range 120−145
kcal·mol-1, arising essentially from
the
electrostatic interaction. Charge transfer contributions are less
important, only 14−17% of the electrostatic
interaction.
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