Theoretical studies on the electronic structure, charge distribution and vibrational spectra of diglyme–M+-AsF6− (M = Li, Na, K)

Electronic structure and the vibrational spectra of CH3(OCH2CH2)2OCH3–M+-AsF6− (M = Li, Na, K) have been obtained using the density functional theory. Lithium ion exhibits a pentavalent coordination via 3 oxygens from diglyme and two fluorines of AsF6− whereas Na+ and K+ exhibit coordinate number 6 with 3 fluorines of the anion binding to alkali metal in these complexes. Analysis of calculated spectra reveal that the CH2 wag (840–1120 cm−1) vibrations in the complex are sensitive to metal ion coordination. A frequency downshift relative to the free anion has been predicted for the vibrations of AsF6− anion when the fluorines are directly bonded (denoted by F*) to metal ion. Consequent reorganization of electron density in the complex engenders a frequency shift in the opposite direction for As–F vibrations wherein the fluorine atoms are not coordinating to the alkali metal ion. An approach based on the molecular electron density topography coupled with the difference electron density map explains the direction of the frequency shifts of C–O–C and the As–F stretchings compared to those of free diglyme or AsF6 anion. A new method, which includes the color-mapping function for the difference molecular electron density (MED), superimposed on the bond critical points in MED topography has been suggested to explain the direction of the frequency shifts in a single attempt.