Ion association of alkali and alkaline earth metal azides in dimethylsulfoxide. Infrared spectrometry and abinitio calculations

Abstract

The 1:1 species formed in dimethylsulfoxide (DMSO) solution between the azide anion and alkaline and alkaline earth metal cations M^z+ are investigated by infrared spectrometry. The ν₃ vibration of N₃^- at 2000 cm^-1 is shifted to higher wavenumbers by ion pairing. Shift values are not significantly different from those observed in MNCO^(z-1)+ although azide ion pairs are less stable. The forbidden ν₁ vibration of N₃^- at 1315 cm^-1 becomes allowed in ion pairs. Δν₃ and Δν₁ wavenumber shifts are both proportional to the polarizing power of the cation. Longitudinal force constants are calculated by assuming that in an N₁N₂N₃M ion pair the force constant of the N₂N₃ bond is insensitive to ion pairing. The mean polarizability of N₃^- and the anisotropy of polarizability are measured by refractive index and by Rayleigh diffusion measurements. From these results the integrated intensity of the ν₁ band is interpreted by calculating the variation of dipole moments induced by the M^z+ in N₃^-. Theoretical calculations of equilibrium geometry, polarizability, wavenumbers and IR intensity are made by the DFT (Density Functional Theory) method on some typical azide species: free N₃^-, unsolvated ion pairs LiN₃, NaN₃ and KN₃, solvent shared ion pair LiN₃.3H₂O and triple anion N₃LiF^-.