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

The 1:1 species formed in dimethylsulfoxide (DMSO) solution between the azide anion and alkaline and alkaline earth metal cations Mz+ 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 Mz+ 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-.