Ion association of alkali and alkaline earth metal azides in dimethylsulfoxide.: Infrared spectrometry and ab initio 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 nu(3) vibration of N-3(-) 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 nu(1) vibration of N-3(-) at 1315 cm(-1) becomes allowed in ion pairs. Delta nu(3) and Delta nu(1) 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 N-3(-) and the anisotropy of polarizability are measured by refractive index and by Rayleigh diffusion measurements. From these results the integrated intensity of the nu(1) band is interpreted by calculating the variation of dipole moments induced by the Mz+ in N-3(-). 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 N-3(-), unsolvated ion pairs LiN3, NaN3 and KN3, solvent shared ion pair LiN3.3H(2)O and triple anion N3LiF-.