The structures, vibrational frequencies, infrared and Raman intensities of hexafluorophosphate anion (PF6-) and M+PF6- (M+ = Li+, Na+, K+, Rb+ and Cs+) ion pairs have been studied by ab initio, calculations. It is shown that the tridentate coordination of cation by PF6- is the most stable structure in gas phase. The vibrational spectroscopies of the most stable geometries were calculated and the changes in band position were used to probe the ion associations. (c) 2005 Elsevier Ltd. All rights reserved.