3-21G and 6-31 + G ab initio molecular orbital calculations have been performed on the (CK3CNCl- complex with three different configurations. A comparison of calculated harmonic frequencies and IR intensities of acetonitrile in the CH stretching region thus obtained in the gas phase with the experimental results for (CH3CN)X(-) systems in the liquid phase suggests that the halide ions interact with acetonitrile linearly through its CH bonds in the C-5 configuration.