Symmetry breaking of the triiodide ion in acetonitrile solution

The triiodide ion shows evidence of symmetry breaking due to spontaneous polarisation and distortion when dissolved in hydrogen-bonding solvents, but in polar solvents such as acetonitrile with no hydrogen bonds the symmetry appears to be D-infinityh. In this Letter we use Monte Carlo simulations to show that there is a symmetry breaking in model triiodide ion in acetonitrile solution but the barrier between the two equivalent forms is less than kT. This result is contrasted with our previous work on aqueous solutions. The instantaneous vibrational frequencies of the triiodide ion in acetonitrile solution were determined. In agreement with experiment we find that the mean value of the antisymmetric mode is less shifted to the blue and the spread of frequencies is less than in aqueous solution. (C) 2001 Elsevier Science B,V. All rights reserved.