A model for the description of the electronic ground state of the triiodide; ion in solution is developed. It is based on the "diatomics in molecules" technique and is parametrized from experimental data. The solvent molecules are treated by classical intermolecular potentials. The solvent-ion interaction, which depends on the instantaneous positions of the solvent molecules, enters into the Hamiltonian matrix elements as a spatially varying external electrostatic potential. We use the model to investigate the distribution of the bond lengths of a linear triiodide ion in water at 300 K using Monte Carlo calculations. We find that under these conditions the molecule is significantly distorted with considerable redistribution of charge and bond lengths of 2.95 Angstrom and 3.38 Angstrom. The free energy barrier to switching bond lengths at room temperature is quite high (of the order of 10 kT) so that the distortion is predicted to have a long lifetime. The distribution of instantaneous vibrational frequencies is investigated and shows that the solvent has a greater effect on the frequency of the antisymmetric stretch than on that of the symmetric stretch vibration.