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Instantaneous normal mode analysis of correlated cluster motions in hydrogen bonded liquids
We analyze the correlated motions of hydrogen bonded clusters in liquid hydrogen fluoride, methanol, and water using the Instantaneous Normal Mode approach. In the case of hydrogen fluoride and methanol, which form a topologically linear hydrogen bond network, the relevant cluster is a triplet formed by a molecule and its two neighbors. In the case of water, whose hydrogen bond structure has a local tetrahedral symmetry, the basic unit considered is the pentamer formed by a molecule and its four neighbors. For each of these clusters we identify, using symmetry arguments, the a priori modes describing the relative motions of the cluster molecules and introduce suitable projections in order to evaluate how much these modes contribute to the actual motions at different frequencies. In the case of hydrogen fluoride we confirm the assignment of a 50 rad/ps peak observed in the single and collective correlation function spectra to the stretching of the hydrogen bonded network. In the case of methanol the analysis of the correlated motions of the triplets shows that in the intermediate frequency range (around 25 rad/ps) a picture similar to what is observed in hydrogen fluoride applies, whereas the high frequency properties of the liquid (beyond 50 rad/ps) are mostly due to the asymmetric stretching motion. In the case of water we demonstrate that the a priori modes, based on the full tetrahedral symmetry of the water pentamer, do indeed mix strongly under the effect of the interaction with the neighbors. The results are related to the spectroscopic measurement.