The spectroscopy of phase transitions in clusters. A molecular dynamics study of infrared frequency shifts and line shapes in small acetonitrile clusters

A combination of molecular dynamics and perturbation theory has been used to investigate the changes in infrared peak shape and frequency shift molecular clusters may exhibit as they undergo a phase transition from a solidlike to a liquidlike state. Four A1 vibrational modes of the acetonitrile molecule in (CH3CN)n clusters with n in the range 2–9, have been studied as a function of kinetic temperature. The calculated response of each vibrational mode to a phase transition is quite different: in some cases neither a frequency shift nor a change in linewidth are observed and, at the other extreme, the linewidths can double and the vibrational frequencies either red- or blueshift by 2–3 cm−1 as clusters move from a solidlike to a liquidlike-state. However, sharp changes in linewidth and frequency shift at the transition temperature are only seen in the larger clusters. Certain of the calculated spectral features are found to be very sensitive to the presence of molecular rotational motion in ‘‘warm’’ solidlike clusters; an observation which could have important implications in other areas of cluster spectroscopy.