On the Asymmetry of Vibrational Energy Flow

A simple model to predict the directionality of vibrational energy flow at molecular level is presented. This model is based on a vibrational energy propagation analysis using ab intio molecular dynamics and the Fukui function and local softness reactivity indexes derived from DFT. By using this simple conceptual model we are giving a cogent rationale to previous theoretical and experimental reports of asymmetrical vibrational energy diffusion in proteins and energetic materials. We proposed here three basic rules for the Vibrational Energy Relaxation in molecules: (i) the vibrational energy flows form a soft site to a hard site but not in the opposite direction, (ii) when vibrational energy is injected directly to a hard site, it is trapped in this site, and (iii) if the vibrational energy is pump in a polarizable site and two sites with different softness are available, the energy will propagate to the softest one.