Preferential stabilization of HeI2 van der Waals isomers: the effect of energetics and temperature

Abstract

The populations of the two different HeI conformers (linear and T-shaped) were calculated as a function of temperature using a simple thermodynamic model and the quantum mechanical partition functions for each conformer. Variational quantum calculations were performed for angular momentum values J up to 15, and by analyzing the rovibrational energies and functions, all states up to dissociation were assigned. On the basis of the vibrational and rotational partition functions calculations, it was found that the relative populations of the isomers have a strong dependence on the temperature. The population of the linear isomer (the most stable one according to the ab initio CCSD(T)/CBS potential used) decreases relative to that of the T-shaped, as the temperature increases, and at temperatures around 1 K the two populations are equal, with the T-shaped isomer being more abundant for higher temperatures. The temperature effect on the relative population was also investigated as a function of the difference in the binding energy values of the two isomers, including those determined from the experimental observations with the T-shaped being energetically most stable. In this case, even though the ratio of the T-shaped/linear populations decreases rapidly for temperatures below 1 K, the T-shaped isomer was the most abundant at all temperatures. The system evolves between both T-shaped and linear arrangements, with no significant changes at temperatures 1.5 K. The disagreement at low temperatures between theoretical predictions and experimental data available indicates that further refinement is still needed for controlling the isomers' formation, and various possible sources of errors are extensively discussed.