Nature and role of the weak intermolecular bond in enantiomeric conformations of H2O2–noble gas adducts: a chiral prototypical model

Abstract

In this work the role of the weak intermolecular interaction between the hydrogen peroxide molecule (H₂O₂) and noble gas atoms Ng (Ng = He, Ne, Ar, Kr, Xe and Rn) was investigated. This is mainly important to determine the spectroscopic features of two-body interacting systems found in the bulk at temperatures confined in the thermal range. The stability of the formed adducts was inferred by their lifetimes evaluated as a function of temperature. The lifetime analysis suggests that only the H₂O₂–He complex is unstable in the 200–500 K temperature range. Moreover, exploiting the combination of several advanced theoretical methods, particular attention has been paid to characterizing the nature and anisotropy of the related intermolecular interaction, focusing our attention on the selected and representative conformations of the H₂O₂–Ng systems that were obtained in a previous study by our research group. Such conformations involve two basic structures of the enantiomeric H₂O₂ moiety and the barriers between the two chiral structures. The interaction analysis was performed using Charge Displacement (CD), Natural Bond Orbitals (NBO), and Symmetry-Adapted Perturbation Theory (SAPT). For the selected conformations, it was observed that the polarization effects were more pronounced in complexes involving Xe and Rn atoms. CD analysis shows, especially in the cis-barrier conformation, a small but non-negligible charge transfer (CT) from Ng to H₂O₂, suggesting that the CT is enhanced by the two symmetrical interactions between the two hydrogens of H₂O₂ and Ng. This stimulates the formation of a weak intermolecular hydrogen bond. The NBO analysis confirms the CD results, further showing that the largest electronic donation occurs from the valence lone pair orbitals of the Ng to the two O–H antibonding orbitals. The selectivity of CT is expected to appreciably reduce the adiabatic energy barrier height due to the torsional mode of the H₂O₂ moiety within the complex, making the torsion slightly favored with respect to the case of the isolated molecule. Finally, SAPT and non-covalent interaction (NCI) analysis indicate that all systems are essentially van der Waals complexes, where the dispersion forces determine the most important attractive contribution to the interaction.