Exploiting Weak Noncovalent Cation···π Interaction for Designing a Molecular Container for Storage of Methane Molecules with Lithiated Carbene Superbases

摘要

This study reports the multiprotonation sites exploiting noncovalent interactions to achieve hyperbasicity of an uncharged organic base. A new class of carbene based organic superbase has been designed to attain the proton affinity (PA) value of 301.9 kcal/mol at M06-2X/6- 311+G**//B3LYP/6-31+G* level of theory. These designed scaffold molecules can possess three independent protonation sites, which is not available in the literature to date. The stabilization of the protonated forms of the superbases by noncovalent interactions such as C—H~+ ···π interaction is responsible for augmenting the basicity of such carbene systems. The DFT calculated results show that such carbene systems can be suitable candidate for tris-protonation with pK_a(MeCN) value of 43.3 and proton affinity value (MeCN) of 312.1 kcal/mol. Molecular electrostatic potential (MESP) analysis shows the existence of electron density at the reactive sites and the role of substituents to enhance the electron density in such designed systems. There is a good correlation with the absolute minima of the MESP (V_(min)) located for such carbene systems with their calculated proton affinity values. The frontier molecular orbital energy of such carbenes also shows a good relationship with their proton affinity results. These carbene systems can be utilized further for selective binding of lithium ions over sodium ions. Such lithiated organic superbases can be used as a molecular container for the storage of hydrogen and methane molecules. The energy decomposition analysis (EDA) has been performed to explain the role of various factors which contribute to the total binding strength of the hydrogen or methane molecules with lithiated carbene superbases. The localized molecular orbital energy decomposition analysis reveals that the electrostatic and polarization effects are the major contributing factors in interactions with methane and hydrogen molecules. This is the first report where the organic superbase has been exploited as efficient methane storage materials.