In silico design of adamantane derived organic superbases with an extended hydrogen bond network and their use as molecular containers for the storage of H2 and CO2

Abstract

Density functional (DFT) calculations predicted that amine substituted adamantane derivatives can function as organic superbases in the gas phase and in acetonitrile solution. The designed superbases have shown much higher basicity compared to the prototype DMAN. The basis set superposition error (BSSE) and ZPVE corrected proton affinity (PA) value calculated for N-methylamine system 6 with B3LYP/6-311+G(d,p) level is 261.0 kcal mol⁻¹ in the gas phase. In acetonitrile, the ZPVE corrected proton affinity was found to be 296.4 kcal mol⁻¹ with the B3LYP/6-311+G(d,p) level of theory. The role of extended hydrogen bonding networks as a strategy for enhancing the basicity of such adamantane derivatives has been explored. These superbases can have multiple protonation sites and the second proton affinity calculated for 2 in acetonitrile is 283.8 kcal mol⁻¹. The pK_a value calculated for N-methylamine superbase 6 was found to be much higher (∼27) than the pK_a calculated for DMAN (∼17.4). These designed molecules can bind lithium ions more strongly than the sodium ion. We have exploited the lithiated adamantane derivative 8 for H₂ storage and separation of CO from CO₂. The designed adamantane have bis-binding sites and can bind two lithium ions favorably. The lithium ion bound at each site can accommodate three hydrogen molecules with an average adsorption energy of −3.4 kcal mol⁻¹. The lower desorption energy (ΔE_DE) of H₂ adsorbed lithiated adamantane derivative 8 suggests good recyclable property. Such lithiated adamantane derivative 8 can also adsorb six CO₂ molecules with an average adsorption energy of −10.5 kcal mol⁻¹, which is higher than the average adsorption energy for six CO −7.40 kcal mol⁻¹.