A Noncovalent Binding Strategy to Capture Noble Gases, Hydrogen and Nitrogen

Abstract

A molecular design strategy to develop receptor systems for the entrapment of noble gases, H2 and N2 is described using M06L-D3/6-31111G(d,p)//M06L/6-31111G(d,p) DFT method. These receptors made with two-, three-, four- and fivefluorinated benzene cores, linked with methelene units viz. RI, RII, RIII and RIV as well as the corresponding non-fluorinated hydrocarbons viz. RIH, RIIH, RIIIH and RIVH show a steady and significant increase in binding energy (Eint) with increase in the number of aromatic rings in the receptor. A stabilizing “cage effect” is observed in the cyclophane type receptors RIV and RIVH which is 26–48% of total Eint for all except the larger sized Kr, Xe and N2 complexes. Eint of RIV. . .He, RIV. . .Ne, RIV. . .Ar, RIV. . .Kr, RIV. . .H2 and RIV. . .N2 is 4.89, 7.03, 6.49, 6.19, 8.57 and 8.17 kcal/mol, respectively which is 5- to9-fold higher than that of hexafluorobenzene. Similarly, compared to benzene, multiple fold increase in Eint is observed for RIVH receptors with noble gases, H2 and N2. Fluorination of the aromatic core has no significant impact on Eint ( 60.5 kcal/mol) for most of the systems with a notable exception of the cage receptor RIV for N2 where fluorination improves Eint by 1.61 kcal/mol. The Eint of the cage receptors may be projected as one of the highest interaction energy ranges reported for noble gases, H2 and N2 for a neutral carbon framework. Synthesis of such systems is promising in the study of molecules in confined environment.