Using density functional theory (DFT) methods, we have investigated two possible
mechanisms for atmospheric CO2 fixation in the cavity of the dinuclear zinc (II) octaazacryptate, and the subsequent reaction with methanol whereby this latter reaction
transforms the (essentially) chemically inert CO2 into useful products. The first mechanism (I) was proposed by Chen et al. [Chem. Asian J. 2007, 2, 710], and involves the attachment of one CO2 molecule onto the hydroxyl-cryptate form, resulting in the formation of a bicarbonate-cryptate species and subsequent reaction with one methanol molecule. In
addition, we suggest another mechanism that is initiated via the attachment of a methanol
molecule onto one of the Zn-centres, yielding a methoxy-cryptate species. The product is
used to activate a CO2 molecule and generate a methoxycarbonate-cryptate. The energy
profiles of both mechanisms were determined and we conclude that, while both mechanisms
are energetically feasible, free energy profiles suggest that the scheme proposed by Chen et al. is most likely.
