Potential of Sumanene Modified with Boron and Nitrogen Atoms for Adsorption of Carbon Dioxide: DFT and SAPT Study ^†

Buckybowls are organic, curved and bowl-shaped molecules whose main representatives are corannulene and sumanene. Aside of specific bowl shape, these molecules are characterized by symmetry (C5 and C3, respectively) [1,2,3]. Corannulene is molecular structure known for more than five decades, since it was originally synthetized in 1966. by Lawton and Barth [4]. Sumanene (Figure 1), however, is much “younger” representative of the buckybowls, firstly synthetized in 2003. by Sakurai, Daiko and Hirao [5].

There are several differences between corannulene and sumanene. Sumanene has higher bowl depth, which is also followed by the higher charge polarization (namely, sumanene possess higher dipole moment than corannulene). Sumanene is also characterized by the presence of three benzylic carbon atoms, which enables further modifications. Owing to their specific structural, reactive and adsorption properties, buckybowls are considered as possible future sensors of greenhouse and other gases [6,7,8,9,10].

Thanks to the relatively high dipole moments, buckybowls, especially sumanene, are suitable for interactions with other molecules on the basis of electrostatic interactions. This enables the physisorption mechanism of adsorption, which allows both adsorption and desorption [2,11,12].

In this work density functional theory (DFT) and symmetry-adapted perturbation theory (SAPT) calculations have been utilized for the computational investigation of carbon dioxide (CO₂) adsorption by sumanene molecule modified with boron and nitrogen atoms. Within DFT calculations, representative functionals, known for their applicability in cases when noncovalent interactions take part, have been used for calculations of binding energies. These calculations indicate that pure sumanene molecule is able to bind from its concave side CO₂ molecule with high binding energy of around −5.50 kcal/mol. This binding energy is much higher than in case of the adsorption of hydrogen molecules by sumanene, which was calculated to be −3.25 kcal/mol and −2.86 kcal/mol, depending on the orientation of H₂ molecule with respect to the hub ring of sumanene [9]. Calculations also show that, contrary to H₂ molecules, CO₂ molecules are adsorbed in neither parallel nor normal orientation with respect to the hub ring of sumanene, no matter what is the starting orientation. Calculations also showed that modifications of sumanene with boron and nitrogen atoms influence the binding energies in both directions.

DFT calculations have been also used for detailed identification and quantification of noncovalent interactions formed between sumanene-based structures and CO₂. The analysis of electron density between sumanene derivatives and CO₂ molecules revealed that number of noncovalent interactions improves thanks to the aforementioned modifications (Figure 2).

Selected spectroscopic properties have been obtained with DFT and time-dependent DFT calculations as well, with the goal to identify the significant changes induced by the presence of CO₂ molecule. Herein we present the comparison of IR spectra for pristine sumanene and sumanene adsorbing the CO₂ molecule, Figure 3. A clear difference originating from the adsorbed CO₂ molecule can be observed.

SAPT calculations have been employed in order to decompose interaction energy into different components, which enabled us to identify the physical origins of interactions between sumanene-based structures and CO₂. Total SAPT interaction energies completely follow the trend of binding energies obtained by DFT calculations. Adsorption of CO₂ by coronene, sumanene’s planar relative and frequently employed model structure for studying adsorption of carbon materials, has also been considered by the above mentioned calculations in order to understand the effects of curvature to adsorption properties. Calculations indicate that CO₂ binds to sumanene and its derivatives with significantly higher binding energies than to coronene.