CO2 Activation Within a Superalkali-Doped Fullerene.

With the aim of finding a suitable synthesizable superalkali species, using the B3LYP/6-31G* density functional level of theory we provide results for the interaction between the buckminsterfullerene C60 and the superalkali Li3F2. We show that this endofullerene is stable and provides a closed environment in which the superalkali can exist and interact with CO2. It is worthwhile to mention that the optimized Li3F2 structure inside C60 is not the most stable C2v isomer found for the "free" superalkali but the D3h geometry. The binding energy at 0 K between C60 and Li3F2 (D3h) is computed to be 119 kJ mol-1. Once CO2 is introduced in the endofullerene, it is activated, and the O C O ^ angle is bent to 132°. This activation does not follow the previously studied CO2 reduction by an electron transfer process from the superalkali, but it is rather an actual reaction where a F (from Li3F2) atom is bonded to the CO2. From a thermodynamic analysis, both CO2 and the encapsulated [Li3F2⋅CO2] are destabilized in C60 with solvation energies at 0 K of 147 and < -965 kJ mol-1, respectively.

Introduction

In 1985, Kroto and co-workers discovered an extremely stable cluster consisting of 60 carbon atoms during a study of long-chain carbon molecules. (Kroto et al., 1985). This cluster, called fullerene, has a football shape with 12 pentagonal and 20 hexagonal rings. (Kroto et al., 1985). Shortly after, a study presenting successful formation of fullerenes with a lanthanum atom trapped in the cavity of C60, called endofullerene, C60La, was published. (Heath et al., 1985). Another consequent experiment proved the stability of C60La+ against H2, O2, NO, and NH3. (Weiss et al., 1988). This suggested that the lanthanum atom can be “protected” by being encapsulated in the fullerene. Since then, a number of studies focusing on novel properties of C60 and its interactions with other species have been carried out. (Ruoff et al., 1993; Diederich and Gómez-López, 1999; Bakry et al., 2007; Mignolet et al., 2013; Wang et al., 2014; Elliott et al., 2018; Kouřil et al., 2018) Some investigations concentrated on medical applications, hydrogen storage, and various endofullerene. (Wang et al., 2013; Srivastava et al., 2016; Srivastava et al., 2017; Elliott et al., 2018; Kouřil et al., 2018). Due to the fullerene's unique cage-like cavity, a procedure called molecular surgery can be performed to entrap an atom or molecule. (Murata et al., 2006; Krachmalnicoff et al., 2016). Utilizing this technique, encapsulation of molecular hydrogen and HF were successfully achieved. (Murata et al., 2006; Krachmalnicoff et al., 2016). In addition, a recent paper by Jana and Chattaraj (2020) describes the effects of a molecular reaction environment, dodecahedrane, on the He dimer bonding. Also, theoretical studies of a new type of endofullerenes with superalkali have been performed. (Srivastava et al., 2016; Srivastava et al., 2017) Superalkalis are clusters with very low adiabatic ionization energies. (Gutsev and Boldyrev, 1982; Gutsev and Boldyrev, 1983; Gutsev and Boldyrev, 1987; Lia et al., 1988; Gutsev and Boldyrev, 1990; Tong et al., 2013a; Tong et al., 2013b). The first and most common superalkalis have the formula Mk+1L, where M is an alkali atom with valence k and L is an electronegative atom. (Gutsev and Boldyrev, 1985; Gutsev and Boldyrev, 1987; Zhao et al., 2017). Nambiar et al. (2021) showed the importance of these compounds through a density functional computational study to improve the efficacy of redox reactions.

The concentration of carbon dioxide (CO2) in the atmosphere has been increasing constantly since the early 20th century with rapid industrialization. (D’Amato and Akdis, 2020; Kuzovkin and Semenov, 2020). This is a globally recognized issue as CO2 contributes significantly to the greenhouse effect and the acidification of the oceans. (Crowley and Berner, 2001; Hönisch et al., 2012). Increment of atmospheric temperature causes climate change that threatens the overall ecosystem. To capture CO2 molecules present in the air, various methods have been employed such as packed column of monoethanolamine and metal-organic frameworks. (Lv et al., 2015; Li et al., 2019; Li et al., 2020). The next step is activating CO2 molecules and converting them to value-added chemicals such as hydrocarbon fuels. (Hu et al., 2013). The activation of CO2 is extremely complex due to its stability and much efforts have been made by researchers to directly convert it into liquid hydrocarbons, useful for the aviation sector as novel jet fuels (Boreriboon et al., 2018; Vogt et al., 2019; Yao et al., 2020) or into oxygenates, such as ethanol. (Song et al., 2016; Bai et al., 2017; Wang et al., 2018). This conversion, whether it involves a direct CO2 hydrogenation route or not, entails the usage of metal-based catalysts to ensure an overall reasonable efficiency. (Yao et al., 2020).

In our previous studies, successful activation of CO2 with a superalkali species, Li3F2, were presented. (Park and Meloni, 2017). The computational study showed charge transfer from Li3F2 to CO2, which indicates migration of the unpaired electron from Li3F2 to CO2. The activated CO2 showed geometric change such as bent angle. The activated CO2 then can be transformed to other organic molecules with catalysis. (Liu et al., 2016; Luc et al., 2017). Removing the unpaired electron from [Li3F2⋅CO2] cluster weakens the interaction between Li3F2 and CO2 and geometry of CO2 returns back to the linear form. (Park and Meloni, 2017). The superalkali Li3F2 was observed and characterized experimentally. (Yokoyama et al., 2000; Haketa et al., 2002). They also confirmed three stable Li3F2 structures through a computational density functional approach. (Haketa et al., 2002). In this investigation, the Li3F2-doped fullerene and its endo-reaction with CO2 has been characterized using the B3LYP/6-31G* level of theory. These results are explained in terms of energetics and molecular orbitals of the species involved. In addition, these findings will be beneficial in providing insights for CO2 reduction and in helping the exploration of new materials with tailored properties.

Computational Methods

Geometries and total electronic energies of the investigated species were calculated at the B3LYP/6-31G* level of theory (Becke, 1988; Lee et al., 1988) using the computational software Gaussian09. (Frisch et al., 2016). B3LYP is one of the most commonly used density functional theory (DFT) methods that employs a three-parameter exchange functional developed by Becke (1992) and Becke (1993) with a correlational functional proposed by Lee, Yang, and Parr (LYP) Becke (1988) to approximate the exchange-correlation energy. The B3LYP/6-31G* level has been employed to study endofullerene systems because it yields reliable geometries and energies. (Wang et al., 2013; Srivastava et al., 2016; Srivastava et al., 2017). Partial atomic charges are calculated based on the Mulliken population analysis (Mulliken, 1955) and natural bond orbital (NBO) population analysis. (Reed et al., 1985).

The adiabatic ionization energy (AIE) is calculated by taking the zero-point energy corrected electronic energy difference between the optimized neutral and cation, whereas the adiabatic electron affinity (AEA) is obtained by subtracting the zero-point-energy corrected electronic energy of the optimized anion and neutral. All the optimized structures have real vibrational frequencies and their Cartesian coordinates have been reported in the Supplementary Material.

Results and Discussion

The main intent of this computational investigation is to study the interactions relevant to the reduction of CO2 by the superalkali Li3F2 inside our molecular reaction vessel, i.e., C60, and see how this environment affects the CO2 activation. The system is fairly large and, therefore, computationally challenging to investigate. We have analyzed the possible interactions between the fullerene and the two reactants, CO2 and Li3F2. All the computed energetics are reported in Table 1 together with the available literature (experimental and computed) values.

TABLE 1

Energetics of all the species relevant to this study calculated at the B3LYP/6-31G* level of theory. The zero-point-energy corrected total electronic energy (E0) is in Hartree, the adiabatic ionization energy (AIE) and adiabatic electron affinity are in eV, and the binding energy at 0 K is in kJ mol−1. [SA⋅CO2] stays for the endo superalkali⋅CO2 complex.

Species	E0	AIE	AIE liter	AEA	AEA liter	BE	BE liter
CO 2	−188.569349	13.6	13.78 Herzberg (1966)	−1.27	−0.60 Wang et al. (1988)	−	−
					−1.60 de Vries et al. (1992)
Li 3 F 2 (C 2v )	−222.473494	3.91	3.80 Hartman and Hisatsune (1966)	0.63	0.59 Hartman and Hisatsune (1966)	−	−
Li 3 F 2 (D 3h )	−222.459703	4.24	3.86 Hartman and Hisatsune (1966)	0.36	0.75 Hartman and Hisatsune (1966)	−	−
C 60	−2285.799198	7.08	7.54 Sikorska and Gaston (2020)	2.25	2.68 Knapp et al. (1986)	−	−
Li 3 F 2 (C 2v )⋅CO 2	−411.112817	5.42	5.21 Park and Meloni, (2017)	0.97	0.36 Park and Meloni (2017)	184	163 Park and Meloni (2017)
Li 3 F 2 (D 3h )⋅CO 2	−411.096882	5.25	−	0.80	−	178	−
C 60 ⋅CO 2	−2474.312373	7.08	−	2.27	−	−147 a	−
C 60 ⋅Li 3 F 2 (D 3h )	−2508.304346	5.64	−	2.45	−	119 a	−
C 60 ⋅[SA⋅CO 2 ]	−2696.601627	5.73	−	2.56	−	a	−

This binding energy at 0 K corresponds to the negative solvation energy at 0 K that C60 exerts on the encapsulated species, reactants, Li3F2 and CO2, and product SA⋅CO2 (see text).

Figure 1 reports the optimized geometries for CO2 and CO2 −, Li3F2(C2v), Li3F2(D3h) and their cations. The structures of CO2 and CO2 − reproduce well the literature experimental values for both bond distances and bond angles. In fact, for CO2 we have rC-O = 1.17 Å (1.16 Å) (Herzberg, 1966) and for CO2 − we have rC-O = 1.25 Å (1.25 Å) (Hartman and Hisatsune, 1966) and ∠OCO° = 134° (127 ± 7°) . (Hartman and Hisatsune, 1966). Both the geometries of the lowest energy Li3F2(C2v) isomer, trigonal bipyramidal Li3F2(D3h), and their cations are in agreement with our previous work. (Park and Meloni, 2017; Cochran and Meloni, 2014). Figure 2 shows the two superalkali isomers reducing the CO2. The geometry for the previously studied C2v isomer interacting with CO2 is in agreement with our previous results, (Park and Meloni, 2017), whereas the D3h⋅CO2 species is reported for the first time. When the D3h structure reacts with CO2, the trigonal bipyramidal geometry is distorted by increasing two “equatorial” Li-Li distances, maintaining only the Li(1)-Li(2) distance of 2.30 Å, with Li(3) being closer to the CO2, Li(3)-O(7) = Li(3)-O(8) = 2.03 Å and increasing the axial F-F distance from 2.40 to 2.68 Å. The ∠OCO° bond angle is 128° and the C-O bond length is 1.26 Å. The Li3F2 isomers have similar binding energy with CO2, with the C2v isomer presenting a stronger interaction of 184 kJ mol−1. These clusters can be defined as “free” or “naked” because they are isolated in the gas phase. The presented energy values are calculated at the B3LYP/6-31G* level and are within 10% from the literature reported quantities, whether they are experimental or computed at very high level of theory.

FIGURE 1

B3LYP/6-31G* optimized geometries of (A) CO2, (B) CO2 −, (C) Li3F2(C2v), (D) Li3F2 +(C2v), (E) Li3F2(D3h), (F) Li3F2 +(D3h).

FIGURE 2

B3LYP/6-31G* optimized geometries of (A) Li3F2(C2v), and (B) Li3F2(D3h) reducing CO2.

When a molecule is inserted in the fullerene (yielding an endofullerene), the chemical system is not free, but it will be subjected to the interactions with the carbon cage (“solvation effects”). In Figure 3, the two endofullerenes with CO2 and Li3F2 are shown. In the case of carbon dioxide, it is clear from the energetics presented in Table 1 that CO2 is destabilized by C60 having a negative binding energy at 0 K of −147 kJ mol−1 or a solvation energy at 0 K of 147 kJ mol−1, calculated as E0(CO2) + E0(C60)—E0(C60⋅CO2). The CO2 occupies the center of the C60, aligned with the C3 axis passing through a hexagonal face, minimizing its interactions with the C cage. The solvation energy is more properly defined as the Gibbs free energy change associated with the transfer of a molecule from the gas phase into a solvent, i.e., it provides the relative equilibrium populations of a species between gas phase and the solvent. Therefore, we should also know the entropy change connected with this process. The values that we are reporting in this investigation are at 0 K, so that , from which we can see that negative binding energies correspond to positive solvation energies (destabilizing effect). For the encapsulated superalkali two main findings can be noticed. First, the superalkali inside the fullerene is “forced” to assume a D3h geometry, a structure almost identical to the free D3h cluster but less stable than the free C2v cluster. Despite having started the Li3F2 geometry optimization from different initial configurations, the optimized structure inside the fullerene resulted in the trigonal bipyramidal geometry. The Li-F distances are shortened in C60 from 1.83 (free superalkali) to 1.77 Å, which corresponds to a compression along the F-F distance from 2.40 to 2.21 Å, and the two Li-Li bonds elongate to 2.40 Å. The second result is that C60 interacts strongly with Li3F2 with a binding energy at 0 K of 119 kJ mol−1 or solvation energy at 0 K of −119 kJ mol−1, calculated as E0(Li3F2(D3h)) + E0(C60)—E0(C60⋅Li3F2(D3h)). This interaction is not a reduction of C60, where the electron from the superalkali is transferred to the fullerene. In fact, upon ionization of C60·Li3F2, the encapsulated Li3F2 retains its trigonal bipyramidal structure, just slightly distorted (as described above) due to the interactions with C60. In addition, looking at the Mulliken population and the natural orbital population neither the endo-Li3F2(D3h) nor the C60 show an increase or change of electron charges. The C60·Li3F2 HOMO, the main contribution of which is given by C 2p AO’s, is delocalized almost entirely on the fullerene (Figure 4). The fact that the C60·Li3F2 AIE is much lower than C60 AIE, 5.64 vs. 7.08 eV, respectively, can be explained using a molecular orbital character argument. The interaction of C60 with the superalkali makes the HOMO of fullerene less bonding, and consequently, most of the C-C bonds are elongated by 0.1–0.2 Å. Upon ionization, the C-C bonds in C60·Li3F2 are shortened on average by 0.1 Å, which can be interpreted as the removal of an electron from a HOMO with antibonding character.

FIGURE 3

Two different views of B3LYP/6-31G* optimized structures of (A)-(B) C60 · CO2 and (C)-(D) C60 · Li3F2(D3h).

FIGURE 4

HOMO of C60 · Li3F2(D3h).

The insertion of CO2 within C60⋅Li3F2(D3h) produces an unexpected result (Figure 5). In previous computational studies, (Zhao et al., 2017; Park and Meloni, 2017; Sikorska and Gaston, 2020), naked superalkali have been shown to be capable of reducing carbon dioxide by transferring an electron and yielding an activated bent CO2 −. In this investigation, we show that our molecular vessel C60 forces the reaction to proceed in a different way (Figure 6). From an analysis of the optimized geometries inside fullerene, it is clear that CO2 is activated by showing a ∠OCO° bond angle of 132° and C-O bond lengths of 1.20 Å, 0.03 Å longer than rC-O in CO2 but 0.06 Å shorter than rC-O in the free Li3F2(D3h)⋅CO2 species. The activation of CO2 is achieved by a F transfer from Li3F2 to CO2 with the formation of a C-F bond of 1.38 Å, almost identical to the rC-F of 1.382 Å in CH3F. (Demaison et al., 1999). This moiety FCO2 does not resemble either the fluorocarboxyl radical FCO2 for which rC-O is 1.234 Å, rC-F is 1.310 Å, and ∠OCO° bond angle is 118.8°, (Zelinger et al., 2003), or the fluoroformate ion FCO2 − for which rC-O is 1.234 Å, rC-F is 1.46 Å, and ∠OCO° bond angle is 135.9°. (Arnold et al., 1995; Thomas et al., 2018). In addition, both fluorocarboxyl radical and fluoroformate ion are planar, whereas the endo-reaction species (Figure 6) resembles a non-planar (trigonal pyramidal) FCO2 that interacts with what it looks like a FLi3 species. All the attempts to optimize this structure outside C60 as free endo-Li3F2⋅CO2 returned a Li3F2(D3h)⋅CO2 geometry. Unfortunately, this prevents us from quantifying the interaction of Li3F2(D3h) with CO2 inside C60. In fact, the reaction we need isfrom which the interaction of Li3F2(D3h) with CO2 can be derived if we were able to find the [SA⋅CO2] reaction product as a free species and then its binding energy (or negative solvation energy) with C60, i.e.,

FIGURE 5

Two different views of B3LYP/6-31G* optimized geometry of C60 · Li3F2(D3h) · CO2.

FIGURE 6

B3LYP/6-31G* optimized geometry of the Li3F2 .CO2 complex inside C60.

In fact, the interaction of Li3F2(D3h) with CO2 inside fullerene can be calculated as:

In other words, this expression tells us that the interaction between endo-Li3F2(D3h) and endo-CO2, i.e., the BE of superalkali-CO2 in fullerene, is equal to the enthalpy of reaction (1) plus the solvation energies of CO2 and Li3F2(D3h) minus the solvation energy of [SA⋅CO2]. Because we cannot derive this last solvation energy absolute value due to the impossibility of optimizing the free endo-[SA⋅CO2] species, we can estimate this interaction by performing a single-point energy calculation of the free [SA⋅CO2] optimized inside C60. This structure necessarily represents a higher energy structure than a real minimum and, therefore, the estimated BE of superalkali-CO2 in fullerene would denote an upper bound providing us some insights on this interaction. From this computation we get an upper bound for of −965 kJ mol−1, which tells us that this endo-product is highly destabilized by C60!

Conclusion

The activation of CO2 by the Li3F2 superalkali within C60 has been investigated at the B3LYP/6-31G* level of theory. C60 has been utilized as a reaction vessel and its interaction with the reactants, superalkali and carbon dioxide, have been computed. C60 is capable of forcing a superalkali geometry, which does not present the global minimum in the gas phase. Specifically, Li3F2 takes the D3h structure. C60 has a stabilizing effect on the superalkali but a destabilizing effect on the CO2, as it can be deduced by the binding energies of these two systems, BE(C60⋅CO2) = −147 kJ mol−1 and BE(C60⋅Li3F2) = 119 kJ mol−1. Upon interaction of Li3F2(D3h) with CO2 inside fullerene, CO2 is clearly activated showing a ∠OCO° bond angle of 132° and C-O bond lengths of 1.20 Å, 0.03 Å longer than rC-O in CO2 but 0.06 Å shorter than rC-O in the free Li3F2(D3h)⋅CO2 species. The activation of CO2 is achieved by a F transfer from Li3F2 to CO2 with the formation of a C-F bond of 1.38 Å. Due to the impossibility of optimizing a free superalkali-CO2 complex, [SA⋅CO2], resembling the one optimized within the C60, a single-point energy calculation has been performed on the free [SA⋅CO2]. This energy has been utilized to provide an upper bound for the binding energy of Li3F2(D3h) with CO2 within C60 of −965 kJ mol−1, showing that C60 destabilizes the reaction product.