Strong chemisorption of CO on M@B_n⁻ (M = Co, Ir, Rh, Ru, Ta, Nb, n = 8–10) clusters: an implication for wheel boron clusters as CO gas detectors

Abstract

In this study, the adsorption behavior of carbon monoxide (CO) gas molecules on anionic M@B_n⁻ (M = Co, Ir, Rh, Ru, Ta, Nb, n = 8–10) clusters has been systematically investigated by employing density functional theory (DFT). It was found that CO adsorption on boron clusters proceeds spontaneously and easily, accompanied with a dramatic structural deformation for the corresponding M@B_n⁻ clusters. Large adsorption energies ranging from −22.82 to −27.38 kcal mol⁻¹ have been observed for CO on boron clusters. The kinetic stabilities of the formed complexes have been verified by ab initio molecular dynamics. The IR spectra and adiabatic detachment energy of the M@B_n⁻ clusters have been discussed before and after CO adsorption. In addition, the adsorption behavior of the other small gas molecules, such as CO₂, N₂, CH₄, H₂O, and O₂, have also been explored. The potential applications of these wheel boron M@B_n⁻ clusters in the detection of CO gas have been proposed for the first time.

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1. Introduction

As a new class of novel boron-rich systems, several transition-metal-centered monocyclic boron wheel clusters M@B_n/M@B_n⁻ (M = Co, Ir, Rh, Ru, Ta, Nb, n = 8–10) have been recently characterized experimentally and theoretically.^1–9 Their good stabilities are determined by the perfect combination of a ring cavity and electronegativity, electronic configuration, and volume of the central transition-metal atom.⁷ Especially for those corresponding anionic M@B_n⁻ clusters, they are extraordinary in chemistry because of their perfect planar structures with high coordination numbers. Despite their high stabilities and multiple aromaticity, however, their potential functionalities remain unclear to the best of our knowledge.

More recently, it was reported that boron-rich systems, such as boron nitride/carbon nanotube, boron fullerene, and solid boron,^10–13 can be used as potential CO₂ adsorbents through the C⋯B and O⋯B intermolecular interactions. Motivated by those studies, we have found that these M@B_n/M@B_n⁻ (M = Co, Ir, Rh, Ru, n = 8–9) clusters can be use as potential advanced materials for CO₂ capture and separation from flue gas and natural gas mixtures.¹⁴ To further explore the potential functionalities of these wheel boron clusters, in this study, we expand our studies on the adsorption of CO molecule on these anionic M@B_n⁻ clusters. Obviously, to clarify this point, it is crucial to get full knowledge of the intermolecular interactions between M@B_n⁻ clusters and CO. Herein, on the basis of the theoretical results of density functional theory and ab initio molecular dynamics, we demonstrate, for the first time, the strong chemisorption of CO gas molecule on the M@B_n⁻ clusters, which can be applied to detect CO gas in the fields of preventing CO poisoning and protecting environments. In addition, for comparison, the adsorption of other small gas molecules, such as CO₂, N₂, CH₄, H₂O, and O₂, have also been explored.

2. Computational details

All the geometries of the species have been fully optimized employing the density functional method B3LYP. Here, the reliability of the selected method has been proven in deal with many systems involving boron clusters.^6,15–18 The LanL2DZ and 6-311+G*(6-311++G**) basis sets have been used for the transition metal atoms and the other atoms (H atom), respectively. Subsequently, frequency calculations have been performed at the same level to confirm whether the optimized species is a minimum or a saddle point. For the calculated transition states, intrinsic reaction coordinate (IRC) calculations^19,20 have been further carried out to confirm the truth of the located transition states indeed connecting the initial reactants and products.

To better clarify the existence and nature of the intermolecular interactions, topological analyses have been performed on the basis of the optimized geometries employing the atoms in molecules (AIM) theory.²¹ The electron density (ρ_bcp) and the corresponding Laplacian (∇²ρ_bcp) and energy density (H_bcp) at the bond critical points (BCPs) have been obtained.

To characterize the adsorption strengths of the CO molecule on the boron clusters, the adsorption energy ΔE_ads has been defined as the energy difference between the formed complexes and the corresponding monomers. Furthermore, zero-point energy corrections and basis set superposition errors (BSSEs) have also been considered, where the Boys–Bernardi counterpoise method has been employed to evaluate the latter.²²

To assess the structural changes of the boron clusters and CO molecule upon complexation, the deformation energy ΔE_Def has been calculated for them, which is defined as the energy difference between the selected fragment in the optimized complex and its corresponding isolated state.

To investigate the bonding characters and the electron transfer behavior between boron clusters and CO molecule upon complexation, bonding and population analyses have been carried out employing the natural bond orbital (NBO) method on the basis of the optimized geometries.²³

All the calculations mentioned above have been carried out using the Gaussian 03 program.²⁴

To obtain the rate constants for the adsorption processes quantitatively, the conventional transition-state theory (TST) has been employed in combination with the Wigner tunneling correction.²⁵ All of them have been completed using KiSThelP program.²⁶

To gain a more detailed insight into the nature of the interaction between CO and boron clusters, the energy decomposition analysis (EDA) is carried out employing the methods of Morokuma^27,28 and Ziegler et al.^29–31 Generally, the instantaneous interaction energy (ΔE_int) between two fragments in the complex can be divided into three physically meaningful components:

ΔE_int = ΔE_elstat + ΔE_Pauli + ΔE_orb

where ΔE_elstat, ΔE_Pauli, and ΔE_orb are the electrostatic interactions, repulsive interactions, and orbital interactions, respectively. The energy decomposition analyses have been carried out at the B3LYP/TZ2P level of theory on the basis of the above optimized geometries.

To assess the kinetic stabilities of the formed complexes, ab initio molecular dynamics has been performed at the BLYP/DNP level of theory employing the DMol³ program on the basis of the optimized complexes characterized by the direct C⋯B interactions.^32,33 Constant temperature simulations at 298.15 K are achieved using the Nosé–Hoover chain, where the total simulation time is 1.0 ps with a time step of 1 fs. The stabilities of the complexes can be predicted from the radial distribution functions (RDFs) of the selected contact distance between boron clusters and CO molecule.

3. Results and discussion

As mentioned above, all the formed complexes between M@B_n⁻ clusters and CO molecule have been discussed firstly. It was found that CO can interact with the boron or metal atom of the M@B_n⁻ clusters through its C or O atom depending on the selected M@B_n⁻ clusters. Especially, all the C atom of CO can interact with the B atom of M@B_n⁻ clusters commonly. Thus, the representative complex involving Co@B₈⁻ cluster has been mainly discussed below since those complexes possess similar structural characters. The corresponding results of the other boron clusters have been given in the ESI for reference.†

For the sake of simplicity, the symbol X–M-n has been employed to stand for the formed complex. Here, X and M refer to the gas molecule and the M@B_n⁻ cluster involved, respectively. The symbol n refers to the numbers of the formed complexes. For example, CO–Co-1 stands for the first complex formed between CO and Co@B₈⁻ cluster.

3.1 Adsorption of CO on M@B_n⁻ clusters

3.1.1 The formed complexes characterized by the C⋯B interaction. As displayed in Fig. 1, the representative complex of Co@B₈⁻ cluster upon CO adsorption is characterized by the direct interaction of the C atom of CO with the B atom of boron clusters, where all the formed complexes involving other boron clusters have been given in Fig. S1 of the ESI† for simplicity. Note that the calculated B⋯C contact distances vary from 1.430 to 1.456 Å depending on the specific boron clusters, suggesting the strong interaction between CO and boron clusters. The presences of the bond critical points (BCPs) between the C and B atoms further validate the formation of the B–C bond. As given in Table 1, the positive Laplacians of the electron density (∇²ρ_bcp) and negative energy density (H_bcp) at the BCP of the B–C bond suggest that it should be partly covalent interactions. Moreover, as shown in Fig. 2, the selected molecular orbital and natural bond orbital (NBO) associated with the favorable interactions between CO and boron clusters have been located on the basis of the corresponding orbital analyses. Therefore, the strong interaction between CO and boron clusters can be confirmed.

Fig. 1 The molecular graph of the CO–Co-1 complex, where the BCP and RCP are denoted as small red and yellow dots, respectively.

Table 1 The calculated topological parameters at the BCPs of the B–C and C–O bonds for the formed complexes^a

Complexes	ρ bcp	∇^2ρbcp	V bcp	G bcp	H bcp
a The results of the C–O bond in complexes are denoted with italics. The ρbcp, ∇^2ρbcp, Vbcp, Gbcp, and Hbcp is electron density, the Laplacian of the electron density, potential energy density, kinetic energy density, and energy density at the BCP, respectively. The ρbcp at the BCP of the C–O bond is 0.4878 in free CO molecule.
CO–Co-1	0.1713	0.5074	−0.4145	0.2707	−0.1438
	0.4592	0.3506	−1.6881	0.8879	−0.8002
CO–Ir-1	0.1853	0.4583	−0.4484	0.2815	−0.1669
	0.4444	0.3009	−1.6031	0.8392	−0.7639
CO–Rh-1	0.1845	0.4653	−0.4471	0.2817	−0.1654
	0.4457	0.3033	−1.6105	0.8432	−0.7673
CO–Ru-1	0.1725	0.4999	−0.4166	0.2708	−0.1458
	0.4620	0.3701	−1.7074	0.9000	−0.8075
CO–Nb-1	0.1730	0.5154	−0.4208	0.2748	−0.1460
	0.4611	0.3619	−1.7008	0.8956	−0.8052
CO–Ta-1	0.1722	0.5146	−0.4184	0.2735	−0.1449
	0.4614	0.3649	−1.7036	0.8974	−0.8062

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Fig. 2 The schematic graphs of the selected molecular orbital (left) and NBO orbital (right) associated with the strong interaction of Co@B₈⁻ and CO molecule. The isodensity contours are 0.02 electron per bohr³.

As expected, the structural deformation and electron redistributions should occur due to the strong interaction mentioned above. As shown in Fig. 1 and S1 of the ESI,† the original planarity of the boron clusters has been destroyed upon CO adsorption, where the transition metal atoms have deviated from the boron ring plane. Compared with the free CO molecule, the C–O bond has been elongated by about 0.024–0.039 Å, implying the weakening of the C–O bond. This point can be further supported by the decrease of the electron density at the BCP of C–O bond after adsorption. As can be seen from the calculated deformation energies in Table 2, larger geometric deformations occur for boron clusters relative to that of CO molecule. As shown in Fig. 3, significant electron redistributions can be reflected from the electron density difference map of the formation of the CO–Co-1 complex. Obviously, the increased electron density within the regions between C and B atoms should be favorable to the interaction between CO and boron clusters. Similar phenomena have also been observed for the other metal boron complexes as shown in Fig. S2 of the ESI.† As a result, electron transfers ranging from 0.106 to 0.279 from CO to boron clusters have been observed as presented in Table 2.

Table 2 The calculated adsorption energies, thermodynamic parameters, barrier heights, rate constants, deformation energies, and the magnitudes of electron transfer during the formation of the complexes^a

Complexes	ΔEads	ΔH	ΔG	ΔE*	k	ΔEDef1	ΔEDef2	ΔQ
a All the energy units are in kcal mol^−1. ΔE* refer to the barrier heights in the adsorption processes of the first and second CO molecules in parentheses. The unit of rate constant is cm^3 per molecule per s. ΔEDef1 and ΔEDef2 refer to the deformation energies of boron clusters and CO occurring in the formation of the complexes, respectively. Positive value of ΔQ suggests the electron transfer from CO molecule to M@Bn^− clusters.
CO–Co-1	−24.51	−26.60	−17.48	2.79 (1.80)	7.21 × 10^−15	18.53	0.92	0.244
CO–Ir-1	−27.28	−30.04	−16.75	4.56 (1.64)	3.85 × 10^−17	23.30	1.98	0.106
CO–Rh-1	−27.38	−29.85	−17.50	4.38 (0.97)	1.67 × 10^−16	20.56	1.87	0.127
CO–Ru-1	−23.48	−25.48	−15.79	4.19 (1.86)	1.09 × 10^−15	22.88	0.76	0.272
CO–Nb-1	−22.82	−24.82	−15.08	5.07 (1.99)	2.69 × 10^−16	25.05	0.82	0.273
CO–Ta-1	−22.85	−24.90	−15.08	5.10 (1.98)	2.66 × 10^−16	24.42	0.80	0.279

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Fig. 3 Electron density difference map for the formation of the CO–Co-1 complex. The blue and purple regions represent the depleted and increased electron density, respectively. The isodensity contours are 0.004 electron per bohr³.

As can be seen from Table 2, the calculated adsorption energies range from −22.82 to −27.38 kcal mol⁻¹ depending on the specific boron clusters. Therefore, the adsorption of CO on boron clusters should belong to the chemisorption type. Moreover, as shown in Table 3, energy decomposition analyses show that the interactions between CO and boron clusters are predominated by the orbital interactions and electrostatic interactions, where the contributions of the former are larger than those of the latter.

Table 3 The energy decomposition analyses of the interaction energies for the formed complexes^a

Complexes	ΔEPauli	ΔEelstat	ΔEorb	ΔEint
a All the energy units are in kcal mol^−1. The data in parentheses refer to the percentage of the electrostatic interaction and orbital interaction to the total attractive interactions.
CO–Co-1	289.00	−139.63 (40.89)	−201.84 (59.11)	−52.47
CO–Ir-1	401.11	−188.14 (40.67)	−276.64 (59.33)	−61.47
CO–Rh-1	382.06	−181.04 (41.02)	−262.49 (58.98)	−59.24
CO–Ru-1	290.69	−142.01 (41.06)	−203.83 (58.94)	−55.15
CO–Nb-1	289.70	−141.54 (40.70)	−206.22 (59.30)	−58.06
CO–Ta-1	296.25	−144.44 (40.78)	−209.71 (59.22)	−57.89

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Thermodynamically, the whole chemisorption processes are exothermic and spontaneous, which can be reflected from the negative enthalpy and Gibbs free energy changes as shown in Table 2. Moreover, as shown in Fig. 4, low temperature and high pressure are favorable for the adsorption of CO, where the adsorption process is more sensitive to temperature than pressure. Kinetically, as shown in Table 2, the barrier heights required to overcome in the adsorption processes are relatively low, which are in the range of 2.79–5.10 kcal mol⁻¹. Correspondingly, the calculated rate constants for the CO adsorption processes are in the ranges of 3.85 × 10⁻¹⁷ to 7.21 × 10⁻¹⁵ cm³ per molecule per s. Note that these rate constants can be comparable to that of the prototype reaction CH₄ + H → CH₃ + H₂, which is 1.77 × 10⁻¹⁶ cm³ per molecule per s at the B3LYP/6-311++G** level of theory. Moreover, as displayed in Fig. 5 and S3 of the ESI,† the calculated rate constants exhibit a typical Arrhenius behavior. Namely, the dependence of the rate constants versus temperatures is well-fitted by two-parameter formula (k = A × exp(−ΔE*/(RT))) over the temperature range of 200–1200 K, where the correlative coefficients R² are all above 0.99.

Fig. 4 The temperature and pressure dependences of ΔG during the formation of the CO–Co-1 complex.

Fig. 5 The dependence of the rate constant versus temperature ranging from 200.0 to 1200.0 K for the CO adsorption process by Co@B₈⁻ cluster.

To further explore the possibility of the adsorption of more CO molecules on M@B_n⁻ clusters, as displayed in Fig. S4 of the ESI,† a second CO has been considered on the basis of the formed CO–M-1 complexes. Similar to those of CO–M-1 complexes, the second CO molecule can interact with the boron clusters through its C atom, which can be confirmed by the presence of the corresponding BCP. Moreover, the strong B⋯C interaction can be further reflected from the positive ∇²ρ_bcp and negative H_bcp at the BCP. As presented in Table S2 of the ESI,† the calculated adsorption energies range from −25.13 to −34.40 kcal mol⁻¹ for the adsorption of the second CO molecule, corresponding to the chemisorption. Moreover, the adsorption processes are favorable thermodynamically from the negative enthalpy and Gibbs free energy changes. Kinetically, the corresponding barrier heights have been further decreased to below 2.0 kcal mol⁻¹ if a second CO molecule is introduced as presented in Table 2, accompanying with the increase of the rate constants to the order of magnitude of 10⁻¹⁵ to 10⁻¹⁴ cm³ per molecule per s. Especially, it should be noted that the designed possible transition states in the simultaneous adsorption of two CO molecules have been collapsed to the transition state of the second CO adsorption as shown in Fig. 6. Therefore, the CO adsorption process on the boron clusters should proceeds easily and stepwisely.

Fig. 6 The optimized complex upon adsorption of two CO molecules (left) and the related transition state (right).

To confirm the kinetic stabilities of the formed complexes, ab initio molecular dynamics has been performed on the basis of the optimized geometries. As shown in Fig. 7, the radial distribution functions (RDF) of the B–C bonds are within 1.37–1.57 Å in all the complexes. Therefore, the formed complexes after CO adsorption are stable during the available time scale.

Fig. 7 Radial distribution functions of the B–C distance in the formed complexes.

To facilitate the experimental identification of the formed complexes by means of the gas-phase vibrational spectra, we have provided the IR spectra of the selected species before and after CO adsorption. Taking Co@B₈⁻ cluster for example, as shown in Fig. 8, significant peak changes have been observed before and after CO adsorption. Red-shift of 135.2 cm⁻¹ for the stretching vibration of the C–O bond occurs compared with the free CO state, which is consistent with its elongation mentioned above. Similarly, the same is also true for the other boron clusters. Especially, two new adsorption peaks involving the stretching vibration of the B–C bond appear at 1234.5 and 1436.7 cm⁻¹, providing the direct evidence for the formation of the B–C bond.

Fig. 8 The IR spectra of the CO–Co-1 complex, CO, and Co@B₈⁻ cluster.

Additionally, to investigate the changes of the electronic properties of the boron clusters upon CO adsorption, the adiabatic detachment energies (ADEs) of them have been calculated before and after CO adsorption. As shown in Table 4, the calculated ADEs of the M@B_n⁻ clusters are in good agreement with the corresponding experimental results, reflecting the reliability of the level of theory employed here. After CO adsorption, all the ADEs have been decreased by 0.16–0.49 eV except for that of Ir@B₉⁻ cluster, where the latter has been increased by 0.18 eV. Therefore, the electronic properties of the boron clusters have been changed upon CO adsorption.

Table 4 The calculated adiabatic detachment energies (in eV) of M@B_n⁻ clusters before and after CO adsorption

Complexes	Before adsorption	After adsorption	Expert. results
a Ref. 6. b Ref. 3. c Ref. 5.
CO–Co-1	3.57	3.41	3.84^a
CO–Ir-1	2.49	2.67	2.59 ± 0.03^b
CO–Rh-1	2.82	2.63	2.86 ± 0.03^b
CO–Ru-1	3.70	3.22	3.83 ± 0.02^a
CO–Nb-1	4.02	3.62	4.10 ± 0.03^c
CO–Ta-1	3.99	3.68	4.04 ± 0.03^c

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3.1.2 The formed complexes characterized by the other interaction modes. Besides the direct interaction of C atom of CO molecule with the B atom of M@B_n⁻ clusters, other interaction modes have also been observed depending on the specific boron clusters. As shown in Fig. S1 of the ESI,† the O atom of CO molecule can interact with the B atom of Ir@B₉⁻ and Rh@B₉⁻ clusters. At the same time, both the C and O atoms of CO can interact with the central metal atoms of the M@B_n⁻ clusters. Similar to those of CO–M-1 complexes, as shown in Table S1 of the ESI,† the partly covalent interactions between CO and boron clusters can be reflected from the positive ∇²ρ_bcp and negative H_bcp of the O⋯B, C⋯M, and O⋯M BCPs except for CO–Nb-3 and CO–Ta-3 complexes. However, as given in Table S2 of the ESI,† most of the calculated adsorption energies are positive except for Nb@B₁₀⁻ and Ta@B₁₀⁻ clusters, suggesting that it is unfavorable for these interaction modes adopted here compared with that of the C⋯B interaction. Moreover, this point can be further verified by their corresponding positive Gibbs free energy changes in the formation processes. As for the Nb@B₁₀⁻ and Ta@B₁₀⁻ clusters, the calculated adsorption energies are −30.48 and −36.84 kcal mol⁻¹, corresponding to the interaction of the C atom of CO with the central metal atom of the boron clusters.

3.2 Adsorption of CO₂, N₂, CH₄, H₂O, and O₂ on M@B_n⁻ clusters

For comparison, the adsorption behavior of other small gas molecules, such as CO₂, N₂, CH₄, H₂O, and O₂, have also been investigated. The corresponding molecular graphs for the formed complexes have been shown in Fig. S5 of the ESI.† The topological parameters at the BCPs and the energetic parameters involving adsorption energy and thermodynamic parameters have been summarized in Tables S1 and S2 of the ESI,† respectively.

Similar to the other boron clusters,¹⁴ as shown in Fig. S5 of the ESI,† Nb@B₁₀⁻ and Ta@B₁₀⁻ clusters can also adsorb CO₂via the C⋯B and O⋯B interactions. Here, the C⋯B interaction belongs to the covalent interactions from the negative ∇²ρ_bcp at the corresponding BCP. As for the O⋯B interaction, it has partial covalent nature. The calculated adsorption energies are −11.86 and −11.50 kcal mol⁻¹, which are smaller than those of the other boron clusters and the case of the CO adsorption. For the N₂ and CH₄ adsorption, both of them are difficult to be adsorbed by boron clusters as can be seen from the calculated positive adsorption energies and thermodynamic parameters.

For the H₂O adsorption, three types of interaction modes have been observed depending on the specific boron clusters. As shown in Fig. S5 of the ESI,† the H atom of H₂O can not only interact with the B atom of boron clusters in H₂O–Ir-2, H₂O–Ru-1, H₂O–Nb-2, and H₂O–Ta-2 complexes, but also can interact with the central metal atoms in H₂O–Co-1, H₂O–Ir-1, and H₂O–Rh-1 complexes. As can be seen from the large H⋯B and H⋯M distances, the adsorption energies ranging from −4.00 to −5.24 kcal mol⁻¹ are relatively small. On the other hand, the O atom of H₂O can interact with the metal atoms directly in H₂O–Nb-1 and H₂O–Ta-1 complexes. Moreover, the adsorption energies are −16.85 and −20.45 kcal mol⁻¹, respectively. The corresponding negative enthalpy and Gibbs free energy changes also suggest that it is favorable for H₂O adsorption on Nb@B₁₀⁻ and Ta@B₁₀⁻ clusters thermodynamically.

As for the O₂ adsorption, it is more complex due to the introduction of the triplet O₂. On the one hand, the spin multiplicity of the formed complexes still remains unchanged as those of the boron clusters. On the other hand, the spin multiplicity of the formed complexes has been changed. For example, for the formed complexes involving Co@B₈⁻ and Ir@B₉⁻ clusters, the spin multiplicity of them can be 3 and 4 if the both unpaired electrons in O₂ are still parallel to each other, respectively. At the same time, 1 and 2 are also possible for them. As a result, it was found that the ground states are triplet for O₂–Co-1, O₂–Ru-1, and O₂–Ta-1 complexes, where the corresponding triplet complex has not been located for Nb@B₁₀⁻ cluster. However, the ground states are still doublet for O₂–Ir-1 and O₂–Rh-1 complexes. As shown in Fig. S5 of the ESI,† the O atom of O₂ interacts with the B atom of boron clusters as can be seen from the presence of the BCP between O₂ and boron clusters. The O⋯B interaction should be partly covalent from the positive ∇²ρ_bcp and negative H_bcp at the BCP as shown in Table S1 of the ESI.† All the adsorption energies are smaller than those of the CO adsorption except for those of Ir@B₉⁻ and Rh@B₉⁻ clusters. Here, larger adsorption energies of −42.32 and −41.43 kcal mol⁻¹ have been observed in the O₂–Ir-1 and O₂–Rh-1 complexes, respectively. Therefore, it is possible that competitive adsorption may occur for CO and O₂ on the Ir@B₉⁻ and Rh@B₉⁻ clusters.

4. Conclusions

In this study, the adsorption behavior of CO molecule on the wheel M@B_n⁻ clusters has been systematically investigated theoretically. It was found that strong chemisorption of CO molecule on the M@B_n⁻ clusters has been validated. The whole adsorption processes can proceed spontaneously and easily thermodynamically and kinetically. The kinetic stabilities of the formed complexes have been confirmed by ab initio molecular dynamics. The IR spectra and ADEs of the boron clusters have been changed upon CO adsorption. On the basis of the above findings, we propose that the M@B_n⁻ clusters in this study can be used as potential candidates for CO gas detection in the fields of preventing CO poisoning and protecting environments. In addition, the adsorption of the other small gas molecules, such as CO₂, N₂, CH₄, H₂O, and O₂, have also been explored. It was found that all the anionic born clusters are insensitive to N₂ and CH₄. Similarly, the same is also true for the H₂O adsorption except for the Nb@B₁₀⁻ and Ta@B₁₀⁻ clusters. On the other hand, relatively strong adsorption has been observed for CO₂ and O₂ depending on the specific M@B_n⁻ clusters. Expectedly, further experimental studies are highly desirable to confirm these points.

Acknowledgements

This work is supported by NSFC (21577076, 21303093, and 21003082), the NSF of Shandong Province (ZR2014BM020), and the Project of Shandong Province Higher Educational Science and Technology Program (J11LB06). The State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (KF2013-05) is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: The molecular graphs of the formed complexes of boron clusters with the CO, CO₂, N₂, CH₄, H₂O, and O₂ gas molecules and their topological parameters at the selected BCPs, the electron density difference maps for the formation of the CO–M-1 complexes, the dependence of the rate constants versus temperatures in the adsorption processes, and the adsorption energies and thermodynamic parameters during the formation of the various complexes. See DOI: 10.1039/c5ra15151a

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