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Theoretical structural study of van der Waals complexes between oxazole and atmospheric gases CO 2 and N 2 for capture applications.

A. Belasri
,
F. Tahiri
,
O. Douass ,
N. Inostroza-Pino
,
M. Belmouden
,
H. Bahmann
,
M. Mogren Al-Mogren
,
M. L. Senent ,
S. Dalbouha

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January 8, 2025

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https://doi.org/10.12688/openreseurope.18925.1

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September 22, 2025 (Version of Record 3.0)
September 11, 2025 (Version of Record 2.0)
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January 8, 2025 (Published Manuscript Under Review 1.0)

European Commission

Grant ID(s): 872081

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Adsorption, CO2 capture, N2, atmosphere, ab initio, MOF’s.

Abstract

Background

The objective of this study is to explore the potential of oxazole (C ₃H ₃NO), a fascinating heterocyclic compound naturally present, which is a potential ligand in the construction of Metal Organic Frameworks (MOF’s) for the selective capture of CO ₂ in a nitrogen-rich atmosphere, by using both molecular and solid-state simulation techniques.

Methods

This study investigates the equilibrium structures and binding energies of van der Waals aggregates formed by an oxazole molecule with nonpolar molecules such as CO ₂ and N ₂, considering both two-body systems (oxazole@CO ₂ and oxazole@N ₂) and three-body systems (oxazole@CO ₂@N ₂ and oxazole-CO ₂/N ₂@Au ₆/Cu ₆/Zn ₃O ₃). Molecular computations for these systems are conducted using ab initio calculations at the MP2/aug-cc-pVXZ level of theory, where X = (D, T). Additionally, solid-state simulations analyze the adsorption behaviors and energies of oxazole@CO ₂ and oxazole@N ₂ on metallic surfaces:Au, Cu, ZnO(111) through Monte Carlo methods.

Results

We find that the oxazole exhibits more adsorption selectivity for CO ₂ than for N ₂. Adding a second gas to the most stable complexes, oxazole@CO ₂ and oxazole@N ₂, the oxazole capture ability does not vary. On the contrary, it strengthens the adsorption energy of three-body complexes compared to two-body complexes. The addition of metallic clusters (Au ₆, Cu ₆, Zn ₃O ₃) and metallic surfaces (Au, Cu, ZnO) enhances the adsorption capacity, where Cu ₆ is particularly highly efficient. Both ZnO and Cu surfaces offer significant adsorption advantages while remaining economically feasible.

Conclusions

This study demonstrates that oxazole exhibits a strong selectivity for CO ₂ over N ₂, with the addition of metallic clusters and surfaces significantly enhancing its adsorption capacity. These findings highlight the potential of oxazole-based materials for effective gas capture and separation, with positive implications for environmental sustainability.

Plain Language Summary

This research project focuses on gas capture and separation, especially for molecules like carbon dioxide (CO ₂) and nitrogen (N ₂), which are crucial in both industrial and environmental contexts. Metal-organic frameworks (MOFs) show promise for efficient CO ₂ capture due to their adjustable structures and large surface areas. However, the use of oxazole, a specific molecule, for CO ₂ capture in these structures has been little explored. This study examines how oxazole interacts with CO ₂ and N ₂ molecules, forming aggregates bound by van der Waals forces (a type of molecular attraction). Results show that oxazole favors binding with CO ₂ over nitrogen, making it potentially useful for gas separation. By adding metallic clusters, such as gold, copper, or zinc oxide, the adsorption capacity of oxazole further improves, especially with copper, while remaining affordable.

This study highlights oxazole's potential for effective gas capture, contributing to more sustainable and environmentally friendly technologies.

Corresponding author

S. Dalbouha

s.dalbouha@uiz.ac.ma

Grant information

This project has received funding from the [European Union’s Horizon 2020 research and innovation programme][European Union’s Horizon Europe research and innovation programme] under the Marie Skłodowska-Curie grant agreement No [872081] (ATMOS).

The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

© 2025 A. Belasri (Co-Author); F. Tahiri (Co-Author); O. Douass (Co-Author); N. Inostroza-Pino (Co-Author); M. Belmouden (Co-Author); H. Bahmann (Co-Author); M. Mogren Al-Mogren (Co-Author); M. L. Senent (Co-Author); S. Dalbouha (Corresponding Author, Co-Author)

This work is licensed under a Creative Commons Attribution 4.0 International License.

First version published

January 8, 2025 (https://doi.org/10.12688/openreseurope.18925.1)

Latest version published

September 22, 2025 (https://doi.org/10.12688/openreseurope.18925.4)

Introduction

The exploration of carbon dioxide capture and separation from gas mixtures by adsorption based on porous materials searching for low-costs and high efficiency, has been a focal point in research^1–7, aiming to address environmental concerns and enhance industrial processes^5,6. Gas adsorption and separation, particularly involving quadrupolar species such as carbon dioxide (CO₂) and nitrogen (N₂), have gained considerable interest due to their importance in industrial and environmental contexts. Metal-organic frameworks (MOF’s) are porous crystalline materials formed by organic structures that act as ligands⁸ connected by metal ions or clusters^9,10. They have been considered to be efficient for CO₂ capture tools owing to their adjustable and tunable structures², extensive surfaces^11,12, large surface areas^13,14, customizable functionalities^15,16, and high mechanical and thermal stabilities¹⁷. Specific organic ligands, such as heterocycles like carboxylic acids^18,19, amines²⁰, imidazoles^21–25, triazoles²⁶, pyridines²⁷, benzenes²⁸, naphthalenes²⁹, benzimidazoles³⁰, etc., enables the regulation of MOF’s properties, thereby enhancing their capability for selective adsorption and thermal stability³¹, making them particularly suitable for gas adsorption and separation processes³². MOF’s based on imidazole have gained interest due to the ability to design various types of architectures such as rigid, flexible, interpenetrating, etc., to influence the size of cavity pores and different types of supramolecular interactions within the pore^33,34.

In our previous study^21–24, we highlighted the significant ability of imidazole to form van der Waals complexes with several greenhouse gases, including CO₂, CH₄, and SF₆, as well as with pollutants such as SO₂ and NH₃ and gas such as CO. This interaction is particularly relevant for selectively capturing CO₂ from mixtures containing these gases. Although oxazole (C₃H₃NO) is a fascinating heterocyclic compound naturally present³⁵, its capacity as a ligand in the construction of CO₂ capturing MOF’s has not been thoroughly investigated. This provides an opportunity for further research aimed to valorize the properties of oxazole in developing efficient MOF’s for CO₂ capture, contributing to advancements in environmental sustainability and gas separation technologies. Our goal is the theoretical characterization of van der Waals aggregates formed with one oxazole molecule and nonpolar molecules such as carbon dioxide (CO₂) and nitrogen (N₂). CO₂ is known to have significant environmental impacts on global weather patterns and human life³⁶. Anthropogenic emissions of N₂ can indirectly lead to an increase in NO_x levels in the atmosphere. When N₂ reacts with atmospheric oxygen, it converts into nitrogen oxides (NO_x) such as NO and NO₂. These NO_x compounds have consequences on air quality, human health, and the environment, contributing to smog formation, acid rain, and causing harmful effects on respiratory systems and lung tissues³⁷.

In this work, we perform a systematic evaluation of ligands containing both nitrogen and oxygen for their interaction energies with CO₂ and N₂ by using both molecular and solid-state simulation techniques. In the first approach we use the possible accurate ab initio methods to calculated the binding energies (BEs) of the tow body systems (CO₂-Oxazole-BEs & N₂-Oxazole-BEs). The analysis of the ternary systems (CO₂-oxazole-N₂-BEs) and (oxazole-CO₂/N₂@Au₆/Cu₆/Zn₃O₃) using the same method was conducted to understand the interactions among CO₂, oxazole, and N₂ within these systems, to examine their influence on CO₂ separation and capture strategies in gas mixtures and predict the capacity of new materials to selectively capture and store CO₂ over N₂ in N₂-rich atmospheres. Indeed, the presence of N₂ can significantly influence CO₂ capture performance in MOF’s, taking into account their common presence in atmospheric and industrial environments, emphasizing the crucial role of computational methods in designing nanomaterials for carbon capture technologies.

On the other hand, the Monte Carlo solid-state simulations were employed to investigate the interactions between oxazole and the (111) surfaces of Au, Cu, and ZnO. These materials were selected for their unique electronic properties and practical relevance. Gold (Au) is frequently used as a reference due to its stability and well-characterized surface chemistry³⁸, Copper (Cu) is chosen for their economic viability and their proven effectiveness in enhancing adsorption processes³⁹. While the zinc oxide ZnO, was included in this study for their unique semiconducting, piezoelectric, and photoelectric properties and high surface area⁴⁰.

Computational details

All the electronic computations were performed using both the Gaussian 09 Revision C.01 software⁴¹ and the Material Studio (BIOVIA, 2020) program⁴². An appropriate free alternative software for this purpose are GAMESS (US), a powerful tool for electronic structure calculations that supports HF, DFT, MP2, and other methods, and ORCA, a versatile quantum chemistry program renowned for its computational efficiency and support for DFT, HF, and post-HF techniques.

We have combined the most accurate possible ab initio method, and Monte Carlo solid-state simulations⁴³, to investigate the interaction between CO₂ and N₂ and oxazole (Oxa), either isolated, attached to a gold, copper and ZnO cluster or surface.

The study employed the second-order Møller–Plesset theory (MP2)^44,45 within the Gaussian software to compute the equilibrium structures and spectroscopic properties of both isolated species and complexes included two-body systems like oxazole@CO₂ and oxazole@N₂. Additionally, the three-body systems such as oxazole@CO₂@N₂ and oxazole-CO₂/N₂@Au₆/Cu₆/Zn₃O₃ were investigated. For accurate depiction of small systems. In these computations, the atoms were described using Dunning and co-workers’ aug-cc-pVXZ basis set (abbreviated as AVXZ, where X=D, T)⁴⁶, augmented with diffuse atomic orbitals to capture long-range interactions. Specifically, AVTZ was employed for oxazole@CO₂ and oxazole@N₂, while AVDZ was used for oxazole@CO₂@N₂ and oxazole@CO₂/N₂@ cluster complexes.

Monte Carlo is a powerful computational technique used to model the adsorption of molecules on surfaces and to determine the adsorption energy associated with them. In this work, the adsorption energies of oxazole, oxazole@N₂, and oxazole@CO₂ on a gold, copper, or zinc oxide surface were researched by employing the software Materials Studio (BIOVIA, 2020)⁴². As a free alternative to Material Studio, I recommend the open-source software Quantum ESPRESSO and CP2K. The first program, Quantum ESPRESSO, is a comprehensive suite for quantum simulations, ideal for materials modeling and the simulation of solids, liquids, and nanostructures.

https://www.quantum-espresso.org/

The second program, CP2K, is also free and open-source software, widely used for molecular dynamics simulations and quantum chemistry calculations, particularly for systems with large numbers of atoms.

https://www.cp2k.org/

The Au, Cu, and ZnO crystal structures have been imported from the Materials Studio database. Surfaces were built by clicking 'Build', then 'Surface', and then 'Cleave Surface'. In the following menu, the cleaving plane (1 1 1) has been chosen, and then afterward, the 'Symmetry' option was used to construct a supercell. All the surface structures were prepared with three layers, as illustrated in Figure 5.

The structures of oxazole, oxazole@N₂, and oxazole@CO₂ were well optimized following the then possible lowest energy using the COMPASS II force field as implemented in the Forcite Module. This same force field was used in the Monte Carlo simulations. The simulations were done utilizing the Adsorption Locator Module in the materials studio 2020 package from BIOVIA. The COMPASS II force field is useful in handling adsorption processes; therefore, it works best in catalysis, materials science, and environmental science.

Therefore, a detailed investigation of adsorption behaviors and energies of the fixed molecules on the chosen surfaces gave valuable insight into their eventual interactions and stability.

The minimum character of all the optimized structures is evidenced by the presence of all positive harmonic frequencies, indicates that the system is at its energy minimum, reflecting a stable and optimized configuration. The binding energy (BE) between the oxazole and CO₂ or N₂ was evaluated for optimized geometries using Second order Möller-Plesset theory (MP2) in connection with aug-cc-pVTZ basis set (denoted in this paper by AVTZ). The binding energies (BEs) were calculated using the following energy expression:

BE= E _Complex - (E _oxazole+ E _gaz)

where: BE is the binding energy, E_complex is the total energy of the oxazole-gas complex, E_Oxazole is the total energy of the isolated oxazole molecule, and E_gaz is the total energy of the isolated gas molecule (CO₂ or N₂). For a more accurate estimation of the interaction energy, the basis set superposition error (BSSE) was corrected using the Boys and Bernardi method⁴⁷. Additionally, the zero-point vibrational energy correction (ZPVE) was also taken into account for all geometries⁴⁸.

For the complexes between the oxazole and CO₂ or N₂ @ cluster (Au₆, Cu₆ or Zn₃O₃) we employed the MP2/AVDZ/LanL2DZ⁴⁹., and for the adsorption of CO₂ and N₂ on Au, Cu, and ZnO (111) surfaces we used the Monte Carlo computational technique.

The binding energies (BEs) were calculated using the following energy expression:

BE = E _Complex - (E _Oxa@CO2/N2+ E _{Cluster/Surface})

where: BE is the binding energy E_complex, is the total energy of the Oxa CO₂/N₂@ cluster (cluster = Au₆/Cu₆ or Zn₃O₃) or Surface Au, Cu or ZnO (111) complexes, E_{Oxa@CO₂/N₂} is the total energy of the complex Oxazole+CO₂ or Oxazole+N₂, and E_{Cluster/Surface} is the total energy of the isolated gas cluster (Au₆, Cu₆ or Zn₃O₃) or Surface Au(111), Cu (111) or ZnO (111). For a more accurate estimation of the interaction energy of Oxazole CO₂/N₂@ cluster (cluster= Au₆/Cu₆ or Zn₃O₃) the zero-point vibrational energy correction (ZPVE) was also taken into account for all geometries.

Results and discussions

Equilibrium structure of the complexes

To examine the molecular-level adsorption of CO₂ and N₂ on oxazole, we first located the CO₂ and N₂ molecules at the N1 site of oxazole (refer to Figure 1), and then we oriented them around it to explore all possible equilibrium structures. It's worth noting that the number of stable structures increases the likelihood of gas capture. Despite multiple attempts, the optimization results at the MP2/AVTZ level consistently converge towards one of the structures presented in Figure 1A and B, which characterize the complexes formed by oxazole with CO₂ and N₂.

Figure 1.

The atom labelling of the complex Oxazole + CO₂ (A), Oxazole + N₂ (B) and Oxazole and the five principal complexation pathways, I1, I2, I3, I4 and I5 (C).

The study carried out at the MP2/AVTZ level on the CO₂-Oxazole complex reveals the existence of two stable structures labeled I1 and I2 (cf. Figure 2A, Table 1). In structure I1 (E_r = 0.0 kcal/mol), identified as the global minimum, the bond is established through an interaction between the electron donor-acceptor (EDA) sites of CO₂ and oxazole (RO10-H6 = 2.6956 Å and RC9-N1 = 2.8418 Å). However, in structure I2 (E_r = 1.31 kcal/mol), the bond is maintained through a donor-acceptor electron exchange (EDA), where the oxygen atom of CO₂ interacts with H8 of oxazole (RO10H8 = 2.7362 Å) and the carbon atom of CO₂ interacts with the oxygen of oxazole (RC9-O4 = 2.8340 Å).

Figure 2.

The equilibrium geometries and the MP2/AVTZ relative energies of the oxazole +CO₂ (A) and oxazole + N₂ (B) Van-der-Waals complexes.

Table 1. Total electronic energies (E, in a.u.), relative energies (E_r, in cm^-1) and structural parameters (Distances in Å; angles in degrees) of the two equilibrium structures of the Oxazole-CO₂ complexes calculated with MP2/aug-cc-pVTZ.

Oxazole (C1)		I1 (C1)		I2 (C1)
		$\frac{ROH}{RCN}$	$\frac{2.6956}{2.8418}$	$\frac{ROH}{RCO}$	$\frac{2.7362}{2.8340}$
E	-245.639836	E	-433.968056	E	-433.965972
		E_R	0.0	E_R	457.2
C2N1	1.2996	C2N1	1.3011	C2N1	1.2987
C3N1	1.3862	C3N1	1.3858	C3N1	1.3869
O4C2	1.3552	O4C2	1.3531	O4C2	1.3570
C5C3	1.3596	C5C3	1.3597	C5C3	1.3593
H6C2	1.0755	H6C2	1.0757	H6C2	1.0757
H7C3	1.0752	H7C3	1.0752	H7C3	1.0752
H8C5	1.0739	H8N4	1.0739	H8N4	1.0741
C3N1C2	104.0	C3N1C2	104.2	C3N1C2	104.1
O4C2N1	114.7	O4C2N1	114.5	O4C2N1	114.6
C5C3N1	109.1	C5C3N1	109.0	C5C3N1	109.3
H6C2N1	128.6	H6C2N1	128.2	H6C2N1	128.8
H7C3N1	122.1	H7C3N1	122.1	H7C3N1	122.0
H8C5O4	116.7	H8C5O4	116.8	H8C5O4	116.6
CO₂
E	-188.321641
CO	1.1702	O10C9	1.1718	O10C9	1.1709
		O11C9	1.1686	O11C9	1.1690
<OCO	180.0	<OCO	177.6	<OCO	179.3
		O10H6	2.6956	O10H8	2.7362
		O10H6C2	104.2	O10H8C5	111.2
		C9O10H6	114.7	C9O10H8	113.6
		C9N1	2.8418	C9O4	2.8340
		C9N1C2	106.8	C9O4C2	142.8
		O10C9N1	86.1	O10C9O4	85.8

Additionally, the study of the complex between N₂ and oxazole, still at the MP2/AVTZ level, revealed the existence of four adsorption sites as outlined in Figure 2B and Table 3. These sites are labeled as follows: I1A (E_r = 0.0 kcal/mol), I1B (E_r = 0.01 kcal/mol), I2A (E_r = 0.28 kcal/mol), and I5 (E_r = 0.57 kcal/mol). These distinct structures depict various stable configurations of the N₂-oxazole complex. Predominantly, the key interactions observed in the stable structures occur where hydrogen bonds link N₂ to oxazole. Additionally, the equilibrium distances within the van der Waals system RN9Hx (X=6,7,8) exhibit the following variations: RN9H6 = 2.7302 Å for I1A, RN9H7 = 2.7437 Å for I1B, RN9H6 = 2.7529 Å for I2A, and RN9H8 = 2.5885 Å for I5.

Table 1 and Table 3 present the structural parameters at the MP2/AVTZ level, while Table 2 and Table 4 provide the MP2/AVTZ harmonic frequencies for each examined complex. The parameters of the van der Waals aggregates are compared to those of the corresponding isolated components.

Table 2. Harmonic fundamental frequencies (ω, in cm^-1) of CO₂, oxazole, and the two equilibrium geometries of the oxazole-CO₂ complexes calculated at the MP2/AVTZ level of theory.

ω
	Oxazole		CO₂		I1		I2
ω₁	a	3330			a	3331	a	3330
ω₂		3311				3312		3309
ω₃		3299				3300		3299
ω₄			σ_u	2402		2402		2404
ω₅		1546				1546		1547
ω₆		1508				1506		1507
ω₇		1355				1355		1354
ω₈			σ_g	1326		1327		1328
ω₉		1272				1269		1270
ω₁₀		1185				1189		1182
ω₁₁		1141				1144		1138
ω₁₂		1109				1111		1108
ω₁₃		1086				1086		1082
ω₁₄		915				917		917
ω₁₅		905				909		904
ω₁₆		864				865		868
ω₁₇		827				836		827
ω₁₈		763				766		771
ω₁₉		665				667		664
ω₂₀			π	659		662		660
ω₂₁			π	659		637		651
ω₂₂		628				628		626
ω₂₃						132		100
ω₂₄						88		80
ω₂₅						52		39
ω₂₆						44		38
ω₂₇						29		21

Table 3. Total electronic energies (E, in a.u.), relative energies (E_r, in cm^-1) and structural parameters (Distances in Å; angles in degrees) of the four equilibrium structures of the Oxazole-N2 complex calculated with MP2/aug-cc-pVTZ.

Oxazole (C1)		I1A (C1)		I1B (C1)		I2A (C1)		I5 (C1)
		R_NH	2.7302	R_NH	2.7437	R_NH	2.7529	R_NH	2.5885
E	-245.639836	E	-355.007379	E	-355.007363	E	-355.006923	E	-355.006474
		E_R	0.0	E_R	4	E_R	101	E_R	203
C2N1	2.9962	C2N1	1.3003	C2N1	1.30006	C2N1	1.2997	C2N1	1.2998
C3N1	1.38622	C3N1	1.3860	C3N1	1.3867	C3N1	1.3859	C3N1	1.3863
O4C2	1.35519	O4C2	1.3549	O4C2	1.3547	O4C2	1.3565	O4C2	1.3550
C5C3	1.35957	C5C3	1.3597	C5C3	1.3597	C5C3	1.3597	C5C3	1.3599
H6C2	1.07554	H6C2	1.0754	H6C2	1.0756	H6C2	1.0752	H6C2	1.0755
H7C3	1.07521	H7C3	1.0752	H7C3	1.0751	H7C3	1.0752	H7C3	1.0752
H8C5	1.07390	H8C5	1.0739	H8C5	1.07388	H8C5	1.0733	H8C5	1.0738
C3N1C2	103.99	C3N1C2	104.0	C3N1C2	104.0	C3N1C2	104.0	C3N1C2	104.0
O4C2N1	114.70	O4C2N1	114.7	O4C2N1	114.7	O4C2N1	114.6	O4C2N1	114.7
C5C3N1	109.13	C5C3N1	109.1	C5C3N1	109.1	C5C3N1	109.1	C5C3N1	109.2
H6C2N1	128.59	H6C2N1	128.4	H6C2N1	128.6	H6C2N1	128.8	H6C2N1	128.7
H7C3N1	122.09	H7C3N1	122.1	H7C3N1	122.0	H7C3N1	122.1	H7C3N1	122.0
H8C5O4	116.75	H8C5O4	116.9	H8C5O4	116.8	H8C5O4	116.8	H8C5O4	116.7
N₂
E	-109.364800
NN	1.11401	NN	1.11387		1.11408		1.11396		1.11401
		N9H6	2.7302	N9H7	2.7437	N9H6	2.7529	N9H8	2.5885
		N9H6C2	109.2	N9H7C3	113.3	N9H6C2	114.9	N9H8C5	167.7
		N10N9H6	147.3	N10N9H7	144.1	N10N9H6	114.9	N10N9H8	172.1

Table 4. Harmonic fundamental frequencies (ω, in cm^-1) of N₂, oxazole and the four equilibrium geometries of the oxazole-N₂ complexes calculated at the MP2/AVTZ level.

Ω
	Oxazole		N₂		I1A		I1B		I2A		I5
ω₁	a	3330			a	3330	a	3330	a	3329	a	3336
ω₂		3310				3314		3310		3316		3310
ω₃		3298				3298		3301		3298		3299
ω₄			σ_g	2187		2186		2185		2186		2186
ω₅		1545				1544		1545		1545		1546
ω₆		1507				1506		1507		1506		1508
ω₇		1355				1354		1354		1355		1356
ω₈		1271				1271		1271		1271		1273
ω₉		1185				1186		1186		1182		1186
ω₁₀		1141				1142		1140		1141		1141
ω₁₁		1109				1109		1110		1108		1113
ω₁₂		1085				1086		1086		1085		1085
ω₁₃		915				914		915		914		915
ω₁₄		904				905		904		904		905
ω₁₅		863				864		870		865		866
ω₁₆		826				834		827		833		827
ω₁₇		763				765		764		763		784
ω₁₈		664				666		665		665		665
ω₁₉		627				628		628		627		630
ω₂₀						84		82		77		58
ω₂₁						63		58		54		57
ω₂₂						46		46		42		49
ω₂₃						40		43		26		22
ω₂₄						21		20		13		9

During the formation of weak intramolecular bonds, the structure of the oxazole ring and N₂ undergoes minimal changes (see Table 1 and Table 3). However, a significant deformation is observed in the CO₂ molecule, resulting in the loss of its symmetry. This deformation is particularly pronounced in the oxazole-CO₂ complexes I1 and I2 (see Table 1), where the C-O bond in CO₂ tends to shorten ( O11C9: 1.1686 Å for I1, 1.1690 Å for I2) while the distance O10C9 is elongated (1.1718 Å for I1, 1.1709 Å for I2), compared to free CO₂ (CO=1.1702 Å). Additionally, the <OCO angle is bent away from the linear configuration of a free CO₂ molecule, measuring 177.6 degrees for I1 and 179.3 degrees for I2, respectively. This deformation is attributed to the formation of the van der Waals complexes between CO₂ and oxazole, leading to electron redistribution and strengthening of the C-O bond.

Harmonic frequencies were computed to ensure minimal energy for all structures. Notably, in Table 2 and Table 4, certain low frequencies (5 frequencies for Oxazole-CO₂, ω₂₃-ω₂₇, and for Oxazole-N₂, ω₂₀-ω₂₄) emerge, which were not observed for the monomers. This situation allows for the use of a Born-Oppenheimer-type approach, which distinguishes between intramolecular and intermolecular vibrations. Intermolecular vibrations more closely resemble hindered rotations or tunneling motions. Generally, there is a strong coupling between different intermolecular degrees of freedom. The deformation vibrations of O–C–O in complexes I1 and I2 occur at frequencies of 637 cm^-1 and 651 cm^-1, respectively. These modes experience significant frequency shifts compared to the CO₂ monomer, approximately 8 cm⁻¹ for complex I2 an 22 cm^-1 for complex I1. Furthermore, the symmetric and antisymmetric intermolecular O-C stretching modes with frequencies ω₈ = 1328 cm^-1 and ω₄ = 2402 cm^-1 for complex I2 have shifted by 2 cm^-1 from frequencies of ω₈ = 1326 cm^-1 and ω₄ = 2404 cm^-1for the CO₂ monomer. In the oxazole-N₂ complex, the stretching of N-N is not influenced by the formation of the hydrogen bond between the N of N₂ and the H of oxazole. Thus, the intermolecular stretching vibrations H..N–N(ω₂₁) in the optimal structure are observed at frequencies of 63 cm⁻¹, 58 cm⁻¹, 54 cm⁻¹, and 57 cm⁻¹ for I1A, I1B, I2A, and I5, respectively.

Binding energies

Two body complexes type oxazole-CO₂ and oxazole-N₂

The binding energy is crucial for understanding, predicting, and enhancing the performance of adsorbent materials in greenhouse gas and pollutant capture processes. It determines the interaction between the host and the guest, as well as the properties of the adsorbent material. A more negative adsorption energy generally indicates a stronger adsorption interaction. For this reason, the binding energies of all previously mentioned stable forms, are computed via the MP2/AVTZ approach and including the zero-point vibrational energy (ZPE) along with as well as the Boys and Bernardi correction (BSSE), are listed in Table 5. Our calculations indicate that oxazole is favored energetically as an adsorbent for both our adsorbates although CO₂ shows a higher capacity to bind oxazole rings. The range of CO₂ binding energies spans from -4.13 to -2.82 kcal/mol, while for N₂, the energies range from -1.77 to -1.19 kcal/mol. Based on the adsorption energies, the nitrogen site of the oxazole is the most stable site for CO₂ adsorption, and Hydrogen 6 is the most stable site for N₂ adsorption. The zero-point vibrational energy (ZPE) correction ranges from 0.39 to 0.51 kcal/mol for the CO₂ complexes and from 0.32 to 0.39 kcal/mol for the N₂ complexes. On the other hand, the BSSE correction is between 0.02 and 0.09 kcal/mol for CO₂ and 0.05 kcal/mol for N₂.

Table 5. MP2/AVTZ binding energies of the oxazole complexes (E_b, kcal/mol).

Complex	E_int	E_int +BSSE	E_int +ZPVE	E_int +BSSE +ZPE
Oxazole-CO₂
I1	-4.13	-4.22	-3.62	-3.71
I2	-2.82	-2.84	-2.43	-2.45
Oxazole-0N₂
I1A	-1.77	-1.72	-1.38	-1.33
I1B	-1.76	-1.71	-1.39	-1.37
I2A	-1.48	-1.43	-1.16	-1.11
I5	-1.19	-1.14	-0.85	-0.8

Three body complexes type CO₂-Oxazole-N₂

Since this article focuses on the CO₂ separation properties relative to N₂, the most abundant gas in Earth's atmosphere (78.084%)⁵⁰, our aim is to understand, at the molecular level, whether oxazole-based MOF’s can simultaneously capture two different gases and how the CO₂ capture capacity varies in the presence of N₂, which a third body can alter the interaction between two bodies. To achieve this, we performed geometry optimizations on three-body systems, namely N₂-oxazole-CO₂, starting from various configurations where CO₂ and N₂ occupy the equilibrium positions of the two-body systems, oxazole-CO₂ and oxazole-N₂. Then, we examined the capture of the second gas. The resulting structures are presented in Table 6 and illustrated in Figure 3, in Figure 3A, the equilibrium structures of three systems are shown, obtained using the equilibrium configurations of the oxazole-CO₂complex and rotating N₂ around oxazole. However, in Figure 3B, the equilibrium structures of three other systems are displayed, obtained using the equilibrium configurations of the oxazole-N₂complex and rotating CO₂ around oxazole. The structures in Figure 3A are identified by the symbol "InIm," where "In" and "Im" respectively represent the main complexation pathways (I1, I2, I3, I4, and I5) of CO₂and N₂ with oxazole (Figure 1C), while in Figure 3B, they are identified by the symbol "ImIn". The binding energies in Table 6 are determined by combining the AVDZ basis set with MP2 correlation, which is an appropriate method for three-body systems. We opted for AVDZ over AVTZ due to the size of the systems, which makes the use of AVTZ impossible. These values are then compared to those of two-body complexes (oxazole+X, where X= CO₂, N₂) calculated using the same level of theory. It is observed that the presence of a second gas appears to be beneficial, thereby enhancing the oxazole's ability to capture both CO₂ and N₂without causing competition.

Table 6. MP2/AVDZ binding energies of the two-body and three-body complexes (E_b, kcal/mol).

Complex	E_int	E_int +ZPVE	Complex	E_int	E_int +ZPVE
Oxazole-CO₂			Oxazole-CO₂+N₂
I1	-4.48	-3.95	I1I5	-5.94	-5.13
I2	-3.13	-2.74	I1I2A	-6.18	-5.40
			I1AI2B	-6.23	-5.44
			I1AI2B	-6.57	-5.70
			I1AI1A	-6.57	-5.71
			I1AI3	-8.40	-7.31
Oxazole-N₂			Oxazole-N₂ +CO₂
I1A	-1.91	-1.51	I1AI1A	-4.85	-4.09
I1B	-1.94	-1.56	I1A I4	-3.93	-3.26
I2A	-1.61	-1.28	I1B I4	-4.81	-3.98
I5	-1.40	-1.03	I2A I4	-3.60	-2.99
			I1B I2	-5.03	-4.354

Figure 3. Three-body complexes composed by CO₂-imidazole-N₂.

A) CO₂-imidazole + N₂ complexes, they are labeled using the symbol ‘‘InIm”.‘‘n” denotes the location of CO₂ and ‘‘m” refers to the position of N₂. B) N₂-imidazole + CO₂ complexes, they are labeled using the symbol ‘‘ImIn”.‘‘ m ” denotes the location of N₂ and ‘‘n ” refers to the position of CO₂.

The interaction energies of the three-body systems increase, is higher than in the case of the two-body oxazole + CO₂ complexes. In the case of the oxazole-N₂+CO₂ complexes, the binding energy ranges between -3.60 kcal/mol and -5.03 kcal/mol, compared to the two-body systems, oxazole+N₂, whose binding energies are respectively between -1.40 kcal/mol and -1.94 kcal/mol. So, for the oxazole-CO₂+N₂ complexes, the binding energy ranges between -5.94 kcal/mol and -8.40 kcal/mol, compared to the two-body systems oxazole+CO₂, which exhibit binding energies ranging from -3.13 kcal/mol to -4.48 kcal/mol. This is explained by the formation of an additional intramolecular bond between the molecules of the three systems (π-stacking and Van der Waals interactions), which can stabilize the systems. The inclusion of the ZPE vibrational correction results in a decrease in the energy quantities of three systems, ranging from -0.49 kcal/mol to -1.09 kcal/mol.

Oxa@Au₆, Cu₆ and Zn₃O₃ and CO₂/N₂–Ox@Au₆, Cu₆ and Zn₃O₃

Recently, various technologies have emerged for CO₂ capture, utilizing methods such as chemisorption and physisorption, due to the development of nanodevices such as metal nanoclusters and metal oxide nanoclusters⁵¹. Metal and metal oxide nanocomposite materials have been particularly used in a wide range of applications, including energy storage⁵², drug delivery⁵³, and the adsorptive removal of a diverse range of pollutants from contaminated environments⁵⁴. These applications benefit from the materials' high stability, ease of synthesis, and ability to be modulated into various shapes and sizes. This section focuses on investigating the mechanisms of adsorption and activation of CO₂ and N₂by metal clusters, Au₆ and Cu₆, as well as on the metal oxide Zn₃O₃. To bolster the CO₂ and N₂ adsorption capability, this choice of cluster was motivated by the tendency of clusters with an even number of particles to demonstrate higher stability compared to their odd-sized counterparts, along with possessing a larger HOMO-LUMO gap^55–57. Moreover, these clusters exhibit a rapid increase in binding energies with increasing cluster size. Notably, these nanoparticles possess the ability to bind with various biological or medicinal molecules containing relevant heteroatoms like nitrogen and oxygen, all while exhibiting low toxicity⁵⁸. The CO₂/N₂–Oxa@Au₆, Cu₆, or Zn₃O₃ complexes were initially formed by attaching the stable structure of two distinct complexes, namely I1 (Oxa@ CO₂) and I1A (Oxa@ N₂), to the most stable form of Au₆, Cu₆, and Zn₃O₃ (a planar triangle with D3h symmetry), using electron-rich positions such as N^56,57,59,60. Harmonic vibration frequencies were also calculated at the same theoretical level to identify the obtained structures as local minima. Figure 4 displays the optimized structures for Oxa@Au₆, Cu₆ and Zn₃O₃ and for CO₂/N₂–Oxa@Au₆, Cu₆ and Zn₃O₃. This figure shows that in the Oxazole@cluster structure, the gold, copper, or zinc atoms situated at the apex of the octahedron forms a covalent bond interaction at the O4 positions of the oxazole molecule. As seen in Figure 4, both gases are physically adsorbed, and the I1 and I1A structures of the Oxa@CO₂and Oxa@N₂ complexes are moderately perturbed by the presence of the cluster. Consequently, in the structure I1@Au₆, I1@Cu₆, the hydrogen bond between the oxygen O10 and H6 of the oxazole breaks, and a new hydrogen bond forms between O11 and H7 of the oxazole. The calculated MP2/AVDZ/ LanL2DZ BEs of the CO₂/N₂–Oxa@Au₆,Cu₆ and Zn₃O₃ complexes are listed in the Table 7. Our results show that metallic and metal oxide clusters favor the adsorption of CO₂ and N₂ gases. In hexametallic clusters, the adsorption energies (E_b) reach a maximum with the Cu₆ cluster, at -6.46 kcal/mol for CO₂ and -10.25 kcal/mol for N₂. However, the Au₆ and Zn₃O₃ clusters exhibit similar binding energy values: E_b(Au₆@I1) = -4.58 kcal/mol and E_b(Zn₃O₃@I1) = -4.56 kcal/mol for CO₂, and E_b(Au₆@I1) = -2.19 kcal/mol and E_b(Zn₃O₃@I1) = -2.22 kcal/mol for N₂. This is highly advantageous as it allows for the replacement of expensive gold clusters with more cost-effective materials such as Zn₃O₃.

Figure 4. Optimized structures of Oxa@Au₆, Cu₆ and Zn₃O₃ and of CO₂/N₂–Oxa@ Au₆,Cu₆ and Zn₃O₃ complexes at MP2/ AVDZ/LanL2DZ method.

Table 7. MP2/AVDZ/LanL2DZ binding energies of the CO₂/N₂–Oxa@Au₆, Cu₆ and Zn₃O₃ complexes (E_b, kcal/mol).

		Au₆	Cu₆	Zn₃O₃
Complexes	I1	I1	I1	I1
Oxazole@CO₂	-4.48	-4.58	-6.46	-4.56
Oxazole@N₂	-1.77	-2.19	-10,25	-2.22

Figure 5. Optimized structures of Oxa@ Au (111) /Cu (111) and ZnO (111) and of CO₂/N₂–Oxa@ Au (111)/ Cu (111) and ZnO (111) complexes, derived from the MC simulation.

Adsorption of CO₂ and N₂ in the metal and metal oxide surfaces (111)

The Molecular simulation (Monte Carlo) was conducted to examine the adsorption of CO₂ and N₂ on Au, Cu, and ZnO (111) surfaces. The choice of Au and Cu surfaces (111) is motivated by the interest in the interaction of small molecules Au (111) surface. Furthermore, adsorption on noble metals (such as Cu, Ag, and Au) is considered a promising avenue for materials in futures technologies and applications⁶¹, however, the selection of ZnO aims to ensure high efficiency in the low-cost adsorption of pollutants. The investigation commenced with optimizing the geometries of the Oxa@CO₂ and Oxa@N₂ complexes using the COMPASS II force field in the Forcite Module. Subsequently, we constructed three-layer models for each surface (Au(111), Cu(111), and ZnO(111)). In the Adsorption Locator Module, we selected one of these surfaces and added the adsorbate, either Oxa@CO₂ or Oxa@N₂, before running the calculations. This process was repeated for all surfaces. The optimized geometries and their corresponding adsorption energies are depicted in the Figure 5. The calculations show spontaneous physisorption of CO₂ and N₂. Furthermore, the ZnO surface is the most favorable for N₂ adsorption compared to CO₂, with an adsorption energy reaching -151.62 kcal/mol for N₂, while for CO₂, it reaches a value of -148.01 kcal/mol. However, for the two other surfaces, Cu (111) and Au (111), the adsorption of CO₂ is more favorable than that of N2, with respective adsorption energy values of -36.97 kcal/mol and -32.88 kcal/mol for Au, and -22.27 kcal/mol and -19.52 kcal/mol for Cu (111). Overall, our calculations demonstrate that the surfaces of gold (Au), copper (Cu), and zinc oxide (ZnO) are capable of adsorbing N₂ and CO₂ from gas mixtures. Moreover, ZnO and Cu offer advantages in terms of adsorption capacity for both gases, without introducing additional cost-related issues.

Conclusion

In summary, our study focused on two-body van der Waals complexes Oxa@CO₂ and Oxa@N₂, as well as three-body complexes CO₂@Oxa@N₂ and Oxa@CO₂@clusters (Au₆, Cu₆, and Zn₃O₃), using the Møller-Plesset perturbation theory (MP2) method with AVTZ and AVDZ basis sets. Our calculations reveal that oxazole exhibits adsorption capacity for both gases. Examination of the binding energies shows a significantly higher CO₂ adsorption capacity compared to N₂. Furthermore, the presence of a second gas does not alter the oxazole's ability for simultaneous capture of both gases, but may enhance adsorption through hydrogen bonding formation among the three systems. Adding clusters Au₆, Cu₆, and Zn₃O₃ to the most stable structures I1 of the Oxa@CO₂ complex and I1A of the Oxa@N₂ complex increases the adsorption capacity for both gases, with Cu₆ exhibiting the highest capacity compared to Au₆ and Zn₃O₃. Finally, the investigation of interactions between the most stable structures of the Oxa@CO₂ and Oxa@N₂ complexes with the three (111) surfaces of Au, Cu, and ZnO demonstrates that gold (Au), copper (Cu), and zinc oxide (ZnO) surfaces are capable of adsorbing N₂ and CO₂ from gas mixtures. Moreover, ZnO and Cu offer advantages in terms of adsorption capacity for both gases, without introducing additional cost-related issues.

Ethics and consent

Ethics and consent were not required.

Data availability statement

No data are associated with this article

Acknowledgements

The authors acknowledge the CTI of Universidad Autónoma de Chile and CSIC for computing facilities. And they express their gratitude to the Researchers Supporting Project (RSPD2024R808) of King Saud University, Riyadh, Saudi Arabia.

Faculty Opinions recommended

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Research Article

Published

January 8, 2025

DOI

https://doi.org/10.12688/openreseurope.18925.1

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September 22, 2025 (Version of Record 3.0)
September 11, 2025 (Version of Record 2.0)
July 29, 2025 (Version of Record 1.0)
January 8, 2025 (Published Manuscript Under Review 1.0)

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European Commission

Grant ID(s): 872081

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Keywords

Adsorption, CO2 capture, N2, atmosphere, ab initio, MOF’s.

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A. Belasri

Laboratory of Organic and Physical Chemistry, Research Team: Molecular Modeling, Materials, and Environment, Department of Chemistry, Faculty of Sciences Agadir, Ibn Zohr University, Agadir, Morocco, B.P. 8106, Morocco

Roles

Co-Author, Data Curation, Investigation

abdessamad.belasri1999@gmail.com

F. Tahiri

Roles

Co-Author, Investigation, Visualization

fatimatahiri249@gmail.com

O. Douass

Optics, Material and Systems Team, , Faculty of Sciences, Abdelmalek Essâadi University, B.P. 2121, M’Hannech II, 93030 Tétouan, Morocco

Roles

Co-Author, Data Curation, Visualization

0000-0001-8988-265X

oumaima.douass@etu.uae.ac.ma

N. Inostroza-Pino

Facultad de Ingeniería, Núcleo de Astroquímica & Astrofísica, Universidad Autónoma de Chile, Av. Pedro de Valdivia 425, Providencia, Santiago, Chile

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Co-Author, Formal Analysis, Software

natalia.inostroza@uautonoma.cl

M. Belmouden

Roles

Co-Author, Formal Analysis, Methodology

m.belmouden@uiz.ac.ma

H. Bahmann

Physical and Theoretical Chemistry, University of Wuppertal, Wuppertal, Germany

Roles

Co-Author, Supervision

bahmann@uni-wuppertal.de

M. Mogren Al-Mogren

Department of Chemistry, College of Sciences, King Saud University, P.O. Box 2455, Riyadh, 11451, Saudi Arabia

Roles

Co-Author, Formal Analysis, Visualization

mmogren@ksu.edu.sa

M. L. Senent

Departamento de Química y Física, Instituto de Estructura de la Materia, Madrid, Unidad Asociada GIFMAN, CSIC-UHU, Spain

Roles

Co-Author, Methodology, Project Administration, Validation, Writing – Original Draft Preparation

0000-0003-1878-7377

ml.senent@csic.es

S. Dalbouha

Roles

Corresponding Author, Co-Author, Conceptualization, Data Curation, Investigation, Supervision, Writing – Original Draft Preparation

s.dalbouha@uiz.ac.ma

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Theoretical structural study of van der Waals complexes between oxazole and atmospheric gases CO 2 and N 2 for capture applications.

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A. Belasri

F. Tahiri

O. Douass

N. Inostroza-Pino

M. Belmouden

H. Bahmann

M. Mogren Al-Mogren

M. L. Senent

S. Dalbouha

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