Hydroxy-substituted aromatic N-heterocycles as high-affinity CO₂ adsorbents: a DFT study

Abstract

Hydroxy-substituted aromatic N-heterocycles, including hydroxy pyridine (py), dihydroxy naphthyridines (nt), and trihydroxy pyridonaphthyridines (pn), have been investigated for their potential as CO₂ adsorbents using density functional theory (DFT) calculations. Building on the pioneering work of Luo et al., who demonstrated exceptional CO₂ capture capacities in pyridine-containing anion-functionalized ionic liquids, this study extends the exploration to a broader range of N-heterocycles. These N-heterocycles exhibit exceptional CO₂ capture capabilities, driven by cooperative interactions between nitrogen and oxygen centres with CO₂. The adsorption capacity increases with the number of nitrogen centres and hydroxy groups, with py, nt, and pn systems binding one, two, and three CO₂ molecules, respectively. Notably, anionic N-heterocycles exhibit dramatically improved CO₂ adsorption compared to their neutral counterparts, forming covalent bonds with CO₂. The presence of counter cations, such as lithium or tetramethylphosphonium ions, further stabilizes CO₂ adsorption, resulting in shorter interaction distances and higher exergonic free energy values. Solvent effects modeled using monoethanolamine (MEA) indicate a modest reduction in interaction energies for neutral and anionic systems, while ion-paired systems exhibit enhanced CO₂ affinity in solution. Additionally, molecular electrostatic potential (MESP) analysis highlights the key adsorption sites and charge delocalization mechanisms that facilitate CO₂ capture. The study also finds that enol–keto transformations, which could lead to CO₂ conversion into carboxylates, are energetically unfavorable due to the loss of aromatic stability. These findings underscore the potential of hydroxy-substituted N-heterocycles, particularly in their anionic and cation-stabilized forms, as promising candidates for efficient CO₂ capture. The insights gained from this study provide valuable guidelines for the design of next-generation CO₂ sequestration materials and highlight new directions for experimental validation and real-world applications.

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Introduction

The rising levels of atmospheric CO₂ have driven global efforts to develop efficient capture and conversion strategies.^1–7 Traditional methods, such as sequestration and storage face significant challenges related to cost, efficiency, and environmental impact.^8–10 In contrast, adsorption-based techniques employing nitrogen-rich molecules and materials present a promising alternative due to their operational simplicity and lower energy demands.^11–15

Ionic liquids (ILs) have gained attention as versatile media for both CO₂ capture and conversion. The evolution of ILs, from conventional to amine-functionalized and multi-sited ILs, has led to remarkable improvements in CO₂ absorption efficiency.^16–27 Among these advancements, the work by Luo et al. stands out as a significant contribution to the field. In their seminal study, Luo et al. demonstrated that pyridine-containing anion-functionalized ionic liquids exhibit exceptional CO₂ capture capacities, achieving up to 1.60 mol CO₂ per mol IL through multiple-site cooperative interactions.²⁸ Their work revealed that the presence of nitrogen and oxygen atoms in the anion, along with π-electron delocalization in the pyridine ring, significantly enhances CO₂ binding. This cooperative interaction between the electronegative oxygen and nitrogen atoms in the anion and CO₂ was shown to be the key driver of the high CO₂ capture capacity.²⁸

Building on the insights from Luo et al.,²⁸ this study aims to further explore the potential of hydroxy-substituted aromatic N-heterocycles as high-affinity CO₂ adsorbents. While Luo et al. focused on the role of pyridine-containing ILs, we extend this approach to investigate a broader range of N-heterocycles, including hydroxy pyridines (py), dihydroxy naphthyridines (nt), and trihydroxy pyridonaphthyridines (pn), to understand how the strategic incorporation of nitrogen and hydroxy groups within these molecular structures can enhance CO₂ capture through cooperative binding mechanisms.

Recent studies have highlighted the significance of multiple-site cooperative interactions in enhancing CO₂ capture.^28–38 Inspired by Luo et al.'s findings,²⁸ we hypothesize that the presence of multiple nitrogen and hydroxy groups in these N-heterocycles will lead to enhanced CO₂ capture through similar cooperative interactions. Specifically, we aim to elucidate the electronic and structural factors governing CO₂ interaction and conversion in these systems, providing valuable insights for the design of efficient CO₂ capture materials.

This study will assess the CO₂ capture capacity of neutral and anionic forms of these molecules, identify the preferred binding sites for CO₂, investigate the thermodynamics of CO₂ capture and conversion processes, explore the influence of molecular structure, charge and counter cation on CO₂ interaction and conversion efficiency. By unravelling the intricate relationship between molecular structure, charge, and CO₂ capture capabilities, this study aims to contribute to the development of innovative solutions for mitigating the environmental impact of greenhouse gas emissions.

In this study, we focus on the adsorption of CO₂ onto functionalized organic molecules, rather than its absorption into bulk liquid or solid phases. Adsorption refers to surface-level binding at active sites, whereas absorption involves uptake into the bulk. The molecules studied here are potential precursors for integration into extended frameworks such as MOFs or ZIFs, or for surface immobilization on porous substrates to enhance CO₂ capture.⁸ For clarity, the term adsorption is used throughout this work to describe CO₂ binding to molecular surface.

Effective CO₂ capture requires optimizing adsorption energy and viscosity. A moderate adsorption energy (e.g., −5 to −10 kcal mol⁻¹) is often ideal-it indicates sufficiently strong binding for efficient capture but also reversible desorption, enabling regeneration.³⁹ Viscosity also influences capture performance, as lower viscosity improves mass transfer. Nitrogen-functionalized molecules, as explored in this study, offer an advantageous balance of moderate binding strength and favourable rheological properties, making them promising candidates for efficient CO₂ adsorption.^40,41

Methodology and molecular design strategies

All geometry optimizations were performed using density functional theory (DFT) at the M06-2X/6-311++G(d,p) level, as implemented in the Gaussian 16 suite of programs.⁴² The inclusion of diffuse functions in the basis set was essential for accurately describing the electronic structure of anionic complexes. All optimized geometries were confirmed to have only real vibrational frequencies, ensuring they represent true minima on the potential energy surface, while transition states exhibited a single imaginary frequency, validating their nature as first-order saddle points. The M06-2X functional is well-regarded for its accuracy in modelling intermolecular non-covalent interactions, crucial in understanding CO₂-adsorbent complexes.⁴³ To identify the most stable equilibrium structures, various initial guess geometries were considered, placing CO₂ near electron-rich sites of the adsorbent. The lowest energy structures from these optimizations were used for further analysis. The adsorption energy (ΔE_ad) of CO₂ was calculated using the supermolecule approach as follows (eqn (1)):

ΔE_ad = E_complex − (E_adsorbent + E_CO2) + E_BSSE (1)

Here, E_complex, E_adsorbent, E_CO2 are the zero-point energy (ZPE)-corrected total energies of the complex, the isolated N-heterocycle (Nhet) and CO₂, respectively (Unless stated otherwise, all energies reported hereafter include ZPE corrections). The basis set superposition error (E_BSSE), was evaluated using the counterpoise correction method of Boys and Bernardi⁴⁴ to account for artificial stabilization arising from basis set overlap. To account for solvent effects, which significantly influence chemical reactivity, selectivity, and product stability,⁴⁵ single-point energy calculations were performed at the M06-2X/6-311++G(d,p) level using the gas-phase optimized geometries. Solvation by monoethanolamine (MEA), a commonly used amine-based solvent in post-combustion CO₂ capture systems, was modeled using the SMD (solvation model based on density) approach within the self-consistent reaction field (SCRF) framework.^46,47 MEA was chosen not to model absorption into the solvent itself but to serve as a reference medium, enabling a comparative assessment of the CO₂ adsorption performance of the N-heterocyclic systems relative to a well-established industrial benchmark.

In addition to adsorption energies (ΔE_ad), Gibbs free energies of adsorption (ΔG_ad) were computed to offer a more complete thermodynamic perspective. These calculations were performed in the gas phase at 293 K and 1 atm, consistent with ambient CO₂ capture conditions.

To further elucidate electronic features relevant to CO₂ binding, topological analysis of the molecular electrostatic potential (MESP), V(r), was carried out. The MESP as defined in eqn (2) was computed at the M06-2X/6-311++G(d,p) level for all optimized geometries using Gaussian 16. This analysis identifies electron-rich regions such as lone pairs and π-systems – critical for understanding interaction sites in the hydroxy-substituted N-heterocycles, their anionic counterparts, and the corresponding CO₂ complexes.

In eqn (2), ρ(r′) is the electron density, Z_A is the charge on nucleus A located at R_A, and r′ is a dummy integration variable.^48,49 The MESP value at a (3, +3) critical point observed in electron rich regions such as lone pairs and π-bonds in topology analysis is called V_min. The V_min analysis is useful to understand the subtle variations and delocalization in electron density distribution due to the interactive behaviour of molecules.^49–52

The molecular design strategy focuses on how introducing nitrogen and oxygen atoms affects the CO₂ capture capacity of N-heterocycle. To explore this, we studied a series of hydroxy-substituted molecules – pyridine (py), naphthyridine (nt) and pyridonaphthyridine (pn) – featuring both two- and three-membered rings with systematically increasing numbers of carbon, nitrogen, and oxygen atoms. This enables evaluation of enhanced cooperative interactions due to the increased number of potential CO₂ binding sites.

The choice of these molecular systems is grounded in prior research: hydroxy pyridines (py) have been examined for CO₂ adsorption, notably by Luo et al. in the context of ionic liquids.²⁸ While naphthyridines (nt) have been extensively studied for their pharmacological properties,^53–57 their potential for CO₂ capture remains largely unexplored. Similarly, some pyridonaphthyridine derivatives have shown therapeutic promise,^58,59 but their abilities for CO₂ adsorption have not been thoroughly investigated.

Additionally, the study extends to anionic forms of hydroxy-N-heterocycles (deprotonated form) to assess the impact of electron density on CO₂ binding.^60–76 We analyze how electron distribution evolves with the addition of aromatic rings and heteroatoms as the molecules transition from monoanionic to dianionic and trianionic states.

By combining the molecular design strategies with DFT calculations and MESP analysis,^{50–52,77,78} this study aims to provide a comprehensive understanding of the structure–activity relationships governing CO₂ capture and conversion in these systems.

Results and discussion

N-Heterocycles and CO₂ complexes

The chemical structure of hydroxy pyridine (py), the selected set of dihydroxy naphthyridines (nt to nt12), and the set of trihydroxy pyridonaphthyridines (pn1 to pn36) are presented in Fig. 1 (see ESI† for their IUPAC names, Fig. S1). Collectively, these compounds are referred to as N-heterocycles (Nhet). In all Nhet systems, the hydroxy groups are strategically positioned adjacent to nitrogen atoms to maximize the potential for synergistic interactions with CO₂. This design rationale is based on calculated adsorption energies (ΔE_ad) for positional isomers of hydroxypyridine: 2-hydroxypyridine showed the most favourable interaction energy (−11.3 kcal mol⁻¹), compared to −10.3 kcal mol⁻¹ for 3-hydroxypyridine and −10.4 kcal mol⁻¹ for 4-hydroxypyridine. The enhanced performance of 2-hydroxypyridine is attributed to cooperative binding between the adjacent nitrogen and hydroxyl functionalities, which strengthens both the N⋯CO₂ and O–H⋯OCO interactions.

Fig. 1 Chemical structures of hydroxy pyridine (py), 12 dihydroxy naphthyridines (nt), and 36 trihydroxy pyridonaphthyridines (pn).

The minimum energy structures for each Nhet compound were identified, along with stable configurations of their CO₂ complexes, denoted as Nhet⋯(CO₂)_n complexes, where n = 1 for py, n = 1 and 2 for nt and n = 1, 2, and 3 for pn systems. The relative energy of each structural isomers of nt and pn (ΔE_rel) was calculated using the equation (eqn (3)):

ΔE_rel = E_sample − E_standard (3)

where E_sample is the energy of the isomer under consideration, and E_standard is the energy of the most stable isomer. By this convention, the most stable isomer has ΔE_rel = 0.0 kcal mol⁻¹, and positive values denote less stable structures (Table 1 and 2).

Table 1 V _min, ΔE_rel, BSSE corrected ΔE_ad and ΔG_ad values of dihydroxy naphthyridines at M06-2X/6-311++G(d,p) level of theory. All values in kcal mol⁻¹

Molecules	V min		ΔErel	ΔEad1	ΔEad2	ΔGad1	ΔGad2
	N1	N2
nt1	−53.1	−53.1	0.0	−10.5	−10.7	−3.7	−2.7
nt2	−47.0	−42.8	0.5	−10.9	−10.1	−3.1	−3.7
nt3	−43.3	−43.2	1.5	−10.5	−10.0	−3.4	−3.5
nt4	−43.8	−43.7	2.9	−10.7	−10.1	−3.6	−3.6
nt5	−44.0	−43.9	3.5	−10.8	−10.3	−3.9	−3.4
nt6	−44.5	−42.4	4.0	−11.0	−10.1	−3.8	−3.4
nt7	−51.4	−47.6	5.1	−10.2	−10.1	−3.9	−3.6
nt8	−44.5	−42.0	5.5	−10.6	−9.9	−3.6	−3.7
nt9	−45.2	−44.9	6.1	−10.8	−10.3	−3.4	−3.4
nt10	−44.2	−40.9	6.5	−10.8	−10.0	−3.7	−3.4
nt11	−41.2	−41.0	6.5	−10.6	−10.2	−3.7	−3.6
nt12	−40.2	−40.1	10.3	−10.5	−10.0	−3.8	−3.7

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Table 2 ΔE_rel, BSSE corrected ΔE_ad and ΔG_ad values of trihydroxy pyridonaphthyridines at M06-2X/6-311++G(d,p) level of theory. All values in kcal mol⁻¹

Molecules	ΔErel	ΔEad1	ΔEad2	ΔEad3	ΔGad1	ΔGad2	ΔGad3
pn1	0.0	−10.5	−10.8	−6.4	−4.1	−2.0	−3.1
pn2	2.5	−10.8	−10.2	−8.1	−3.1	−3.8	−3.0
pn3	4.2	−11.6	−6.3	−9.4	−4.5	−3.2	−2.4
pn4	4.7	−10.5	−10.8	−9.0	−1.7	−3.8	−4.7
pn5	4.7	−10.5	−10.2	−7.9	−1.7	−5.0	−3.4
pn6	5.0	−10.6	−10.0	−7.8	−3.0	−1.9	−2.1
pn7	5.2	−10.8	−10.0	−9.6	−3.2	−3.7	−3.5
pn8	5.2	−10.5	−10.7	−9.1	−3.3	−3.7	−3.6
pn9	5.3	−10.8	−10.9	−9.0	−3.2	−3.6	−3.5
pn10	5.5	−10.2	−9.2	−7.8	−2.0	−4.9	−3.5
pn11	5.7	−9.9	−10.7	−6.2	−1.9	−5.1	−0.9
pn12	6.2	−10.8	−10.1	−9.4	−3.5	−3.7	−3.6
pn13	6.4	−10.5	−10.1	−8.2	−3.6	−3.7	−3.5
pn14	6.6	−10.5	−10.6	−8.6	−3.4	−3.7	−3.6
pn15	6.9	−10.5	−10.5	−9.0	−3.4	−3.7	−3.5
pn16	7.4	−10.5	−10.7	−9.0	−3.7	−3.6	−1.8
pn17	7.5	−10.9	−10.0	−9.5	−3.8	−3.4	−3.6
pn18	7.8	−9.7	−10.8	−9.2	−3.7	−3.7	−3.5
pn19	8.0	−10.6	−9.8	−6.7	−3.7	−3.5	−3.7
pn20	8.0	−10.7	−10.1	−9.4	−3.7	−3.6	−3.6
pn21	8.8	−10.9	−10.1	−9.2	−3.7	−3.7	−2.0
pn22	8.9	−10.6	−9.9	−9.4	−3.8	−3.7	−3.3
pn23	9.0	−10.4	−10.0	−9.2	−3.7	−3.6	−3.5
pn24	9.2	−10.6	−9.8	−9.5	−3.7	−3.5	−3.4
pn25	9.4	−10.8	−9.8	−9.5	−3.6	−3.5	−3.4
pn26	9.6	−10.8	−10.0	−9.4	−3.8	−3.5	−3.5
pn27	9.7	−10.6	−10.9	−7.2	−3.6	−3.5	−0.4
pn28	9.7	−9.7	−10.3	−9.8	−3.5	−3.9	−1.7
pn29	9.8	−10.6	−9.8	−9.4	−3.4	−3.7	−1.5
pn30	10.0	−10.3	−10.0	−8.9	−3.5	−3.5	−3.1
pn31	10.2	−9.8	−10.0	−9.6	−4.9	−1.0	−3.7
pn32	10.3	−9.9	−10.6	−9.4	−3.9	−2.5	−2.1
pn33	10.9	−10.3	−10.0	−9.3	−3.8	−2.6	−3.4
pn34	12.4	−10.5	−9.6	−9.4	−4.2	−2.1	−3.5
pn35	12.6	−10.3	−9.9	−9.2	−3.8	−2.5	−3.6
pn36	15.5	−10.8	−10.1	−9.5	−4.0	−2.2	−3.8

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The Nhet⋯CO₂ complex is characterized by two key interactions: the nitrogen-to-carbon (N⋯CO₂) interaction and the CO₂ oxygen-to-hydroxy hydrogen (CO₂⋯HO) interaction. The N⋯CO₂ interaction enhances electron density at the oxygen atoms of CO₂, thereby strengthening the CO₂⋯HO interaction. This synergistic effect results in positive cooperativity, where an improvement in N⋯CO₂ interaction leads to enhanced CO₂⋯HO interaction. Consequently, the CO₂ adsorption efficiency of Nhet can be attributed to the combined influence of these mutually reinforcing interactions.

MESP analysis of N-heterocycles and CO₂ complexes

Molecular electrostatic potential (MESP) topology analysis uses the minimum electrostatic potential (V_min) to map lone pair electron density around relevant atoms.⁵² As illustrated in Fig. 1, nitrogen centres in Nhet are identified based on their V_min values and are numbered in decreasing order of magnitude, with N1 being the most electron rich, followed by N2 and N3. These nitrogen atoms serve as the primary interaction sites for CO₂.

The V_min distribution of a representative set of Nhet systems is shown in Fig. 2. In py, the V_min value for the nitrogen lone pair is −47.9 kcal mol⁻¹. Among nt systems, nt1 exhibits the most negative V_min (−53.1 kcal mol⁻¹) while nt12, with a V_min −40.1 kcal mol⁻¹, comparatively less electron-dense. The V_min values of N1 and N2 within the same nt molecule show minimal variation, with an average difference of approximately 10 kcal mol⁻¹. In pn systems, V_min varies significantly across N1, N2 and N3, ranging from −53.1 to −31.3 kcal mol⁻¹, depending on the position of nitrogen centres (Table S2, ESI†). Based on the MESP analysis, nitrogen centres are identified as the most probable sites for CO₂ interaction. The affinity of CO₂ for different nitrogen centres follows a general trend, where a more negative V_min corresponds to stronger interaction potential. MESP plotted on 0.005 a.u. isodensity surface provides a visual representation of electron reorganization during CO₂ complexation with N-heterocycles (Fig. 3).⁷⁹ When the carbon atom of CO₂ accepts electron density from the nitrogen centre of Nhet, a counterbalancing electron density donation occurs from the oxygen of CO₂ to the hydroxy hydrogen of Nhet. This process leads to delocalization of electron density around the lone pairs of nitrogen and oxygen atoms due to the N⋯CO₂ and CO₂⋯HO interactions. Because of the mutually compensating effect of these interactions, apart from the noncovalently bonded regions, the overall electron distribution in the Nhet⋯CO₂ complex remains largely unchanged outside the noncovalently bonded regions. For instance, in pn systems, the oxygen atoms of hydroxy groups exhibit only minor changes in V_min even with successive CO₂ adsorption. Similarly, nitrogen atoms not directly involved in noncovalent bonding retain nearly the same V_min values in Nhet⋯CO₂ and Nhet⋯(CO₂)₂ complexes. This consistency suggests a steady interaction energy value for CO₂ adsorption, allowing Nhet to effectively capture up to three CO₂ molecules.

Fig. 2 MESP topography of representative cases of py, nt and pn system at M06-2X/6-311++G(d,p) level of theory. The black dots represent V_min positions. MESP isosurface value at −12 kcal mol⁻¹. All values in kcal mol⁻¹. (Color key: green-C, grey-H, blue-N, red-O).

Fig. 3 MESP on 0.005 a.u. isodensity surface for CO₂, N-heterocycles and CO₂ complexes of N-heterocycles. V_min values (kcal mol⁻¹), N⋯CO₂ (d_NC) and CO₂⋯HO (d_OH) distances (Å) at M06-2X/6-311++G(d,p) level of theory. Colour code: blue (electron-rich) to red (electron-deficient).

Transition state (TS) calculations

To further understand the CO₂ capture process, transition state (TS) calculations were performed to evaluate the activation energies for the reaction pathways involving different CO₂ binding configurations. For the py system, the transition state for the first CO₂ binding (TS1) is located at −0.7 kcal mol⁻¹, with the corresponding acid formation at −2.9 kcal mol⁻¹. In the case of nt5, the activation energy for CO₂ binding decreases as additional CO₂ molecules are added, with values of −1.3 kcal mol⁻¹ for the first binding, −5.0 kcal mol⁻¹ for the diacid formation (TS2). The pn36 system exhibits a similar trend, with transition state energies of −2.2 kcal mol⁻¹ for TS1 and −5.1 kcal mol⁻¹ for TS2, with a significant drop in activation energy observed at −0.9 kcal mol⁻¹ for the triacid formation (TS3). These calculations provide valuable insight into the energetic barriers for CO₂ binding and highlight the influence of molecular structure on the activation energies and stability of the intermediates.

CO₂ adsorption and enol–keto transformation in N-heterocycles

The nitrogen-to-carbon interaction distance (d_NC) serves as an initial indicator of interaction strength. Notably, all captured CO₂ molecules bind to the nitrogen centre at distances ranging from 2.70 to 2.90 Å. The complexes are further stabilized by CO₂⋯HO hydrogen bonding, with hydrogen bond distance (d_OH) ranging from 1.98 to 2.12 Å (Table S3, ESI†). Representative examples of Nhet⋯(CO₂)_n complexes are illustrated in Fig. 4. The noncovalent interactions slightly reduce the OCO bond angle from 180°.

Fig. 4 Optimized structures of Nhet⋯(CO₂)_n complexes at M06-2X/6-311++G(d,p) level of theory, with bond distances in Å and bond angle in degrees. (Color key: green-C, grey-H, blue-N, red-O).

The py⋯CO₂ complex demonstrates an interaction energy (ΔE_ad) of −10.8 kcal mol⁻¹, with a corresponding free energy of interaction (ΔG_ad) of −3.8 kcal mol⁻¹, confirming its exergonic nature. For nt systems, ΔE_ad1 represents the reaction nt + CO₂ → nt⋯CO₂, corresponding to CO₂ adsorption at the N1 site. Similarly, ΔE_ad2 refers to the energy released upon CO₂ adsorption at the N2 site of the nt⋯CO₂ complex. In pn systems, ΔE_ad1, ΔE_ad2 and ΔE_ad3 correspond to CO₂ adsorption energies at N1, N2 and N3 atoms, respectively. The data presented in Tables 1 and 2 suggest that the combined effect of N⋯CO₂ and CO₂⋯HO interactions result in nearly equivalent CO₂ adsorption strength across N1, N2 and N3 in all Nhet systems. The interaction energy for the first CO₂ adsorption is approximately −10.5 kcal mol⁻¹, for the second CO₂ adsorption is −10.1 kcal mol⁻¹, and for the third CO₂ adsorption is −8.8 kcal mol⁻¹. Moreover, each stage of the CO₂ adsorption is exergonic, with average ΔG_ad1, ΔG_ad2 and ΔG_ad3 values of −3.5, −3.4 and −3.0 kcal mol⁻¹, respectively (Tables 1 and 2). These thermodynamic data confirm that the CO₂ adsorption capacity of naphthyridine is nearly double that of hydroxypyridine, while that of pyridonaphthyridine is nearly triple.

The CO₂⋯HO hydrogen bond interaction in Nhet⋯CO₂ can lead to a proton transfer, potentially converting the adsorbed CO₂ into a carboxylate functionality. In other words, the CO₂⋯HO–C interaction can transition to a CO₂H⋯O=C interaction, corresponding to an enol–keto transformation of the Nhet system. The schematic representation of CO₂ complex formation and enol–keto transformation is illustrated in Fig. 5 for a representative pn5 system. This transformation simultaneously alters the N⋯CO₂ noncovalent interaction into a covalent N–CO₂H bond. However, the enol–keto transformation is expected to be energetically unfavourable, as it significantly disrupts the π-electron distribution within the ring, eliminating the alternating single and double bond pattern necessary for aromatic stabilization. In Fig. 5, the regions with reduced aromatic character are highlighted in red. When one hydroxy group is converted to the keto form, the nt moiety is denoted as nt′, while the notation nt′′ represents the conversion of both hydroxy groups to the keto form. Similarly, py′ refers to the keto form of hydroxypyridine, while pn′, pn′′, and pn′′′ represent the keto forms of pyridonaphthyridines.

Fig. 5 Schematic illustration of CO₂ complex formation with pn5 and subsequent enol–keto transformations. A ring that cannot be represented by alternate single and double bonds is shown in red.

Although transformation of py⋯CO₂ to py′-COOH is exothermic by −2.9 kcal mol⁻¹, it is endergonic by 7.8 kcal mol⁻¹, indicating that py′-COOH formation is highly unlikely due to the loss of aromaticity in the ring structure. This trend is consistently observed across most nt and pn systems (Fig. S2, ESI†). In some cases, the formation of carboxylated products is even endothermic. As shown in Fig. 6, the CO₂ adsorption is energetically more favourable than its conversion to carboxylates across all systems due to the partial loss of aromatic character in the keto form. However, certain nt′-COOH and pn′-COOH (the keto forms) exhibit relatively favourable energy values compared to the others. For instance, in nt systems, the formation of nt2′-COOH, nt5′-COOH, nt6′-COOH, nt9′-COOH, and nt10′-COOH is exothermic, while the rest display endothermic behavior. Similarly, among monocarboxylated pn derivatives, pn2′–COOH, pn7′–COOH, pn9′–COOH, pn12′–COOH, pn17′–COOH, pn19′–COOH, pn20′–COOH, pn21′–COOH, pn25′–COOH, pn26′–COOH and pn36′–COOH show better energy data than the rest of the monocarboxylic acids. Compared to pn′–COOH, the relative energy of pn′′–(COOH)₂ is more negative only in two cases: pn21′′–(COOH)₂ and pn36′′–(COOH)₂. In contrast, all tricarboxylated pn′′′–(COOH)₃ systems have significantly higher energy compared to the corresponding pn⋯(CO₂)₃ complexes. Thus, the energy data presented in Fig. 6 strongly support the efficient capture of CO₂ by Nhet, while its conversion to carboxylates remains energetically demanding due to the loss of aromatic stabilization in the core structure.

Fig. 6 Relative free energy of the CO₂ complexes of nt and pn molecules. All values in kcal mol⁻¹.

To provide a more thermodynamically meaningful comparison of the different CO₂ adsorption processes, we calculated the relative Gibbs free energies (ΔG_rel) for all systems studied. ΔG_rel values offer insight into the spontaneity and favourability of each reaction under standard conditions (293 K, 1 atm), incorporating both enthalpic and entropic contributions. These values complement ΔE_rel and allow for a more accurate assessment of the adsorption–desorption equilibrium, which is particularly important for evaluating practical CO₂ capture applications (Fig. S2, ESI†). By analyzing ΔG_rel, we identify systems where CO₂ binding remains favourable not only in terms of interaction strength but also in terms of thermodynamic feasibility, thereby guiding the selection of optimal candidates for efficient and reversible CO₂ capture.

Interaction of anionic N-heterocycles with CO₂

Luo et al. explored the application of pyridine-containing anion-functionalized ionic liquids (ILs) for CO₂ capture.²⁸ The ILs were synthesized via the neutralization of hydroxypyridine using an ethanol solution of trihexyl(tetradecyl)phosphonium hydroxide. Their study proposed a plausible mechanism for CO₂ interaction, identifying the 2-pyridonate ion as the key active species. This ion engages in a two-site interaction with CO₂, where both the oxygen and nitrogen atoms within the anion participate in coordinating and binding CO₂. Building on this mechanism, this study delves into the interaction behavior of the 2-pyridonate ion (py⁻) with CO₂. Additionally, as a representative case, the naphthyridonate system (nt5⁻) is examined to explore the CO₂ adsorption properties of both its monoanionic (nt5⁻) and dianionic (nt5²⁻) forms. Similarly, for the pyridonaphthyridine system, a representative case, pn36, is selected to investigate the complexation behaviour of its monoanionic (pn36⁻), dianionic (pn36²⁻) and trianionic (pn36³⁻) forms with CO₂.

Among the various hydroxy-substituted N-heterocycles examined, nt5 and pn36 emerged as the most promising candidates for CO₂ capture based on their computed interaction energies. The nt5 system, representing the naphthyridine class, demonstrated the highest CO₂ adsorption energy among the twelve derivatives studied, suggesting a strong and favourable interaction with CO₂. Similarly, pn36, a trihydroxy-substituted pyridonaphthyridine, exhibited superior CO₂ binding performance compared to its analogues. These results highlight the importance of both the number and position of hydroxy groups relative to nitrogen centres in facilitating cooperative binding effects. Consequently, nt5 and pn36 were selected for detailed analysis in their anionic forms to further explore the impact of charge and substitution patterns on CO₂ adsorption efficiency.

Fig. 7 illustrates the MESP characteristics of these anions. The MESP minimum (V_min) in the vicinity of oxygen atom is consistently more negative than that near than the nitrogen centre. Relative to their neutral counterparts, the monoanions exhibit a significant increase in the magnitude of V_min, specifically by 134.0.0, 122.5 and 113.1 kcal mol⁻¹, respectively for py⁻, nt⁻, and pn⁻, respectively. Similarly, the dianions exhibit even more negative V_min values than monoanions, with increases of 76.4 and 83.2 kcal mol⁻¹ for nt²⁻and pn²⁻, respectively. The V_min of pn³⁻ is 79.5 kcal mol⁻¹ more negative than that of pn²⁻. This data suggests that the electron-donating interactive behavior of these anionic systems with CO₂ follows the order: trianion > dianion > monoanion » neutral.

Fig. 7 MESP isosurface at −125.0 kcal mol⁻¹ for anions, at −190.0 kcal mol⁻¹ for dianions and at −250.0 kcal mol⁻¹ for trianion. The location of the MESP minimum point (V_min) is shown with black dot and also the corresponding V_min value is depicted. (Color key: green-C, grey-H, blue-N, red-O).

The complexation of py⁻ with CO₂ is more stable when CO₂ binds via the nitrogen centre rather than the oxygen centre, with a stability difference of 0.9 kcal mol⁻¹. In both cases, CO₂ forms a covalent bond, as indicated by N–CO₂ and O–CO₂ bond distances within the covalent range. The nitrogen-bound covalent complex (py–CO₂⁻)_N and the oxygen-bound covalent complex (py–CO₂⁻)_O (Fig. 8) were further analysed for interactions with a second CO₂ molecule. In the scenario where one CO₂ binds at nitrogen and the second at oxygen, the total interaction energy (ΔE_ad) is −26.9 kcal mol⁻¹. This complex is denoted as (py⋯(CO₂⁻)₂)_N&O wherein the dotted line indicates that CO₂ molecules are connected noncovalently to N and O. In the case of (py–CO₂⁻)_O, the second CO₂ interacts noncovalently with the oxygen of the already covalently bound CO₂. This complex (py–CO₂⁻)_O⋯CO₂. A similar structure, (py–CO₂⁻)_N⋯CO₂ is also identified. Among the complexes the two-CO₂ complexes, the most stable configuration is (py–CO₂⁻)_N⋯CO₂ with an ΔE_ad of −31.4 kcal mol⁻¹.

Fig. 8 Optimized structures of the CO₂–adsorbed by py⁻, at M06-2X/6-311++G(d,p) level of theory. Distances in Å and angles in degrees. (Color key: green-C, grey-H, blue-N, red-O).

For the nt5⁻ anion, the nitrogen-bound (covalent) complex (nt5–CO₂⁻)_N exhibits an ΔE_ad of −13.6 kcal mol⁻¹, which is 1.5 kcal mol⁻¹ more stable than the oxygen-bound (noncovalent) complex (nt5⁻⋯CO₂)_O. Notably, an oxygen-bound covalent complex is not observed. When two CO₂ molecules are adsorbed, one binds covalently to nitrogen while the other interacts noncovalently with oxygen, forming the complex (CO₂⋯nt5–CO₂⁻)_N&O, with an ΔE_ad of −29.4 kcal mol⁻¹. Another identified structure, (nt5–CO₂⁻)_N⋯CO₂ describes the interaction of the second CO₂ with the oxygen of the initially adsorbed CO₂ molecule (Fig. 9).

Fig. 9 Optimized structures of the CO₂–adsorbed by nt5⁻, at M06-2X/6-311++G(d,p) level of theory. Distances in Å and angles in degrees. (Color key: green-C, grey-H, blue-N, red-O).

For the nt5²⁻ dianion interacting with two CO₂ molecules, among the various optimized structures (Fig. S3, ESI†), the most stable configuration is (nt5–(CO₂⁻)₂)_N&N. Here, both CO₂ molecules form covalent bonds with nitrogen, acquiring carboxylate character and delocalization of the excess negative charge. The ΔE_ad of this complex is −59.3 kcal mol⁻¹, meaning the interaction energy per CO₂ molecule adsorbed is −29.7 kcal mol⁻¹-more than twice that of the nt5⁻ anion and nearly three times that of neutral systems. Further CO₂ adsorption occurs via an oxygen connection (noncovalent), forming (CO₂⋯nt5–(CO₂⁻)₂)_N&N&O, which stabilizes the system by an additional 17.7 kcal mol⁻¹ (Fig. 10).

Fig. 10 Optimized structures of the CO₂-adsorbed by nt5²⁻, at M06-2X/6-311++G(d,p) level of theory. Distances in Å and angles in degrees. (Color key: green-C, grey-H, blue-N, red-O).

For the pn36⁻ anion, the oxygen-bound (noncovalent) complex (pn36⋯CO₂⁻)_O is slightly more stable than the nitrogen-bound (covalent) complex (pn36–CO₂⁻)_N (Fig. S4, ESI†). However, the most stable configuration with two CO₂ molecules originates from (pn36–CO₂⁻)_N. The complex (pn36–CO₂⁻)_N⋯CO₂, where a second CO₂ interacts with the oxygen of the first adsorbed CO₂, has an energy of −29.4 kcal mol⁻¹-more than twice the interaction energy of (pn36–CO₂⁻)_N, indicating strong positive cooperativity.

For the pn36²⁻ dianion, the nitrogen-bound (covalent) CO₂ complex (pn36⁻–CO₂⁻)_N is more stable than the oxygen-bound (covalent) complex (pn36⁻–CO₂⁻)_O by 8.8 kcal mol⁻¹ (Fig. S5, ESI†). The most stable configuration for two pn36²⁻ dianion with two CO₂ molecules is (pn36–(CO₂⁻)₂)_N&N, with an ΔE_ad of −50.0 kcal mol⁻¹, meaning that the ΔE_ad per CO₂ molecule adsorbed is −25.0 kcal mol⁻¹-approximately 12.0 kcal mol⁻¹ higher than that of the monoanion. When a third CO₂ molecule is adsorbed through an oxygen connection (noncovalent), forming (CO₂⋯pn36–(CO₂⁻)₂)_N&N&O, the interaction energy further improves by 17.6 kcal mol⁻¹ (Fig. 11).

Fig. 11 Optimized structures of the CO₂-adsorbed by pn36⁻ and pn36²⁻, at M06-2X/6-311++G(d,p) level of theory. Distances in Å and angles in degrees. (Color key: green-C, grey-H, blue-N, red-O).

For the pn36³⁻ trianion, the nitrogen-bound (covalent) CO₂ complex (pn36²⁻–CO₂⁻)_N, is the most stable among all configurations, with ΔE_ad = −46.1 kcal mol⁻¹ (Fig. 12). Similarly, the nitrogen-bound (covalent) complex with two CO₂ molecules, (pn36⁻–(CO₂⁻)₂)_N&N, shows the highest stability with ΔE_ad = −85.8 kcal mol⁻¹. Furthermore, when three CO₂ molecules are adsorbed in a nitrogen-connected (covalent) manner, forming (pn36–(CO₂⁻)₃)_N&N&N, the ΔE_ad reaches −115.1 kcal mol⁻¹, indicating that interaction energy per CO₂ molecule adsorbed is −38.4 kcal mol⁻¹.

Fig. 12 Optimized structures of the CO₂–adsorbed by pn36³⁻, at M06-2X/6-311++G(d,p) level of theory. Distances in Å and angles in degrees. (Color key: green-C, grey-H, blue-N, red-O).

Across all anionic, dianionic, and trianionic species, the CO₂ adsorption preferentially occurs at nitrogen rather than oxygen, except in one case (Tables S4 and S5, ESI†). Unlike neutral systems, where N to CO₂ interactions are noncovalent, the anionic systems exhibit covalent N–CO₂ bonding. For dianions, the interaction energy nearly doubles with the adsorption of two CO₂ molecules, while for trianions, it almost triples with three CO₂ molecules. In all cases, carboxylate formation facilitates charge delocalization, centralizing the anionic charge within carboxylate groups, which in turn promotes further CO₂ interactions. As a result, multiple CO₂ molecules are effectively adsorbed, demonstrating significantly enhanced CO₂-binding affinity for anionic species compared to their neutral counterparts. With increasing anionic charge, a substantial increase in ΔE_ad is observed, with the trianionic system exhibiting the highest interaction energy. Notably, exergonic CO₂ adsorption involving up to five CO₂ molecules is reported here (Fig. 13 and Table 3). The exergonic nature of these systems is further confirmed by the calculated free energies (Table S6, ESI†).

Fig. 13 Schematic illustration of CO₂ complex formation with pn36 trianion. A ring that cannot be represented by alternate single and double bonds is shown in red.

Table 3 ΔE_ad values of selected anions at M06-2X/6-311++G(d,p) level of theory. All values in kcal mol⁻¹

Molecule	ΔEad1	ΔEad2	ΔEad3	ΔEad4	ΔEad5
py^−	−16.6
nt5^−	−13.6
nt5^2−	−32.5	−59.3
pn36^−	−13.3
pn36^2−	−27.5	−50.0
pn36^3−	−46.1	−85.8	−115.1	−133.5	−148.7

---

To assess the practical relevance of CO₂ capture, we computed the CO₂ uptake in terms of weight percent (wt%) using the molar mass ratio of adsorbed CO₂ to the to the total mass of the anion–CO₂ complex (Table S9, ESI†). The calculated values reveal substantial uptake across all anionic systems studied. For instance, the anion of 2-hydroxypyridine captures up to 48% CO₂ by mass with two CO₂ molecules, while the dianion of dihydroxy naphthyridine reaches 45% with three. The trianion of trihydroxy pyridonaphthyridine exhibits the highest uptake, achieving nearly 49 wt% upon binding five CO₂ molecules. These results underscore the strong potential of these N-heterocyclic systems as high-capacity CO₂ adsorbents.

Influence of counter cations on CO₂ adsorption

To evaluate the impact of counter cations on CO₂ adsorption, the py, nt, and pn systems were studied in the presence of lithium chloride (LiCl), leading to the formation of Li⁺-incorporated complexes and HCl (Scheme 1). In these reactions, CO₂ forms nitrogen-bound (covalent) complexes (Fig. 14), which are further stabilized by bidentate Li⋯O interactions involving both the oxygen of the adsorbent system and the oxygen of the adsorbed CO₂. The energy of reactions (E_rea1) given in Scheme 1 was calculated using the equation (eqn (4)):

E_rea1 = ΣE_products − ΣE_reactants (4)

Scheme 1 Reaction of N-heterocycle with LiCl and CO₂.

Fig. 14 Optimized structures of CO₂ complexes of Li⁺-incorporated anionic systems at M06-2X/6-311++G(d,p) level of theory. Distances in Å and E_rea1 in kcal mol⁻¹. (Color key: green-C, grey-H, blue-N, red-O, pink-Li).

The E_rea1 serves as a measure of the adsorption efficiency in the presence of Li⁺. These reactions are exothermic, with E_rea1 values of −12.4, −29.3, and −37.5 kcal mol⁻¹, indicating that, compared to neutral systems, the presence of Li⁺ significantly enhances covalent CO₂ binding.

To further investigate the effect of a larger countercation on CO₂ adsorption, tetramethylphosphonium hydroxide ((CH₃)₄P⁺(OH)⁻) was selected as a model for the bulky trihexyl(tetradecyl)phosphonium hydroxide used by Luo et al.²⁸Table 4 presents the reaction energies for (CH₃)₄P⁺(OH)⁻ with py, nt, and pn molecules both in the absence and presence of CO₂. The reaction energy (E_rea3) in the presence of CO₂ (entries 7–12 in Table 4) is significantly more favourable than in its absence (E_rea2; entries 1–6). In both cases, the Gibbs free energy of reaction (G_rea2 and G_rea3) confirms exergonic behaviour. The enhancement of E_rea3 compared to E_rea2 (by 40–80%) highlights the strong positive influence of the large counter cation on CO₂ adsorption. The structural analysis of cation-incorporated complexes indicates covalent CO₂ binding at the nitrogen centre (Fig. 15). The acquired anionic character of CO₂ in these complexes leads to stabilizing electrostatic interactions with the surrounding cation. Notably, E_rea3 improves in the order: complexes with three CO₂ > complexes with two CO₂ > complexes with one CO₂, suggesting that CO₂ adsorption efficiency follows the trend pn > nt > py.

Table 4 E _rea and G_rea values of selected anions with and without CO₂ in the presence of counter cation at M06-2X/6-311++G(d,p) level of theory. All values in kcal mol⁻¹

No.	Reaction	E rea2 (1–6)	G rea2 (1–6)
		E rea3 (7–12)	G rea3 (7–12)
1	py + (CH3)4P^+(OH)^− → py^−⋯^+P(CH3)4 + H2O	−24.9	−22.8
2	nt5 + (CH3)4P^+(OH)^− → nt5^−⋯^+P(CH3)4 + H2O	−24.5	−23.8
3	pn36 + (CH3)4P^+(OH)^− → pn36^−⋯^+P(CH3)4 + H2O	−24.3	−23.5
4	nt5 + 2(CH3)4P^+(OH)^− → nt5^2−⋯^+(P(CH3)4)2 + 2H2O	−42.8	−39.2
5	pn36 + 2(CH3)4P^+(OH)^− → pn36^2−⋯^+(P(CH3)4)2 + 2H2O	−56.2	−52.7
6	pn36 + 3(CH3)4P^+(OH)^− → pn36^3−⋯^+(P(CH3)4)3 + 3H2O	−72.7	−69.9
7	py + (CH3)4P^+(OH)^− + CO2 → py–(CO2)^−⋯^+P(CH3)4 + H2O	−37.1	−24.6
8	nt5 + (CH3)4P^+(OH)^− + CO2 → nt5–(CO2)^−⋯^+P(CH3)4 + H2O	−41.6	−29.6
9	pn36 + (CH3)4P^+(OH)^− + CO2 → pn36–(CO2)^−⋯^+P(CH3)4 + H2O	−41.6	−29.3
10	nt5 + 2(CH3)4P^+(OH)^− + 2CO2 → (nt5–(CO2)2)^2−⋯^+(P(CH3)4)2 + 2H2O	−80.7	−56.7
11	pn36 + 2(CH3)4P^+(OH)^− + 2CO2 → (pn36–(CO2)2)^2−⋯^+(P(CH3)4)2 + 2H2O	−78.9	−55.0
12	pn36 + 3(CH3)4P^+(OH)^− + 3CO2 → (pn36–(CO2)3)^3−⋯^+(P(CH3)4)3 + 3H2O	−116.9	−79.8

---

Fig. 15 CO₂ complexes of anionic N-heterocycles in the presence of trimethyl phosphonium cation. Distances in Å. (Color key: green-C, grey-H, blue-N, red-O, magenta-P).

Solvation effect and Gibbs free energy changes

For neutral and anionic Nhet species such as hydroxypyridine (py), dihydroxy naphthyridines (nt), and trihydroxy pyridonaphthyridines (pn), solvation generally led to a modest reduction in the CO₂ adsorption energies (ΔE_ad). This reflects the stabilizing influence of the polar solvent on individual charged and polar components (Table S10, ESI†). For example, the strength of CO₂ adsorption in py weakened with ΔE_ad shifting from −10.8 kcal mol⁻¹ in the gas phase to −7.6 kcal mol⁻¹ in MEA. Similar reductions were observed across nt and pn systems (e.g., ΔE_ad for nt5 changed from −10.3 to −7.0 kcal mol⁻¹). As a result, the Gibbs free energy of adsorption (ΔG_ad) in the solvent phase hovered around thermoneutrality, ranging from −1.6 to +3.2 kcal mol⁻¹ for all Nhet⋯(CO₂)_n complexes (Table S13, ESI†). For anionic species, ΔG_ad values were more favourable, falling between −8.7 and 0.1 kcal mol⁻¹, while dianionic systems exhibited ΔG_ad values in the range of −23.4 to −1.8 kcal mol⁻¹ (Tables S14 and S15, ESI†). Notably, trianions showed the most exergonic adsorption, with ΔG_ad values between −26.3 and −7.8 kcal mol⁻¹ (Table S16, ESI†).

In comparison to the gas phase, a substantial reduction (71–81%) of the reaction energy (E_rea1) is observed in the solvent phase for the reactions described in Scheme 1, which involve the formation of CO₂ complexes of Nhet in the presence of Li⁺. A similar trend is noted for the formation of ion pairs between Nhet and the bulky cation (CH₃)₄P⁺(OH)⁻ with a marked reduction in exothermicity (E_rea2) in solution—by 52% for py, and more modest reductions for nt (16%) and pn (6%) systems (entries 1–6, Table 4). These observations are consistent with the strong solvation of the small, highly charged Li⁺ ion, which stabilizes the free cation more than the associated complex, thereby lowering the net driving force for complexation. In contrast, the bulky cation (CH₃)₄P⁺(OH)⁻ is less strongly solvated due to its diffuse charge distribution, resulting in a smaller energetic penalty upon complex formation. The nt and pn systems may further resist tight ion pairing due to steric or electronic factors, contributing to the relatively lower impact of solvation.

Interestingly, a moderate enhancement (6–9%) in exothermic character (E_rea3) is observed in the solvent phase for the formation of CO₂ complexes from pre-formed Nhet⋯(CH₃)₄P⁺(OH)⁻ ion pairs (entries 7–12, Table 4). This likely arises from solvent-induced weakening of the cation–anion interaction, which renders the anion more accessible for CO₂ binding, thereby enhancing the overall adsorption energy in the solvated environment. The corresponding Gibbs free energies of reaction G_rea3 in solution range from −26.8 kcal mol⁻¹ for the monoanionic complex py–(CO₂)⁻⋯⁺P(CH₃)₄ to −81.2 kcal mol⁻¹ for the trianionic complex (pn–(CO₂)₃)³⁻⋯⁺(P(CH₃)₄)₃.

The strongly negative ΔG_ad values observed in solution for many dianionic and trianionic systems along with the highly exergonic character of G_rea3 suggest that these Nhet-based framework systems have the potential to bind more CO₂ molecules than currently explored. These findings highlight the promising CO₂ uptake capacity of such systems and underscore the critical role of solvation in modulating both the thermodynamics and the cooperative effects involved in CO₂ capture. They emphasize that solvent-specific interactions must be carefully considered in the rational design of efficient and regenerable CO₂ adsorbents for solution-phase applications.

Conclusions

This comprehensive DFT study highlights the exceptional CO₂ capture capabilities of hydroxy-substituted aromatic N-heterocycles, revealing their potential as powerful adsorbents for mitigating greenhouse gas emissions. Through density functional theory (DFT) calculations, we have demonstrated that strategic incorporation of nitrogen and hydroxy groups within the molecular structures of pyridines, naphthyridines, and pyridonaphthyridines fosters cooperative binding mechanisms, leading to enhanced CO₂ adsorption. The findings reveal that the adsorption CO₂ capacity increases with the number of nitrogen and hydroxy centres.

A key breakthrough in this study is the significantly enhanced CO₂ adsorption observed in the anionic forms of these N-heterocycles. Compared to their neutral counterparts, anionic species exhibit significantly stronger CO₂ interactions, with the formation of covalent bonds between nitrogen and CO₂. As the anionic charge increases from monoanion to dianion and trianion, the CO₂-binding affinity improves dramatically due to charge delocalization, resulting in more exergonic adsorption energies. This trend suggests that designing anionic or negatively charged frameworks could be an effective strategy for developing highly efficient CO₂ capture materials. Importantly, solvation effects modeled using monoethanolamine (MEA) show that while polar solvents modestly reduce adsorption energies in neutral and anionic systems, they can enhance CO₂ binding in ion-paired complexes by disrupting ion-pairing and increasing CO₂ accessibility.

Energetic analysis further supports these findings, showing that CO₂ adsorption is generally exergonic under post-combustion conditions (293 K, 1 atm). Notably, the interaction energy per CO₂ molecule in trianionic species is nearly three times higher than in neutral systems, underscoring the role of charge modulation in enhancing CO₂ capture efficiency. The study also establishes that enol–keto transformations, which could potentially convert the adsorbed CO₂ into carboxylate groups, are energetically unfavourable. This is attributed to the loss of aromaticity in the core structure of N-heterocycles, reinforcing the importance of preserving aromatic stability for maintaining efficient CO₂ adsorption.

Additionally, the influence of counter cations, such as lithium and tetramethylphosphonium, in stabilizing CO₂ adsorption has been explored. The presence of these cations further strengthens CO₂ binding by reducing interaction distances and increasing exergonicity, thereby improving the overall adsorption performance. These results suggest that incorporating appropriate cations into CO₂ capture materials could enhance their efficiency, offering another avenue for material optimization.

Overall, this study provides valuable molecular-level insights into the structure–activity relationships governing CO₂ capture in hydroxy-substituted N-heterocycles. The findings suggest that these molecules, particularly in their anionic and cation-stabilized forms, hold significant promise for developing next-generation CO₂ capture technologies. Future research should focus on experimentally validating these computational predictions and exploring the practical implementation of these materials under real-world conditions. Furthermore, extending this approach to solid-state frameworks, such as functionalized porous materials or ionic liquids, could offer additional pathways for enhancing CO₂ capture efficiency and selectivity.

Author contributions

CHS conceptualized the project, supervised the work, and contributed to the writing of the manuscript. PKA conducted research, curated the data, and contributed to the writing of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

PKA is thankful to DST, Inspire, for a Senior Research Fellowship. CHS acknowledges CSIR-NIIST, Trivandrum for his deputation assignment as Director to SRIBS, Kottayam.

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Footnote

† Electronic supplementary information (ESI) available: SCF energies, zero-point correction, thermal correction to Gibbs, free energies and BSSE (Table S1), relative interaction energies of all complexes (Table S7), Rrelative free energies of all complexes (Table S8), Ssolvation energies of anion (Table S11), Ssolvation energies of anions in presence of counter cation (Table S12) and Cartesian coordinates of all the systems (Table S17). See DOI: https://doi.org/10.1039/d5cp01075f

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