CO2 Complexes with Five-Membered Heterocycles: Structure

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CO Complexes with 5-Membered Heterocycles: Structure, Topology and Spectroscopic Characterization Ángel Vidal-Vidal, Olalla Nieto Faza, and Carlos Silva López J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09394 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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The Journal of Physical Chemistry

CO2 Complexes with 5-Membered Heterocycles: Structure, Topology and Spectroscopic Characterization ´ Angel Vidal-Vidal,† Olalla Nieto Faza,‡ and Carlos Silva López∗,† †Universidade de Vigo. Departamento de Qu´ımica Org´ anica. Campus Lagoas-Marcosende, 36310, Vigo, Spain ‡Universidade de Vigo. Departamento de Qu´ımica Org´ anica, Facultade de Ciencias, Universidade de Vigo. Campus As Lagoas, 32004, Ourense, Spain E-mail: [email protected] Phone: +34 986812632

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ACS Paragon Plus Environment


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Abstract In a first step towards the rational design of macrocyclic structures optimized for CO2 capture, we have systematically explored the potential of 30 five-membered aromatic heterocycles to establish coordinating complexes with this pollutant. The interactions between the two moieties are studied in several orientations and the obtained complexes analyzed in terms of its electron density and vibrational fingerprint. The former is aided to provide an in depth knowledge of the interaction whereas the latter should help to select structural motifs that have not only good complexation properties but also diagnostic spectroscopic signals.

Introduction Fast rising of sea levels due to ice melting at the Earth’s poles or radical changes in terrestrial ecosystems are expected if anthropogenic global warming, caused by excessive amounts of greenhouse gases in the atmosphere, is not addressed. 1–3 Carbon dioxide (CO2 ) one of the main greenhouse gases present in the atmosphere. 4,5 Its concentration has increased by 30% since the 19th century, mainly due to intensive use of fossil fuels such as coal, oil or natural gas (80% CO2 emissions worldwide 6 ) and other human activity. The International Panel on Climate Change (IPCC) has predicted that CO2 levels in the atmosphere could reach up to 590 ppm by 2100 entailing a temperature rise of almost 2 ◦ C in the globe. 7 To prevent this, a two-pronged approach to the reduction of greenhouse emissions can be proposed: 8 finding and using sustainable sources of energy with a low carbon footprint and capturing these emissions so that they don’t reach the atmosphere. For the latter, scientists have been developing several strategies to capture and sequester polluting gases. 9–11 Apart from being a pollutant, carbon dioxide is at the same time a renewable resource of carbon and represents a very interesting building block in organic chemistry. 12,13 Once the main drawback for its use (low chemical reactivity) is overcome by using chemical catalysts, it can be converted into diverse valuable organic molecules. 14–16 Moreover, the use of CO2 2


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instead of carbon monoxide or oxalyl chloride in chemical synthesis constitutes an attractive alternative that avoids the use of hazardous and toxic reagents. As a result, the development of novel materials that capture and store CO2 selectively is highly desirable. 17 At present, CO2 is commercially separated using absorption processes in which chemical reactions may or may not be involved. The most common procedures are based on the use of basic aqueous solutions of NaOH and KOH or amine scrubbing. 18–20 The main drawback of these processes is the energy that they require, which translates into high economic and environmental cost. 21–23 Other limiting factors of this technology are the corrosive and environmentally unfriendly character of the reagents involved, the engineering of large and complex absorption units 24 and also the thermal degradation of solvents such amines. 25,26 To reduce the degradation problem of amines, aromatic molecules can be used to enhance their stability. It was demonstrated that substituted aromatic or heteroaromatic systems can have enhanced CO2 binding energies. 27,28 The urgency for materials that can be used for CO2 capture has prompted the exploration of several technologies such as: boron nitride nanotubes, 29 ionic liquids, 30,31 porous inorganic membranes (PIMs), 32 polymers with light organic functional groups (like polythiophene and poly-pyrrole), 27,33–35 inorganic-organic interface composites 36 and covalent organic framewoks (COFs), 37–39 metal organic frameworks (MOFs) 10,40–43 and zeolitic imidazolate frameworks (ZIFs) 44–46 among others 47 tested recently. In order to have high CO2 uptake materials, it is necessary to facilitate the establishment of strong interactions between the greenhouse gas and the solid framework. Until now experimentalists focused on CO2 adsorption experiments rather than prioritizing a deep knowledge of the mechanism and nature of interactions between guests and hosts in MOFs. Type and mode of interaction of CO2 with organic molecules is a broad topic of investigation in which it is necessary to gain deeper insight in order to understand how these systems behave at the molecular level. The main contributions of the few studies published on this topic are summarized below:

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• Chen et al. 48 performed high level ab-initio calculations of π-π interactions between benzene, pyridine and pyrrole with CO2 benchmarking different computational methods with respect to CCSD(T)/CBS. They stated that this kind of non-covalent interaction plays an important role on CO2 adsorption and also highlighted the necessity of performing a correct treatment of electron correlation to obtain an accurate description of interactions, binding energies (BEs) and charge transfer (CT) in these systems. • Hernández-Mar´ın et al. 49 studied complexes formed by imidazole, 2-methylimidazole, benzimidazole and pyrazine with CO2 using density functional theory (DFT). In-plane and top-on systems were characterized concluding that the most favorable interaction is obtained when carbon dioxide is parallel to the plane defined by the ring due to electrostatic and Lewis acid-base interactions. Dispersion correction terms in density functionals was proven to be essential to optimize complexes because of the existence of Van der Waals interactions. • Lee at al. 50 studied the interaction of CO2 with various molecules using dispersion corrected functionals and high level wave function theory (SI-MP2, SCS-MP2, CCSD(T)) for the computation of the binding energies of the complexes. The work is focused on tautomerizable strong multi-N-containing bases such as guanidine, triazabicyclodecene, melamine and 7-azaindole. In addition, DFT-SAPT was used to gain deeper insight into the nature of the interactions stablished. • Ab initio and DFT benchmarks were done by Boulmène et al. 51 DFT functionals with and without dispersion were included to study the adsorption mechanism and mutual preferential sites in the complex formation between CO2 and different isomers and tautomers of triazole. Potential energy surfaces along the intermonomer coordinates were also mapped. The results suggested that M05-2X+D3 leads to binding energies and equilibrium parameters as close as those derived using coupled cluster techniques. • The nature of the binding or CO2 and organic heterocyclic molecules was examined by 4


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Vogiatzis et al. 52 using high-accuracy ab-initio methods. Pyrimidine, purine, adenine or imidazopyridamine are only four of the thirteen systems studied. An special attention is given to different adsorption positions in the pyridine-CO2 complex. Electron density redistribution plots and electrostatic potential maps are used to rationalize the interaction mechanism. In the present work we report a thorough and systematic study of CO2 interacting with N-substituted five-membered ring heterocycles derived from pyrrole, furan and thiophene using high-level DFT calculations. Molecular complexes between CO2 and a complete set of heterocycles with different main atoms and substitution patterns (see Figure 1) were optimized with no constraints. With these structures at hand, full characterization of the complexes with CO2 and the host-guest interactions were carried out by computing: accurate binding energies (BE), detailed charge distribution analysis based on Natural Bonding Orbital (NBO), electronic density redistribution and topological analyis of the electron density based on the Quantum Theory of Atoms in Molecules (QTAIM) among other fundamental properties.

Computational methods Previous ab-initio and DFT benchmarks suggested the necessity of taking into weak interactions and, in particular, dispersion, for the study of non-covalent interactions in complexes between CO2 and other aromatic and non-aromatic molecules. 52 Usually DFT functionals unerestimate dispersion effects so a careful selection of the calculation method is need. Dispersion correction terms have been proved to be essential, for instance, to describe complexes where π-π and Van der Waals interactions are relevant. 49,52 There are at least two approaches to include these interactions: 1- density functionals with empirical corrections for the dispersion forces, such as DFT-D3 53 and 2- density functionals explicitely parametrized taking into account non-covalent interactions like PWB6K, 54 M05-2X, 55 M06 56 and M06-2X. 56 5



6

LVRWKLD]ROH 6,

1 WKLD]ROH 6 ,,

+

WULD]ROH

6 1 1 WKLDGLD]ROH 6,

1 1 R[DGLD]ROH 2,

2

1,

+ 1 1 1 +

1 WULD]ROH

2

1 ,,

+ 1

6

1 WKLDGLD]ROH 6 ,,

1

1 R[DGLD]ROH 2 ,,

1

1 +

WULD]ROH

1

2 1

1 ,,,

+ 1

6 1

WKLDGLD]ROH 6 ,,,

1

R[DGLD]ROH 2 ,,,

1

1 +

1 1 WKLDGLD]ROH 6 ,9

6

1 1 R[DGLD]ROH 2 ,9

2

1 ,9

1 1 WULD]ROH

+ 1

6 1 1 1 WKLDWULD]ROH 6,

1 1 1 R[DWULD]ROH 2,

1

1

1 1 WKLDWULD]ROH 6 ,,

6

1 1 R[DWULD]ROH 2 ,,

1 ,, 2

1, 2

+ 1

1 1 + WHWUD]ROH 1

+ 1 1 1 1 + WHWUD]ROH

6 1 1 1 1 WHWUDQLWURJHQ ,,, PRQRVXOILGH 6,

1 1 1 1 WHWUDQLWURJHQ ,,, PRQRR[LGH 2,

2

1,

+ 1 1 1 1 1 + SHQWD]ROH

Figure 1: Representation of the chemical structure of the 30 systems whose interaction with CO2 will be studied in this work. In the first row there are heterocycles derived from pyrrole, in the second group, those derived from furan and in the third one, heterocycles derived from thiophene. Besides the IUPAC nomenclature, the notation that will be used for naming them throughout the discussion and results is included. In this naming scheme the first figure indicates the number of heteroatoms contained in the system, the letter written next corresponds to the main heteroatom of the heterocycle from which the systems are derived and finally, the Roman numerals are only used to differentiate the different possible positional isomers. Numeration with Roman numerals is random, so, it does not follow any chemical criteria.

WKLRSKHQH 6,

6

6

6

1

1 R[D]ROH 2 ,,

1

LVR[D]ROH 2,

2

1 ,,

1 + LPLGD]ROH

+ 1

IXUDQ 2,

2

1,

1,

2

+ S\UD]ROH

+ 1

+ S\UUROH

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

+ 1

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Density Functional Theory in the Kohn-Sham formulation as implemented in Gaussian 09 57 was used to locate minimum structures on the potential energy surfaces of the complexes under study. All calculations were done with the dispersion corrected hybrid-meta GGA functional M06-2X in combination with the Ahlrich’s triple-ξ quality basis set including polarization functions def2-TZVPP. 58,59 This basis set uses 2p1 d polarization for H, and 2d1 f for atoms B-Ne and Al-Ar. M06-2X functional was selected because of the good performance in the study of electrostatics, hydrogen bonds, H-π, π- π, electrostatic interactions both in neutral and charged dimeric systems. 50,60 To achieve results with high accuracy, tight self-consistent field (SCF) criteria and an ultrafine pruned grid (99,590) for the numerical integration were used. This consists on 99 radial shells and 590 angular points per shell. All calculations were carried out in gas phase and the stability of the wavefunction has been checked for all stationary points found. 61 Harmonic analysis of the second derivatives of the energy with respect to the nuclear displacements was also performed to ensure that a minimum structure and not a transition state had been found. Binding energies (BEs) were computed within the supermolecule approach (Eq 1), this means, the BE is obtained subtracting from the energy of the complex the energy of the heterocycle and the energy of CO2 maintaining the same geometry as in the complex. The counterpoise procedure described by Boys and Bernardi 62 was used to correct the basis set superposition error in the calculation of BEs.

BE = EComplexHet−CO2 − (EHet − ECO2 )

(1)

Quantum chemical topological analysis is gaining relevance in the study of interacting molecules, fragments and in bond characterization. 63 Most of studies are based on the mathematical analysis of a three-dimensional quantum mechanical function such as the electronic localization function, 64 the electrostatic potential, 65–67 the electronic density 68 or its Laplacian. 69–71 In the present work we will focus on the analysis of the electronic density (ρ) of 7



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the complexes formed between CO2 and 5 membered-ring heterocycles by using Quantum Theory of Atoms in Molecules (QTAIM) 72–74 which has already been successfully applied to the analysis of non-covalent interactions in different systems. 75–78 QTAIM analysis has been performed applying the open source software Multiwfn 79 to the wavefunctions obtained using Gaussian 09. 57 QTAIM partitions the electron density (ρ) through the concept of gradient path. 63 By doing that, the principal objects of molecular structures, such as atoms and bonds, can be naturally expressed using the characteristics of ρ. The topology of ρ is dominated by the appearance of local maxima at nuclear positions and then values decreasing with the distance. As a consequence, if we consider the boundaries determined by the planes of minimum ρ between nuclei in a given molecule, we can assign a definite spatial region to each nucleus (nucleus plus an electronic basin around), which provide us with a definition of the atom in the molecule based on a physical observable. With the purpose of analyzing aromaticity, nucleus independent chemical shifts (NICS) have been calculated, based in the association between aromaticity and diatropic ring currents. 80,81 The NICS tensor is defined as the negative value of the isotropic shielding constant at a given point in space. NICS have been calculated at the center of the rings and at a range of distances from the center over a line crossing it, perpendicular to the ring plane. These values have been used to characterize the aromaticity of heterocycles involved on the complex formation. 82 Analysis of atomic charges has been performed using the natural bond orbital method (NBO) 83 at M06-2X/def2-TZVPP level of theory using the NBO 3.1 program. 84 The Natural Population Analysis (NPA) 85 involves partitioning the charge into natural atomic orbitals, constructed by dividing the electron density matrix into sub-blocks with the appropriate symmetry. Finally, Multiwfn software is also used for the computation of the electronic density redistribution plots (EDRP) for the complexes. EDRP is defined as the subtraction from the electronic density of the complex ρcomplex the electronic densities of both the heterocycle

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(ρhetero ) and CO2 (ρCO2 ). The quantum resonance effect known as Fermi Dyad or Fermi Resonance (FR) is treated according to an analytical procedure with negligible computational cost after the assumption that the real system can be described using a perturvative treatment is done. The FR effect has been described in detail for the particular case of isolated CO2 by McCluskey and Stoker 86 as well as Vidal-Vidal et al. 87 This last work analyzes the FR of a supramolecular system filled with CO2 and emphasizes the ability of this analytical procedure to provide accurate results even when compared with other costly computational procedures. The magnitude of the repulsion between the states | 100 0 > and | 020 0 > in FR depends on the matrix element Wni of the perturbation function W that is acting on the system.

Wni =

Z

ψn0 W ψi0∗ dτ

(2)

Because the integral W must take a non-zero value, the functions ψn0 and ψi0 (that are the zero approximation eigenfunctions of the two vibrational levels that perturb each other) must have the same symmetry, fact that only occurs between the levels | 100 0 > and | 020 0 > (Σ+ g symmetry). Note that the level | 020 > is formed by two sub-levels | 020 0 > and | 022 0 > of 0 symmetries Σ+ g and ∆g respectively, so that only sub-level | 02 0 > has adequate symmetry

to disturb at level | 100 0 >. 88 If the resonance is fairly close the magnitude of the shift can be obtained according to the first-order perturbation theory from the secular determinant. E100 − EF Wni = 0, W E − E ni 020 F

(3)

where EF stands for the Fermi dyad energy and Wni stands for the mixing interaction energy. Some calculations over the determinant show that the solutions of EF , E− and E+ ,

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(or equivalently the spectral bands) must satisfy the following two conditions:

E− + E+ = E100 + E020 2 (E− − E+ )2 = (E100 − E020 )2 + 4Wni ,

(4)

If harmonic levels are calculated, this mathematical treatment allows to compute the FR once the Eint is known. Since the FR can be measured experimentally, one of the ways to obtain the value of Eint is based on the substitution of the measured experimental values in the given equations. For the calculation, we used the Raman spectroscopy values measured by Howard-Lock 89 for 12 CO2 , the same experimental values selected by McCluskey and Stoker, 86 in his work: Eint = - 51,232 cm−1 .

Results and Discussion Carbon dioxide is a linear non polar molecule (D∞h ) in which the carbon atom with sp hybridation is bonded to two sp2 oxygens. Although this geometry confers to CO2 a zero dipole moment, it exhibits a high charge separation in the C=O bonds. Oxygen atoms has negative Mulliken charge (-0.259 each) and the carbon atom is positively charged (+0.518). Those charges have been computed at MP2-FC/cc-pV(T+d)Z level. This charge separation provides CO2 with a significant quadrupole moment 90–93 through which it can establish higher order electrostatic interactions such as quadrupole-dipole, quadrupole-quadrupole, quadrupole-octapole, among others, with a wide variety of molecules.

Flexible optimization By virtue of the charge separation, CO2 can at the same time act as a Lewis acid (LA) and as a Lewis base (LB). 52 The ability of charge-deficient atoms (like the positively charged carbon atom in CO2 ) to stabilize interactions with electron-rich sites of other molecules to

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Table 1: Summary of the main properties computed for the CO2 -heterocycle complexes (fully optimized). It includes binding energy (BE, kJ/mol). The NICS (0) and NICS (1) (in ppm) have been selected for the aromaticity study (see Supporting Information for the remaining values). ∆QC (in a.u.) represents the NPA charge transfer to the carbon in CO2 due to complexation. ΘXX and ΘY Y (expressed in D˚ A) correspond to the main diagonal XX and YY elements of the quadrupole tensor. Likewise, for furan-derived heterocycles, ΘXXX and ΘY Y Y are included, which are the equivalent elements of the octapolar tensor. Finally, A and angles in degrees) of electron the most relevant geometric parameters (distances in ˚ donor-acceptor (EDA) complexes are shown. R represents the centre of the heterocyclic ring, O the oxygen of the CO2 interacting with the ring, O(2) the non-interacting oxygen in CO2 , and H corresponds to the main heteroatom of the heterocycle. The notation used in Figure 2 is also shown to facilitate interpretation. BE (kJ/mol) NICS (0) (ppm) NICS (1) (ppm) ∆QC (a.u.) ΘXX (D·˚ A) ΘY Y (D·˚ A) A) (R1 ) R-C (CO2 ) (˚ (R2 ) H-C (CO2 ) (˚ A) (R3 ) R-O (CO2 ) (˚ A) A) (R4 ) H-O (CO2 ) (˚ d (◦ ) (θ) OCO d (◦ ) (α) HOR d (◦ ) (β) ORC d (◦ ) (γ)HRC d (δ)RCO (2) (◦ )

1N-I -15,42 -13,43 -10,50 0,018 -23,78 -26,94 3,139 3,593 3,066 3,130 177,85 21,37 21,42 103,86 106,43

2N-I -13,52 -13,75 -11,86 0,017 -21,51 -30,63 3,309 3,451 3,242 3,125 177,99 19,85 20,29 87,97 105,31

2N-II -14,68 -13,85 -11,36 0,017 -22,90 -29,48 3,146 3,706 3,027 3,228 178,11 20,94 21,47 110,20 108,35

3N-I -11,76 -13,93 -13,07 0,013 -23,13 -30,53 3,168 3,679 2,988 3,143 178,58 20,55 21,39 109,03 110,73

3N-II -11,99 -13,93 -12,67 0,013 -20,86 -33,17 3,137 3,576 3,020 3,128 178,93 20,69 21,53 104,26 107,53

3N-III -9,78 -14,48 -13,52 0,010 -20,55 -33,20 3,159 3,534 3,057 3,070 179,07 19,82 21,33 102,11 106,56

3N-IV -15,33 -13,16 -12,28 0,018 -25,35 -28,63 3,189 3,854 2,963 3,284 178,12 20,59 21,30 116,80 113,56

4N-I -12,53 -14,22 -14,03 0,014 -26,06 -29,43 3,218 3,785 2,923 3,163 178,77 20,59 21,03 112,42 116,28

4N-2 -9,13 -15,10 -14,61 0,009 -20,8 -34,68 3,182 3,659 2,987 3,123 179,18 19,78 21,29 108,56 110,79

5N-I -10,33 -16,13 -15,88 0,012 -24,23 -33,13 3,254 3,775 2,887 3,089 179,31 20,16 20,63 111,35 119,21

BE (kJ/mol) NICS (0) (ppm) NICS (1) (ppm) ∆QC (a.u.) ΘXX (D·˚ A) ΘY Y (D·˚ A) ΘXXX (D· ˚ A 2) ΘY Y Y (D· ˚ A 2) (R1 ) R-C (CO2 ) (˚ A) (R2 ) H-C (CO2 ) (˚ A) (R3 ) R-O (CO2 ) (˚ A) (R4 ) H-O (CO2 ) (˚ A) d (◦ ) (θ) OCO d (◦ ) (α) HOR d (β) ORC (◦ ) d (◦ ) (γ)HRC d(2) (◦ ) (δ)RCO

1O-I -11,96 -11,97 -9,77 0,013 -24,18 -28,04 -24,71 -31,97 3,142 3,587 3,071 3,122 178,74 21,61 21,38 103,01 105,40

2O-I -10,23 -12,14 -11,15 0,011 -24,25 -29,41 -22,85 -33,39 3,265 3,430 3,116 3,085 178,56 21,03 20,69 88,67 108,78

2O-II -11,44 -11,96 -10,72 0,013 -22,60 -31,43 -29,18 -28,10 3,178 3,742 3,063 3,273 178,89 20,84 21,24 110,15 107,26

3O-I -7,12 -12,53 -12,56 0,006 -27,34 -27,89 -23,20 -43,58 3,194 3,719 3,005 3,168 179,33 21,04 21,20 108,74 110,13

3O-II -8,80 -12,89 -12,16 0,009 -21,69 -34,06 -17,20 -44,27 3,127 3,533 3,022 3,111 179,67 21,29 21,57 101,45 106,20

3O-III -5,30 -12,61 -12,88 0,003 -29,97 -25,27 -20,29 -48,89 3,193 3,583 3,221 3,239 179,97 19,56 20,70 101,84 99,01

3O-IV -12,04 -12,14 -11,718 0,014 -22,77 -33,08 -25,55 -26,97 3,237 3,905 2,968 3,298 178,81 20,67 20,94 116,73 114,87

4O-I -8,54 -13,60 -13,63 0,010 -25,06 -32,30 -21,18 -42,15 3,304 3,904 2,938 3,220 179,49 20,78 20,30 113,49 118,81

4O-II -4,28 -14,16 -14,38 0,002 -25,78 -31,39 -15,22 -50,33 3,212 3,718 3,080 3,258 179,45 19,78 21,01 108,90 106,47

5O-I -6,90 -16,15 -15,82 0,013 -29,87 -29,32 -17,34 -47,35 3,836 4,165 2,861 3,046 179,82 21,34 10,75 99,48 152,69

BE (kJ/mol) NICS (0) (ppm) NICS (1) (ppm) ∆QC ΘXX (D·˚ A) ΘY Y (D·˚ A) (R1 ) R-C (CO2 ) (˚ A) (R2 ) H-C (CO2 ) (˚ A) (R3 ) R-O (CO2 ) (˚ A) (R4 ) H-O (CO2 ) (˚ A) d (◦ ) (θ) OCO d (◦ ) (α) HOR d (◦ ) (β) ORC d (◦ ) (γ)HRC d (δ)RCO (2) (◦ )

1S-I -11,44 -12,67 -11,22 0,012 -34,73 -31,55 3,227 3,927 3,149 3,404 178,65 25,21 20,81 108,04 105,53

2S-I -9,65 -12,76 -11,77 0,028 -32,05 -35,39 4,013 4,135 4,358 4,098 177,32 18,61 15,13 85,17 82,99

2S-II -11,30 -12,56 -12,48 0,012 -37,31 -31,40 3,254 4,079 3,127 3,553 178,80 24,21 20,74 114,28 107,68

3S-I -9,25 -12,61 -13,38 0,009 -34,04 -35,63 3,239 3,995 3,039 3,387 179,03 24,45 20,90 112,67 110,92

3S-II -9,29 -13,30 -13,05 0,009 -34,79 -35,26 3,243 4,006 3,100 3,485 179,37 23,63 20,83 113,15 107,98

3S-III -7,75 -13,61 -12,49 0,006 -29,33 -39,61 3,239 3,868 3,105 3,326 179,63 23,44 20,83 108,49 107,33

3S-IV -11,94 -12,85 -14,10 0,014 -41,46 -29,70 3,305 4,249 3,028 3,592 178,69 23,85 20,47 120,52 115,17

4S-I -10,40 -13,89 -15,23 0,012 -38,23 -34,08 3,306 4,140 2,957 3,424 179,14 24,30 20,34 116,63 118,24

4S-II -7,86 -14,32 -14,17 0,007 -31,41 -39,98 3,246 3,960 3,018 3,344 179,39 23,59 20,86 112,68 111,80

5S-I -9,76 -16,06 -16,45 0,011 -35,05 -38,82 3,330 4,101 2,923 3,338 179,47 23,89 20,00 115,43 120,80

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: Geometrical parameters used to characterize the geometry of the complexes obtained through non-constrained geometry optimizations. The image shows a random model system. Distances between fragments are written in red while angles are represented in blue. form electron donor-acceptor (EDA) complexes (LA-LB) is well known. 94 At the same time, negatively charged oxygen atoms may participate in attractive interactions with electrondeficient molecular systems (LB) forming LB-LA complexes. There are several experimental and theoretical studies confirming this behaviour for CO2 95–97 so, in order to take into account the possibility of forming different types of complexes with heterocycles, a relaxed potential energy surface exploration was performed. To ensure that the complexes obtained are the absolute minima, structures with several random orientations of carbon dioxide with respect to heterocycles were constructed, hence exploring a wide range of interaction modes, they were used as initial configurations and were subjected to geometry optimization. Once the optimal geometries for the complexes had been obtained, the most stable systems were characterized by multiple methods. First, the calculated BEs show that the pyrrole derivatives present higher BEs and that, for the three series studied, adding nitrogens creates a deleterious effect in the BEs. The mean value of BE in these systems is -12.45 kJ/mol, being the most favourable case 1N-I (-15.42 kJ/mol) and the least favourable 4N-II (-9.13 kJ/mol). If the 30 systems are ordered according to their BE, the first 5 are pyrrole derivatives. For the sulphur family, its average BE is -9.86 kJ/mol and it varies between

12


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-9.86 kJ/mol in the least favourable case and -11.94 kJ/mol in the most stable configuration. Globally, they occupy a position in the middle of the list of systems. Furan derivatives present less systematic behavior. With an average energy of -8.66 kJ/mol it has stable configurations like 3O-IV with -12,04 kJ/mol or 1O-1 with -11,96 kJ/mol BE, however, the four systems with lower BE are also derivatives from furan: -7,12, -6,90, -5,30 and -4,28 kJ/mol (3O-I, 4O-I, 3O-III and 4O-II). These large energy disparities show that furanderived systems are very sensitive to the degree of substitution with nitrogen atoms in the structure and to their relative positions. For instance, the oxygenated heterocycle is the only one in which 3X-IV shows a BE that is more than twice the BE of its 3X-III isomer (-5.30 vs. -12.04 for 3O-III and 3O-IV, respectively). The BE is systematically higher for all the 3X-IV systems, but for the furan analogue, this increase is particularly large. We associate this effect to the symmetrical geometry with the two nitrogen atoms forming an N-N bond in the region opposite to the main heteroatom. Deeper analyis of this and other unexpected observations in terms of BEs will be provided below in terms of geometric and electronic characteristics. It is worth noting that the BEs of these complexes are such that they would be thermally unstable at room temperature. A strategy to efficiently trap CO2 would combine several of these interactions in a single macromolecular structure. Results exploring this strategy will be communicated in due course. Chen et al. 48 in their study of the interactions between CO2 and benzene, pyridine and pyrrole suggest the possibility that an increase in the aromaticity of the heterocycles used as linkers in MOFs and ZIFs could strengthen the π-π interactions and may be an effective approach to increase the CO2 uptake in these materials. Taking that statement into account, we have carried out NICS evaluations along an axis perpendicular to the ring structure (between -5 ˚ A and +5 ˚ A from the ring centre) for all the heterocycles involved in this study. Only NICS(0) and NICS(1) are shown in Table 1 but the remaining data can be found in the SI. All cycles are aromatic according to these values. The mean value of NICS(0) for N heterocycles is -12.98 ppm while -13.01 and -13.46 ppm is obtained on average for furan

13



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and thiophene derivatives. NICS values and BEs are found to poorly correlate in a linear fit (0.60 R2 correlation coefficient) suggesting that aromaticity could have some contribution to the final complex stability but is not the main driver for the computed BEs. A set of geometrical parameters are used to characterize the structure of the CO2 heterocycle complex (Figure 2). In Table 1 these geometrical parameters are compiled for the 30 systems under study along with other computed properties. In all complexes the carbon dioxide bond angle is close to linear, however it is not exactly 180◦ . The degree of variation of the CO2 angle is nothing more than the distortion of the geometry due to the complexation and can be used as a diagnostic value to measure the strength of the interd angle of CO2 is 178.6◦ action between the two molecules. The average value of the OCO

for heterocycles with N, 179.3◦ for those with O and 178.9◦ for heterocycles derived from

thiophene. The greater the deviation of linearity, the greater the interaction between the two molecules. This observation is in good agreement with the average BE values discussed above. Despite the qualitative agreement, again, there is no linear correlation between the CO2 bond angles and BEs. We have however found a linear correlation between the charge variation (∆QC , Table 1) in the CO2 carbon atom and the binding energy. In an EDA-type complex there is charge transfer from the electron-donor unit to the electron-deficient unit. The average charge transfer of the heterocycle to the carbon of CO2 is 0.014 a.u. for the pyrrole derivatives, 0.009 a.u. for the furan derivatives and 0.012 a.u. for sulfur-containing heterocycles. The correlation coefficients (R2 ) are in the three cases higher than 0.96 (see the SI). In furan analogues, the 5O-I system slightly diverges from the linear trend, the 2S-I system is also off-line in the linear fit of the thiophene derivatives. Both systems have more charge transfer to the carbon atom of CO2 than the one that corresponds for the binding energy that they present. Apart from these exceptions, we can observe a global trend of stability in terms of binding energy. Table 1 also shows selected values of the main diagonal elements of the quadrupole tensor

14


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(ΘXX and ΘY Y ) for all the systems as well as the values of the octapolar moment (ΘY Y Y and ΘY Y Y ) for furan derivatives. There is a considerable linear dependence when considering the components of the quadrupole tensor versus binding energy (R2 ∼0.90) for the systems with N and S. However, when studying the systems that derive from furan, it is necessary to resort to the components of the octapolar tensor to obtain satisfactory correlations. These trends suggest that for the former systems (with S and N) the quadrupolar component has a great influence in the stabilization of the systems, whereas for the case of O, higher order electrostatic interactions (octapolar) are to a greater extent responsible for stabilizing that complexes.

QTAIM and NPA charge analyis When looking at the most stable complex geometries obtained in this study we noticed that different geometries arise depending on the heterocycle that is involved in the complex. This suggests that CO2 preferentially stablishes different interactions depending on the heterocycle. We considered that hydrogen bonds could be responsible for some extra stabilization in some of the complexes. In order to shed light onto this issue we employed QTAIM. The topology of the electron density for all complexes was analysed and the interaction modes of CO2 with the heterocycles is summarized in Figure 3. A way to classify the global interaction stablished between CO2 and the heterocycles can be based on the number of singular bonds between the two fragments located through QTAIM and on the atoms that participate in them. For the complexes analysed only two situations arised in terms of the number of bonds between fragments: a single interaction, or two interactions (Figure 3). An example of the QTAIM analysis for a system in each of these families is provided in Figure 4. This Figure includes a representation of the molecular structure, critical points and bond paths for a single interaction system (A, 3N-II) and for a two-interaction system (B, 3O-III). In the single interaction family there is either an interaction between the electron-deficient 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Page 16 of 41

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Figure 3: Diagram that sumarizes interaction modes of CO2 with heterocycles. We divided the systems in two sets (CO2 featuring one and two interactions with the heterocycle). Within each of these groups, subfamilies are made depending on the atoms involved in the interaction. carbon atom of CO2 with an electron-rich moiety, or interaction between one of the CO2 oxygens and a heteroatom. Three groups can be formed with the former: Nitrogen ((C(CO2 )N), carbon (C(CO2 )-C) and a π orbital (C(CO2 )-π) group. All the systems that interact with CO2 through a secondary N atom contain a single nitrogen with the only exception of 3N-I, which contains two (Figure 3). The position in which the nitrogen atom is positioned in the structure seems to be not crucial for the establishment of this interaction as half of the systems interact through a nitrogen atom in position 2 of the heterocycle and the other half interacts via a nitrogen atom in position 3. In all cases when the interaction is stablished there is a charge redistribution and nitrogen loses in average 0.019 a.u. The highest charge donation is 0.037 a.u. in the 2S-I complex, probably motivated by the presence of a less electronegative and easily polirazible sulphur atom adjacent to the nitrogen. 16


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The other two groups involving a single interaction between the carbon atom of CO2 and the heterocycle contain only one system. An interaction between the carbon of the CO2 and a carbon atom of the heterocyclic structure (C(CO2 )-C) occurs in the structure 4O-I. In this case the carbon that binds CO2 loses 0.010 a.u. Perhaps, the most interesting interaction in this group is the one that is established with π electron density of the aromatic system in 2O-I (C(CO2 )-π). The bond is found between the carbon of CO2 and a critical bond point between C3 and C4 in furan. Although only one interaction between the CO2 fragment and the heterocycle is established in the systems shown so far, there are two systems with a peculiar behaviour: 3N-I and 1O-I. In these two cases one of the oxygen atoms of the carbon dioxide is oriented toward the centre of the heterocyclic ring. This arrangement creates a topology of the electron density with a critical cage point (3, +3) in addition to two critical ring points (3, +1), one belonging to the heterocycle itself (present in all other systems) and a second point located in the region between the heterocycle and the oxygen atom of CO2 (see SI). Due to the geometry, distance and electronic characteristics of the system, no bond path is detected between the critical box point and the CO2 oxygen atom which is lying parallel to the plane defined by the heterocycle. These topological features (ring and cage critical points) will be common in systems having two interactions between CO2 and the heterocyclic ring (see below). Within the systems that establish only one interaction there is also a group (10 of the 30 under study) in which the interaction is found between an oxygen atom of the CO2 and one of the nitrogen atoms that is doping the structure of the heterocycle (O(CO2 )-N). This interaction always involves the nitrogen atom adjacent to the main heteroatom. The average charge transfer in this set is relatively low (0.014 a.u.) probably due to the electronegativity of nitrogen. We found that the potential energy of the critical point V(r)BCP correlates reasonably well (R2 = 0.86) with the BEs. The higher potential energy at the critical bond point (3,-1) the higher the binding energy. This correlation is found to be general for all

17



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tween the oxygen atom of CO2 and the sulphur atom of 1,2,5-thiadiazole. In this case, the S atom increases its charge with respect to the isolated heterocycle in 0.017 a.u. In addition, there is an interaction between the electropositive carbon of CO2 and a carbon atom of the heterocyclic structure which has negative NPA charge. The potential energy at the critical point V(r)BCP of the latter is larger than that of the O—S interaction (-4.44·10 -3.48·10

−3

−3

a.u. and

a.u., respectively), which suggests a counterintuitive greater strength for the C—C

interaction. There is also an interesting group (formed by 1S-I, 1N-I) in which two interactions are established: one between the oxygen and the main heteroatom of the heterocycle, and the other between the carbon atom of CO2 and π electron density delocalized on the C—C bond opposite to the heteroatom. The V(r)BCP for the C(CO2 )-π interaction are very similar for the 1N-I (-4.86·10

−3

a.u.) and the 1S-I systems (-4.37·10

interaction a larger difference is observed in V(r)BCP (-4.37 ·10

−3

−3

a.u.). For the O(CO2 )-X for 1N-I and -3.20·10

−3

for

1S-I). This is consistent with the difference in BE between this two systems (-15.42 kJ/mol for 1N-I and -11.40 kJ/mol for 1S-I). The remaining members of the O(CO2 )-X / C(CO2 )-X group feature C—C interactions between the two fragments in addition to an interaction of one oxygen atom in CO2 with the heterocycle (at a C or a heteroatom site). As indicated in Figure 3, within the O-C / C-C subgroup there is only one system: 3O-II, with a binding energy of -8.80 kJ/mol. Both carbon atoms in the ring possess positive NPA charge: +0.360 a.u. for the carbon located between O and N and +0.120 a.u. the one located between the two nitrogen atoms. The variation of charge that they experience due to complexation is very small (∼0.01 a.u.) and in opposite directions (the carbon that is located between O and N increases its positive charge whereas the carbon placed between the two nitrogen atoms of 1,2,4-oxadiazole decreases it). The last subgroup of systems featuring two intermolecular bonds within the QTAIM framework involve only oxygen atoms of the CO2 molecule. In 3O-III one of the oxygen atoms of CO2 interacts with the oxygen of the furan derivative, and the other interacts with

19



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one of the carbons found at the opposite C—C bond. Upon complexation the heteroatom becomes more negative with respect to the uncomplexed heterocycle (-0.123 a.u. complexed versus -0.119 a.u. isolated). Both of the carbon atoms in the heterocycle increase their charge (they becomes more negative) in 0.007 a.u. The V(r)BCP reveals that the interaction between the CO2 and the heterocyclic carbon is twice intense as that established with the heteroatom. These values are not surprising if the large electronegativity and low polarizability of the oxygen atoms is considered. Higher order electrostatic interactions such as quadrupolar and octapolar must overcome the repulsion between both negatively charged atoms. Within the O(CO2 )-X / O(CO2 )-X subgroup two subgroups can be considered. Both have in common a particularity not found in the other systems. It is the same oxygen atom of CO2 that interacts with the heterocycle at two points. The first subgroup includes the interaction with a N atom and the S heteroatom on the poly-N-substituted thiophenes 4S-II and 5S-I. In both systems the interaction of one of the oxygen atoms of CO2 occurs with the nitrogen atom that lies two bonds away from S. The charge of the nitrogen atom becomes more negative with respect to isolated heterocycle (about -0.014 a.u.). The interacting oxygen and sulphur atoms show similar values of charge variation: the oxygen is charged more negatively with respect to the isolated heterocycle whereas the sulphur atom increases its positive charge in a similar magnitude (around 0.018 a.u.). However, the absolute values of the charge vary substantially between the nitrogen atom of the 5S-I system (-0.048 a.u.) and the 4S-II system (-0.231 a.u.). This marked difference is also manifested in the nitrogen adjacent to that establishing the interaction with CO2 . There is also a large difference in the charge variation of the oxygen atom at CO2 : while in the 4S-I system it decreases by 0.014 a.u. in the 5S-I system it decreases more than twice that value (0.032 a.u.). Finally there is a subgroup including 3O-IV and also both fully-N-substituted complexes 5O-I and 5N-I. Both 5O-I and 3O-IV share the structural characteristic of having interactions with the two nitrogen’s that are opposite to the main heteroatom. In 5N-I, however, the interaction occurs between the N of the pyrrole and one of the non-adjacent nitrogens.

20


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All N atoms except for the main N of the pyrrole gain electron density upon complexation. The same happens with the O atom of the CO2 that is coordinated, it becomes more negative changing the charge around 0.031 a.u. on average. These atoms gain electrons at the expense of the carbon atom in CO2 and the remaining atoms in the heterocycle. The only charge parameter that does not follow the trend is the one belonging to the N of the pyrrole in the 5N-I system. Once interacting with CO2 , N decreases electron population from -0.177 a.u. in the isolated heterocycle to -0.170 a.u. in the complex. The H attached to this N becomes also slightly depleted (in 0.003 a.u.). Analysis of the bond critical point reveals that V(r)BCP of the O(CO2 )—N bond is 14% higher than that of the O—N bond, suggesting a stronger bond strength for the former. The average value of V(r)BCP for the two bonds for these three systems shows a good degree of correlation with the binding energy: the higher the V(r)BCP the greater the BE. Finally, if the we perform the EDRP isosurfaces analysis (see Figure 4 and SI for more information), it can be observed that in all cases there is a decrease of electronic density in the intermonomeric region (the space separating both fragments) and an increase of the electron density in the vicinity of the CO2 molecule. In heterocycles, once the charge transfer occurs to stabilize the EDA complex with CO2 , there is a local redistribution of electron density. After this redistribution, there are regions of the rings that gain electron density while other regions lose it. If we consider systems that have one or two interactions with CO2 , there is a difference in the extent of electron density depletion, being higher in systems with 2 interactions. What both modes of complexation have in common is a considerable decrease in the electronic density inside the ring.

Vibrational analysis Vibrational spectroscopy is a technique that is commonly employed to analyze the formation and nature of numerous complexes. 98–104 In the field of CO2 capture Boulmène et al. 51 studied the shifts of the different vibrational modes of CO2 when its is coordinated to different isomers 21



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of triazole. These triazoles are well-known as typical N-based organic linkers in MOFs and ZIFs. Vitillo et al. 105 studied the interaction between CO2 and different aliphatic and aromatic amines (including those commonly found in the MOFs) making use of the Möller Plesset (MP2) perturbation theory. Carbon dioxide in the gas-phase has four vibrational modes: 106 a symmetrical C—O stretching (ν1 , Σg symmetry) located at 1333.0 cm−1 and active in Raman, two degenerate bending modes ν2 (Πg ) which are IR active at 667.0 cm−1 and an asymmetric C-O stretch (ν3 , Σu symmetry) mode located at 2349.0 cm−1 , which is also active at IR. These bands can switch activity if their symmetry is modified, and this is what makes vibrational analysis an extremely valuable tool to characterize CO2 uptake. 51,107–109 The most characteristic signal in the vibrational spectrum of CO2 is the Fermi Resonance (FR) or Fermi dyad doublet formed by the νc− (lower level of FR) and νc+ (upper level of FR) bands. 110 This quantum resonance effect, was studied experimentally by multiple authors, 111–114 occurs due to the proximity of energies between the first overtone of the bending mode (2ν2 , two quanta of energy in bend) and the stretching band ν1 . The FR effect has been described in detail for the particular case of isolated CO2 by McCluskey and Stoker 86 as well as Vidal-Vidal et al. 87 which recently, have used a similar procedure for the analysis of FR for CO2 when it is forming a hydrogen bond stabilized complex in gas hydrates. Several spectroscopic studies of CO2 forming complexes with heterocycles have been reported, but, as far as we know, Fermi resonance has never been addressed. Different ways of computing FR can be classified according to the mathematical treatment, for example, Andersson et al. 115 and Pinto et al. 116 used computationally demanding alternatives as iterative numerical procedures to deal with this issue while McCluskey and Stoker 86 as well as Vidal-Vidal et al. 87 used less computational demanding analytical procedures. This last analytical approach shown on the computational methods section is the one followed in this work.

22


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1360,0

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K s cm)

1355,0 1350,0 1345,0 1340,0 1335,0 r

s

t

u

v

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1450,0 1445,0 1440,0 1435,0 r

s

t System

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Figure 5: Graphical representation of average νc− and νc+ values for the systems containing from zero to four nitrogen atoms (not considering the main heteroatom). In subfigure A νc− is represented (systems derived from pyrrole in blue, in red those derived from furan and in green thiophene analogues). The Subfigure B the average value in νc+ for the systems under study is plotted with the same color code.

Results are summarized in Tables 2 and 3, which include the complexation-induced shifts with respect to the vibrational spectrum of isolated CO2 in the gas phase for each of the 30 systems under study. The particular value of CO2 bands adopted by each system will depend on the degree of interaction with the structure, but also on the position of the nitrogen atoms. For example, the reciprocal modification of CO2 and heterocycle vibration modes is not the same if the CO2 -heterocycle interaction occurs between the carbon of CO2 with a N atom of the heterocycle which has a contiguous CH group, or if it has another nitrogen atom. The same happens if the interaction is stablished between one oxygen atom of CO2 with one or two atoms of the heterocyclic structure instead of having interaction by the C atom of CO2 as told before. As reported by Boulmène et al. 51 similar spectra are expected for different

23



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Page 24 of 41

Table 2: Computed average of νc− , νc+ and ν3 νc− , vibrational bands for the 5 general groups based on the number of nitrogen’s substituting the heterocycle structure made with the systems under study. The label 1X represents the group of systems having 0 nitrogen atoms doping the structure, 2X is the group of systems with 2 N and so on. Values are expressed in terms of wavenumbers (cm−1 ). (νc− )N (νc+ )N (ν3 )N

1X 1342,43 1450,72 2453,09

2X 1344,40 1451,04 2450,67

3X 1348,27 1453,96 2453,39

4X 1349,81 1455,28 2455,52

5X 1355,21 1459,06 2457,45

(νc− )O (νc+ )O (ν3 )O

1347,75 1453,91 2454,04

1348,92 1454,60 2454,48

1351,38 1456,07 2453,67

1351,88 1456,00 2451,19

1355,98 1460,91 2463,93

(νc− )S (νc+ )S (ν3 )S

1344,29 1450,54 2447,06

1348,02 1454,10 2455,47

1350,62 1455,43 2452,57

1351,60 1456,48 2455,50

1355,21 1459,20 2458,12

Table 3: Individual values of νc− , νc+ and ν3 νc− bands expressed in cm−1 for each of the 30 systems under study. Complexation induced shifts (cm−1 computed as the subtraction of the computed vibration band minus the corresponding signal in the gas phase are also shown. νc− (cm−1 ) νc+ (cm−1 ) ν3 (cm−1 ) Shift νc− (cm−1 ) Shift νc+ (cm−1 ) Shift ν3 (cm−1 )

1N-I 1342,43 1450,72 2453,09 -12,96 -8,54 -5,62

2N-I 1345,53 1451,47 2450,55 -9,85 -7,80 -8,16

2N-II 1343,28 1450,61 2450,79 -12,11 -8,65 -7,92

3N-I 1349,50 1455,05 2454,91 -5,89 -4,21 -3,80

3N-II 1347,03 1452,78 2451,85 -8,35 -6,49 -6,86

3N-III 1349,08 1454,88 2455,29 -6,31 -4,38 -3,42

3N-IV 1347,49 1453,14 2451,49 -7,89 -6,12 -7,22

4N-I 1350,03 1454,97 2453,54 -5,36 -4,29 -5,17

4N-2 1349,60 1455,59 2457,49 -5,79 -3,67 -1,22

5N-I 1355,21 1459,06 2457,45 -0,17 -0,21 -1,26

νc− (cm−1 ) νc+ (cm−1 ) ν3 (cm−1 ) Shift νc− (cm−1 ) Shift νc+ (cm−1 ) Shift ν3 (cm−1 )

1O-I 1347,75 1453,91 2454,04 -7,64 -5,35 -4,67

2O-I 1347,64 1453,53 2453,41 -7,75 -5,73 -5,30

2O-II 1350,19 1455,67 2455,54 -5,19 -3,60 -3,17

3O-I 1353,00 1457,49 2455,53 -2,38 -1,78 -3,18

3O-II 1353,04 1457,44 2454,95 -2,34 -1,83 -3,76

3O-III 1349,44 1454,16 2453,01 -5,94 -5,11 -5,70

3O-IV 1350,02 1455,20 2451,18 -5,37 -4,06 -7,53

4O-I 1351,38 1455,62 2451,88 -4,01 -3,64 -6,83

4O-II 1352,38 1456,38 2450,50 -3,01 -2,88 -8,21

5O-I 1355,98 1460,91 2463,93 0,59 1,65 5,22

νc− (cm−1 ) νc+ (cm−1 ) ν3 (cm−1 ) Shift νc− (cm−1 ) Shift νc+ (cm−1 ) Shift ν3 (cm−1 )

1S-I 1344,29 1450,54 2447,06 -11,10 -8,72 -11,65

2S-I 1345,61 1452,09 2453,99 -9,77 -7,17 -4,72

2S-II 1350,42 1456,12 2456,95 -4,96 -3,15 -1,76

3S-I 1352,13 1456,93 2455,41 -3,26 -2,33 -3,30

3S-II 1348,77 1453,73 2449,18 -6,61 -5,54 -9,53

3S-III 1350,23 1455,35 2453,27 -5,16 -3,91 -5,44

3S-IV 1351,35 1455,69 2452,43 -4,04 -3,57 -6,28

4S-I 1352,38 1456,79 2455,17 -3,01 -2,47 -3,54

4S-II 1350,83 1456,16 2455,83 -4,55 -3,11 -2,88

5S-I 1355,21 1459,20 2458,12 -0,18 -0,06 -0,59

24


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heterocycles independent of the adsoption energy, so it is not possible to obtain a simple relationship between the shifts of the CO2 modes and the individual BEs. However, it is possible to analyze the trends that exist for each of the five sets formed taking into account the number of nitrogen atoms that are doping the structure. The mean values for each of the groups (0N, 1N, ... 4N) are shown in Figure 5. Trends are clear when analyzing this plot, the highest values (in terms of wavenumbers) within each group are for furans, then thiophenes and finally pyrroles. Increasing the degree of substitution with N atoms in the heterocyclic structure promotes a shift of the band value towards higher values. The behavior of the complexation-induced shift is similar to that of the band values. Conversely, the greatest displacements are experienced by systems with N as the main heteroatom, then with S and finally with O. In all cases the greater variation of complexation-induced shifts are obtained for the unsubstituted systems (-12.96 cm−1 for 1NI, -11.10 cm−1 for 1S-I and -7.64 cm−1 for 10-I). As the substitution degree with nitrogen atoms increases, this shift decreases and the νc− band shows closer values to that of isolated CO2 . In terms of discrete values for the displacements, that experimented by 5O-I is peculiar in the sense that is the only one of all studied that adopts a positive value of the complexationinduced shifts in both νc− and νc+ and also ν3 . The behavior of the νc+ band (Figure 5B) is similar in all respects to that of νc− except for two particular cases: the average in the 3N systems adopts a very similar value (1456.00 cm−1 ) to that obtained in the 2N systems (1456.07 cm−1 ) instead of increasing as would be expected. On the other hand, the value of the position of the νc− band in the systems without any substitution with N is very similar in 1N-I (1450.72 cm−1 ) and 1S-I (1450.54 cm−1 ). Based on the differences in both the position of the FR and the shifts, results suggest the possibility of identifying and characterizing the complexation in different macromolecular systems such as MOFs, ZIFs and COFs.

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Conclusions With the goal of contributing to the efficient design of CO2 capturing systems incorporating heterocyclic motifs, we have performed a systematic study of CO2 interacting with N-substituted five-membered ring heterocycles derived from pyrrole, furan and thiophene using high-level DFT calculations. We have performed full geometry optimizations with no constraints, in order to get the structure of complexes that we can confidently assume that are global minima, and computed accurate binding energies (BE), aromaticity parameters, detailed charge distribution analysis based on Natural Bonding Orbital (NBO), electronic density redistribution and topological analyis of the electron density based on the Quantum Theory of Atoms in Molecules (QTAIM) among other fundamental properties. From these data we have found that the interaction modes between CO2 and the heterocycles studied can be clasified in two broad groups. One interaction systems include 18 of the 30 systems studied. In them either the carbon or an oxygen in CO2 interacts with a secondary nitrogen in the heterocycle or the carbon in CO2 interacts with a carbon or the π system on the ring (just two furan derivatives follow these modes). The systems with two nitrogens (besides the main atom in pyrrole derivatives) tend to prefer the C(CO2 )–N interaction and the systems with three and four nitrogens prefer a O(CO2 )–N interaction. Two-interaction systems are less clustered and offer more variety, and these interactions can occur between O(CO2 )–X and O(CO2 )–X or O(CO2 )–X and C(CO2 )–X (being X an heteroatom on the ring). In terms of binding energies, it is found that the pyrrole derivatives present higher BEs and that, for the three series studied, adding nitrogens creates a deleterious effect in the BEs. The correlation of aromaticity with BEs is poor, and it seems that, although having a contribution, aromaticity is not the main factor affecting BEs. Although there is a qualitative relationship between the bond angles in the CO2 fragment and BEs, there is no linear correlation. We have however found a very good (R2 larger than 0.96) linear correlation between the charge variation (∆QC ) in the CO2 carbon atom and the binding energy: 26


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the average charge transfer of the heterocycle to the carbon of CO2 is 0.014 a.u. for the pyrrole derivatives, 0.009 a.u. for the furan derivatives and 0.012 a.u. for sulfur-containing heterocycles. Analysis of the diagonal terms of the quadrupolar moment and their relationship with the binding energies shows a linear correlation in the case of nitrogen and sulfur heterocycles; however, for the furan derivatives, such a correlation exists with the octapolar moment terms. In all cases, upon complexation there is a decrease of electronic density in the intermonomeric region and an increase of the electron density in the vicinity of the CO2 molecule. In heterocycles, once the charge transfer occurs to stabilize the EDA complex with CO2 , there is a local redistribution of electron density. Detailed analysis of the Fermi resonance bands, the results about both the position of the FR and the shifts, suggest the possibility of identifying and characterizing the complexation in different macromolecular systems such as MOFs, ZIFs and COFs through Raman/IR spectroscopy.

Acknowledgement The authors thank the Centro de Supercomputación de Galicia (CESGA) for the generous allocation of computer time, and the Ministerio de Econom´ıa y Competitividad (MINECO, PCTQ2016-75023-C2-2-P) for funding. A. Vidal-Vidal is grateful to the Universidade de Vigo for a predoctoral fellowship.

Supporting Information Available SCF energies, Cartesian coordinates, number of imaginary frequencies for all computed structures. All this material is available as supporting information.

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CO2 Complexes with Five-Membered Heterocycles: Structure

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