**1. Introduction**

Carbon dioxide (CO2 ) causes negative effect on global climate due to its "greenhouse effect" property multiplying by its high level in the atmosphere. Currently, its concentration is increasing due to the fossil fuels combustion process by human activities [1]. Nevertheless,

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it is usually regarded as an environmentally benign solvent because of its less hazardous property. Furthermore, it is an attractive solvent owing to the ease of its removal capacity, abundance, inexpensive, and flexibility of the solvent parameters [2]. Consequently, supercritical CO2 (sc-CO2 ) is well known as an efficient solvent over conventional organic ones in many chemical processes, and is expected to be useful in many applications of green chemistry such as extraction, separation, chemical reaction, and material processes [3–6]. Recently, sc-CO2 has been employed in direct sol-gel reactions for the synthesis of oxide nanomaterials, oligomers, and polymers, etc. [7–10]. It is noteworthy that due to a lack of polarity, sc-CO2 is a very feeble solvent for most polar solutes [11]. Nevertheless, due to the possession of a substantial quadrupole moment and a polar C═O bond, the majority of materials attached by carbonyl functional or fluoride groups are soluble in sc-CO<sup>2</sup> . In the context, continuing efforts have been reported for the purpose of enhancement in applicability of sc-CO<sup>2</sup> solvent through the use of "CO<sup>2</sup> -philes", which makes insoluble or poorly soluble materials becoming more soluble in sc-CO2 at an acceptable level of low temperature and pressure conditions [3, 12]. Experimental works have aimed at better understanding of behavior of the sc-CO<sup>2</sup> as solvent for organic compounds [13–18]. It might be assumed that CO2 is a green yet feeble solvent because its full potential could not be realized without a thorough understanding of its solvent behavior at molecular level. Accordingly, numerous results on the CO2 -philes have been reported during the 1990s [19, 20]. These CO2 -philes are less favorable and effective both economically and environmentally because most of them are fluorinated polymers. Thus, in attempts to avoid expensive cost and environmental impacts of the fluorous materials, during the last three decades, large-scale studies have focused on the design of nonfluorous CO<sup>2</sup> philes, specifically hydrocarbon-based and oxygenated hydrocarbon-based polymers [21, 22]. In 1996, Kazarian et al. discovered the formation of Lewis acid-base (LA-LB) type of interaction between CO2 with the O atom of a number of carbonyl compounds (>C═O) for the first time [23]. Soon later, Beckman et al. successfully synthesized copolymer of nonfluorousete-carbonate in sc-CO2 at low pressure [21]. Interaction of CO2 with delocalized π aromatic systems in the gas phase have been theoretically studied for the purpose of ranking in a database of a large variety of organic ligands, which would be valuable candidates for designing new metal-organic framework materials with enhanced affinity for CO<sup>2</sup> adsorption at low pressure. Accordingly, some extensive studies have been reported on the interactions of CO2 with π-systems at level of theory and experiment [24–29]. In recent years, interactions of simple functionalized organic molecules, including CH3 OH, CH3 CH2 OH [30–32], CH3 OCH3 , CH3 OCH2 CH2 OCH3 [33–35], HCHO, CH3 CHO, CH3 COOCH3 , CH3 COCH3 , CH3 COOH [36– 41], and XCHZ (X = CH3 , H, F, Cl, Br; Z = O, S) [42] CH3 SZCH3 (Z = O, S) [43] with CO2 have been carried out using quantum chemical methods. Today, the interest in CO2 computational chemistry is the interaction capacity of a solute molecule surrounded by a number of CO2 molecules. Despite the fact that numerous studies have been performed, a full understanding of the CO2 characteristics as a solvent remains a challenging task [1]. It is therefore clear that we need more systematic studies to gain a better understanding on the nature of the interactions involved, rather than considering the origin for a few disparate systems. Furthermore, there is also a great interest in deep understanding of the origin of the interactions between different types of organic compounds with CO<sup>2</sup> at molecular level for an effective use of CO<sup>2</sup> in different states.

This chapter focuses on evaluating interaction capacity of CO2

the investigation of interactions between CO2

parts: (3.1) interactions of CO2

**2. Computational details**

**3. Interaction capacity of CO2**

utilizing liquid and supercritical CO2

**3.1. Interaction of CO2**

*3.1.1. Interaction of CO2*

been currently used as CO2

at level of molecule using theoretical approaches. Section 2 provides us with a brief summary of theoretical methods and computational techniques. Section 3 reviews the remarkable results of

and their substituted derivatives, and modeled aromatic hydrocarbons; (3.2) interactions of CO2 and modeled organic compounds with different functional groups and their substituted deriva-

used as a replacement for toxic classical organic solvents in industrial applications. In addition,

Geometrical parameters of all the considered structures including monomers and complexes are optimized using suitable quantum-chemical methods such as the molecular orbital theory (MO) and density functional theory (DFT) and large basis sets, depending on investigated systems, such as 6-311++G(2d,2p), 6-311++G(3df,2pd), aug-cc-pVDZ, aug-cc-pVTZ, which have succeeded in investigating noncovalent interactions, especially hydrogen bonds [44, 45]. Harmonic vibrational frequencies are subsequently calculated at investigated level of theory to ensure that the optimized structures are local minima on the potential energy surfaces, and to estimate their zero-point energy (ZPE). The stabilization energy of each complex is calculated using the supermolecular method as the difference in total energies between that of each complex and the sum of the relevant monomers at the selected level of theory. The interaction energy is corrected by zero-point energy (ZPE) and basis set superposition errors (BSSE). The latter is computed using the function counterpoise procedure of Boys and Bernardi [46]. The "atoms-in-molecules" (AIM) [47] analyses are applied to identify critical points and to calculate their characteristics including electron density (ρ(r)), Laplacian, electron potential and kinetic energy density, and total energy density. The GenNBO 5.G program [48] is used to perform NBO calculations, which is extensively applied to investigate chemical essences of hydrogen bonds and other weak interaction, and can provide information about natural hybrid orbitals, natural bond orbitals, natural population, occupancies in NBOs, hyperconjugation energies, rehybridization, and repolarization.

 **with organic compounds**

 *with saturated hydrocarbons and their substituted derivatives*


as a "green" alternative to conventional organic solvents

Saturated hydrocarbons are a primary energy source for our civilization. Fluorocarbons have

 **with model hydrocarbons**

tives. Obtained results presented in Section 4 support for the enhancement of sc-CO2

revealed, which could be attached on the surface of materials to absorb and store CO<sup>2</sup>

the functional groups that present the more stable interaction with CO2

with model organic compounds

http://dx.doi.org/10.5772/intechopen.71878

solvent

107

at molecular level are

gas.

with organic compounds and is divided into two

with selected models of saturated and unsaturated hydrocarbons

Understanding Interaction Capacity of CO2 with Organic Compounds at Molecular Level:…

This chapter focuses on evaluating interaction capacity of CO2 with model organic compounds at level of molecule using theoretical approaches. Section 2 provides us with a brief summary of theoretical methods and computational techniques. Section 3 reviews the remarkable results of the investigation of interactions between CO2 with organic compounds and is divided into two parts: (3.1) interactions of CO2 with selected models of saturated and unsaturated hydrocarbons and their substituted derivatives, and modeled aromatic hydrocarbons; (3.2) interactions of CO2 and modeled organic compounds with different functional groups and their substituted derivatives. Obtained results presented in Section 4 support for the enhancement of sc-CO2 solvent used as a replacement for toxic classical organic solvents in industrial applications. In addition, the functional groups that present the more stable interaction with CO2 at molecular level are revealed, which could be attached on the surface of materials to absorb and store CO<sup>2</sup> gas.
