Triclosan Adsorption on C60 Nanocage: NBO, Thermodynamic, Structural and Electronic Properties

INTRODUCTION
Triclosan (TCS)[1–3], which is antimicrobial [4] and has low toxicity [5], is used as an additive in care products and medical disinfectants [6-7].  Strong degradation of TCS in short term has resulted in increasing amounts of it in nature [8][9]. Triclosan (TCS) is used widely in antimicrobial and fungicidal ingredient such as soaps , detergents [10], toothpastes, and cosmetics [11]. Many methods have recently been proposed to remove TCS effectively including biodegradation [12-13], photolysis [14], adsorption [15], and advanced oxidation [16-17]. Triclosan, which is a special and popular antifungal agent and antibacterial [18][19], has drawn public attention due to its wide usage in personal care products and possible removal and disposal in water [20-22]. High technologies such as biological oxidation [23-24], treatment [25], and adsorption [26] by activated carbon (AC)[27-28] and carbon nanotubes [29, 12] have  been applied to treat water [30] containing triclosan. However, the adsorption of triclosan by high-silica zeolites has not yet been reported in the literature. 
Bucky ball (C60) was applied as absorbant [31-32] for some toxic and nontoxic chemical ingredient [33]  in dies [34] and wastewater [7, 22][35][36]. Density functional theory [37-41] and molecular dynamic calculation [42, 9, 43-44] were used for theoretical calculation in all fields of study [45] such as nanocage[31].
In this study, bucky ball(C60) was used as an absorbant of triclosan in gas and water phases by density functional theory with 6-311++G(2d,2p) basis set. 

Computational method
The chemical quantum calculations were applied due to the Gaussian 03 (46) package which  was run on a supercomputer. The full geometry optimization, electric field gradient, NBO analysis, thermodynamic properties, electronical parameters, and IR spectrum were carried out by the density functional theory (DFT) with three parameter hybrid functionals of Becke B3LYP and the 6-311++g(2d,2p) basis set. Bucky ball C-C (C60 ih) was applied as an absorbant in this study. Adsorption energy was calculated by the following formula:

Eads=E(B-T)-(Etriclosan+Ebackyball)

RESULTS AND DISCUSSION
In previous studies, results of calculations have shown that the adoption of nanocages with metallic atoms can efficiently modify their electrical properties, chemical activity, and reaction potential [47]. Therefore, in this study, all structures such as triclosan, bucky ball (C60, fullerene), and adopted triclosan on bucky ball were optimized by B3LYP/6-311++G(2d,2p). Fig. 1 shows the optimized structure of triclosan in which there are seven active sites in triclosan including three Cl atoms, an OH and an O functions, and two phenyl rings. These seven places have different chemical and electrochemical positions in triclosan. Fig. 2 illustrates all active positions in triclosan. The density of electron in each position has different effects on the chemical reaction. The presence of two phenyl rings with different functional groups can lead to better chemical reactivity in gas phase and solvent phases. HOMO and LUMO density electron is demonstrated in Fig. 3 and showed positive and negative position in triclosan. The red color shows negative side and green color shows positive side for reaction. Negative side is electron donating place and positive side is electron philicity place in triclosan.  Based on Fig. 3, phenyl ring, which has OH and Cl functions, acts as a donating place in triclosan and phenyl ring which has two chlorine atoms react as positive position in triclosan. The HOMO energy, which is -7.89 and -4.37 eV in water and gas phases, respectively, demonstrates the strong reactivity of triclosan.
Important characters in chemical reactivity are HOMO and LUMO energies. The large amount of LUMO shows that a molecule is more electron accepting. HOMO and LUMO energies can recognize and predict the strength and stability of chemical compounds. According to the energies of these frontier orbitals, which are the nearby orbitals with various levels of energy, the HOMO-LUMO band gap energy is where the most electron excitations can occur. In particular, when there is a large chemical and aromatic system, small HOMO-LUMO band gaps enable the  π electrons to jump to the next higher level of energy. The higher mobility of π electrons in large conjugated π orbital systems leads to the greater distribution of the energy throughout the molecule which increases stability. Hence, smaller HOMO-LUMO gaps correspond to better stability. In addition, this mobility of the π electrons means that large aromatic systems (like graphene nanoribbons) have good conductivity and can make great semiconductors since their conduction band gap is small, HOMO and LUMO are fascinating aspects of chemistry which can provide a remarkable insight into the working of reactions based on how orbitals interact to control the outcome of reactions. HOMO energy provides weak electron to react in chemical reactions. Due to its amount of HOMO energy, triclosan participate strongly in chemical reactions; it also has an electron donating role in all chemical reactions due to its electronegative atoms (O, Cl). Table 1 shows the data regarding the triclosan frontier (HOMO and LUMO) orbitals. 
A natural bond orbital or NBO is a calculated bonding orbital with maximum electron density. Red placed atoms refer to the negative side and green placed atoms denote the positive side in the structure of triclosan (Fig. 4). According to NBO electron contribution all three Cl are active sides for electron donating and makes the close neighborhood a negative side for chemical reactions.
Fig. 5 compares the predicted IR spectrum with the real one. O-H function appears strongly in 3000 to 3500 cm-1, and C-H aromatic appears weakly in 1650- 2000 cm-1.  
Chemical parameters (Table 1) and thermodynamic properties (Table 2) can help chemists to decide about the reactivity of chemical compounds. The dipole momentum of chemical compounds introduce polarity of compounds in reaction media. Based on the data in Table 1,  the dipole momentum of triclosan is 38.105 D in water solution which is higher than the dipole momentum in gas phase (7.722 D). It shows the polarity of triclosan because of the electronegative atoms in its chemical structure (Cl atom). The -OH functional group can make hydrogen bonding with water molecules. Hydrogen bonding and high dipole momentum lead to higher strength and reactivity. Table 2 demonstrates thermodynamic parameters which shows the stability of triclosan.  
Fig. 6 illustrates that triclosan has seven active positions for adopting C60 which is (ih) a symmetric system, and all places has the same situation to participate in chemical reactions. Fig. 7 shows the main and optimized structure of bucky ball. 
Energy is the capacity of doing work and supplementing heat. Chemical potential energy is saved energy in chemical bonds. Chemical potential is a an important thermodynamic property and an applicable concept to many materials sciences such as chemistry, physics, biology and chemical engineering. All of the thermodynamic parameters of a chemical material at a specific pressure and temperature can be calculated from chemical potential. Under the condition of constant pressure and temperature, chemical potential determines the stability of substances, chemical compounds, and solutions. The value of chemical potential (µ) for triclosan is -260.11 and -396.08 eV in gas and water phases, respectively, and triclosan is a stable chemical compound in both phases. Negative energy shows the hidden energy in the chemical bonds of triclosan.  The energy of an atom is determined when it loses or gains energy through chemical reactions that results in losing or gaining electrons. A chemical reaction that releases energy is called an exothermic reaction and a chemical reaction that absorbs energy is called an endothermic reaction. Energy from an exothermic reaction is negative, thus energy is given a negative sign; whereas energy from an endothermic reaction is positive, and energy is given a positive sign. When an electron is added to a neutral atom (i.e., first electron affinity) energy is released; thus, the first electron affinities are negative. However, more energy is required to add an electron to a negative ion (i.e., second electron affinity) which overwhelms the release of energy from the electron attachment process; therefore, second electron affinities are positive. Triclosan has positive electron affinity energy (EA) which means that it does not want to give electron, rather it wants to donate it. Reactivity parameters of a molecule such as electronegativity (χ), softness (σ), hardness (η), and electrophilicity index (ω) have been extracted from the theory of Koopman by the DFT popular method. Electronegativity (χ) is the value of attraction power of an atom or molecule achieved from HOMO and LUMO. η is responsible for the stability and reactivity of a chemical molecule. σ illustrates the capacity of chemical molecules for accepting electrons. Table 3 shows reactivity parameters of triclosan in gas and water phases.  According to these results, it was found that triclosan has high stability and reactivity in chemical reactions. Triclosan was adopted on C60. Table 4 demonstrates the energy of each complex in Fig. 6. In addition, the dipole momentum of each complex indicates that all active sides can act in polar media as well. The relative adopting energy shows that all active sides are stable structures in the chemical media, and structure 5 is the most stable one. The energy of an electron in a single atom can be determined solely by the principal quantum number. However, the energy of an electron in multi-electron atoms depends on both its principal quantum number (n) and its azimuthal quantum number. This difference in energy of various subshells residing in the same shell is mainly attributed to the mutual repulsion among the electrons in a multi-electron atom. In multi-electron atoms, there is a repulsive force acting between various electrons apart from the attractive force between the nucleus and the electrons. Thus, the stability of an electron in a multi-electron atom depends on the total attractive and repulsive interactions. The electron in an atom is only stable when the total attractive interaction is more than the total repulsive interaction. For bigger atoms, due to the presence of electrons in the inner shells, the electrons in the outer shell are deprived of experiencing the full positive charge of the nucleus. This effect is known as the shielding of the outer shell electrons from the nucleus by the inner shell electrons. The net positive charge experienced by the outer shell electrons is termed the effective nuclear charge.

CONCLUSION
The present work considered the chemical reactivity of triclosan and adopting it on bucky ball (C60) which was applied by DFT B3LYP/6-311++G(2d,2p) in gas and water phases. The results of chemical structure, thermodynamic properties, electronic parameters, and IR spectrum demonstrated that triclosan is an active molecule which can be adsorbed on C60 as a stable compound. Triclosan has seven active sides all of which are thermodynamically stable, and structure 5 is the most stable one among other structures.