INTRODUCTION:

Corrosion is an electrochemical process by which metallic structures are destroyed gradually through anodic dissolution¹. Corrosion inhibitors are best option to protect the metal by electrochemical attack. A large number of organic compounds containing hetero atoms N, O and S have been reported to be effective corrosion inhibitors^2-4. The presence of oxygen, nitrogen, sulphur and phosphorous in these molecules has a major effect on the inhibition efficiency and consequently on the phenomenon of adsorption on metals surface. These inhibitors have extended π -electron systems and functional groups (such as -C=C-, -OR, -OH, -NR₂, -NH₂ and -SR). The functional groups provide electrons that facilitate the adsorption of the inhibitor on the metal surface^2,5-7.

The reactivity of inhibitors is depends on the molecular structure of inhibitors such as the heteroatoms, the functional group and electronic density at the donor or acceptor atom and π orbital character. The adsorption of inhibitors on the metal surface is shows it’s reactivity⁷.

Recently, the corrosion inhibition of mild steel in 0.1 M HCl solution by Ciprofloxacin drug has been studied using weight loss method by Inemesit et al. 2013⁸. The inhibition efficiency increases with the concentration of Ciprofloxacin Drug to attain 86% at 2.570 × 10⁻³ M. Therefore, theoretical studies will be a good set to explain these results.

The objective of the present paper is to correlate the effect of structural parameters of Ciprofloxacin drug through their quantum parameters. Now a days Density functional theory was successfully applied to study the reaction mechanisms and to interpret experimental results as well as to solve chemical ambiguities⁹. DFT is also a most powerful tool to investigate the reaction mechanism in a molecule and its electronic structure levels^10,11. Such computations have been used to analyze the molecular electronic structures of organic inhibitors using some quantum chemical descriptors ¹². Advances in computer hardware and software and in theoretical chemistry have brought high-performance computing and graphical tools within the reach of many academic and industrial laboratories. Quantum chemical methods are particularly significant in the study of electrochemistry and provide researchers with a relatively quick way of studying the structure and behavior of corrosion inhibitors. Recently more corrosion publications contain substantial quantum chemical calculations¹³. The chemical and optimized structure of studied compound is given in Figure 1.

FIGURE 1: Basic Chemical structure of Ciprofloxacin drug

FIGURE 2: Optimized structure of Ciprofloxacin drug

MATERIAL AND METHODS:

In the present study all the calculations were performed with Gaussian 03 for windows¹⁴. The molecular structure of Ciprofloxacin drug was fully and geometrically optimized using the functional hybrid B3LYP (Becke, three-parameter, Lee-Yang-Parr exchange-correlation function)¹⁵ Density function theory (DFT) formalism with electron basis set 6-31G (d,p) for all atoms¹⁶. The quantum chemical parameters obtained were E_HOMO, E_LUMO, Energy Difference (ΔE), Dipole Moment (μ), the total energy (TE) and Mulliken charges on heteroatoms¹⁷.

RESULTS AND DISCUSSION:

The computed quantum chemical properties such as energy of highest occupied molecular orbital (E_HOMO), energy of lowest unoccupied molecular orbital (E_LUMO), ΔE energy gap, dipole moment (μ), Total energy (TE) are summarized in Table 1.

Table 1: Calculated quantum chemical parameters of the studied compounds

Quantum chemical parameters	Ciprofloxacin drug
E_HOMO	-8.19 (eV)
E_LUMO	-7.29 (eV)
ΔE (_LUMO-HUMO)	0.898 (eV)
Dipole Moment (μ)	2.67 Debye
Total Energy (TE)	-30933.697 (eV)

Table 1 presents the calculated energy levels of the HOMO and LUMO for the studied compound. According to Wang et al.¹², Frontier orbital theory is useful in predicting adsorption centers of the inhibitor molecules responsible for the interaction with surface metal atoms. The reactive ability of the inhibitor is considered to be closely related to their frontier molecular orbital, the HOMO and LUMO. Higher HOMO energy (E_HOMO) of the molecule means a higher electron-donating ability to appropriate acceptor molecules with low-energy empty molecular orbital and thus explains the adsorption on metallic surfaces by way of delocalized pairs of π-electrons. E_LUMO, the energy of the lowest unoccupied molecular orbital signifies the electron receiving tendency of a molecule. Accordingly, the difference between E_LUMO and E_HOMO energy levels (ΔE = E_LUMO- E_HOMO) and the dipole moment (µ) were also determined.

According to the frontier molecular orbital theory (FMO) of chemical reactivity, transition of electron is due to interaction between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of reacting species (Figure 3).

HOMO

LUMO

FIGURE 3: Schematic representation of the HOMO and LUMO of the studied molecule

The higher value of E_HOMO indicate the greater its ability of offering electrons to unoccupied d-orbital of the metal and higher the corrosion inhibition efficiency through better adsorption. On other hand the inhibition efficiency also increases with decreasing E_LUMO. It is known that E_LUMO indicates the ability of the molecule to accept electrons. Excellent corrosion inhibitors are usually those organic compounds, which are not only offer electrons to unoccupied orbital of the metal, but also accept free electrons from the metal¹⁸. In our study, the highest value of E_HOMO -8.19 (eV) and lowest value of E_LUMO (-7.292 eV) of compound indicates the better inhibition efficiency, As for the values of ΔE (E_LUMO-E_HOMO) concern; lower values of the energy difference ΔE will cause higher inhibition efficiency because the energy to remove an electron from the last occupied orbital will be low^19,20. As ΔE decreases the reactivity of the molecule increases leading batter inhibition efficiency²⁰. The results as indicated in table 1 shows that tested compound has the lowest energy gap (ΔE) 0.898 (eV), this means that the molecule could have better performance as corrosion inhibitor. Analysis of Fig. 3 shows that the distribution of two energies HOMO and LUMO localized in the atoms of Ciprofloxine cycle, consequently this is the favorite sites for interaction with the metal surface. The estimation of the total energy gives good information, the higher Total Energy -30933.697(eV) confirms the higher stability of inhibitor molecule⁷.

Dipole moment is the measure of polarity of a polar covalent bond. The dipole moment (μ in Debye) is another important electronic parameter that results from non uniform distribution of charges on the various atoms in the molecule^13,21,22.

Literature survey reveals that several irregularities appeared in case of correlation of dipole moment with inhibitor efficiency²³. In general, there is no significant relationship between the dipole moment values and inhibition efficiencies¹³.

FIGURE 4: Mulliken charges population analysis of Ciprofloxacin using DFT at the B3LYP/6-31G (d,p) basis set level

In the present study, the value of dipole moment (μ) for Ciprofloxacin drug molecule is 2.67 Debye and the dipole moment is not correlated with the experimental results.

The Mulliken charges of drug molecule are shown in Table 2 and Figure 4. The highest negative charge is domiciled in Nitrogen atoms as well as Oxygen atoms. There is a general consensus by several authors that the more negatively charged heteroatom is, the more is its ability to adsorb on the metal surface^23,24.

TABLE 2: Calculated Mulliken charges of Ciprofloxacin Drug

Atom Positions	Atoms	Mulliken Charges
1	O	-0.324222
2	C	0.534571
3	C	-0.146946
4	C	0.684717
5	N	-0.615705
6	C	0.117198
7	C	0.214276
8	C	-0.102809
9	C	0.125913
10	C	0.048030
11	C	-0.036970
12	C	0.248817
13	F	-0.208870
14	C	0.228533
15	N	-0.560831
16	C	0.111960
17	C	0.148360
18	N	-0.347370
19	C	0.093438
20	C	0.183168
21	C	0.185299
22	C	0.407512
23	O	-0.447459
24	O	-0.540611

CONCLUSION:

The correlation between the quantum chemical parameters and inhibition efficiency of Ciprofloxacin Drug compound was investigated using DFT/B3LYP calculations. The inhibition efficiency of the inhibitor are closely related to the quantum chemical parameters, the highest occupied molecular orbital (E_HOMO), energy of lowest unoccupied molecular orbital (E_LUMO), energy gap (ΔE), and Total energy (TE). The calculated HOMO energy, energy gap (ΔE) show reasonably good correlation with the efficiency of corrosion inhibition. The total energy (TE) determined is good tool to explain inhibition efficiency of studied inhibitor. Quantum chemical approach is adequately sufficient to predict the structure and molecule suitability to be an inhibitor.

ACKNOWLEDGEMENT:

It gives me great pleasure to Dr. R. K. Tugnawat, Principal, Government Holkar Science College, Indore for providing Computer lab facilities.

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Received on 12.04.2014 Modified on 25.04.2014

Asian J. Research Chem. 7(4): April 2014; Page 386-389