Molecular-Level Understanding of the Inhibition Efficiency of Some

Article pubs.acs.org/IECR

Molecular-Level Understanding of the Inhibition Efficiency of Some Inhibitors of Zinc Corrosion by Quantum Chemical Approach Gökhan Gece*,† and Semra Bilgiç‡ †

Department of Chemistry, Bursa Technical University, Osmangazi 16200, Bursa, Turkey Department of Chemistry, Ankara University, Beşevler 06100, Ankara, Turkey

‡

S Supporting Information *

ABSTRACT: 2-[4-(Methylthio) phenyl] acetohydrazide (HYD) and 5-[4-(methylthio)benzyl]-4H-1,2,4-triazole-3-thiol (TRD) are compounds that contain methylthiophenyl moiety and substantially inhibit corrosion of zinc in acidic medium. The relationship between molecular structures and inhibition efficiencies of these intriguing inhibitors remains the subject of intense speculation, as introduction of thiophenyl group in strategic positions of many molecules alters their activity. We have performed density functional theory calculations at the B3LYP/6-311++G(d,p) level to analyze the inhibition mechanisms proposed for these compounds. In agreement with experiments, the results afford a full explanation of the highest efficiency observed for TRD in terms of electronic and structural characteristics.

1. INTRODUCTION The resistance of zinc to corrosion under natural conditions is largely responsible for the several applications of the metal. The property that gives zinc this valuable corrosion resistance is its ability to form a protective layer consisting of zinc oxide and hydroxide, or of various basic salts, depending on the nature of the environment. The formation of the protective layers is governed largely by the pH of the environment. Because zinc forms an amphoteric oxide, both acid and alkaline conditions adversely affect its corrosion behavior by interfering with the formation of the protective layers. Zinc has been reported to dissolve in liquids whose pH is below about 5 and above 12.5.1 To improve its corrosion resistance, surface pretreatments, coatings, and additives are applied to the corrosive media. Numerous compounds of varied chemical nature have been tested as potential corrosion inhibitors of zinc in acidic media;2,3 among them, methylthio phenyl moiety-containing compounds are particularly interesting. They constitute a group of compounds employed in pharmaceutical products.4 Shylesha et al.5 have recently reported considerable inhibition efficiencies of such two compounds, that is, 2-[4(methylthio) phenyl] acetohydrazide (HYD) and 5-[4(methylthio)benzyl]-4H-1,2,4-triazole-3-thiol (TRD) (Figure 1), for zinc corrosion in HCl medium. The results of their

electrochemical and weight loss measurements showed that both compounds reduce the corrosion rate of zinc substantially in HCl medium. The inhibition efficiency of TRD reached about 99%, a higher rate than that of HYD (∼95%), at a concentration of 0.2 μM. In addition, the study of surface morphology of the zinc samples suggested that the depth attack was more prominent with zinc surfaces exposed to 0.1 M HCl than those exposed to the same medium supplemented with the inhibitors. A recondite explanation was suggested that the inhibitor molecules hinder the dissolution of zinc by forming an organic film on the surface. Although their experimental work has shown that the inhibitor molecules act by adsorption at the metal/solution interface, the degree to which direct relationships exist between inhibition efficiency and structural/electronical features of these inhibitors is still uncertain. The advancements in computer simulation techniques hold promise that questions regarding the inhibitive properties of such compounds can be addressed at the atomic level. Therefore, there is a wealth of theoretical studies concerning the associations between molecular structure and inhibition effect.6−15 In this Article, prompted by the successful application of theoretical calculations in corrosion inhibitor research, an attempt is devoted to elucidate the inhibition mechanism of HYD and TRD compounds and to give a suitable explanation to the experimental results by using density functional theory (DFT) method, which has proven to be a very efficient method for the prediction of different molecular properties.

2. COMPUTATIONS All computations have been carried out with the Gaussian 09 program package.16 B3LYP, a hybrid functional of the DFT Received: Revised: Accepted: Published:

Figure 1. Molecular structure of (a) HYD and (b) TRD. © 2012 American Chemical Society

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The phenomenon of electrochemical corrosion takes place in the liquid phase, so it is relevant to include the effect of solvent in the computations. Self-consistent reaction field (SCRF) theory,20 with Tomasi’s polarized continuum model (PCM), was used to perform the calculations in solution. These methods model the solvent as a continuum of uniform dielectric constant (ε = 78.5) and define the cavity where the solute is placed as a uniform of series of interlocking atomic spheres. The molecular volume (MV) was defined as inside a contour of 0.001 electrons/bohr3 density that was evaluated using Monte Carlo integration.

method, which consists of the Becke’s three parameters exact exchange functional B3 combined with the nonlocal gradient corrected correlation functional of Lee−Yang−Parr (LYP), has been used. The standard triple split valence basis set supplemented by polarization and diffuse functions, 6-311G+ +(d,p), is used throughout the computational process. In the process of geometry optimization for the fully relaxed method, convergence of all of the calculations and the absence of imaginary values in the wave numbers confirmed the attainment of local minima on the potential energy surface. According to DFT-Koopmans’ theorem,17 the ionization potential I can be approximated as the negative of the highest occupied molecular orbital (HOMO) energy: I = −E HOMO

3. RESULTS AND DISCUSSION It has been established experimentally that the methylthiophenyl group possesses a notable effect on intermolecular interactions.21 HYD and TRD molecules share the same methylthiophenyl moiety but differ with respect to the core structure, which incorporates an acetohydrazide fragment in HYD (Figure 1a), whereas a 1,2,4-triazole-3-thiol component is present in TRD (Figure 1b). The main difference between these compounds is not only the molecular structures they embody but what other behaviors they exhibit, like their rapid interconversions by chemical reactions and/or their protonation characteristics under acidic conditions. It has been reported that hydrazides are protonated in strong acidic media (Figure 2) where the majority of them (pKa ≈ 3)

(1)

The negative of the lowest unoccupied molecular orbital (LUMO) energy is similarly related to the electron affinity A:

A = −E LUMO

(2)

The proper vertical ionization potentials Ivert were evaluated as the energy difference between the neutral species and the positive ion at the neutral equilibrium geometry: I vert = Eneutral − Epositive ion

(3)

The global reactivities include electronegativity, χ, which is identified in the finite difference approximation as the negative of the chemical potential μ: χ = −μ = (I + A)/2

(4)

with the global hardness η defined as: η = (I − A)/2

(5)

Figure 2. The protonation mechanism of HYD under acidic conditions.

(6)

remain neutral only under mildly acidic conditions (e.g., pH ≈ 5).22 On the other hand, the thione−thiol tautomerism of TRD remains as a matter of controversy, because it is not straightforward to determine which of the two forms is present. As shown in Figure 3, for the parent molecule of TRD (i.e., 1,2,4-triazole-3-thione), two tautomeric forms may exist: the thione (NH) and thiol (SH) forms. The first has a CS double bond, and the latter has the endocyclic double bond CN. The respective population of the thione−thiol tautomeric forms of the molecule is expected to vary with the pH of the solution. A preponderance of the thione form of the molecule at acidic pH has been suggested in accordance with Raman spectroscopic measurements.23 Moreover, in an extremely acidic media, N21 and N23 atoms of the thiol form and the N21 atom of the thione form of TRD will be the probable active sites of protonation (Figure 4). Because the experimental data used in this study were based upon the results of electrochemical measurements in 0.1 M HCl solution, we performed DFT calculations for both neutral and protonated forms of these compounds. To determine the dominant form of TRD responsible for the corrosion inhibition, we have extended the calculations to its thione and thiol tautomers. The B3LYP/6-31++G(d,p) fully optimized structures of neutral forms of HYD and TRD are illustrated in Figure 4. To validate the optimized geometry of HYD in the gas phase, some selected geometrical parameters for the neutral form of HYD have been compared to those calculated for the structure of acetohydrazide (see Table S1 in

and the global softness σ as its inverse: σ = 1/η

During the interaction of the inhibitor molecule with bulk metal, electrons flow from the lower electronegativity molecule to the higher electronegativity metal until the chemical potential becomes equalized. The fraction of the transferred electron, ΔN, was estimated according to Pearson:18 χm − χi ΔN = 2(ηm + ηi) (7) where the indices “m” and “i” refer to metal atom and inhibitor molecule, respectively. The reactivity of the compounds was analyzed through an evaluation of the Fukui indices,17 which is a measurement of the chemical reactivity, as well as indicative of the reactive regions and the nucleophilic and electrophilic behavior of the inhibitor molecules. The regions of a molecule where the Fukui function is large are chemically softer than the regions where the Fukui function is small, and by invoking the HSAB principle in a local sense, one may establish the behavior of the different sites with respect to hard or soft reagents. For electrophilic and nucleophilic attacks, the condensed Fukui functions, f i − and f i + , based on the finite difference approximation,19 are given by: f i− = qNi − qNi − 1 and f i+ = qNi + 1 − qNi

qiN,

qiN − 1,

(8)

qiN + 1

where and are the atomic charges on the ith atom in the neutral, cationic, and anionic species, respectively. 14116

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Figure 3. The mechanism proposed for tautomerization of 1,2,4-triazole-3-thione to 1,2,4-triazole-3-thiol and corresponding transition structure (TS) in TYD.

Figure 4. Optimized structures using the B3LYP/6-311++G(d,p) level for the neutral form of (a) HYD and (b) TRD (left, thione; right, thiol) in the aqueous phase.

the Supporting Information) at the MP2/6-311+G(d,p) level by Badawi.24 It is clear that the syn orientation of the amino group greatly stabilizes the near cis form of the HYD molecule due to the intramolecular attraction between one of the amino hydrogen atoms and the carbonyl oxygen. The significance of this hydrogen bonding in HYD molecule can be realized upon analyzing the calculated Mulliken charges where the N20 atom was predicted to have a relatively high partial negative charge (−0.198 e). Although reports on the crystal structure of 3-(alkyl or aryl)1,2,4-triazole-5-thione compounds are still rare, the recent experimental work of Jing et al.25 on 3-ethyl-1H-1,2,4-triazole5(4H)-thione provides sufficient information on structural parameters that can be used to make direct comparisons to the distribution of the S−C, N−N, and C−N bond lengths and angles of TRD (see Table S2 in the Supporting Information). Despite the minor differences, calculated geometric parameters represent a reasonable approximation and can provide a starting point to calculate other parameters, such as the energies of the frontier molecular orbitals, as will be described below. The inhibition effect of inhibitor compound is usually ascribed to adsorption of the molecule on metal surface. If molecules adsorb at solid surfaces, this can occur either by chemical or by physical bonding. Chemical adsorption (chemisorption) thereby refers to bonds of chemical strength, whereas physical adsorption (physisorption) refers to unspecific adsorption based on the dispersion interaction. In general, the chemical bond between molecules and the surface can be considered a combination of the interactions of doubly HOMOs and LUMOs. In the case of chemisorbed atoms, the interaction with the LUMOs usually dominates. For molecular adsorption, both the HOMO and the LUMO interactions often are of comparable strength and tend to have opposing behavior

when differences in coordination or different metals are compared. A doubly occupied HOMO of low energy participates in adsorbate−surface orbital fragments with a relative high electron occupation of antibonding orbital fragments. An unoccupied LUMO of high energy participates in adsorbate−surface orbital fragments with no or low electron occupation of antibonding orbitals. Mainly bonding fragment orbitals are occupied. Because of the dominance of Pauli repulsion, a strong interaction of an adsorbate HOMO orbital with a highly occupied d-valence electron band pushes the molecule to a one-fold position. The interaction with a LUMO, however, always tends to favor high coordination sites, in which bonding orbital fragments dominate. The corresponding molecular orbital energies allow one to estimate stability and reactivity of the studied molecules. Thus, the difference between HOMO and LUMO energies, the so-called HOMO−LUMO gap, is an important stability index. Reportedly, excellent corrosion inhibitors are usually those organic compounds that not only offer electrons to the unoccupied orbital of the metal, but also accept free electrons from the metal.6 A high value of the HOMO energy corresponds to copious donation of electrons to congruent molecules with low energy, empty molecular orbitals. Increasing values of EHOMO lead to an increment in adsorption and exalts the efficiency of inhibition.6 The energy of the LUMO indicates the ability of the molecule to accept electrons. The lower is the value of ELUMO, the more probable it is that the molecule would accept electrons. Accordingly, lower values of the energy difference (ΔE) will render good inhibition efficiency, because the energy to remove an electron from the last occupied orbital will be low.6 On the basis of the explanations above, it is useful to compare the electronic properties of HYD and TRD. Certain 14117

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hardness value increases from the thione form of TRD through the thiol form to HYD. The fraction of transferred electron given in the table is calculated for zinc metal, and the experimental work function of polycrystalline Zn (3.63 eV)26 was employed for electronegativity, and a global hardness of zero was used due to the I = A approximation for a bulk zinc. The thione form of TRD has the largest fraction of transferred electron to the zinc metal, closely followed by the thiol form, while HYD has a slightly smaller value. The calculated global reactivity parameters follow the order TRD (thione) > TRD (thiol) > HYD, in agreement with the above ordering supported by electronic parameters and also the ones reported by other researchers.27−29 A well-defined separation was evidenced in the distribution of HOMO and LUMO energies, which are located in two distinct parts of the molecule (Figure 6). The HOMO levels show greater contributions of π-like orbitals at the benzene ring of the molecules, with smaller contributions from p-orbitals at the N and O atoms close to the benzene ring. On the other hand, the LUMO is to a large extent delocalized over the triazole ring. The greater contributions of the LUMO orbitals come from p-orbitals of the nitrogen atoms, with further significant contributions from the p-orbitals of the sulfur atom around the triazole ring in thione form of TRD. This trend is also mirrored in the Mulliken atomic charges. For carbon atoms, the negative charges in HYD range from −0.191 to −0.760 e. This is in contrast to the thiol form of TRD where we determined negative charges in the range of −0.195 to −0.926 e. For the thione form of TRD, there are two types of sulfur atoms (S25 and S11) with negative charges of −0.669 and −0.218 e, respectively. Three nitrogen atoms (N21, N23, and N24) have positive charges in the range of 0.077−0.202 e, while the remaining carbon atoms exhibit negative net charges in the range of −0.164 to −0.947 e. Therefore, nitrogen atoms in the two tautomers of TRD exist as electron donors with positive net Mulliken charges, while the majority of carbon atoms behave as electron acceptors. The changes in the molecular volume also provide an alternative insight into the charge redistribution. Thus, Table 2 reports the molecular volumes (MV) in gas and in solution. As is seen, the solvent leads to an important shrinking on the molecular volume, due to the movement of electrons to the

quantum chemical parameters related to the molecular electronic structures of HYD and TRD, such as EHOMO, ELUMO, and ΔE= ELUMO − EHOMO, have been used for the sake of comparison. As can be seen from the data in Table 1, no Table 1. Quantum Chemical Parameters Calculated at the B3LYP/6-311++G(d,p) Level for the Neutral Forms of HYD and TRD in Gas and Aqueous Phases inhibitor

phasea

EHOMO (eV)

ELUMO (eV)

ΔE (EL − EH) (eV)

IE (%)b

HYD

G A G A G A

−6.216 −5.911 −5.949 −6.042 −6.613 −6.029

−1.033 −0.789 −1.196 −0.802 −1.059 −0.789

5.183 5.122 4.753 5.240 5.554 5.240

71.34

TRDc TRDd

95.99

a

G, gas phase (ε = 1.0); A, aqueous phase (ε = 78.5). bExperimental data from ref 5. cThione form. dThiol form.

correlation has been found between inhibition efficiency and calculated parameters of the compounds in their neutral forms. It should not be construed that these energetic parameters are no longer worthless. As detailed herein, this lack of correlation is likely attributable to underestimation of drastic pH effect on the structural properties of these inhibitors. It thus appears that under such strong acidic conditions only protonated forms of the compounds (Figure 5) give rise to robust inhibition performance. The results reported in Table 2 also show that there is a considerable change both in HOMO and in LUMO energies with respect to the gaseous phase situation. The effect of the electrostatic interactions between solute and solvent continuum is therefore non-negligible. Both in gas and in aqueous phases, the thione tautomer of TRD has lower HOMO and LUMO energies than the other two compounds. In particular, we noted that the energy gap of HOMO−LUMO (ΔE) of thione tautomer is smaller (4.120 eV) than the corresponding ΔE values of the other two compounds (HYD, with ΔE = 4.842 eV and thiol tautomer, with ΔE = 4.610 eV) in aqueous phase. These results are also supported by the global reactivity parameters. The thione form of TRD has the lowest and HYD the highest electronegativity. The electronegativity of thiol form is very close to that of thione. Similarly, the global

Figure 5. Optimized structures using the B3LYP/6-311++G(d,p) level for the protonated form of (a) HYD and (b) TRD (left, thione; right, thiol) in the aqueous phase. 14118

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Table 2. Electronic Parameters (Orbital Energies for HOMO and LUMO, HOMO−LUMO Energy Gap (ΔE), Vertical Ionization Energies (Ivert), Dipole Moment (μ)) and Global Reactivity Parameters (Electronegativity (χ), Global Chemical Hardness (η), Global Softness (σ), the Fraction of Transferred Electron (ΔN), Molecular Volumes (MV)) Calculated at the B3LYP/6-311++G(d,p) Level for the Protonated Forms of HYD and TRD Tautomers in Gas and Aqueous Phases inhibitor

phasea

EHOMO (eV)

ELUMO (eV)

ΔE (EL − EH) (eV)

Ivert (eV)

μ (D)

MV (cm3/mol)

χ

η

σ

ΔN

IE (%)b

HYD

G A G A G A

−8.744 −6.105 −8.950 −6.249 −8.745 −6.215

−5.295 −1.263 −5.981 −2.129 −5.383 −1.605

3.449 4.842 2.969 4.120 3.362 4.610

2.68

18.03 22.44 6.66 8.60 10.68 13.55

166.54 143.64 186.63 138.71 164.42 166.95

7.02 3.68 7.47 4.19 7.06 3.91

1.72 2.42 1.48 2.06 1.68 2.31

0.58 0.41 0.68 0.49 0.59 0.43

−0.99 −0.01 −1.29 −0.14 −1.02 −0.06

71.34

TRDc TRDd a

3.94 3.45

95.99

G, gas phase (ε = 1.0); A, aqueous phase (ε = 78.5). bExperimental data from ref 5. cThione form. dThiol form.

Table 3. Condensed Fukui Function of HYD and TRD Tautomers in Aqueous Phase f i−

a

atom

HYD

O25 N20 N21 N22 N23 N24 S25

0.01 0.50

f i+ a

TRD

TRD

b

HYD

TRDa

TRDb

0.05

0.25

0.05 0.04 0.07

0.17 0.04 0.05

0.03 0.13 0.04

0.05

0.55

0.12 0.04 0.11 0.19

0.07 0.07 0.09

Thione form. bThiol form.

electrophilic attack. For HYD, N20 and N22 are the preferred reactive sites. Turning to nucleophilic attack, the most reactive site of thione tautomer is on S25, while, for thiol tautomer, the N21 atom is the most reactive center. In summary, Fukui analysis predicts that all species possess more than one attack center, which enables multicenter adsorption of the inhibitor molecules on a metal surface.

Figure 6. Contour plots of the frontier molecular orbitals for the protonated form of (a) HYD, (b) TRD (thione), and (c) TRD (thiol) in aqueous phase (left, HOMO; right, LUMO).

inner regions. The volume of HYD is significantly larger than that of the thione form of TRD and obviously smaller than that of the thiol form of TRD. In summary, water has a significant and quite specific effect on these molecules. In addition to this, another parameter pertaining to inhibition effectiveness is dipole moment (μ), which reflects the molecular charge distribution. Although there is no agreement in the literature concerning the correlation between the dipole moment and the inhibition efficiency, it can be used as a descriptor to depict the charge movement across the molecule. Lower values of μ are reported to favor accumulation of the inhibitors in the surface layer.30,31 As a result of B3LYP calculations (Table 2), the highest dipole moment was observed for HYD, whereas the smallest one was observed for the thione form of TRD. The local reactivity behavior was analyzed by means of Fukui indices and is presented in Table 3. The atomic sites of molecule, which possess the largest condensed Fukui functions, favor the higher reactivity. Thus, the molecular sites with the maximum value of f i− are the preferred sites to which the inhibitor molecule will donate charge when attacked by an electrophilic reagent. On the other hand, a large value of f i+ is assigned to the sites where the inhibitor molecule will receive charge, when attacked by a nucleophilic reagent. The sulfur atom of triazole ring S25 is the most favorable site on the thione tautomer for an electrophilic attack, and the N24 position shows only slightly lower local reactivity. The identical values of the f i− indices for the nitrogen atoms N23 and N24 of thiol tautomer indicate that both centers are equivalent when it undergoes an

4. CONCLUSIONS The general importance of our theoretical results for the understanding of inhibition processes through the use of methylthio phenyl moiety containing compounds may be summarized by the two following statements: (1) In all cases, the ground state of the protonated forms of the compounds yields the electronic and global reactivity parameters that are in best agreement with the experimental data, giving considerable credence for the identified structures in acidic medium. On the basis of the data obtained, we can confirm that the most stable tautomer of TRD in aqueous phase is the thione form, which is responsible for its higher inhibition efficiency as compared to HYD. Apart from this, the solvent effect produced significant changes in the molecular and electronic structure of the inhibitor molecules. (2) The results based on Fukui indices demonstrate that the inhibition mechanism depends on multicenter adsorption of these compounds on the metal surface. In particular, in the thione tautomer of TRD, an electrophilic reaction may be expected to occur preferentially on the S25 and N24 sites, while the N21 and N23 centers are preferred for a nucleophilic attack. 14119

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ASSOCIATED CONTENT

S Supporting Information *

Selected structural parameters for the neutral form of HYD and the thione form of TRD in the gas phase. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +90 224 3141618. Fax: +90 224 3141650. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ie302324b | Ind. Eng. Chem. Res. 2012, 51, 14115−14120