## 3 Results and Discussion
This means either that the diffusion of the soluble corrosion product on the electrode's surface slows down, or that the barrier properties of the protective film blocking the aggressive elements of the solution increase. \[\eta_{\text{inh}}\ =\ \left(1-\frac{R_{\text{P,0}}^{EIS}}{R_{\text{P,inh}}^{EIS} }\right)\text{100\%} \tag{6}\]  
The inhibition effectiveness is calculated using the values of charge transfer resistance without (\(R_{\text{P,0}}^{EIS}\)) and with the inhibitor (\(R_{\text{P,inh}}^{EIS}\)), is about 100% (Table 5) for all the studied inhibitors, when \(C_{\text{inh}}\geq\) 0.10 mM. This agrees well with the results of the regression analysis of the polarization curves carried out using Mansfeld method (Table 2). The results of the electrochemical studies and corrosion testing were interpreted within the framework of the quantum chemical approach to the inhibition effect assessment. Optimized molecular structures of 3-mercaptoalkyl-5-amino-1_H_-1,2_4-triazolees are given in Figure 6. Table 6 gives the values of the ionization potential, electron affinity, electronegativity, absolute hardness and softness in eV calculated at B3 LYP/6-311 + G (d, p), and optimized geometry. It is known that the ability of inhibitors to interact with a metal surface increases with the growth of HOMO and decrease in LUMO [74]. The smaller the HOMO LUMO gap (HLG), the higher the reactivity of the inhibitor to the metal surface and the stronger the inhibition effect. In Table 6, of all the three triazole molecules, 3-mercapto(3-methylbutyl)-5-amino-1_H_-1,2_4-triazolee, i.e., molecule C, has the smallest HLG and hardness value of 5.73 eV, \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Molecule** & **HOMO** & **LUMO** & **HLG** & **IP** & **EA** & \(\chi\) & \(\eta\) & \(\sigma\) \\ \hline A & –6.36 & –0.61 & 5.75 & 6.36 & 0.61 & 3.49 & 2.88 & 0.35 \\ B & –6.38 & –0.61 & 5.77 & 6.38 & 0.61 & 3.50 & 2.89 & 0.34 \\ C & –6.35 & –0.62 & 5.73 & 6.35 & 0.62 & 3.48 & 2.87 & 0.35 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Calculated energies of HOMO, LUMO, HOMO LUMO gap (HLG), Ionization Potential (IP), Electron Affinity (EA), Electronegativity (\(\chi\)), Absolute Hardness (\(\eta\)) and Softness (\(\sigma\)) in eV at B3 LYP/6-311 + G (d, p) level. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Inhibitor** & \(C_{\text{inh}}\)**, **mM** & \(R_{\text{O}_{\text{r}}}\) & \(C_{\text{DL}}\) & \(R_{\text{P}}\)**, & \(R_{\text{P}}\)**, & \(R_{\text{P}}\)**, & \(R_{\text{P}}\)**, & \(R_{\text{P}}\)**, & \(R_{\text{P}}\)**, \\  & & \(\Omega\)**-**cm**2** & \(\mu\)**F**-**cm**2** & \(k\Omega\)**-**cm**2** & \(k\Omega\)**-**cm**2** & \(BW\)**, & \(k\Omega\)**-**cm**2** & \(BW\)**, & \(BW\)**, & \(\eta_{\text{inh}}\)**, \%** \\ \hline - & 0.00 & 240 & 26.2 & 0.176 & 1.28 & 0.20 & - \\ \hline \multirow{3}{*}{A} & 0.01 & 256 & 1.57 & 72 & 73 & 1.77 & 99.76 \\  & 0.10 & 250 & 1.00 & 141 & 298 & 6.00 & 99.88 \\  & 1.00 & 258 & 1.54 & 473 & 135 & 0.26 & 99.96 \\ \hline \multirow{3}{*}{B} & 0.01 & 200 & 2.56 & 0.9 & 13 & 0.81 & 79.86 \\  & 0.10 & 255 & 1.58 & 149 & 151 & 23.27 & 99.88 \\  & 1.00 & 244 & 1.02 & 35 & 9336 & \(10^{-6}\) & 99.50 \\ \hline \multirow{3}{*}{C} & 0.01 & 275 & 0.86 & 52 & 239 & 2.87 & 99.66 \\  & 0.10 & 275 & 1.42 & 158 & 20605 & \(10^{-6}\) & 99.89 \\ \cline{1-1}  & 1.00 & 275 & 0.79 & 111 & 23526 & \(10^{-6}\) & 99.84 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Equivalent circuit elements and degree of protection of brass electrode in 10 mM NaCl solutions with various derivatives of 5-amino-1_H_-1,2_4-triazole. and 2.87 eV, respectively. While, 3-mercaptoethanolpropyl-5-amino-1\(H\)-1,2,4-triazole (molecule A) and 3-mercaptoethanolbutyl-5-amino-1\(H\)-1,2,4-triazole (molecule B) have the same LUMO energy level of 0.61 eV. Destabilization of HOMO level 0.02 eV was observed upon changing the substituent from propyl (\(A\)) to butyl (\(B\)).