It is well known that the lower the value of E
LUMO
, the more probable it is that the
molecule would accept electrons from the metal. Our calculations show that
investigated molecules has the lowest E
LUMO
, so it has the greatest ability to interact
with the metal surface. The ranking of the molecules according to their ability to accept
electrons from mild steel is: 2-Phenylacetaldehyde, Hexanal, (9Z)-Octadec-9-enoic
acid, Hexadecanoic acid. Table 3 shows the calculated values of the energy of the crack
the molecules under study. The seperation energy is parameter which determines the
reactivity of a molecule towards a metallic surface and is a very important stability
index. As ∆ε decreases, the reactivity of the molecule increases leading to increase
adsorption on the metal surface and hence high inhibition efficiency, since the energy
to remove an electron from the last occupied molecular orbital will be low.
Table 3 Calculated quantum chemical properties for the most stable
conformations of the major effective components of extracts pomaces the apricot
Molecule
E
HOMO
(eV)
E
LUMO
(eV)
ΔεН-L (eV)
Hexanal
-10.771
0.864
-11.63
2-Phenylacetaldehyde
-9.908
0.125
-10.03
(9Z)-Octadec-9-enoic acid
-9.883
0.907
-10.79
Hexadecanoic acid
-11.183
1.066
-12.24
The calculations from Table 4, shows the following increasing reactivity abilities
of the molecules towards steel: Phenylacetaldehyde, 3,7-dimethylocta-2,6-dienal,
(9Z)-Octadec-9-enoic acid, Butanal, Nonanal, Hexanal, Hexadecanoic acid.
Consequently, the first substances of the specified series are likely to be more potent
inhibitors. Corrosion inhibition of mild steel by organic inhibitors (present in plant
extracts) is a complex process and is mainly influenced by charge transport.
Electrophilicity(ω) is an index which measures the propensity of chemical specie to
accept electrons.
Table 4 Calculated quantum chemical properties for the most stable
conformations of the major effective components of extracts pomaces the grapes
Molecule
,(eV)
, (eV)
ΔN
ω
ΔE
Hexanal
-4.95
5.81
0.175
1.238
0.00077
2-Phenylacetaldehyde
-4.89
5.01
0.210
1.222
0.00025
(9Z)-Octadec-9-enoic acid
-4.48
5.39
0.232
1.220
0.00510
Hexadecanoic acid
-5.05
6.12
0.158
1.264
0.00232
The higher the value of ω, the higher the capacity of the molecule to accept
electrons. Thus, a good nucleophile is characterized by low values of µ and ω; whereas
a good electrophile is characterized by high values of µ and ω. It is clear in Table 4 that
the molecules have low electrophilicity index values and are good nucleophiles. The
amount of charge transfer, ∆N, between the molecules and the mild steel Fe surface as
calculated is given in Table 4. A positive value of ∆N indicates that the molecules act
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