)
(
2
1
A
I+
=
−
=
,
(1)
(2)
The global electrophilicity index (ω) was introduced by Parr et al. [13-15] and is
given by:
(
)
(
)
A
I
A
I
+
+
=
8
2
,
(3)
In considering the adsorption process from the point of view of the acid-basic
interaction, the important quantitative parameters characterizing this process are the
degree of charge transfer ΔN, that is, the interaction strength of acid-acceptor
molecules (iron surface) and the donor base (investigated substances of the vapor phase
of the extract) and the magnitude of the energy change that accompanies the formation
of the complex. The electron charge transfer, ∆N, from base B to acid A, and the
associated energy change ∆E is given as:
)
(
2
B
A
A
B
N
+
−
=
,
(4)
(
)
(
)
В
А
А
В
Е
−
−
=
2
2
,
(5)
χ = -μ – electronegativity, еV; η –chemical hardness, еV; A, B – Indices in
formulas (3,4); A – characteristics of the molecule of the test substance; B –
characteristics of the elemental lattice of the surface of iron.
The electronegativity of metal surface is replaced by the work function for Fe
(110) surface, theoretically equals 4.82 eV and the hardness metal which equals 0 eV
for bulk metals [13-15].
These parameters will provide insights into the mechanism of the interaction of
the extract components with mild steel surface.
The results of quantum-chemical calculations of the energy parameters of
molecules are presented in Tables 3 and 4. The HOMO energy (E
HOMO
) is often
associated with the electron donating ability of the molecule, whereas the E
LUMO
indicates the ability of the molecule to accept electron. Therefore, high values of the
E
HOMO
indicate an increased tendency of the inhibitor to donate electron to the vacant
d orbital of Fe in mild steel. Namely the HOMO regions for the molecules are the sites
at which electrophiles attack and represent the active centers with the utmost ability to
interact with the metal surface atoms. The highest E
HOMO
values are obtained for 2-
Phenylacetaldehyde.
On the other hand, the LUMO orbitals predict the regions where the molecules
can accept electrons from the metal using antibonding orbitals to form feedback bonds.
),
(
2
1
A
I−
=
- 1451 -