Chemistry Reference
In-Depth Information
•
Highest occupied molecular energy (ε
HOMO
) is related to the EI and is a
measure of nucleophilicity.
•
Lowest unoccupied orbital energy (ε
LUMO
) is related to the EA and it is a
measure of electrophilicity.
•
The energy gap between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) represents the differ-
ence between HOMO and LUMO energies:
GAP = ε
LUMO
− ε
HOMO
.
(3.17)
GAP is an important stability index. A large GAP means a high stability and low reac-
tivity in chemical reaction. The HOMO-LUMO energy fraction (f
H/L
) is a stability
index defined as the ratio of HOMO and LUMO energies,
=
ε
ε
.
(3.18)
HOMO
LUMO
f
H/L
From the frontier orbitals energies, an approximated absolute hardness is also
obtained,
EI
−
EA
GAP
2
=
ε ε
LUMO
HOMO
η=
=
(3.19)
2
2
where EI is the ionization energy, EA is the electron affinity, GAP denotes the
HOMO-LUMO energy difference, and ε
LUMO
and ε
HOMO
are the energies of the low-
est unoccupied orbital and the highest occupied orbital, respectively.
The total softness can be also calculated by
1
S
=
.
(3.20)
EI
−
EA
3.2.2 E
lEctropositivity
Electropositivity is a measure of an element's ability to donate electrons, and
consequently, to form positive ions with a stable configuration. Electropositivity
reflects a tendency that is opposite that of electronegativity. Metals are electro-
positive elements: the greater the metallic character of an element, the greater the
electropositivity. The alkali metals are the most electropositive of all as is related to
their low ionization energies. Electropositivity decreases along periods (from left
to right) and increases down groups.
3.2.3 o
xidation
n
umbEr
(o
xidation
s
tatE
)
Each atom tends to have its outer energy level complete, and also to gain, lose, or
share its valence electrons to achieve a stable configuration. The absolute number
