Chemistry Reference
In-Depth Information
TABLE 4.4
Reactivity Fukui Indices for Heavy Atoms in Guanine Isomers
Isomer
Guanine A
Guanine B
Type
I
N
I
E
I
R
I
N
I
E
I
R
1
C
1.27
11.63
4.42
3.32
27.31
11.12
2
N
0.83
7.29
2.82
1.84
12.09
5.52
3
C
4.52
9.36
7.48
3.93
0.16
0.94
4
N
21.54
0.22
2.47
19.95
9.51
16.09
5
C
7.76
21.98
15.01
4.64
13.95
9.40
6
C
20.99
13.38
19.27
19.24
1.07
5.30
7
N
12.63
16.15
16.42
14.23
10.29
14.14
8
C
5.93
18.46
12.03
3.50
25.81
11.11
9
N
5.46
0.05
0.61
6.79
2.95
5.23
10
O
5.64
5.23
6.25
2.31
3.70
3.42
11
N
12.84
2.02
5.86
21.23
0.06
1.34
The Fukui Equations (4.16), (4.17), and (4.18) reactivity indices are considered
a measure of the reactivity of the molecule (as nucleophil, electrophil, or radical)
when the reaction center is the atom for which the indices have been calculated. The
values of the Fukui indices, calculated for different atoms, allow the comparison of
the atoms and the identification, as a rule, of the reaction center in different types of
chemical reactions. The reactivity estimated in this way does not consider steric fac-
tors. Table 4.4 presents the values of the Fukui reactivity indices for the isomers of
guanine that are analyzed here. The indices I
N
, I
E
, and I
R
do not reflect the deforma-
tion of the electron clouds of two molecules that are approaching in order to react; 1
therefore they are called
static
. The reactivity indices that consider this factor also
are called
dynamic
. Their calculation requires much more complicated formulas
(Roothaan 1951) and involves the comparison of the energy of the molecular orbitals
of the isolated molecules to the energy of the molecular orbitals of the supermolecule
(i.e., the ensemble of the A and B molecules, situated at very short distances).
4.3.7 h
eat
oF
F
orMation
The heat of formation is the energy released as heat when atoms situated at theoreti-
cally infinite distance approach, bind, and form the molecule of interest. The core
includes, by definition, the atomic nucleus and the electrons that do not participate in
chemical bonds, that is, the nonvalence electrons. The semiempirical method PM6
estimates the heat of formation as the sum of the total repulsion energy of the cores
and the total heat of formation of the atoms. Each semiempirical quantum mechanics
method calculates, in its own manner, the energy of repulsion of the cores and utilizes
a different set of values for the atomic heat of formation. Consequently, the values of
the heat of formation determined for the same molecule by different semiempirical
