Nepal Journal of Science and Technology 2 (2000) 37-40
Molecular Orbital Prediction of Chemical Reactivity
R. N.Tripathil, J. S. Yadav2 and 0. P. Singh3
' ~ e p a r t m e nof
t Physics, Tribhuvan Multiple Campus, Tansen Palpa
2 ~ o l c c u l a rComputational Biology, Rutgers DState, Piscatewny, University $New Jersey, New Jersey U.S.A.
3~epartmentof Physics, Paliwal (PC.)
College, Shikokubad 205135, India
Received November 1998; accepted December 1999
Abstract
Mayer's suggestions (Mayer 1983,1984) i n ab initio SCF (Self' Consistent Field) theory have been
incorporated to the semi-empirical MIND013 method to evaluate valence indices of atoms in some trisubstituted
benzenes. Chemical reactivity of different of atoms in terms of Jug's (Jug 1984 a,b) normal, hyper and
subvalences are discussed in terms of their affinities for covalent bond formation. The traditional view that
valency is directly related to the atomic charge is found to be invalid. The valency of an atom in a molecule is a
function of the orbital occupancies corrected for intraatomic terms, as the affinity of atoms for covalent bond
formation changes from molecule to molecule. It should be mentioned that we are dealing with reactivity as a
tendency for covalent bond formation; nucleophilic or electrophilic reactivity is thus excluded.
Key words: ab initio, excess valence, MIND013 method, orbital occupancies, valence index
Introduction
Semiempirical MIND013 method which is much
cheaper and less time consuming than costly ab initio
method, gives results for several parameters,
comparable to that of ab initio methods for
qualitative purposes.
In an earlier paper, Singh and Yadav (1984)
applied Mayer's suggestions (Mayer 1983, 1984) to
calculate valence numbers in same fluorosubstituted
molecules and discussed the same in the light of its
affinity for covalent bond formation. Tripathi et al..
(1998) evaluated indices of the atoms in some
trisubstituted benzenes with substituents at 1,2,3
positions, using various semiempirical methods and
used these valence numbers to predict chemical
reactivity of the atoms.
According to Gopinathan and Jug (1983 a & b),
in chemical reactions a subvalent atom in a molecule
may form further covalent bond(s) with other
reagents whereas a hypervalent atom may either
break or weaken the exiting bond(s) so as to convert
its sub-or hypervalency to its normal valency. As the
affinity of atoms for covalent bond formation
changes from molecule to molecule and covalent
chemical reactivity has direct relations with the
valency, it was thought adequate to apply Mayer's
method to calculate the valence indices of atoms in
some other trisubstituted benzenes with substituents
at 1,2,4 - positions, and to see how the different
atoms of these molecules behave towards their
tendencies for covalent bond formation
The present paper deals with the results of such
investigations for 1,2,4 - trisubstituted benzenes with
-OH. -NH2 and - CH3 as substituents benzene
derivatives form a large group of medicinal
compounds, therefore, the study of the chemical
reactivities i n such molecules have importance in
molecular biology and medicine.
Method
In ab initio SCF formalism, Mayer (1983, 1984),
defined the actual total valence VA of atom A in the
molecule for a closed shell system as
Where P and S are the density and overlap
matrices respectively. The notation acA indicates that
the summations have to be carried out for all the
basis orbitals centered on the ath basis orbital. In the
present MIND013 calculations. overlap matrix S
being unit matrix the above relation reduces to
Furthermore for MIND013 the density matrix is
R. N. Tripathi et a1.lNepal Journal of Science and Technology 2 (2000) 37-40
Table 1. Valency and covalent chemical reactivity
Molecdes
Atoms
-
124-TrimethylBenzene
Predicted affinity for
covalent bond formation.
C6
07
08
H9
010
HI1
H 12
HI3
HI4
HI5
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Slightly reactive
Slightly reactive
Slightly reactive
C1
C2
C3
C4
C5
C6
N7
N8
H9
N 10
HI 1
HI2
HI 3
HI4
H 15
H I6
H 17
HI8
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Hypervalent
Hypervalent
Subvalent
Hypervalent
Subvalent
Normalvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
C1
C2
C3
C4
C5
C6
C7
C8
H9
C10
HI 1
HI2
HI3
H 14
HI5
HI6
H17
HI8
HI9
H20
H2 1
Su bvalen t
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactrve
Unreactive
Unreacti ve
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
Unreactive
C5
124-Triaminobenzene
A
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
Subvalent
CI
C2
C3
C4
124-Trihydorxy
benzy ne
Excess Valence (%)
and its type
-
R. N. Tripathi et al.lNepal Journal of Science and Technology 2 (2000) 37-40
symmetrical, the above relation can be written as
where P,==P,, is diagonal matrix and Pab=Pba
The percentage "excess valence" of different atoms
in the respective molecules has been evaluaed in
MIND013 method by the equation
where VAr is the reference valence of atom A and
VA is its valence index in the molecule under
consideration. As per Mayer's suggestions (1 983,
1984), we use the term "excess valence" to avoid the
use of free valence in two meanings. The definitions
of normal, hyper and sub-valences are given in
Gopinathan and Jug's papers (1983 a, b)
Results
Table I lists the valence indices of these atoms in
the respective molecules, calculated by Mayer's
method a l o n g with their 'excess valence' a n d
predicted affinity for covalent bond formation. The
values of the valence indices of atoms in these
molecules are found consistent with their classical
magnitudes and close to integers. Each of the carbon
atom to which substituents are bonded, has a smaller
valency than the other ring Carbon atoms in the same
molecule. all the hydrogen atoms are found to have a
valence index and almost equal to unity, as required
c l a s s i c a l l y e x c e p t t h e hydrogen a t o m s of t h e
substituent - OH group. It is observed that the
valency of C a r b o n a t o m in all t h e molecules
considered for the present study, is close to (but less
than) its traditional value 4. In other words, there is
no hyper-valency of carbon and octet rule is not
violated in these molecules. The hydrogen atoms in
these molecules have either have normal valence or
small subvalence values showing that these atoms
are unreactive. T h e atoms with small subvalence
values will not tend to make further covalent bond(s)
and therefore, may be treated as unreactive. All the
N- atoms in 1,2,4 -Triaminobenzene have been
observed to possess the hyper-valency, but their
magnitudes are so small that they are not supposed to
weaken o r break the existing bond(s), and hence
have been taken as unreactive. On the other hand,
some atoms of hydrogen attached with substituentOH, in 1,2,4-Trihydroxy benzene have somewhat
high subvalent values, hence they a r e taken as
slightly reactive.
Discussions
T h e difference between the classical valence
(reference valence) and the actual valence index may
be attributed to delocalization and polarization
effects. In some cases, they may also be caused by
the inadequacy of the basis set used. The "free
valence" defined by Gopinathan and Jug (1983 a,b)
may be possible measure of covalent chemical
reactivity (excluding electrophilic substitutions) but
the name free valence, in general is not suitable,
because the "free valence" for the closed shell
systems is zero. Therefore, as suggested by Mayer
(1985) it is better to call this difference the "excess
valence" o r "valence defect" for subvalence and
hyper valence cases respectively. instead of "free
valence".
Acknowledgements
We are also thankful to the valuable comments
and suggestions of referee1s.The first author is
grateful to UGC, Nepal, for the award of a teacher
fellowship and to Mr. Cholewhwar Sharma, Campus
Chief, T.M.C. Palpa. for encouragements.
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