Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
Theoretical study for degradation of Captan in gas phase Using DFT
S. N. Naman
College of Science, Dohuk University
B.R.J. Muhyedeen
College of Science, Baghdad University
A.A.Drea
College of Science, Babylon University
(NJC)
(Recevied on 27/3 /2011 )
(Accepted for publication
27/9 /2011)
Abstract
Captan Photodegradation have been theoretically studied in gas phase through
reacting with hydroxyl free radical simulating their degradation in aqueous solution
using UV light and some oxidants. Quantum calculation likes Semi-empirical and ab
initio and DFT have been carried out by using two reliable well-known programs
Hyperchem7.5 and Gaussian03W. Chemical reactivity, activation energy and rate
constant of cleavage reaction step are calculated for Captan . Total energy, heat of
formation and vibration spectrum has been calculated for all reaction components that
is probable to be formed during the complete degradation of Captan into essential
minerals.
The activation energy of peroxide dissociation into its two moieties of
hydroxyl radical is equal to 224.304 kJ mol-1.
The first degradation reaction step of Captan is endothermic reaction have been
carried out through nitrogen-sulphur bond to give up three major component THID,
SCCl2 and OCl*. The activation energy of this reaction step is 406.852 kj.mol-1. Rate
constant of cleavage step reaction is 2.50 x 108 s-1. Enthalpy change value of overall
reaction for Captan is -109089.432 kj.mol-1. Forty one moles of hydroxyl radical are
needed to convert Captan completely into simple moieties produced from this reaction
like H2CO3, H2O, H2SO4, HOCl, H2, NH3 and OCl*.
Keyword :
1- pesticides
2- DFT
3- photodegrdation
4- Captan
5- theoretical calculations .
539
‫المجلة العراقية الوطنية لعلوم الكيمياء‪ 2011-‬المجلد الرابع واألربعون‬
‫‪Iraqi National Journal of Chemistry,2011,volume 44‬‬
‫الخالصة‬
‫تم دراسة التكسر الضوئي للكابتان نظريا في الطور الغازي بوساطة التفاعل مع جذر الهيدروكسيل‬
‫الحر ‪,‬محاكاة لتفاعل التكسر الضوئي في المحلول المائي بوساطة األشعة فوق البنفسجية وبعض المؤكسدات‪.‬‬
‫أجريت حسابات الكم ‪,‬مثل الحسابات الشبة التجريبية والحسابات األولية األساسية وحسابات نظرية‬
‫الكثافة الوظيفية المحملة ضمن البرنامجين هايبركم ‪ 7.5‬وكاوسين ‪ .03‬تم احتساب الفعالية الكيميائية وطاقة‬
‫التنشيط وثابت معدل سرعة تفاعل خطوة التجزيء األولى‪ .‬احتسبت الطاقة الكلية وح اررة التكوين والطيف‬
‫االهتزازي لجميع مكونات التفاعل التي ان تتكون خالل تفاعل التجزيء الكامل للكابتان إلى المواد األساسية‪.‬‬
‫وجد من خالل الدراسة ان حاجز الجهد لتفكك البيروكسيد إلى جذري هيدروكسيل يساوي ‪224.304‬كيلو‬
‫جول لكل مول واحد‪ .‬إن الخطوة األولى لتجزيء الكابتان هو تفاعل ماص للح اررة ويتم من خالل اآلصرة الرابطة‬
‫بين ذرتي الكبريت والنيتروجين لتكون بذلك ثالث مكونات رئيسية وهي‪ THID‬و ‪ SCCl2‬و*‪ . OCl‬إن طاقة‬
‫حاجز الجهد لهذه الخطوة تساوي ‪ 406.852‬كيلو جول لكل مول واحد وبثابت سرعة تفاعل مقداره ‪108x2.50‬‬
‫ثانية‪. 1-‬إن قيمة التغير في المحتوى الحراري‬
‫للتجزيء التام للكابتان يساوي ‪ - 109089.432‬كيلو جول لكل‬
‫مول واحد‪ .‬يلزم إحدى وأربعون مول من جذر الهيدروكسيل لتحول جزيئة كابتان واحدة الى مواد كيميائية ابسط‬
‫تركيبا لتنتج من هذا التفاعل مثل ‪H2CO3‬و‪ H2O‬و‪ H2SO4‬و‪, HOCl‬و‪ H2‬و ‪ NH3‬و‪. OCl*.‬‬
‫الكلمات المفتاحية‪:‬‬
‫‪ -1‬نظرية الكثافة الوظيفية‬
‫‪ -2‬التجزيء الضوئي‬
‫‪ -3‬الكابتان‬
‫‪ -4‬الحسابات النظرية‬
‫‪ -5‬طرق ميكانيك الكم‬
‫‪high pesticides contamination levels‬‬
‫‪Introduction‬‬
‫‪are found in ground waters and surface‬‬
‫‪major‬‬
‫‪water: 0.1—0.3 µg/L in US ground‬‬
‫‪pollutants of aquatic environment, and‬‬
‫‪waters, and 0.03-0.5 µg/L in European‬‬
‫‪their presence is of cretin concern‬‬
‫‪ground waters. To protect the quality‬‬
‫‪because of their potential toxicity‬‬
‫‪of drinking‬‬
‫‪towards animals and human being.‬‬
‫‪priority lists of pesticides discharged in‬‬
‫‪They are only partially eliminated by‬‬
‫‪waters,‬‬
‫‪and surfaces‬‬
‫‪constitute‬‬
‫‪Pesticides‬‬
‫)‪(1‬‬
‫‪the aquatic environment have been‬‬
‫‪biological and chemical degradation .‬‬
‫‪established‬‬
‫‪Presently, several hundred pesticides‬‬
‫‪community with very drastic tolerance‬‬
‫‪of various chemical natures are used‬‬
‫‪limits(3).‬‬
‫‪all over the world for agricultural and‬‬
‫‪increasing interest in the developments‬‬
‫‪non-agriculture purposes. Many of‬‬
‫‪of simple cheap and rapid methods for‬‬
‫‪these‬‬
‫‪European‬‬
‫‪an‬‬
‫‪is‬‬
‫‪the‬‬
‫‪there‬‬
‫‪by‬‬
‫‪Therefore,‬‬
‫‪in‬‬
‫‪utilizing‬‬
‫)‪(2‬‬
‫‪are‬‬
‫‪pesticides‬‬
‫‪amounts over 50 ton/year . Relatively‬‬
‫‪540‬‬
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
the destruction of organic pesticides in
tomatoes and lettuce most of the
water.
residue was presence on the surface of
Photochemical reactions have
the
leaves
and
fruit,
mainly as
been extenensively used directly and
unchanged captan. Metabolism in the
indirectly for pesticides degradation.
plants includes cleavage of the thio-
Direct photodecomposition processes
indole bond to form THPI and
can
derivatives of the -SCCl3 side chain.
be
used
whenever
organic
of
[trichloromethyl-14C]
pesticides absorb light sufficiently in
Half-life
the UV region (250-300nm)(4-6).
captan in Greenville sandy loam soil
Captan is synthetic chemical
exposed to sunlight was 15 days while
fungicides. It belongs to the group's
the half-life for the dark control soil
related
was 20 days(9).
chemicals
called
Photochemical half-
"Phthalimides"(7). Half-life, assuming a
life was estimated to be 54 days. Most
quantum
using
of the radioactivity was accounted for
experimental extinction values, was
as captan and carbon dioxide, with the
estimated to be about 880 days(8).
remainder present as bound residues
Metabolism of Captan in goats and
and
hens proceed by cleavage of the N-S
irradiated samples on day 21 captan
bond to form THPI and a derivative of
accounted for 36-40%,
the -SCCl3 side chain. THPI and -
51%, bound residues for 7%, acid-
SCCl3 undergo further metabolism
released
through independent pathways. The
unidentified residues for 1.4% of the
carbon of the side chain becomes
applied radioactivity. In dark control
incorporated into Thio-Tri
soil samples captan accounted for 50-
yield
of
1
and
Chloro
unidentified
residues
14
compounds.
for
14
In
CO2 for 49-
2-5%
and
Methan TTCM and natural products.
56% and
The cleavage partner THPI is oxidized
applied
to form THPI epoxide which is
sunlight photolysis of [cyclohexene-
subsequently hydrolyzed to form 4,5-
14
diOH THPI, or hydroxylated at the
on the surface of Greenville sandy
cyclohexene ring to form 3-OH and 5-
loam soil treated at 4.48 kg ai/ha
OH THPI. The hydrolysis of THPI and
The half-life’s for photolysis were 130
its hydroxylated derivatives results in
and 236 hours for the light and dark
the formation of the corresponding
conditions respectively, resulting in an
THPAM
estimated photochemical half-life of
derivatives. In
apples,
541
CO2 for 33-40% of the
radioactivity.
The
natural
C] captan at 25°C have been studied
(10)
.
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
287 hours. Products present at >3%
method was used for optimization
were identified by HPLC and TLC.
algorithm. The initial guess of the
The major components formed under
molecular orbital coefficients was from
both light and dark conditions were
a projected Hückel, INDO and Harris
THPI,
calculation (11).
THPI
epoxide
,
THCY,
THPAM and THPAL.
Theoretical
vibrational
The aim of the present work tend
frequencies of large molecules were
to study the reaction of Captan with
carried on at PM3 method while the IR
hydroxyl radical theoretically through
H2O2 have been calculated at MP2/6-
the quantum calculation treatments of
311++G(2df,2p)
the
geometrical
addition to calculation of zero point
structure of this pesticide with their
energy, ZPE, to compute the relative
reliable transition states , also to find
quantum
mechanical
out
Theoretical
ultraviolet
electronic
a
and
reasonable
photodegradation
mechanism
reaction
in
of
gas
spectra
are
(5d,7f)
also
level
in
energies.
and
visible
calculated
at
phase.
configuration interaction, CI (10x10),
Theoretical Part
level using 6-31G(d) basis sets. PM3
DFT, ab initio and PM3 methods
and ZINDO/S are also used for
have been used for evaluation of
calculation of UV/VIS spectra of the
electronic energies, heat of formation
copper
and electrostatic potential of Captan
HyperChem 7.52 was used for these
and OH*. The geometries of fifteen
calculations
intermediates and final molecules have
MOPAC2000 program (12) .
been optimized at /6-311++G(2df,2p)
Rate constant calculation using
RMP2/6-311++g(2d,2p)//RHF/631G(D) . Also PM3 method has been
used for IR, UV, and Transition state
calculation(13).
(5d,7f) and PM3 level. Polarized spilt
valence
6-311++G(2df,p)
and
6-
311++G(d,p) basis functions were
addition
to
unpaired electron at oxygen atom,
Single point MP2 calculation were
which involved from hemolytic of O—
performed on the optimized structures
6-311G(d),
in
complexes.
Hydroxyl radical has single
LYP was used for DFT calculation.
6-31G(d),
nickel
Results and Discussion
employed for HF calculation while B3-
using
and
O bond in peroxide or O—H bond of
6-
water.
311++G(d,p) and 6-311++G(2df,2p)
Hypothetically,
one
may
generate the hydroxyl radical by
for high accuracy. The Polak-Ribiere
542
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
simulation of photochemical reaction
similar or higher values as shown in
of peroxides molecules in gas phase.
table 1. Of course the most reliable
Peroxide hemolytic products consisted
quantum method, CBS-Q,
from two OH* radical's moiety. The
123.777 A°.
gave
geometry parameters obtained from the
Different calculation methods
calculation of O—O bond length is
can be used to estimate the energetic
1.4024 οA. The angle of H-O-O bond is
properties of hydroxyl radical as
101.264
ο
and the torsion angle of
shown in Table 2. The physical
atoms H-O-O-H is built to 90o but the
properties of peroxide calculated at
equilibrium value became 112ο which
MP2 6-311++G(2d,2p) level of theory
calculated by MP2 6-311++G(2d,2p).
has been shown below in Figure 1.
Other methods from G03W(14) gave
Table 1. estimate the bond torsion angle of peroxide molecule using different
calculation methods of G03W program .
Method
torsion angle of O—O bond (A°)
CBS-Q
126.777
G2
121.197
MP2(Full)/6-311++G(2df,2p)
115.293
MP2(Full)/6-31++G(d,p)
121.371
543
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
Iraqi National Journal of Chemistry,2011,volume 44
A-Basis set function of calculation
B-Bond Type
a-Ball and stick view
b-bond order
b-bond length( A°)
c-Atomic Charge (coulomb)
Figure 1. Physical properties of peroxide calculated at MP2-6-311++G(2d,2p)
level of theory.
544
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
Iraqi National Journal of Chemistry,2011,volume 44
Table 2. Energy and IR frequencies calculations of H2O2 at different theoretical
methods using G03W.
Method
Energy
*IR- wave No
**IR-
kJ mol-1
(cm-1)
Intensity
km mol-1
CBS-Q
MP2/6-311++G(2df,2p) (5d,7f)*
-633.340
-633.311
371.2188
280.21
1144.7134
2.44
1492.5602
118.32
1671.5184
0.571
4103.3726
17.53
4106.4137
80.33
395.3719
189.45
936.6880
0.823
1345.5774
110.28
1457.0925
0.089
3824.1276
19.98
3824.7258
71.56
MP2/6-311++G(2d,2p)//UHF/6-1G(d,p)
-632.955
-
-
G2MP2
-633.281
-
-
*Normal Mode Frequencies of Vibration in ( cm-1). **Integrated Infrared Band Intensities in( km mol-1).
The theoretical spectrum of vibrational
MP2/6-311++G(2df,2p) (5d,7f)* can
frequencies of H2O2 calculated at
be
displayed
in
Figure
Figure 2. IR frequencies calculated at MP2/6-311++G(2df,2p) (5d,7f)*
545
2.
Iraqi National Journal of Chemistry,2011,volume 44
The
stability
calculated
from
of
O—O
potential
bond
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
illustrates potential energy curve of
energy
this
bond
calculated
surface is approximately equal to -
31++G**level
395931.92 kJ mol-1 which can be
HyperChem 7.52.
of
at
MP2/6-
theory
using
broken down at 2οA . Figure 3.
Figure 3. Potential energy of O—O bond in peroxide.
The proposed reaction in gas phase is
in good agreement with experimental
suggested as follow:
energy value 214.605 kJ mol-1 in
2 OH*
H2O2
(1)
literature.
The energy barrier of this reaction
Of
course,
this
value
of
is equal to 224.304 kJ mol-1, calculated
wavelength from Tungsten lamp is
at configuration interaction of four
very useful for this gas phase reaction.
occupied
The oxygen atom and hydrogen atoms
(15)
orbitals
with
one
PM3/CI(4x1)
unoccupied
of
have equal opposite charges as shown
theory. Also the energy barrier of this
in Figure 4. the oxygen and hydrogen
reaction equal to 258.295 kJ mol-1 at
atoms have charges of -0.336 and
HF/6-31G* level of theory according
0.336 C respectively. Hydroxyl radical
to
can pass into free radicals reaction
transition
state
level
pathway.
The
corresponding wavelength for this
mechanism
energy barrier 224.304 kJ mol-1 is
electrostatic potential to form an
equal to 533.84 nm. These results are
transition state(16) , that’s tend to be in
546
through
an
appositive
Iraqi National Journal of Chemistry,2011,volume 44
minimum
energy
decomposition
into
either
by
fragments,
or
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
rearrangement side reactions.
A-Atomic Symbol
B-Electrostatic potential various 2D
C-Bond Length (A°)
D-Atomic Charge(coulomb)
Figure 4. Physical properties of Hydroxyl radical.
molecules have been calculated at
Electronic Features of Captan
Energies and geometries of
MP2/6-311++G** level of theory as
molecules have a clear relationship to
shown in Figure 5. The atomic charge
chemical phenomena. Other quantities,
of the reactive atoms, which are
such as atomic charge, electrostatic
possibly sharing in the reaction, that’s
potential and bond length are less
atoms of carbon, nitrogen, and oxygen
defined but provide useful qualitative
have a negative charge and the atoms
results. The total energy in a molecular
of sulphur, chlorine, and hydrogen
orbital calculation is the net result of
have a positive charge. It could be
electronic kinetic energies and the
expected that the negative charge of
interactions between all electrons and
oxygen
atomic course in the system(17). The
attached to the positive charge atoms
geometry
in
optimization
of
Captan
547
of
Captan
hydroxyl
molecule,
radical
and
can
same
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
opposing thing for positive hydrogen
is -610749.053 kJ mol-1 calculated at
atoms at hydroxyl radical. The total
MP2/6-311++G** Level of Theory.
surface potential energy of free Captan
A-numbers of atoms
B- symbols of atoms
C-Ball and cylindrical view
D-Atomic charge C
E-Bond length (A°)
F-Electrostatic Potential
Figure 5. Physical properties of Captan calculated at MP2/6-311++G** level of
Theory.
548
Iraqi National Journal of Chemistry,2011,volume 44
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PES
torsion of bond gives us a clear view(19)
(Potential Energy Stability) has been
about the stability of bond S14-C15 .
measured(18) for the main important
They were the torsion value of N10-S14-
bonds. Figure 6. illustrates the stability
C15-Cl17 angle is 61.2ο, and torsion
of N10—S14 bond, and S14—C15 bond.
angle of C11-N10-S14-C15 is 97.2ο.
The S14—C15 is more stable rather than
Another evidence for the cleavage
N10—S14 bond, because the first bond
reaction step begins from N11-S14
can be broken down at longer length. It
bond.
Bond
strength
from
has needed more potential energy. The
KJ mol-1
KJ mol-1
A-N10—S14 Bond potential
B-S14—C15 Bond Potential
KJ mol-1
KJ mol-1
C-Torsion Angle of N10-S14-C15–Cl17 is 61.2 ο
D-Torsion angle of C11-N10-S14-C15 is 97.2ο.
Figure 6. Potential energy stability of main bonds in Captan .
549
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
atom in hydroxyl radical is changed
Chemical Reactivity of Captan
Investigation of the chemical
from-0.336, 0.336 into-0.714, 0.239 C
reactivity has been carried out through
respectively. The same thing occurred
the
between
to Captan atoms, was atomic charge of
charges favours reaction between sites
N10, S14, C15, and Cl17, that’s which
(on the two species) that have extreme
changed from -1.015, 0.599,-0.540,
charge values(20) . The most positive
0.073 into -0.657, 0.148,-0.375,-0.017
charge interacts with the most negative
respectively and so on for the rest
charges, so that calculations of the
atoms. Bond length of N10—S15 bond,
atomic charge and molecular orbital
S14—C15 bond, and C15—Cl17 bond in
have been carried out in order to
Captan was also changed from 1.6727,
studying
interaction
predict the active site
(21)
of reaction in
1.8228, 1.7863 into 7.8368, 1.6166,
the Captan molecules toward hydroxyl
4.3416 οA respectively. Bond length of
radical.
hydroxyl
radical
has
the
same
Thermodynamics force factors
behaviour were bond length of O—H
oriented the reaction to produce the
bond was changed from 0.9907 to
most stable product, which usually
2.0306 οA. Initial assumption from the
take place in one-step(22). Negative
above data, those oxygen atoms of
atoms of hydroxyl radicals attached
hydroxyl radical has preferred to attach
with opposite charges atoms in Captan.
to Cl17, S14 in large ratio than other
Figure 7. described the total change in
active site in Captan. Hydrogen atom
electronic features of Captan through
of hydroxyl radical has preferred to
reaction with hydroxyl radical. The
attach to N10, C15 in large ratio than
overlap interaction investigation of
other active site in Captan molecule.
electrostatic potential for both reactant
and also the most original electronic
properties were changed. The net
atomic charge of oxygen and hydrogen
550
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
A- Interaction of atomic charge C.
B-Bond length Interaction Bond length (οA).
C-Interaction of Electrostatic Potential .
Figure 7. Electronic features interaction of Captan with hydroxyl radical
through the transition state calculated at MP2/6-31+G** Level of Theory.
Reaction of Captan with Hydroxyl
H2O2,
Radical
temperature, and intensity of UV-light.
The reaction of Captan with
All
semiconductor,
these
factors
may
sensitizer,
have
an
hydroxyl radical may be complicated
important rule in the transition state
due to several factors. There are
formation of reaction mechanism. Only
different possibilities for hydroxyl
one radical initiate the degradation
radical to attach at more than one
reaction
active site in Captan. the conditions of
determining step one. By taken into
reactions are like the influence of
account the above results, one may
551
through
cleavage
rate
Iraqi National Journal of Chemistry,2011,volume 44
‫ المجلد الرابع واألربعون‬2011-‫المجلة العراقية الوطنية لعلوم الكيمياء‬
derive some reasonable transition state,
structure of Captan with radical, the
which will be responsible for First
others have abnormal bond length
Degradation Step.
which are expected to yield cleavage
Proposed Transition States
through N10—S14 bond.
Five different transition state have
The investigation of these five
been proposed, that may be one of
transition states have been carried out
these which will give us the real path
through the theoretical calculation.
of degradation Figure 8. shows the
Only TS1 was real TS structures due it
geometries stick view of these states.
is
The
prediction
first
negative
frequencies.
on
Potential energy surface of TS1 has a
different attachments to the possible
minimum value -326471.746 kJ mol-1
sites in Captan for hydroxyl radical to
than other state. Zero point energy
interact with. These sites are N10, S14,
calculation estimation of both TS1 has
C15, Cl17, and O12, or O13. All five
higher ZEP value than other states,
proposed Transition state structures
which tends to be has low value of
have
activation energy than others transition
been
depends
the
studied
through
and
states(23). Theoretical vibration spectra
calculation of their vibration spectra.
have calculated for these structures to
Table 3. describes the features of these
identify transition state structures.
states with their imaginary frequencies.
Structure with one negative frequency
Bond length of N10—S14, and S14—C15
is
bond have the same ratio of length
structure with more than negative one
difference relatively to the interaction
frequency is hilltop structure(24).
optimization
their
geometries
552
transition
state
structure,
the
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Iraqi National Journal of Chemistry,2011,volume 44
A-TS1
B-TS2
C-TS3
D-TS4
E-TS5
Figure 8.Geometries in wire form view of five proposed transition states for
Captan cleavage through N10—S14 bond, Calculated at MP2/3-21G** level of
theory.
553
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Table 3. Optimized Geometry and Vibration spectrum of proposed Transition
state for Captan reaction with Hydroxyl radical Calculate at 3-21G** level of
theory.
R(N10-S14)
R(S14 –C15)
PES
ZPE
Imaginary
Aο
Aο
kJ mol-1
kJ mol-1
Frequency
1
1.744
1.716
-326471.746
450.939
-
2
1.698
2.115
-32621.099
437.950
+
3
1.740
1.818
-326020.961
447.450
+
4
1.791
1.851
-325804.314
429.199
+
5
1.692
1.826
-32619.117
436.057
+
TS
From the above evidence, the first
TS1 structure ,scheme1. represent the
degradation reaction step pass through
suggestion reaction as follow:-
Scheme 1. Suggested first cleavage step of Captan .
The equation of this reaction can be written as in the following:
Captan + OH* →
THID + SCCl2
+ OCl*
∆Hreaction = 104.591 kJ mol-1
554
(2)
Iraqi National Journal of Chemistry,2011,volume 44
The
energy
barrier
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(Activation
component. Table 4. represented the
Energy) of this reaction is 406.856 kJ
thermodynamics values of reaction
mol-1 (294.284 nm), and the enthalpy
component. The enthalpy change of
change formation of this reaction is
reaction is equal to ∆H298.15K= 78.102
104.591 kJ mol-1calculated by CI –
kJ mol-1. The Gibbs free energy of
PM3 methods.
reaction is equal to ∆G298.15K= 19.376
kJ mol-1.
Reaction rate of the cleavage
reaction rate, K value is
equal to 2.50 x 108 s-1.
step can be calculated according to
Arrhenius equation by using G03W
program, which is used to calculate the
thermodynamic properties of reaction
Table 4. thermodynamics calculation of Captan cleavage reaction components,
using G03W program.
Reaction
Sum of electronic and thermal
Sum of electronic and thermal
components
Enthalpies ∆H298.15K( Hartree)*
Free Energies ∆G298.15K
(Hartree)
Captan
-2325.377825
-2325.436450
OH*
-75.369868
-75.390067
THID
-512.205648
-512.247980
SCCl2
-1354.285387
-1354.319100
OCl*
-534.226911
-534.252057
* 1Hartree = 627.5095 kJ mol-1
Reactions of THID with Hydroxyl
suggestion of some structures based on
Radical
chemical intuition as well as for the
Extensive calculations have been
above data. A high level of ab initio
achieved on this pathway to discover
calculation has been used for these
the probable path for the reaction that
reaction steps of THID with more than
may take place to complete the
one hydroxyl radical.
degradation of THID molecule. The
555
Iraqi National Journal of Chemistry,2011,volume 44
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The energy calculations of the
other different products. Full optimized
relative stability and the equilibrium
geometries of the structures obtained
structures
from
of
these
possible
the
subside
reaction
are
configurations will enable us to deduce
calculated. To estimate the reaction
the other reactions which are taking
possibility to take place within the
place to complete the cleavage reaction
cleavage step, the calculations of the
step.
activation energy of reaction barrier
Hydroxylation
process
may
and
enthalpy
changes
for
these
take place at different possible site of
reactions were carried out. There are
Captan molecule. All these subside
some parallel reactions which differ in
reactions may face the same cleavage
activation energy barrier. The lowest
reaction
the
activation energy reaction will occur at
hydroxylated products towards the
high probability with high yielding
complete degradation. The theoretical
percent. Other reactions will have
vibration spectra were used for the
small probability and so on for yield
proposed structures to identify the
percentage. The over all mechanism of
nature of each structure whether it is a
THID can be summarized into four
transition state, hilltop or intermediate
main
structure.
equations.
step,
to
produce
reactions
as
Subside reactions have produced
several compounds. These compounds
may inter further reactions to produce
THID + 13*OH →TOHA + 2H2CO3 + NH3 +4H2O + H*
(3)
∆H reaction = -3540.895(kJ mol-1)
TOHA + 7*OH + H* → 4H2CO3 + 2*CH3
(4)
∆H reaction = -1521.902(kJ mol-1)
2 x {*CH3 + 5*OH → 2H* + 2H2O + H2CO3}
(5)
2 x (∆H reaction = -86.092(kJ mol-1))
2 x {H* + H* → H2}
(6)
556
in
following
Iraqi National Journal of Chemistry,2011,volume 44
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2 x (∆H reaction = -492.042 (kJ mol-1) )
THID + 30 *OH → 8H2CO3 + 8H2O + 2H2 + NH3
(7)
∆Hreaction= -6219.06 kJ mol-1
Equation 7 represents the over all degradation reaction of THID.
Reactions of SCCl2 with Hydroxyl
reaction steps of SCCl2 with more
Radical
hydroxyl radicals to produce simple
Several assumptions have been
chemical moieties. These moieties may
achieved on this pathway to discover,
inter further reactions to produce
what probabilities for the reactions are,
another
that may take place to complete
optimized geometries of the structures
degradation of SCCl2. Some structures
obtained from the subside reaction are
were
calculated at MP2/6-31(2d,2p) level.
suggested
that’s
based
on
different
product.
Full
chemical intuition as well as on the
The over all mechanism of SCCl2
above data. A high level of ab-initio
can be summarized into four main
calculation has been used for these
reactions as in following equations:-
SCCl2 + 3 *OH → (HO)2-C-Cl2 + HO-S *
(8)
∆H rea = -482.164 (kJ mol-1)
(HO)2-C-Cl2 + 3*OH → 2HOCl + H2CO3 + H*
(9)
∆Hrea = -101065.168 (kJ mol-1)
HO-S * + 4*OH → H2SO4 + H2O + H*
(10)
∆Hrea = -664.377 (kJ mol-1)
{H* + H* → H2}
(11)
∆H rea = -492.042 (kJ mol-1)
SCCl2 + 10OH*→ H2SO4 + H2CO3 + 2HOCl + H2O +H2
(12)
∆Hrea = -102871.116(kJ mol-1)
Equation 12 represents the over all degradation reaction of SCCl2.
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Overall reactions of Captan with
components. These components will be
hydroxyl radical
degraded as in equation 7, and
Degradation reactions of Captan in
equation 12 to represent the step two
gas phase by hydroxyl radicals can be
and
generalized into three main steps. First
combination, the last two exothermic
step is the cleavage step of Captan
reaction steps with first step to produce
equation2,
general
which
is
endothermic
reaction to produced main degraded
step
three
respectively.
equation
for
By
Captan
degradation as following:-
Captan + 41*OH →9H2CO3 +9H2O +H2SO4 +2HOCl +3H2 +NH3 +*OCl
-----(13)
∆H overall reaction= -109090.176 kJ mol-1
The enthalpy change value of overall
another molecule of HOCl by energy
reaction
barrier equal to 9.811kJ mol-1.
for
Captan
degradation
indicates that reaction is exothermic
CONCLUSIONS
Activation energy of
reaction, although the first step is
dissociation
endothermic. Finally, simple moieties
produced from this
mol-1 ( 533 nm).
moles
of
First degradation
reaction step of Captan can be written
and OCl* . The radical OCl* may inter
as in the following equation:
further reactions with H* to produce
THID + SCCl2
two
hydroxyl radical is equal to 224.283 kJ
reaction like
H2CO3, H2O, H2SO4, HOCl, H2, NH3,
Captan + OH* →
into
peroxide
+ OCl*
∆Hreaction = 104.587 kJ mol-1
The
activation
energy
of
this
value of overall reaction for Captan is -
endothermic reaction step is 406.852
109090.176 kJ mol-1. Simple moieties
kJ mol-1 (294 nm). The calculated rate
are liberated from this degradation
constant of cleavage step reaction is
reaction according to the following
2.50 x 108 s-1.
equation:
The enthalpy change
558
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Iraqi National Journal of Chemistry,2011,volume 44
Captan+ 41OH* → 9H2CO3 +9H2O +H2SO4 +2HOCl +3H2 +NH3 +OCl*
Abbreviations
CODE
THPI:
3-OH THPI
FULL NAME STRUCTURE
1,2,3,6-tetrahydrophthalimide
cis/trans-3-hydroxycyclohex-4-ene-1,2-dicarboximide
5-OH THPI
4,5diOHHHPI
THPAM
cis/trans-5-hydroxycyclohex-3-ene-1,2-dicarboximide
4,5-dihydroxycyclohexane-1,2-dicarboximide
(cis/trans-1,2,3,6-tetrahydrophthalamic acid)
epoxide7oxabicyclo[4.1.0]heptane3,4dicarboximide
Dichloroviyl dimethyl phosphate
Moller-Plesset Perturbation Second Version
Slater- type orbital
Gaussian type function
Unrestricted Hartree-Fock
Restricted Hartree-Fock
Complete basis set –Quadratic method
3a,4,7,7a-Tetrahydro-isondole-1,3-dione
Thiophosgen
DDVP
MP2
STO
G
UHF
RHF
CBS-Q
THID
SCCl2
8.
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