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simulation study of ozone depletion through photolysis mechanism

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 عباس عبد علي دريع الصالحي 15/03/2017 05:24:28
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JOURNAL OF MULTIFUNCTIONAL MATERIALS & PHOTOSCIENCE
7(2), December 2016, pp. 99-110
Abbas A-Ali Drea and Roaa A-Hussien
Babylon university College of Science Chemistry Department, Hilla – Iraq
E-mail: aadreab22@yahoo.com
Abstract: Quantum calculation methods like DFT, Ab-initio, and semiempirical have been used
to simulate the mechanism of Ozone depletion by HCFC-124 (1-Chloro-1,2,2,2-
tetrafluoroethane). Structural properties such as Geometry optimized and single point
calculation has been done to understand the configuration interaction singly excited state for
all chemical species of the suggested reactions and their transition states. Energetic properties
such as total energy, molecular orbital energies, zero point energy, energy gap has been
calculated with RHF/6-31G* and Beack88LYP/3-21G(d) methods . Potential energy surface
calculation has been determined to evaluate the responsible bond of first initiation cleavage
step of photolysis mechanism reaction.
They found C-Cl bond is the most responsible in the photolysis mechanism reactions which is
produced chlorine radical and other radicals by energy barrier value of 61.305 kCal mol-1 with
enthalpy of reaction equal to 78.419 kCal mol-1. The transition state examination of these reactions
indicated that chlorine radical is the most probable species than other radicals to depleted
ozone by HCFC-124. The depletion reaction is spontaneous and exothermic with enthalpy
change of reaction equal to -252.64kCal mol-1 and free energy equal to -291.376 kCal mol-1 .
Key words: HCFC-124, Ozone, Depletion, Photolysis mechanism, transition state, Calculation
methods, Simulation study.
Introduction
The earth’s atmosphere is protected from much of the sun’s UV-B and UV-C radiation
by a layer of ozone (O3
) in the stratosphere, which is strongly absorbed in the 230 to 290
nm region1
. Ozone is ubiquitous in the atmosphere but its mainly concentrated in the
stratosphere between 19 and 23 km above the surface of the earth2
. Majority of ozone in
the stratosphere layer is formed naturally by photolysis of oxygenic molecules under
UV-radiation3
. Human activities during the last century involving particularly a new
chemical compounds such as chlorofluorocarbons(CFCs), Halons (H), carbontetrachloride
(CTC) which leads to the destruction of earth’s protective stratospheric
ozone layer.4
*Corresponding author: aadreab22@yahoo.com
CFC (chlorofluorocarbon) is a stable compound that is used extensively for air
conditioning and refrigeration, Propellants and solvents also for the production of foams.
chlorofluorocarbons is the main source of ozone depleting substances in the stratospheric
layer, also they contribute to global warming.5
The production of CFC will phase out in
beginning of 1995 under the Copenhagen amendment and the Montreal protocol6
. A new
chemical compounds have been identified to be a replacement to the CFC compound
such as HCFC (hydrochlorofluorocarbons) and hydrofluorocarbons (HFC) that’s have
the same physical-chemical properties to the CFC compound with less stratospheric ozone
depletion7
. HCFC-124 (1-Chloro-1,2,2,2-tetrafluoroethane) is a replacement to CFC-114
one of the most practical CFC compound as refrigeration and fire extinguishing agents .
The HCFC-124 has a molecular weight equal to 136.48, and B.P equal to –12.1C°
, also
ozone depletion Potential relative to CFC-12 equal to 0.04 8.
In the present work quantum calculation methods have been used to estimate the
ozone depletion mechanism by HCFC-124, through determining the geometry optimized
of the parent compound and for the fragments that’s resulted from the photolysis reaction
also the transition state . The structural properties are important to understand the chemical
reactivity during the potential energy surface calculations. Theoretical calculations give
energetic parameters of short-lived reactive intermediate moieties. The calculation of
transition state structures is very sensitive to the level of theory and basis sets that’s be
used 9,10 .
Computational Details
Geometry optimizations were performed at the level of HF/RHF, and density functional
theory with an exchange potential Becke 88 and correlation potential LYP that’s proposed
by lee-yang-parr with 3-21G(d) basis sets. Then, harmonic vibration frequencies were
computed to prove that each of the optimized structures is a local minimum on its potential
surface11. Potential energy surface calculation performed by mapping reactants into
products to calculate the activation energy.12
Results and Discussion
HCFC-124 (1-Chloro-1,2,2,2-tetrafluoroethane) is a polar compound possess one hydrogen
atom, the chemical structure represented in figure 1 . Energetic properties of HCFC-124
have been studied using different quantum calculation methods to estimate the chemical
reactivity. Table 1 shows the energetic properties of HCFC-124. The total energy calculation
by Abinitio method give a value of -583015.8, -585812.9kCal mol-1computed by 3-21G (d),
6-31G* basis set respectively, compared with recent theoretical studies the 6-31G* give
nearest value of total energy calculation13.
The physical properties of HCFC-124 as a display in figure 2,which is approved that
the electrostatic potential give a negative charge density appears on the fluorine atom
(red color) while the carbon atom appears with positive charge density, also the Homo
&Lumo calculation in 2&3 dimension has been computed. The total charge density gives
high density centered on the fluorine atom that connected to carbon atom number two.
Table 1
Energetic properties of HCFC-124 computed by different methods*
DFT/Becke88
LYP
Semiempirical Abinitio
Type of calculation
PM3 microstate/ 4*4 AM1 3-21G (d) 6-31G* 3-21G (d)
Total energy - 53771.57 -59619.32 -583015.8 -585812.85 -584669.69
Binding energy -712.9214 -712.5329 - - -
Heat of formation -214.489 -214.101 - - -
Molecular HOMO -9.8055 -7.7162 -244.546 -307.31 -164.909
orbital
energy LUMO 8.7332 12.2246 121.3469 109.447 57.5026
Eg 18.5387 19.941 365.892 416.757 222.412
Zero point energy 23.05 23.2777 25.4333 25.0256 22.098
Dipole moment D 1.811 1.739 1.4618 1.565 1.383
Energy values in kCal mol-1 unit.
Figure 1: Chemical structure of HCFC-124.
Figure 2: Physical properties of HCFC-124 computed at Abinitio /6-31G* method
The bond parameters such as bond length, bond angle of HCFC-124 have been
computed using DFT, Ab-initio method which is given a good agreement between the
two methods, the C-C bond length computed by the DFT method give nearest value to
the experimental value, as shown in table 2,which is equal to 1.542? and experimental
value equal to 1.534?. The C-F bond length gives a value in the range 1.3-1.35? which is
reaching the experimental value 1.3 ?. The C-Cl bond length equal to 1.74? calculated by
Ab-initio method and the C-H bond length equal to 1.0736 ? which is the same as
experimental value14-16.
Table 2
Structural properties of HCFC-124
Type of calculation Ab-initio DFT/Becke 88 lyp
3-21G(d) 3-21G(d)
C1
-C2 1.5 1.542
C1
-F1 1.3069 1.344
Bond length? C1
-F2 1.31 1.34
C1
-F3 1.3 1.35
C2
-Cl 1.746 1.814
C2
-H 1.0736 1.0972
C2
-F4 1.332 1.3556
C1
-F1
-F2 108.798 109.621
C1
-F1
-F3 109.156 109.082
C1
-F2
-F3 108.897 109.203
Bond angle degree C1
-C2
-F2 108.385 111.472
C1
-C2
-Cl 110.814 109.12
C1
-C2
-F4 107.893 109.757
C2
-Cl-F4 109.724 110.212
C2
-H-Cl 107.936 105.982
C2
-H-F4 111.263 112.471
The vibration calculation of HCFC-124 has been computed by both DFT, Ab-initio
method which is given 18 normal modes with one positive value of imaginary frequency
these normal modes show in table3 which gives good agreement with recent theoretical
studies 13.
The potential energy of bond stability has been computed for five types of bonds in
HCFC-124 C1
-F1
, C1
-C2
, C2
-H, C2
-F4
, C2
-Cl by using semiempirical- PM3 method as shown
in table 4. The C-Cl bond have less value of the dissociation energy with high value of
wavelength equal to 407.8643 nm. The C-F bond required less value of wavelength in the
range 249.5-250.911 nm, that means high value of energy .
Table 3
Vibration calculation of HCFC-124 computed by
different methods
IR- Frequency* Abinitio DFT/Becke 88 LYP
3-21G(d) 3-21G(d)
?1 73.73 58.3
?2 199.84 177.14
?3 242.79 215.07
?4 345.66 301.06
?5 414.16 343.79
?6 488.65 418.62
?7 588.19 505.78
?8 642.3 553.24
?9 773.47 665.97
?10 892.25 741.23
?11 996.09 852.09
?12 1290.79 1136.36
?13 1368.94 1180.39
?14 1459.31 1254.87
?15 1488.47 1275.82
?16 1527.85 1307.66
?17 1602 1391.21
?18 3400.85 3068.54
The C-H bond, and C-C bond needs wavelength equal to 277.326, 348.5226 nm
respectively, with an energy value equal to 96.531, 76.8116 kCal mol-1,these values reached
the experimental value 17. The examination of potential energy for bond length stability is
represented in figure 3.
Figure 3: Potential energy search of bonds length & bond angle stability of HCFC-124 computed by
semiemprical-PM3 /CI(4*4) microstate.
Table 4
Potential energy of bond stability of HCFC-124 computed by semimperical-PM3 /CI (4*4) microstate
Type of Equilibrium Equilibrium Breaking Breaking Dissociation ?
bond energy bond length ? energy bond length ? energy nm
C1
-F1 -702.933 1.247 -595.6659 2.546 107.267 249.569
C1
-C2 -711.835 1.576 -635.023 2.47 76.8116 348.523
C2
-H -711.835 1.147 -614.852 2.55 96.531 277.326
C2
-F4 -703.283 1.25 -596.589 2.54 106.694 250.911
C2
-Cl -699.526 1.57 -633.89 2.67 65.636 407.864
* Energy values in kCal mol-1 unit
The suggested photolysis mechanism of HCFC-124 occurs through three proposed
transition states as represented in figure 4. TS1 through C-Cl bond which leads to the
formation of Cl&C2
F4
H , second proposed transition state TS2 is through C-C bond which
gives CF3
&CFHCl, the last proposed transition state is TS3 C-H bond dissociation to give
two types of radical H & C2
F4
Cl. The energy barrier & enthalpy of reaction of these proposed
transition states given in table 5.
Figure 4: The proposed transition state in photolysis of HCFC-124.
Table 5
Evaluation compares between the proposed transition state of HCFC-124 photolysis,
calculated at semiempirical -PM3quatraic states .
Transition States Energy barrier* Enthalpy change*
TS1 61.3058 78.419
TS2 141.258 63.585
TS3 65.278 78.6245
*Energy values in kCal mol-1 unit.
The calculations of potential energy and energy barrier give up an indication, proof
that the reaction of C2
F4
HCl molecule occurs through C-Cl bond scission with higher
probability than C-C, C-F ,C-H bonds. The photolysis of HCFC-124 under atmospheric
conditions will form the •C2
F4
H and Cl• radicals. The suggested reaction, including •C2
F4
H
radical formation, that’s rapidly react with O2
to form a peroxy radical (C2
F4
HO2
). The
radical C2
F4
HO2
will react initially with NO species to produced C2
F4
BrO and NO2
, the
energetic values of alkoxy radical shows in table 6 18.The most probable fate of alkoxy
radical is a breakdown of C-C bond to form CF3
and CFHO 19.
Table 6
Energetic properties of C2
F4
HO radical computed by different methods
*Energy values in kCal mol-1 unit.
Type of calculation Ab-initio /6-31G* Semiemprical-PM3
Total energy -344438.819 -53204.0208
Binding energy - -741.56
Heat of formation - -211.9988
Dipole D 1.662 1.901
ZPE 25.5937 23.655
HOMO ev -14.798 -7.97
LUMO ev 1.711 -0.1117
Eg ev 16.509 7.858
C1-C2 1.525 1.602
Bond
length ?
C2-H 1.0834 1.1187
C2-O 1.345 1.3253
C1-F1 1.314 1.344
C2-F4 1.346 1.353
Bond C1-C2-O 110.505 113.036
angle
degree
C1-C2-H 109.542 111.418
C1-C2-F4 107.643 108.8
C1-C2-F1 111.158 112.304
Imaginary frequency +
*Energy values in kCal mol-1 unit.
The photolysis mechanism initiated by the reaction of C2
F4
H radical with the ozone
molecule to form alkoxy radical with an energy barrier equal to 8.56 kCal mol-1 computed
at PM3/CI(4*4)microstate and enthalpy of reaction equal to -73.736 kCal mol-1 also negative
value of free energy change(V) which is equal to -71.941 kCal mol-1. The alkoxy radical
undergo secondary photolysis through C-C bond at a wavelength equal to 633.53 nm, this
reaction occurs with less value for the enthalpy of reaction which equal to -7.22 kCal mol-
1
also the rate constant and “G equal to 9.5*103
s-1, -20.413 kCal mol-1 respectively. The
released fragment in photolysis mechanism will react with ozone in different pathways
as represented in scheme1. The rate constant of C-C bond scission reached the experimental
value 1.5*104
s-1 20 . The CF3
radical react with ozone with enthalpy of reaction equal to -
72.077 kCal mol-1& “G equal to -69.7514kCal mol-1, the reaction of CF3 radical with ozone
is most likely to produce the CF3
O and any other product would be thermodynamically
unstable or would not contribute to ozone depletion6
. The release radical *CF3
, *F, *CF3
CFH
will contribute to ozone depletion through *CF3
O, *FO, *CF3
CFHOcycle respectively, as
shown in scheme 2.
Scheme 1: The proposed photolysis mechanism of HCFC-124 computed by PM3 method
Scheme 2: The *CF3
,FO*, *CF3
CFH cycle calculated at Semiempirical- PM3
The geometry optimization of the fragment *CF2
,*CF2
O, *CF3
O, FO*, *CFHO, which
resulted in photolysis of HCFC-124 have been computed using PM3 method as represented
in table 7.
Table 7
Geometry optimization of fragment resulted from photolysis of HCFC-124 calculated at
Semiemprical-PM3 method
*Energy values in kCal mol-1 unit
The vibrational calculation of CFHO (formyl fluoride) has been calculated by the same
method which is given six fundamental frequencies two with high intensity one for C=O
stretching, which is equal to 1990.43 cm-1 and the other one for C-F stretching which is
equal to 1033.8 cm-1 these values reached the experimental value which is equal to 1834
,1049 cm-1 for C=O,C-F stretching respectively 21. The bond length of CFHO found to be
equal to 1.097 ? for C-H bond and 1.34 ? for C-F also 1.202 ? for C=O bond these value
reached the experimental value 1.08?,1.4?,1.15? for C-H,C-F,C=O respectively based on
electron diffraction data for related molecules22.
Conclusion
• Depletion of ozone in the stratospheric layer occurs in the presence of HCFC-124
through photolysis of C-Cl bond by energy barrier equal to 61.31 kcal mol-1.
• The reactions of the fragment FO, CF3
O, CF3
CFHO with ozone are exothermic with
?Hoverall fall in the range –(48.73 - 48.789) kCal mol-1.
• The photolysis net equation of HCFC-124 show that one mole of HCFC-124 deplete
six mole of ozone with enthalpy change of reaction equal to -252.64kCal mol-1and free
energy change equal to -291.376 kCal mol-1.
hv
CF3
CFHCl + 6O3 CFHO+CF2
O*
+ClO*
+FO*
+7O2
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  • وصف الــ Tags لهذا الموضوع
  • HCFC-124, Ozone, Depletion, Photolysis mechanism, transition state, Calculation methods, Simulation study.

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