European Journal of Scientific Research ISSN 1450-216X Vol.16 No.4 (2007), pp.490-505 © EuroJournals Publishing, Inc. 2007 http://www.eurojournals.com/ejsr.htm
DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere
Bahjat R J Muhyedeen Department of Chemistry-College of Science University of Baghdad, Baghdad-Iraq
Abstract
Density Functional Theory, DFT, and ab initio calculations of the potential energy surface of tropospheric ozone with light hydrocarbons (petroleum gas) have been performed which is known to be important in atmospheric chemistry. Reactants, transition states, intermediate species and products are optimized at DFT (B3LYP) and MP2 level of theory using three basis sets 6-31G(d), 6-31++G(d,p) and 6-311++G(2df,2p) for the three possible reaction paths, namely, abstraction reaction through formation of hydrotrioxyl radical (HTR), replacement reaction through formation of hydrodioxyl radical (HDR) and the biradical reaction path. Thermodynamic properties ∆Hº, ∆Sº, and ∆Gº have been calculated for these reactions. Harmonic vibrational frequencies have been calculated for reactants, products, and transition states up to HF/6-311++G(2d,2p) level of theory. The calculated energy barriers for replacement reactions are less than for abstraction reactions. The IRC calculation on ozone reaction with methane gave an energy barrier of 619 Kcal/mol.
Introduction The troposphere is chemically complex and regionally diverse. Many of the important species in the troposphere are short-lived includes the reactive nitrogen oxides, reactive sulfur, ozone, non-methane hydrocarbons (NMHCs) and carbon monoxide. Oxidation of NMHCs produces a variety of oxygenated products including aldehydes, ketones, dicarbonyls, alcohols, phenols, peroxides, organic acids and organic nitrates. NMHCs generally react with OH* to produce O3 such that an increase in their concentration will decrease OH* concentrations in troposphere. The chemistry of these intermediate compounds is imprecisely known, however laboratory and modeling studies are helping to quantify and reduce some of the uncertainties[1-5]. Gramatica et al have been predicted the rate constant for the tropospheric degradations of 125 organic compounds by reaction with ozone using MLR-QSAR modeling [6]. Ozone in troposphere can damage plants and materials and act as a respiratory irritant. In the upper troposphere it is also an important greenhouse gas. The sources of ozone in troposphere are from stratosphere and in situ photochemical production. The main sinks for ozone abundance are photochemical destruction, deposition and its reaction with the alkenes, isoprene and terpenes which directly leads to the removal of O3 from troposphere[7-11]. O3+HO2* OH* +2O2, and/or O3+HO* HO2* +O2 The reactions of tropospheric ozone with saturated hydrocarbons are of less interest from environmental point of view due to their non-spontaneity and improbability. Nevertheless, these reactions are of great interest to the theoreticians to investigate the actual mechanisms for these reactions. DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 491
Few experimental and theoretical studies have been reported on the oxidation of C-H bond in hydrocarbons, organic compounds by ozone and various oxidant molecules such HOO*,OH*,NO*, O* etc[12-14]. The accepted mechanism for ozonation reactions is formation of biradical through hydrotrioxides (HT) formation. Hydrotrioxides, ROOOH, were proposed as intermediates in the reaction of ozone with saturated compounds in the early 1960s[15–18] which include a wide range of O- and Si-containing compounds (alcohols, ethers, acetals, aldehydes, silanes, etc.) that have been synthesized and examined since then[19,20]. The formation of hydrotrioxides on ozonation of hydrocarbons has been proved experimentally only for some alkanes[21-26]. Recently, Shereshovets et al have proved experimentally the formation of HT during the ozonation of adamantine, isopentane, 1,4- dimethylcyclohexane, 1,3-dimethyl-cyclo hexane, decalin and triphenyl methane[27,28]. Shustove and Rauk reported the mechanism of the dioxirane oxidation of the C-H bonds in the light hydrocarbon using DFT and MP2 level of theory. They showed that the reaction ensued through [29] biradical (R*/CH2O*OH) mechanism rather than formation of neutral products (ROH/CH2=O) . Shereshovets et al had studied the gas-phase reactions of ozone with C-H bonds in methane, ethane, propane (secondary C-H bond), and isobutane (tertiary C-H bond) by semiempirical AM1 method[30]. They showed that the formation of hydrotrioxides ROOOH in gas-phase reactions between ozone and hydrocarbons was highly improbable and the reactions proceed through biradical transition state which lead to alkyl R* and hydrotrioxyl HOOO* radicals. They also reported that the HOOO* radicals immediately decomposes into molecular oxygen and hydroxyl radical HO*. In present work we are interested in study the mechanism of ozone reaction with light hydrocarbon such as methane, ethane, propane and butane of troposphere to look into the type of products and the mechanism of transition state through calculation of energy barrier using DFT level and MP2 high level of ab initio calculation. The understanding of the chemical interaction of these pollutants is very important because its composition in the troposphere depends not only on its rate of formation but also on its rate of removal. We will investigate the three possible reaction paths of ozone reaction with light hydrocarbon, namely, abstraction (dehydrogenation) reaction through formation of hydrotrioxyl radical (HTR) and replacement (oxygenation) reaction through formation of hydrodioxyl radical (HDR) and the radical species OH*+O+RO* which result in the final products ROH+O2 with help of some thermodynamic properties ∆Hº, ∆Sº, and ∆Gº. Some structures for transition state are suggested based on the possibility of attack of ozone molecule to the hydrocarbon molecule. We suggested that the attack of ozone to the hydrocarbon molecule might be either to carbon–carbon, C-C, carbon–hydrogen, H-C, or to hydrogen–hydrogen, H-H, atoms of the hydrocarbon.
Computational Details Through this study, various theoretical MO methods have been used. The geometries of the 37 structures of methane, ethane, propane, butane and their complex structures with ozone and hydroxyl radicals have been optimized at hybrid density functional, DFT, (B3-LYP)/ 6-31G(d), 6-31++G(d,p) and 6-311++G(2d,2p) level of theory, based on Becke 3-grid integration and exchange functional[31,32] with the correlation function by Lee et al [33] and MP2 levels of theory[34,35] using 6-31G(d), 6- 31++G(d,p), 6-311++G(2d,2p) and 6-311++G(2df,2p)[36] basis sets. In the hybrid functional, B3-LYP, the exchange part is calculated as an optimized combination of both HF and DFT exchange, in contrast to non-hybrid functional where the exchange part is based only DFT. The Polak-Ribiere and the new SCF convergence algorithm which uses a combination of two Direct Inversion in the Iterative Subspace (DIIS) and Extrapolation methods EDIIS were used as optimization algorithm and Raffenetti method was used for processing two-electron integrals[37] except for DFT calculation. The initial guess of the MO coefficients was either from a projected Hückel calculation or from Harris. Vibrational frequencies of the proposed transition state structures have been calculated at UHF/RHF/6-31++G(2d,2p) level of theory for characterization of the nature of stationary points and zero point energy (ZPE) calculations to compute the quantum energies of these
492 Bahjat R J Muhyedeen reactions. All the stationary points have been positively identified for minimum (number of imaginary frequencies NIMAG=0), transition state (NIMAG=1).IRC calculation have been carried out at 3- 21+G**. All calculations have been carried out using HyperChem 7.52[38], MOPAC2000[39,40], Gaussian03W and GaussianViewW[41].
Results and Discussion 1. Energies, Geometries and IR The energies of reactants and products were calculated at DFT and MP2 levels as shown in Tables 1 and 2 respectively. The geometric structures of reactants and their radicals are shown in Figures-1 except alcohols molecules. The proposed transition state structures are illustrated in Figures 2-4. The structural parameters and harmonic vibrational frequencies of various compounds used in the hydrocarbons ozonation reactions are listed in Table-3. The energies of transition state structures were calculated at MP2 level only as shown in Table-4. The zero-point energies are calculated by two basis sets and listed in Tables 5 & 6. The potential energy calculation showed that the bond length of C-C of hydrocarbon decreases through moving from the neutral molecule to oxygenated then to their corresponding radical species (i.e. 1.528, 1.519 and 1.498 A0 respectively) (see Table-4). The C-H bond length usually decreases at radical carbon atoms (≈ 1.07 A0) due to electron density of the odd electron and increases at other carbons atoms in the same molecule (≈ 1.11 A0) (see Table-4). The calculated O-O bond length at 6-31++G(d,p) in HDR molecule is 1.302 Ao and it is well compared with that of Allen et al {1.309 Aº calculated at 6-31G(d)}.
Table 1: Total Energies of the Species of Abstraction and Replacement Reaction used in the Hydrocarbons + Ozone Reaction Calculated at DFT Level with Three Basis Sets
Compounds DFT 6-31G(d) DFT 6-31++G(d,p) DFT 6-311++G(2d,2p) PG HO* -47616.983 -47637.590 -47694.832 C2v HO2* -94266.646 -94991.378 -95101.559 Cs HO3* -142291.105 -142340.327 -142503.628 Cs O3 -141881.287 -141900.198 -142060.752 D3h CH3* -25079.336 -25102.252 -25130.613 D3h C2H5* -49854.779 -49892.456 -49949.552 Cs C3H7* -74632.533 -74674.929 -74770.088 Cs C4H9* -98634.758 -98687.132 -98812.890 C1 CH4 -25528.406 -25555.421 -25586.287 Td C2H6 -50279.503 -50339.400 -50398.244 D3d C3H8 -75070.033 -75126.969 -75215.885 C2v C4H10 -99048.923 -99105.409 -99195.093 C2h CH3O* -72419.681 -72419.769 -72058. 214 Cs C2H5O* -97179.162 -97228.846 -97314.411 C1 n-C3H7O* -121994.924 -122021.974 -122161.244 Cs i-C3H7O* -121960.830 -122021.994 -122159.593 C1 n-C4H9O* -146729.317 -146804.653 -146970.198 C1 i-C4H9O* -146734.786 -146810.125 -146975.677 C1
DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 493
Table 2: Total Energies (Kcal/mol) of Various Compounds used in the Hydrocarbons + Ozone Reaction Calculated at MP2 Level with Four Basis Sets
Compounds MP2/ 6-31G(d) MP2/6-31++G(d,p) MP2/ 6-311++G(2d,2p) MP2/ 6-311++G(2df,2p) HO* -47391.284 -47404.240 -47448.676 -47461.6727 HO2* -94410.865 -94455.608 -94527.537 -94573.0851 HO3* -141474.824 -141494.149 -141637.131 -141671.7770 O -46989.165 -46992.620 -47035.1790 -47044.5434 O2 -94092.576 -94097.8410 -94155.9878 -94182.4388 O3 -141067.101 -141074.631 -141216.336 -141251.7380 CH3* -24895.159 -24913.036 -24934.788 -24943.8901 C2H5* -49475.632 -49504.736 -49548.679 -49566.7412 C3H7* -74057.933 -74098.180 -74164.421 -74191.3363 C4H9* -98634.758 -98687.132 -98776.420 -98811.4429 CH4 -25311.822 -25333.446 -25356.701 -25367.2455 C2H6 -49889.373 -49922.527 -49967.692 -49987.1834 C3H8 -74469.103 -74513.924 -74581.335 -74609.6242 C4H10 -99048.923 -99105.409 -99195.093 -99232.2062 CH3O* -71969.076 -71991.800 -72059.892 -72080.1287 C2H5O* -96551.688 -96585.814 -96676.190 -96705.0901 n-C3H7O* -121133.314 -121172.218 -121281.050 -121327.5154 i-C3H7O* -121136.851 -121180.739 -121289.060 -121330.7003 n-C4H9O* -145713.579 -145766.010 -146880.028 -145950.31620 i-C4H9O* -145716.926 -145772.151 -146886.039 -145887.76059 CH3OH -72384.573 -72415.299 -72488.143 -72508.5658 C2H5OH -96967.686 -97009.688 -97105.717 -97133.8574 n-C3H7OH -121551.817 -121719.163 -121755.074 -121755.0739 i-C3H7OH -121551.817 -121605.443 -121724.693 -121760.5743 n-C4H9OH -146127.371 -146192.628 -146335.500 -146379.0605 i-C4H9OH -146131.792 -146196.459 -146339.261 -146382.8557
494 Bahjat R J Muhyedeen
Table 3: MP2/6-31++G(2d,2p) Structural Parameters and Harmonic Vibrational Frequencies of Various Compounds used in the Hydrocarbons + Ozone Reaction
Compounds C-C ºA C-O ºA O-O ºA C-H ºA IR-Frequencies HO* 0.9552 (O-H) 4053 HOO* 1.3080 1253, 1598, 4077 HOOO* 1.2614, 1.3943 i-240, 634, 1050, 1313, 1592, 4079 O3 1.3654 984, 983, 1443 CH3* 1.0731 418, 1523, 1524, 3258, 3435, 3438 CH3O* 1.3837 1.0858,1.08 895, 1093, 1197, 1559, 1565, 1640, 3156, 3228, 84 3246 CH4 1.0839 1462, 1462, 1462, 1671, 1672, 3172, 3275, 3276, 3277 C2H5* 1.4976 1.076,1.092 160, 462, 867, 1071, 1110, 1296, 1529, 1590, 1607, 1607, 3135, 3204, 3240, 3293, 3394 C2H5O* 1.5194 1.3881 1.085,1.089 121, 269, 458, 928, 960, 1156, 1196, 1366,1509, 1550, 1575, 1603, 1618, 3160, 3183, 3195, 3248, 3262 C2H6 1.5270 1.086 328, 885, 886, 1058, 1324, 1324, 1526, 1559,1620, 1621, 1622, 1623, 3174, 3179, 3221, 3223, 3249, 3251 C3H7* 1.4994 1.078,1.085 128, 169, 370, 466, 926, 1010, 1023, 1128,1202, , 1.093 1276, 1474, 1543, 1544, 1596, 1602, 1607, 1612, 3119, 3125, 3190, 3194, 3238, 3239, 3307 n-C3H7O* 1.5971 1.4039 1.102,1.098 99,147, 260, 269, 360, 393, 577, 798, 903, 923, 1.5689 1023, 1087, 1112, 1158, 1221, 1311, 1372, 1437, 1445, 1519, 1533, 1543, 1562, 1599, 1612, 1617, 1629, 3135, 3147, 3150, 3157, 3171, 3185, 3198, 3223, 3223 i-C3H7O* 1.5292 1.3952 1.085,1.088 120, 231, 239, 263, 377, 433, 506, 815, 876, 975, 1.5229 1054, 1077, 1085, 1102, 1195, 1283, 1376, 1420, 1464, 1518, 1526, 1539, 1598, 1605, 1617, 1621, 1625, 3139, 3156, 3162, 3176, 3194, 3223, 3236, 3248, 3255 C3H8 1.528 1.087 229, 286, 389, 804, 926, 976, 1006, 1123,1273, 1317, 1421, 1485, 1540, 1548, 1610, 1611, 1617, 1626, 1631, 3161, 3163, 3171, 3187, 3221,3232,3234, 3235 C4H9* 1.4998 1.076,1.088 129, 156, 257, 281, 446, 487, 783, 880, 932, 1.5313 , 1.093 1034, 1091, 1140, 1179, 1266, 1358, 1426,1435, 1.5274 1525, 1545, 1586, 1605, 1613, 1617, 1628, 3111, 3163, 3170, 3176, 3200, 3233, 3235, 3282, 3384 C4H10 1.076,1.088 125, 236, 270, 278, 450, 785, 869, 897,1032, , 1.093 1046, 1088, 1130, 1264, 1312, 1396, 1435, 1439, 1524, 1543, 1545, 1611, 1613, 1619, 1621,1625, 1632, 3155, 3162, 3166, 3167, 3174, 3194,3225, 3230, 3231, 3232
DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 495
Table 4: Total Energies (Kcal/mol) of Transition State Structures used in the Hydrocarbons + Ozone Reaction Calculated at MP2 Level with Three Basis Sets
Compound MP2/6-31G(d) MP2/6-31++G(d,p) MP2/6-311++G(2d,2p)* CH4-O3-TS1 -166332.911 -166375.082 -166541.813 CH4-O3-TS2 -166443.302 -166465.991 C2H6-O3-TS1 -191000.510 -191051.836 C2H6-O3-TS2 -191019.664 -191063.636 C2H6-O3-TS3 -191025.700 -191072.532 C2H6-O3-TS4 -190925.917 -190978.254 -191167.340 C2H6-O3-TS5 -191000.847 -191052.254 C2H6-O3-TS6 -190957.069 -190998.480 C2H6-O3-TS7 -191029.496 -191076.471 C3H8-O3-TS1 -215584.302 -215646.533 C3H8-O3-TS2 -215467.623 -215558.537 C3H8-O3-TS3 -215610.281 -215669.996 C3H8-O3-TS4 -215607.088 -215674.298 C3H8-O3-TS5 -215512.376 -215575.670 C3H8-O3-TS6 -215519.946 -215608.565 -215827.602 n-C3H8-O3-TS7 -215609.367 -215667.638 i-C3H8-O3-TS7 -215612.886 -215671.458 C4H10-O3-TS1 -240212.627 -240263.151 C4H10-O3-TS2 240167.715- -240239.955 C4H10-O3-TS3 -240116.661 -240181.480 C4H10-O3-TS4 -240117.135 -240181.971 -240413.012 C4H10-O3-TS5 -240123.988 -240191.562 * For only reasonable TS molecule
Table 5: Zero-point energies ZEP (Kcal/mol) Calculated at HF/6-31G(d) Level
Molecule Zep Molecule Zep Molecule Zep HO* 5.64227 HOO* 2.87264 HOOO* 6.82622 O 0.00 O2 2.85693 O3 4.53114 CH4 30.16315 CH3* 19.74337 CH3O* 25.83639 C2H6 50.02216 C2H5* 39.71166 C2H5O* 44.28385 C3H8 69.60394 C3H7* 59.18615 C3H7O* 63.39562 C4H10 88.46707 C4H9* 78.41312 C4H9O* 72.44600
Table 6: Zero-point energies ZEP (Kcal/mol) Calculated at HF/6-31++G(2d,2p) Level
Molecule Zep Molecule Zep Molecule Zep HO* 5.89466 HOO* 9.98509 HOOO* 12.08096 O 0.00 O2 2.63788* O3 4.83080 CH4 29.69230 CH3* 19.62343 CH3O* 25.41406 C2H6 49.57887 C2H5* 39.40429 C2H5O* 43.31234 C3H8 68.63106 C3H7* 58.70431 C3H7O* 62.23865 C4H10 87.70424 C4H9* 77.70218 C4H9O* 70.92845 *O2 in triplet state
496 Bahjat R J Muhyedeen
Figure 1: Geometries of Reactants, Products
* Ozone HDR HTR CH3O
* METHANE ETHANE C2H5O
* * PROPANE n-C3H7O i-C3H7O
* BUTANE n-C4H9O* i-C4H9O
DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 497
Figure 2: Geometries of CH4-O3 and C2H6-O3 Transition States Structures
Proposed CH4-O3-TS1 Proposed CH4-O3-TS2 Proposed C2H6-O3-TS1
Proposed C2H6-O3-TS2 Proposed C2H6-O3-TS3 Proposed C2H6-O3-TS4
Proposed C2H6-O3-TS5 Proposed C2H6-O3-TS6 Proposed C2H6-O3-n-TS7
498 Bahjat R J Muhyedeen
Figure 4: Geometries of C4H9-O3 Transition States Structures
Proposed C4H9-O3-n-TS1 Proposed C4H9-O3-n-TS2 Proposed C4H9-O3-n-TS3
Proposed C4H9-O3-n-TS4 Proposed C4H9-O3-n-TS5 Proposed C4H9-O3-n-TS6
The O-O bond in HDR molecule is 1.3080 Aº which is average value of the two O-O bonds length in HTR molecule (1.2614 and 1.3943 Aº). The resonance in O-O bond in HDR molecule due to three–electron bond made it mush shorter than that of HTR molecule. Of course this stabilization energy in HDR molecule increases the activation energy of transition state that lend to HDR molecule formation. The theoretical IR calculations at UHF/HF/6- 31++G(2d,2p) showed that the HDR species were stable molecules while the HTR species were labile and transition state molecules in gas phase which decomposed to a stable O2 and OH radical.
2. Reaction Mechanism The route of ozone reaction with the four hydrocarbon gases RH (methane, ethane, propane and butane) in the troposphere has been examined via the three possible reaction paths, namely, abstraction (dehydrogenation) reaction through formation of hydrotrioxyl radical (HTR) and alkyl radical R*, replacement reaction through formation of hydrodioxyl radical (HDR) and alkoxyl radical RO* and the OH*+O radicals as shown follow: a-) Abstraction reaction (formation of HTR molecule): RH + O3 →R* + HO3* --- then---→ [R* + O2 + OH*]--- then--- → ROH+ O2 b-) Replacement reaction (formation of HDR molecule) RH + O3 → RO* + HO2* ---then---→ [RO*+ O + OH*]--- then--- → ROH+ O2 The enthalpy and Gibbs free energy change and the final energies for these reactions are calculated at MP2 high level and DFT high level using basis sets 6-31++G(2d,2p)) which tabulated in Table-7. This Table concentrated on HDR and HTR species. DFT level of calculation gave more accurate results of energies than MP2 level for configurationally structures and transition state structures as seen in the 3rd and 5th columns of Table-7. DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 499
Table 7: The Quantum mechanical Energies, ∆Gº and ∆Hº of the Abstraction and Replacement Reaction Pathways Calculated at Different Levels (Kcal/mol)
Suggested Reactions Path MP2/F DFT/6-31G(d) DFT/F ∆Go # ∆Ho # Route CH4 + O3 → CH3* + HO3* HTR -30.262 31.127 10.121 -45.373 -44.352 CH4 + O3 → CH3O*+HO2* HDR -189.603 717.375 45.348 -103.217 -102.161 C2H6 + O3 → C2H5* + HO3* HTR -40.762 6.891 3.815 -48.225 -46.707 C2H6 + O3 → C2H5O* + HO2* HDR -216.148 707.585 18.262 -108.562 -107.249 C3H8 + O3 → C3H7* + HO3* HTR -49.794 -19.559 450.363 -48.687 -50.447 C3H8 + O3 → n-C3H7O* + HO2* HDR -179.295 682.082 452.706 -109.437 -110.135 C3H8 + O3 →i-C3H7O*+HO2* HDR -232.129 716.171 453.686 -107.218 -108.882 C4H10 + O3 → C4H9* + HO3* HTR -1.511 -3.412 -24.604 -46.530 -47.991 C4H10 + O3 →n-C4H9O* + HO2* HDR -53.200 -83.532 -802.046 -109.240 -109.805 C4H10 + O3 → i-C4H9O* + HO2* HDR -59.341 -89.000 -807.518 -107.006 -107.942 F=6-31++G(2d,2p) , #∆ Gº and ∆Hº calculated at HF/6-311++G(2d,2p) for high accuracy
The reaction rates of these reactions are also calculated at UHF/HF/6-31++G(2d,2p) level of theory as shown in the Table-8.
Table 8: The Reaction Rate of the Abstraction and Replacement Reaction Pathways Calculated at HF/6- 311++G(2d,2p) (rate in sec-1)
Reaction Suggested Reactions/ rate Transition /rate Products/ rate CH4 + O3 → CH3* + HO3* → CH3* + OH*+O2 CH3OH + O2 1.14 x 1046 2.53 x 1086 115 → CH3O* + HO2* → CH3O* + OH*+O 8.0 x10 2.88 x 1088 4.97 x 10495 for Singlet Oxygen 3.37 x 1092 for Triplet Oxygen C2H6 + O3 → C2H5* + HO3* → C2H5* + OH*+O2 C2H5OH + O2 1.135 x 1046 3.12 x 1088 88 → C2H5O* + HO2* → C2H5O* +OH*+O 3.124 x 10 6.8 x 1091 2.0 x 10485 for Singlet Oxygen 4.72 x 1098 for Triplet Oxygen C3H8 + O3 → C3H7* + HO3* → C3H7* + OH*+O2 48 90 5.95 x 10 1.33 x 10 n-C3H7OH + O2 118 → n-C3H7O* + HO2* → n-C3H7O* + OH*+O 1.59 x 10 92 93 4.1 x 10 3.8 x 104 for Singlet Oxygen i-C3H7OH + O2 98 121 →i-C3H7O* + HO2* 4.77 x 10 for Triplet Oxygen 3.51 x 10 1.04 x 1093 → i-C3H7O* + OH*+O 5.87 x 10500 for Singlet Oxygen 3.96 x 1099 for Triplet Oxygen C4H10 + O3 → C4H9* + HO3* → C4H9* + OH*+O2 47 88 9.43 x 10 2.11 x 10 n-C4H9OH + O2 119 →n-C4H9O* + HO2* → n-C4H9O* + OH*+O 1.0 x 10 91 99 8.39 x 10 1.56 x 104 for Singlet Oxygen i-C4H9OH + O2 95 120 → i-C4H9O* + HO2* 1.22 x 10 for Triplet Oxygen 9.86 x 10 93 1.5 x 10 → i-C4H9O* + OH*+O 3.37 x 10500 for Singlet Oxygen 2.27 x 1099 for Triplet Oxygen
The results of reaction rates calculations showed that all these types of reactions are possible with various probabilities. The formation of HDR (~x1092) is faster than HTR formation (~x1048) but still less probable than OH*+O species formation (~x1099 with triplet state) and (~x10500 with singlet state) in contrast to Shereshovets et al[30] interpretations. The strange thing is that these reactions prefer singlet oxygen atom rather than triplet oxygen atom, whereas the triplet is more stable than singlet by
500 Bahjat R J Muhyedeen
47.44 Kcal/mol calculated by G3 method42 at STP environment. The reaction rates values (see Table- 8) also showed that the reaction of ozone with these hydrocarbons pass through the formation of ↑↓ biradical formation of RO*+ O +OH* to form the final products ROH+O2. The rate of alcohols formation decrease as follow: i-C3H7OH< i-C4H9OH< n-C4H9OH< n-C3H7OH< CH4OH< C2H5OH 121 Of these products, the formation of iso alcohols like i-C3H7OH (~x 10 ) is faster than the 118 [29] normal alcohols n-C3H7OH (~x 10 ) which in accordance with Shustove conclusion. Generally, the reaction rates of HDR and HTR is slower than that of their fragment species (i.e. OH*+O or OH*+O2) as seen in the second column of Table-8.
3. Transition States Calculations Two different techniques have been used to look into the transition state. The first technique is using the quadratic synchronous transit method (QST) of HyperChem 7.52 which searches for a maximum a long a parabola connecting reactants and products, and for a minimum in all directions perpendicular to the parabola. This method will calculate the over all energy barrier between the reactant and product but it can not identify the structure of the transition state. The energy barrier calculated at 6-31G(d) level for the replacement reaction (HDR) of ozone with methane, ethane, propane, and butane are 429.767, 403.861, 525.809 , 606.086 Kcal/mol respectively as listed in Table-9.
Table 9: Energy Barrier for Replacement Reaction (HDR) of Ozone with Hydrocarbon Calculated at HF/ 6- 31G(d) Level Using Transition State Method (QST) (Kcal/mol)
Molecule CH4 C2H6 C3H8 C4H10 Energy barrier 429.767 403.861 525.809 606.086
The second method used for searching transition state structure is based on chemical guess in which one can suppose several possible transition states and then examined by IR calculation. This method is good for a well-known reaction mechanism. For deep investigation, thirty-three transition state structures are proposed for reaction of ozone with ethane, propane and butane as shown in Figures 3-5. These structures show the most probable attack sites of ozone on the hydrocarbon. Among these possible transition state structures only TS1 of methane, TS4 of ethane, TS6 of propane and TS5 of butane are proved to be a real transition state with one negative frequency (NIMAG=1) and gave a positive energy barrier. The ozone prefers to have a ring structure rather than a broken one through formation of transition state. The calculated energy barrier of these transition state structures for the reaction of ozone with methane, ethane, and propane are 32.995, 18.904, and 37.969 respectively. The TS4 and TS5 of butane gave a negative energy barrier (ca≈-2 Kcal/mol) as listed in Tables 10-13. Finally, it seems that none of these thirty-three transition state structures is close to the real transition state configurations. Therefore, the QST could be considered more reasonable treatment for these reactions.
Table 10: Transition State Calculated at MP2 Level with Three Basis Sets*
Compound MP2/ 6-31G(d) MP2/ 6-31++G(d,p) MP2/ 6-311++G(2d,2p) OZONE -141074.631 -141074.631 -141216.336 CH4 -25311.822 -25333.446 -25356.701 TOTAL -166378.923 -166408.077 -166573.037 CH4-O3-TS1 -166332.911 -166375.082 -166541.813 ENERGY BARRIER 46.012 32.995 31.224 *using second method DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 501
Table 11: Transition State Calculated at MP2 Level with Three Basis Sets
Compound MP2/ 6-31G(d) MP2/ 6-31++G(d,p) MP2/ 6-311++G(2d,2p) OZONE -141067.1008 -141074.631 -141216.336 C2H6 -49889.373 -49922.527 -49967.692 TOTAL -190956.474 -190997.158 -191184.028 C2H6-O3-TS4 -190925.917 -190978.254 -191167.340 ENERGY BARRIER 30.557 18.904 16.688 *using second method
Table 12: Transition State Calculated at MP2 Level with Three Basis Sets
Compound MP2/6-31G(d) MP2/6-31++G(d,p) MP2/6-311++G(2d,2p) OZONE -141067.1008 -141074.633 -141216.337 C3H8 -74469.1030 -74513.924 -74581.335 TOTAL -215536.2038 -215588.557 -215797.672 C3H8-O3-TS6 -215519.9456 -215608.565 -215827.602 ENERGY BARRIER 16.2582 37.969 -29.93 *using second method
Table 13: Transition State Calculated at MP2 Level with Three Basis Sets
Compound MP2/6-31G(d) MP2/6-31++G(d,p) MP2/ 6-311++G(2d,2p) O3 -141067.1008 -141074.633 -141216.336 C4H10 -99048.9230 -99105.409 -99195.093 TOTAL -240116.0238 -240180.042 -240411.429 C4H9-O3-TS4 -240117.1350 -240181.971 -240413.013 ENERGY BARRIER -1.1112 -1.929 -1.584 *using second method
502 Bahjat R J Muhyedeen
Figure 3: Geometries of C3H8-O3 Transition States Structures
Proposed C3H8-O3-TS1 Proposed C3H8-O3-TS2 Proposed C3H8-O3-TS3
Proposed C3H8-O3-TS4 Proposed C3H8-O3-TS5 Proposed C3H8-O3-TS6
Proposed n-C3H8-O3 -TS7 Proposed i-C3H8-O3-TS7 Proposed C3H8-O3-TS8 n= oxygen of ozone is attached to Terminal carbon, C1 or C3 i= oxygen of ozone is attached to Middle carbon, C2
Figure 5: The Geometry of TS1 Transition States Structures of CH4+O3 from IRC calculation
The structure of proposed CH4-O3-TS1 at saddle point position for CH4+O3 reaction (energy in a.u.) DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 503
4. CH4+O3 ---> CH3OH+O2 Reaction
In this reaction we consider the CH4+O3 as reactant and the CH3OH+O2 as products. The energy barrier values calculated at 6-31G(d,p) level using QST method of HyperChem 7.52 for the reaction of ozone with methane are -35.44 and -1.89 Kcal/mol respectively for forward and reverse reaction. This method might be not successful for this reaction with this basis set. To reconsider the reaction path leading down from a transition structure for this reaction, we used the intrinsic reaction coordinate, IRC, calculation on TS1. The IRC results for CH4+O3 ---> CH3OH+O2 reaction at 3-21+G** level gave an activation energy of 619 and 68 Kcal/mol for forward and reverse reaction as shown in Figure-5&6. Of course, these results state that although this TS1 structure is a transition state and contact two minima but it does not fall in the path way reaction.
Figure 6: CH4+O3 Æ CH3OH+O2 REACTION at 3-21+G**
5. Conclusion It could be concluded that the reaction of ozone with petroleum gases is possible in the atmosphere indoor and outdoor but with ambiguous mechanism, since all these various thirty-three proposed transition state structures are far from the saddle point structure and the real mechanism at this point is still unclear and need further investigations. The ozonation reaction of these hydrocarbons passes through the formation of biradical species RO*+O+OH* to reach the final products ROH+O2 rather than through formation of HDR or HTR. Of course, these reactions could be utilized in the refining industry to produce alcohols from hydrocarbons in special units which will be mixed later with the reformate gasoline to be of high octane number. It seem also that through reaction path, one of O–O bond of ozone breaks and the two terminal oxygen of ozone immediately attack the two hydrogen’s of hydrocarbon to form oxygenated transition state molecule. This oxygenated molecule decomposes either with less possibility through O-H and C- H bonds breakage to form HO3* and R* radicals or through of O-O bond breakage of oxygenated molecule to give the HO*, O and RO* radicals.
504 Bahjat R J Muhyedeen References [1] I.S.A. Isaksen, V. Ramaswamy, H. Rodhe, and T.M.L. Wigley, “Radiative forcing of climate, in Climate Change”, 1992, The Supplementary Report to the IPCC Scientific Assessment, Ed. by J.T. Houghton, B.A. Callander, and S.K. Varney, Cambridge University Press, 1992. [2] W.B. DeMore, S.P. Sander, D.M. Golden, R.F. Hampson, M.J. Kurylo, C.J. Howard, A.R. Ravishankara, C.E. Kolb, and M.J. Molina, “Chemical kinetics and photochemical data for use in stratospheric modeling”, JPL, 92, 20, Pasadena, California, 1992. [3] R. Atkinson, D.L. Baulch, R.A. Cox, R.F. Hampson Jr., J.A. Kerr, and J. Troe, Evaluated kinetic and photochemical data for atmospheric chemistry: Supplement IV, Atmos. Environ., 26A, 1187-1230, 1992. [4] A.P. Altshuller, “Chemical reactions and transport of alkanes and their products in the troposphere”, J. Atmos. Chem, 12, 19-61, 1991. [5] A.M. Thompson and R.W. Stewart, “Effect of chemical kinetics uncertainties on calculated constituents in a tropospheric photochemical model”, J. Geophys. Res., 96, 13,089-13,108, 1991. [6] Gramatica P., Pillutti P., Papa E., University of Insubria, via Dunant 3, 21100 Varese, Italy; http://disbsf.uninsubria.it/qsar [7] S.A. Montzka, M. Trainer, P.D. Goldan, W.C. Kuster, F.C. Fehsenfeld, “Isoprene and its oxidation products, methyl vinyl ketone and methacrolein, in the rural troposphere”, J. Geophys Res., 98, 1101-1111, 1993. [8] S.E. Paulson, R.C. Flagan, J.H. Seinfeld, “Atmospheric photooxidation of isoprene part II: The ozone-isoprene reaction”, Int. J. Chem. Kin. 24, 103-125, 1992. [9] S.E. Paulson and J.H. Seinfeld, “Development and evaluation of a photooxidation mechanism for isoprene”, J. Geophys. Res., 97, 20,703-20,715, 1992. [10] A.H. Haines, “Methods for the Oxidation of Organic Compounds. Alkanes, Alkenes, Alkynes and Arenes”. Academic Press, London, 1985. [11] (a) W. Lei, A. Derecskei-Kovacs, R. Zhang, J. Chem. Phys., 113, 5354, 2000. (b) W. Lei; R. Zhang, W. S. McGivern, A. Derecskei-Kovacs, S. W. North, Chem. Phys. Lett., 326, 109, 2000. (c) W. Lei; R. Zhang, W. S. McGivern, A. Derecskei-Kovacs, S. W. North, J. Phys. Chem. A, 105, 471, 2001. [12] F. Gilardoni, A. T. Bell, A. Chakraborty, P. Boulet., J. Phys. Chem., B, 104, 12250- 12255, 2000. [13] M. R. Soto, M. Page., J. Phys. Chem., 94, 3242-3246, 1990. [14] 14- D. Zhang, R. Zhang, S. W. North, J. Phys. Chem. A, 107, 11013-11019, 2003. [15] C.C. Price, A.L. Tumolo, J. Am. Chem. Soc.; 86: 4691–4694, 1964. [16] H.M. White, P.S. Bailey. J. Org. Chem., 30: 3037–3041, 1965. [17] M.C. Whiting, A.J.N. Bolt, J.H. Parish. Adv. Chem. Ser. 77, 4, 1969. [18] G.A. Hamilton, B.S. Ribner, T.M. Hellman. Adv. Chem. Ser., 77,15,1969. [19] B. Plesnicˇar, In Organic Peroxides, Ando W (ed.). Wiley: New York,; 479–533, 1992. [20] A. De Meijere, F. Wolf, In Organische Peroxo-Verbindungen, vol. E13, Kropf H (ed.). George Thieme: Stuttgart, 971, 1988. [21] W.A. Pryor, N. Ohto, D.F. Church. J. Am. Chem. Soc., 105:3614–3622, 1983. [22] M. Zarth, A. de Meijere. Chem. Ber., 118, 2429–2449, 1985. [23] Z. Cohen, E. Keinan, Y. Mazur, T.H. Varkony. J. Org. Chem., 40, 2141–2145, 1975. [24] D. Tal, E. Keinan, Y. Mazur. J. Am. Chem. Soc., 101, 502–503, 1979. [25] A. Beckwith, C. Bodkin, J. Duong. Aust. J. Chem., 30: 2177–2188, 1977. [26] E.V. Avzyanova, N.N. Kabal’nova, V.V. Shereshovets., Russ. Chem. Bull., 45, 360, 1996. [27] E.V. Avzyanova, Q.K. Timerghazin, A.F. Khalizov, S.L. Khursan, L.V. Spirikhin, V.V. Shereshovets., Mendeleev Commun., 227, 1997. DFT and Ab Initio Study of the Mechanism of Ozone Reaction with Light Hydrocarbons in the Troposphere 505
[28] E. V. Avzyanova, Q. K. Timerghazin, A. F. Khalizov, S. L. Khursan, L. V. Spirikhin, V. V. Shereshovets, J. Phys. Org. Chem., 13, 87, 2000., [29] G.V. Shustov, A. Rauk, J. Org. Chem. 63(16):5413-5422, 1998. [30] 30. Q.K. Timerghazin, S.L Khursan, V.V. Shereshovets., J. Mol. Struct.(Theochem), 489: 87– 93, 1999. [31] 31- A.D. Becke, J. Chem. Phys., 96, 2155, 1993. [32] 32- A.D. Becke, J. Chem. Phys., 98, 5648, 1993. [33] 33- C. Lee, W. Yang, R.G. Parr, Phys. Rev. B.,37, 785, 1988. [34] 34- M. Head-Gordon, J. A. Pople and M. J. Frisch, Chem. Phys. Lett., 153, 503, 1988. [35] 35- M. S. Gordon, Chem. Phys. Lett., 76, 163, 1980. [36] 36- W. J. Hehre, R. Ditchfield, and J. A. Pople, J. Chem. Phys., 56, 2257, 1972; P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 28, 213, 1973; M. S. Gordon, Chem. Phys. Lett., 76, 163, 1980; M.M. Francl, W.J. Petro, W.J. Hehre, J.S. Binkley, M.S. Gordon, D.J. DeFrees and J.A. Pople, J. Chem. Phys.77, 3654, 1982; V. Rassolov, J.A. Pople, M. Ratner and T.L. Windus, J. Chem. Phys. 109, 1223, 1998. [37] Raffenetti, R.C., Pre-processing two-electron integrals for efficient utilization in many electron self-consistent field calculations, Chem. Phys. Letters 20,335-338, 1973. [38] HyperChem 7.52, Hypercube Inc., FL, 2003. [39] J. J. P. Stewart, J. Comput. Chem. 10, 209-220 (1989). [40] MOPAC2000, J. J. P. Steward, Fujitsu Limited, Tokyo, Japan, (1999). [41] Gaussian03W from Gaussian, Inc. 340 Quinnipiac St., Bldg. 40, Wallingford CT 06492. [42] L. A. Curtiss, K. Raghavachari, P. C. Redfern, V. Rassolov, and J. A. Pople, J. Chem Phys. 109, 7764 (1998); L. A. Curtiss, P. C. Redfern, K. Raghavachari, V. Rassolov, and J. A. Pople, J. Chem. Phys. 110, 4703 (1999).