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Studies on the isomerization of acetone CH3COCH3
Journal of Molecular Structure (Theochem) 638 (2003) 215–228 www.elsevier.com/locate/theochem
Studies on the isomerization of acetone CH3COCH3 X.S. Xu*, Z. Hu, M.X. Jin, H. Liu, D. Ding1 Institute of Atomic and Molecular Physics, Jilin University, Qianwei Road, Number 10, Changchun 130012, People’s Republic of China Received 2 April 2003; revised 28 July 2003; accepted 28 July 2003
Abstract The isomerization reaction processes of acetone molecule CH3COCH3 has been calculated using the DFT-B3LYP method combined with the standard 6-31G(d,p) basis set. In order to obtain more accurate single-point energy the QCISD(T)/cc-pVDZ method is used. Altogether five isomerization reaction channels are confirmed using the IRC method and the corresponding isomerization products are methyl vinyl ether (cis- and trans-), allyl alcohol, and propen-2-ol, respectively. Acetone isomerizes to trans-methyl vinyl ether in one channel, and in the other four channels it isomerizes through the intermediate propylene oxide. In two of the four channels, acetone isomerizes to cis-methyl vinyl ether. Obviously, propylene oxide is an important intermediate in the isomerization process of acetone molecule. From the number of transition states and intermediates and the heights of barriers we infer that the most possible product is allyl alcohol. Based on the isomerization products and intermediates the possible dissociation channels and product fragments are discussed. q 2003 Elsevier B.V. All rights reserved. Keywords: Acetone; Isomerization; Fragment; DFT method
1. Introduction Acetone as the simplest aliphatic ketone plays an important role in photochemistry and environmental science and has been considered for a long time as an excellent model for the ketone chemistry [1 – 4]. To understand its role in photochemical processes in atmosphere, acetone has been extensively investigated experimentally by interacting with photons or electrons. These processes have also been studied by * Corresponding author. Tel.: þ86-431-5168751; fax: þ 86-4315168816. E-mail addresses: [email protected] (X.S. Xu), [email protected] (D. Ding). 1 Tel.: þ86-431-5168819; fax: þ86-431-5168816.
different theoretic calculation. Generally, the interaction with photons or electrons produces some þ fragments such as CHþ 3 , CH3CO . However, several recent experiments show that some fragments of acetone cannot be obtained through a simple dissociation of acetone, implying that an isomerization from excited acetone occurs prior to decomposition. For example, in an electron bombardment experiment, Zhang et al. showed that a small fraction of A state of acetone radical cation may rearrange to propen-2-ol, the enol form of acetone, through a scheme of acetoneþ $ 1,2-epoxypropaneþ $ propen-2-olþ [5]. Multiphoton ionization and fragmentation studies at 355 nm have been performed for acetone molecule using time-of-flight mass spectrometry, showing a peak at m=e ¼ 29 (COHþ) in the case of CH3COCH3
0166-1280/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0166-1280(03)00585-2
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and at m=e ¼ 30 (CODþ) for CD3COCD3, which provided another evidence for partial isomerisation of the keto form to the enol form Ref. [6]. Further observation in an experiment using 355 nm pulsed laser has presented some fragments such as C2Hþ 3, COHþ, CH3Oþ that could not be produced directly from acetone decomposition, suggesting that the isomerization to propen-2-ol, allyl alcohol or methyl vinyl ether should take place before the dissociation of acetone molecules [7]. All these experiments show that acetone isomerization can be resulted from the incomplete randomization of the excited energy in the molecules, which is in agreement with the early suggestion of non-ergodic process by Lifshitz et al. [8]. For carbonyl compounds such as acetone, UV photo absorption leads to a p p ˆ n transition and the nonradiative decay of the excited state (108 – 109 s21) is three orders of magnitude faster than the radiative process (105 – 106 s21) in the case of acetone [9]. The major process in the nonradiative decay is an intersystem crossing from the excited singlet states to the first triplet state that finally opens a dissociation channel from the first triplet state T1 [10,11]. Another possibility for the nonradiative decay is an internal conversion of the photo excitation energy which will result in a vibrational excitation of the molecule in the singlet ground state. Several studies have indicated that the process from this vibrationally excited molecule can become increasingly competitive with intersystem crossing and finally various excitation energies can be converted into an ‘effective temperature’ of the molecule in the ground state of the singlet manifold [12]. Similar temperature effect has been established on large system of organic molecules (i.e. C60) which shows the electron can be ‘boil-off’ from the vibrational ‘hot’ C60 (namely, thermionic emission of this molecular system) [13,14]. In the aspect of theoretical studies, to our best knowledge, there is no research concerning the unimolecular isomerization of acetone molecule, except some studies about relevant organic molecules. For example, the ring-opening reaction behavior of ethylene oxide was calculated and the results indicated that C –O cleavage is the easiest reactive path [15]; this kind of three-membered rings system was also studied by Yamaguchi et al. [16], they investigated the mechanism of the ring-opening reaction mainly concerning the C – C cleavage. Liu
et al. have calculated the photoionization processes with the aid of Gaussian-2 method for ethylene oxide and propylene oxide systems [17,18], which mainly involve ring opening and H atom migration and the dissociation channels proposed for the formation of fragments include simple bond cleavage reactions. Therefore, it is required in the present stage to do some theoretical studies for explanation of acetone isomerization. In this work, we investigate the isomerization process of acetone molecule from its electronic ground state using density function theory (DFT). The equilibrium geometries were optimized at B3LYP/6-31G(d,p) level and the energies corresponding to these structures were calculated at QCISD(T)/cc-pVDZ level. The calculated results show that acetone molecule can isomerize to methyl vinyl ether, allyl alcohol and propen-2-ol via thermal activation, though this process requires a high excited energy in the freedom of vibronic degree in the electronic ground state, which can be accumulated from photoexcitation energy through various ways of the internal conversion [10]. From these theoretical calculations, a general view for understanding the mechanism of acetone isomerization reaction is given and the possible reaction pathways to produce different fragments are also assigned.
2. Computational methods DFT has emerged as a useful tool for calculating molecular structures of larger systems [19,20]. One of its attractive features is that DFT method offers savings in computational time over conventional HF plus MP3 calculations [21], particularly when the local density approximation (LDA) for exchange and/ or correlation is used. Progress in recent years in the development of non-local corrections to the LDA [22 –24] made DFT method to predict molecular geometries more reliably. The structures and frequencies of the reactant, intermediate isomers, transition states, and products are calculated at the B3LYP/6-31G(d,p) level of the theory while the single-point energy calculations are performed at the QCISD(T)/cc-pVDZ level using the B3LYP/6-31G(d,p) optimized geometries. The zero-point vibration energies (ZPVE) at
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the B3LYP/6-31G(d,p) level are also included and they are scaled by the ZPVE scaling factor of 0.9806 [25]. To confirm the correction of the connection of the transition states with the reactants and products obtained, the intrinsic reaction coordinate (IRC) calculations are also carried out at the B3LYP/631G(d,p) level of the theory.
3. Results and discussion The calculated equilibrium geometric structures and potential surfaces show five possible isomerization pathways, corresponding to the production of trans-methyl vinyl ether, cis-methyl vinyl ether, allyl alcohol, and propen-2-ol, respectively. These isomerization reaction pathways include several transition states (TS) and intermediates (INT), and they are: (1) R(acetone) ! TS1 ! INT1 ! TS2 ! P1(transmethyl vinyl ether); (2)R ! TS3 ! INT2 ! TS4 ! INT3 ! TS5 ! P2(cis-methyl vinyl ether); (3) R ! TS3 ! INT2 ! TS6 ! INT4 ! TS7 ! P2(cismethyl vinyl ether); (4) R ! TS3 ! INT2 ! TS8 ! P3 (allyl alcohol) ! TS9 ! P30 (allyl alcohol); (5) R ! TS3 ! INT2 ! TS10 ! INT5 ! TS11 ! P4(propen-2-ol). There are two pathways that can lead to the product cis-methyl vinyl ether and the differences are the number of H-atom migration and the height of energy barriers. The calculated equilibrium structures and the potential energy surfaces of the five isomerizations are shown in Fig. 1 – 5, respectively. The corresponding total energies, zeropoint vibrational energies, relative energies and imaginary frequencies obtained for all of the species of acetone isomerization are listed in Table 1. From the calculation, all of the predicated isomerizations involve the bond cleavage and formation followed by H-atom and/or hydroxyl migration. The cleavage and formation of bonds and the H-atom and hydroxyl migration determine which isomer is produced. The intermediates and transition states are necessary for all of the acetone isomerization obtained. One important intermediate is propylene oxide (INT2) that was involved in four processes of the five predicted channels. As the one-order saddle point on potential energy surface, transition states also plays an important role in reaction pathway.
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3.1. Acetone to trans-methyl vinyl ether The acetone to trans-methyl vinyl ether (P1) isomerization process is shown in Fig. 1. The calculation shows that the surface contains two transition states (TS1 and TS2), indicating that the isomerization does not proceed via a concerted mechanism. Judged by the atomic movement of the normal mode of the imaginary frequency, the reaction coordinate involves a combination of the bend of C2O3C1 angle and the shortening of the length O3C1 in pathway R ! TS1 ! INT1. The C 2O3C 1 angle changes from 32.78 in the reactant, acetone, and to 116.18 in the intermediate INT1 through a transition state TS1 with an angle of 43.68. Enlargement of the C2O3C1 angle is accompanied by stretching of the ˚ in acetone to 1.816 A ˚ in C1 – C2 distance from 1.520 A ˚ TS1 and finally to 2.340 A in INT1, leading to a bond breaking between C1 and C2 atoms. In addition, the distance between O3 atom and C1 atom also changes ˚ in acetone to 1.711 A ˚ in TS1 and finally from 2.394 A ˚ to 1.442 A in INT1, forming a new bond O3 –C1. By this way, the loss of one bond due to the cleavage of the C1 –C2 bond is compensated by the formation of new bond O3 – C1. Furthermore, the reaction process INT1 ! TS2 ! P1 is a process of hydrogen atom shift. H9 atom migrates from C 4 to C2 atom accompanied by the distance C4 –C2 shortened from ˚ to 1.330 A ˚ , and then the C4 – C2 bond changes 1.508 A from single bond to double one, which leads to the product P1. The energy barriers of the transition state TS1 and TS2 are 117.26 and 95.68 kcal/mol, respectively, and the energy of the product P1 is 31.12 kcal/mol higher than that of the reactant acetone. 3.2. Formation of propylene oxide For the formation of propylene oxide from acetone, a combination of two normal modes, O3C2C1 angle bend and H5 shift, can be considered as the reaction coordinate. In the process, the O3C2C1 angle is reduced from 121.88 in acetone to 59.18 in propylene oxide. This large change needs to pass through a transition state, TS3, in which the angle of O3C2C1 is 111.08. The change of the O 3C 2C 1 angle is accompanied by shortening of the O3 – C1 distance ˚ in acetone to 2.287 A ˚ in transition state from 2.394 A
218 X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228 Fig. 1. Potential energy profile of the R ! P1 isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228
Fig. 2. Potential energy profile of the R ! P2 (path 2) isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
219
220 X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228 Fig. 3. Potential energy profile of the R ! P2 (path 3) isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228 Fig. 4. Potential energy profile of the R ! P3 ! P30 isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
221
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Fig. 5. Potential energy profile of the R ! P4 isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
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Table 1 Total energies ET, zero-point vibrational energies ZPVE with zero-point correction, relative energies DE and imaginary frequencies n for all of the species of acetone isomerization calculated at QCISD(T)/cc-pVDZ//B3LYP/6-31G(d,p) level Species
B3LYP/6-31G(d,p)
QCISD(T)/cc-pVDZ
ET (hartree)
ZPVE (kcal/mol)
Reactant and products R(acetone) P1(CH2yCH–O–CH3) (trans) P2(CH2yCH–O–CH3) (cis) P3(CH2yCH–CH2OH) P30 (CH2yCH –CH2OH) P4(CH3 –COHyCH2) (CH3C þ OCH3) (CH2CH þ OCH3) (CH2CH þ CH2OH) (CH3C þ CH2OH)
2193.164213 2193.119296 2193.122851 2193.175398 2193.172830 2193.138009 2192.884371 2192.960903 2192.967096 2192.890564
51.51 51.96 52.61 52.52 52.30 51.94 44.45 45.28 45.62 44.80
Transition states and intermediates TS1 INT1 TS2 TS3 INT2 TS4 INT3 TS5 TS6 INT4 TS7 TS8 TS9 TS10 INT5 TS11
2192.451166 2193.057130 2193.014931 2193.005213 2193.162917 2193.011112 2193.047924 2193.027268 2192.992492 2193.094913 2193.007969 2193.017560 2193.119081 2193.002609 2193.010887 2192.981764
49.07 50.98 48.87 48.70 52.70 49.90 50.54 49.57 48.73 50.62 48.98 48.87 52.02 48.88 50.61 49.95
˚ in propylene oxide, (TS3) and finally to 1.434 A which causes a bonding formation between O3 and C1 atoms and a change of the C2 –O3 bond from double ˚ ) to single (1.435 A ˚ ). On the other hand, (1.215 A while the angle is changing, the hydrogen H5 is moving away from the carbon C1. The distance of H5 ˚ , respectively, in to C1 is 1.096, 1.920 and 2.197 A acetone, transition state TS3, and propylene oxide. With this movement, H5 shifts from C1 to C2, and form another new bond, H5 – C2. A reverse process of this process has been discussed in Ref. [26]. Comparing our results with theirs, the obtained transition state (TS3) has the same structure but different energy (110.17 kcal/mol, instead of 86.93 kcal/mol from Ref. [26]). This difference results from the methods adapted in the calculation. Although
21
n (cm )
2878 21438 2948 2549 21226 2847 21453 22104 2135 21193 2895
ET (hartree)
DE (kcal/mol)
2192.638035 2192.588438 2192.592778 2192.598019 2192.595528 2192.610805 2192.356198 2192.430288 2192.439654 2192.365564
0 31.12 28.40 25.11 26.67 17.09 176.86 130.36 121.86 170.98
2192.451166 2192.531765 2192.485562 2192.462466 2192.587467 2192.490383 2192.517008 2192.496349 2192.466182 2192.518981 2192.478732 2192.486471 2192.593582 2192.477582 2192.486095 2192.454058
117.26 66.69 95.68 110.17 31.73 92.65 75.95 88.91 107.84 74.71 99.96 95.11 27.89 100.69 95.34 113.89
we use the same basis set, the method used in the present work is QCISD(T) not CCSD(T) in Ref. [26]. In general, QCISD(T) method is considered to be slightly superior than other methods particularly for the systems close to equilibrium geometries [27] and has been used widely [28 –30]. Propylene oxide is found to be important as the intermediate product in acetone isomerization since the different transformations of propylene oxide determine which reaction channel can be produced. 3.3. Acetone to cis-methyl vinyl ether There are two reaction pathways for the isomerization from acetone to the product P2, cis-methyl vinyl ether, i.e. R ! TS3 ! INT2 ! TS4 ! INT3 !
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TS5 ! P2 and R ! TS3 ! INT2 ! TS6 ! INT4 ! TS7 ! P2. The difference of these two pathways is that the isormerization takes place through the different intermediates and transition states after the process of acetone to propylene oxide. Two potential energy surfaces are shown in Figs. 2 and 3, respectively. The potential energy surface of the INT2 ! TS4 ! INT3 ! TS5 ! P2 isomerization involves two transition states. The reaction coordinate of the first stage is a C2O3C1 angle bend combined with conrotatory double rotation of the H6C1H7 methylene group with respect to the other two carbon atoms and their hydrogen atoms. The bend of the angle and rotation lead to the formation of the intermediate INT3 via a transition state, TS4. After rearranging, the C2O3C1 angle changes from 61.68 in propylene oxide to 128.98 in INT3 through the transition state with the angle of 101.88. In the meantime, the distance of C2 – ˚ in propylene oxide to C1 changes from 1.470 A ˚ 2.366 A in the intermediate INT3 through a value of ˚ in the transition state. This change of the C2 – 2.090 A C1 distance causes the break of C2 –C1 bond in the transition state. In the intermediate INT3, it is believed that the C2 – C1 bond is completely broken. The calculation also indicates that the bond length of C2 – O3 and O3 –C1 become shortened slightly in the process INT2 ! TS4 ! INT3 comparing with the values of C2 – O3 and O3 –C1 in the case of propylene oxide. The process from INT3 to P2 is a process of hydrogen H10 atom migration. The shift of H10 from C4 to C1 atom produces an isomer with a cis-form of P2 via a transition state TS5. Owing to the shift of hydrogen atom the structure of this transition state resembles a five-membered ring composed of the atoms C4, C2, O3, C1 and H10. In reaction pathway of INT3 ! TS5 ! P2, the distance of C4 –H10 changes ˚ and to 2.822 A ˚ , whereas the from 1.100 to 1.290 A ˚ distance of H10 – C1 changes from 2.880 to 1.655 A ˚ and to 1.098 A, and thus, the H10 atom has shifted from C4 to C1 atom. It is noted that the C4 –C2 bond in INT3, which is a single bond with a distance of ˚ , is shortened in the transition state TS5 1.470 A ˚ ) toward the formation of a C4yC2 double (1.410 A ˚ . Thus the new bond in P2 with a distance of 1.336 A product, cis-methyl vinyl ether (P2) is formed finally. Dubnikova had also calculated the process of INT2 ! P2 [26]. Although the reactant and product
in this stage is identical, there are one less intermediate and one less transition state in our work. Fig. 3 gives another reaction pathway leading to the same product, P2 (cis-methyl vinyl ether). This process is also through two intermediates, INT2 (propylene oxide) and INT4, as discussed above. It takes place through two transition states, TS6 and TS7 in order to reach the stage INT4 and the final stage P2 starting from INT2, propylene oxide. The reaction coordinate of the first stage is a C1O3C2 angle bend combined with H5 shift from C2 to C1. As a result of the angle bend, the C2 – C1 bond extends from a ˚ in the transition state distance of 1.470 to 1.850 A TS6. The C2 –C1 bond is completely broken with a ˚ in INT4, and there is no distance of 2.478 A interaction at all between C1 and C2 atom. The second stage INT4 ! TS7 ! P2 of the process is a H9 atom migration. The intermediate INT4, which still has a cis structure, undergoes a H9 shift from C4 to C2 to produce the cis isomer of P2 via the transition state TS7. The distance of C2 – H9 changes from ˚ in INT4 to 1.290 A ˚ in TS7 and to 1.086 A ˚ 2.089 A finally in P2. Following the same procedure as in the case above, the single bond between C4 and C2 in ˚ , is shortened in the INT4 with a length of 1.512 A ˚ transition state TS7 (1.400 A) and then toward the formation of a C4yC2 double bond in P2 with a length ˚. of 1.336 A It should be pointed out that all of these two pathways from acetone to cis-methyl vinyl ether include three transition states and two intermediates and only the first intermediate is identical (INT2, propylene oxide). The processes from INT2 to P2 is different: in the path (2) the bond C2 –C1 breaks firstly and then the H10 atom shifts from the C4 to the C1 atom, whereas in the path (3) the cleavage of the bond C2 – C1 is accompanied by the H5 migration from the C2 to the C1 atom and then the H9 atom shifts from the C4 to the C2 atom. Therefore the number of the shifted hydrogen atoms is different in these two reaction pathways. The energy barriers from INT2 to P2 in the path (2) are 92.65 and 88.91 kcal/mol and in the path (3) they are 107.84 and 99.96 kcal/mol, respectively. As a result, the energy barriers for the path (3) are higher than those for the path (2). Compared with the proposed processes in Ref. [26], these two processes obtained here do not include the stage of a cis – trans isomerization with respect to the O3 – C2 bond.
X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228
Therefore, even with almost the same heights of the energy barriers, the process in our case might be more practical because the number of energy barriers decrease. The isomerization processes from acetone to cismethyl vinyl ether and to trans-methyl vinyl ether are different and the formation of the latter does not relate to the important intermediate propylene oxide. At the first stage, the same is the formation of C – O bond, and the difference is that there need H atom migration in the former and C – C bond cleavage in the latter. After the first step the formation process of cisproduct require two steps to realize the C – C bond cleavage (adding H atom movement from C2 to C1 atom in path (3)) and H atom migration, and that of trans-product requires only one step to complete the H atom migration. Therefore the isomerization process from acetone to methyl vinyl ether is so complex that it cannot be completed with one step and need to pass through more and higher energy barriers. 3.4. Acetone to allyl alcohol The isomerization of acetone to allyl alcohol with different structures (P3 and P30 ) is shown in Fig. 4. The calculation shows that this process takes place via a stepwise mechanism, i.e. the reaction pathway go through the intermediate product, propylene oxide (INT2), then to P3 through a transition state TS8 with a higher energy barrier (95.11 kcal/mol). P3 can transform to P30 through another transition state with a very low barrier (2.78 kcal/mol, relative to the energy of P3). The isomerization process INT2 ! TS8 ! P3 includes the bond cleavage of C2 – O3 with the migration of H10 atom from C4 to O3. As indicated by the atom’s movement in the normal mode of the imaginary frequency, the reaction coordinate is a combination of two movements. The main movement is a H10 migration, with a rotation of the remaining methylene group H8C4H9. The distance of C2 –O3 ˚ in propylene oxide to 1.940 A ˚ increases from 1.435 A ˚ in the transition state, TS8, and finally to 2.432 A in P3. At the same time the C2C1O3 angle increases gradually from 59.28 in propylene oxide to 84.08 in TS8 and to 112.28 in P3. Thus, after the movement the C2 – O3 bond is broken completely. On the other hand the hydrogen atom H10, which moves from the methyl
225
group toward the oxygen atom in the transition state, is somewhere in the middle between the oxygen and carbon atom (C 4), with the distances of C4 – ˚ and H10 –O3 ¼ 1.398 A ˚ . Whereas the H10 ¼ 1.364 A ˚ in P3, a new bond is distance of H10 – O3 is 0.967 A formed between H10 atom and O3 atom. Note that the C4 – C2 bond in propylene oxide is a single bond with a ˚ and is shortened in the transition length of 1.507 A ˚ , which form a state, TS8, with a length of 1.412 A ˚. C4yC2 double bond in P3 with a length of 1.333 A This reaction pathway of INT2 ! TS8 ! P3 obtained in the present work is the same as described in Ref. [26]. The transformation between different types of allyl alcohol (P3 to P30 ) only requires small amount of the energy to overcome the very low energy barriers (2.78 kcal/mol) via a transition state TS9 since they have little difference only in the structures, i.e. in P3 hydroxyl O3 – H10 lies in front of C4 – C2 – C1 plane whereas behind of C4 –C2 – C1 plane in P30 . Additionally, hydroxyl O3 –H10 in P30 is parallel to C4 –C2 – C1 plane. We believe that, due to the small energy barriers, P3 and P30 can exist simultaneously and transform to each other easily. 3.5. Acetone to propen-2-ol So far we have discussed the isomerization reaction processes that the products aremethyl vinyl ether and allyl alcohol. Besides, another channel leading to product propen-2-ol (P4) is calculated. This process goes also through INT2 as the first step and then takes place via an intermediate (INT5) and two transition states (TS10 and TS11). The optimized geometries of the molecule in different stages and the energy barriers in this isomerization process are shown in Fig. 5. In the process, the hydrogen atom H5 shifts from the C2 to the O3 atom firstly, through a transition state TS10, to produce an intermediate, INT5. The next step of this pathway is a migration of hydroxyl from the C1 to the C2 atom via a transition state TS11. The energy barriers of INT2 ! TS10 ! INT5 and INT5 ! TS11 ! P4 processes are 100.69 and 113.89 kcal/mol. Although P4 is the most stable product among all of products discussed above, it is difficult to perform this pathway due to its higher energy barriers. There are some results [31,32] shown
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that this kind of keto – enol direct isomerization is difficult and the realization of the process need the aid of the reaction molecule as an intermediate in order to complete the proton transfer. 3.6. The most possible isomerization channel and the dissociation of isomers Generally, one can judge the feasibility of an isomerization process according to its relative energies of the transition states in the process and the number of high barriers involved in the pathway [28, 29,33]. More clearly, the pathway that involves higher-energy transition state and more high barriers may be less competitive. From our calculation, it can be seen that the isomerization processes to form the isomer P2 involves three energetically high transition states and two intermediates through the path (2) or the path (3). Furthermore, path (2) is more competitive than path (3). The process to form P3 (the path (4)) involves only two transition states and one intermediate, and product allyl alcohol (P3 and P30 ) can isomerize itself easily. The highest barriers in path (4) and in path (2) are almost equivalent. In addition, the transition state TS1 (117.26 kcal/mol) linking acetone to P1 has the highest energy in all the transition states. Therefore, from the consideration of the energy relation for all isomers and the number of transition states, the results demonstrates that the path (4) may be the most feasible pathway whereas the path (1) may be the most difficult one to perform among all obtained reaction pathways for acetone isomerization. The isomerization of acetone will result in very different fragments during interaction with photons or electrons, as observed in the experiments [5,6]. In general, acetone molecule can dissociate to CH3, CH3CO, CO fragments. After the internal conversion of photoexcitation energy, vibrationally excited acetone molecule can isomerizes to its various isomers before the dissociation takes place. Therefore, this will lead to different bond cleavage processes. The observation of the different fragments can be taken as the evidence of the isomerization of the molecules. The dissociation energies for different possible isomers of acetone have been calculated in the same methods described in Section 2. From the products P1,
P2, P3, P30 , and the intermediates INT1, INT3, INT4, INT5, the dissociation can occurs as follows. For example, there are eight dissociation channels in which the fragments C2H3 and CH3O can be produced, which are INT1 ðCH3 – C – O – CH3 Þ
110:17 kcal=mol
!
CH3 C
þ OCH3
ð1Þ
INT3 ðCH3 – CH – O – CH3 Þ þ OCH3 INT4 ðCH3 – C – O – CH3 Þ
100:91 kcal=mol
!
CH3 C ð2Þ
102:15 kcal=mol
!
CH3 C
þ OCH3
ð3Þ
P1 ðCH2 yCH – O – CH3 Þ ðtransÞ
99:24 kcal=mol
!
CH 2 CH þ OCH3
ð4Þ
P2ðCH2 yCH – O – CH3 Þ ðcisÞ
101:96 kcal=mol
!
CH2 CH
þ OCH3 P3 ðCH2yCH – CH2OHÞ
ð5Þ 96:75 kcal=mol
!
CH2 CH
þ CH2 OH
ð6Þ
P30 ðCH2yCH – CH2OHÞ þ CH2 OH
95:19 kcal=mol
INT5 ðCH3 – C – CH2 OHÞ
75:64 kcal=mol
þ CH2 OH
!
CH2 CH ð7Þ
!
CH3 C ð8Þ
From the energy consideration, the possibility of performing the channels is in the order (8), (7), (6), (4), (2), (5), (3) and (1) from high to low. Ion fragments may come from the ionization of those neutral fragments. Thus, the possible dissociation fragments C2H3 may be from CH3C and CH2CH, and CH3O may be from CH3O and CH2OH. Besides, if P4 dissociates one can find the fragments COH, CH3, CH2, etc. The dissociation of intermediate INT2 can surely produce the fragments CH3 and C2H3O, etc. The fragments with the highest probability may be CH2OH and CH2CH due to the most competitive pathway (4).
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4. Conclusions We have investigated the isomerization reaction process of acetone molecule using the B3LYP and QCISD(T) methods. Five energetically accessible reaction pathways from the electronic ground state have been obtained. The products are methyl vinyl ether (cis- and trans-), allyl alcohol and propen-2-ol. In five pathways there are two channels that the isomerization product is cis-methyl vinyl ether and four channels pass through the isomerization reaction process from acetone to propylene oxide except the isomerization process from acetone to trans-methyl vinyl ether. Obviously, propylene oxide is an important intermediate in isomerization process of acetone molecule. The isomerization process from acetone to transmethyl vinyl ether was proposed via two transition states and one intermediate. Because of the two higher barriers the realization of this process may be difficult. Although the two reaction channels connecting acetone with cis-methyl vinyl ether all include three transition states and two intermediates with the same process from acetone to propylene oxide, the process from propylene oxide to cis-methyl vinyl ether are different with one process including one C – C bond cleavage and one H-atom migration while the other process including one C – C bond cleavage and two Hatoms migration. Because the energies of transition states in path (2) are lower than that in path (3), path (2) is more competitive. The forming process from acetone to allyl alcohol (P3) proposed in this paper is the simplest one which need to pass through only one transition state after the process from acetone to propylene oxide, and the product P3 can isomerize itself easily with a very low energy barrier (2.78 kcal/mol). Another isomerization reaction process calculated is the process from acetone to propen-2-ol. Also it needs to pass through three transition states and two intermediates, which the first step is still the process from acetone to propylene oxide and the next two steps are C – C bond cleavage with H-atom migration and hydroxyl migration, respectively. Due to the three higher energy barriers this isomerization reaction process may be difficult to complete. Judged by the number of transition states and the height of energy barriers the reaction channel from acetone to allyl alcohol is the most feasible one. Based on
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the isomerization reaction channels calculated, we infer that the possible fragments are CH3C, CH2CH, CH3O, CH2OH, COH, CH3, CH2, CH3, C2H3O, etc. Surely, the fragments with the highest probability may be CH2OH and CH2CH because allyl alcohol is the most possible isomerization product. It is hoped that our results may stimulate future studies on the acetone molecule and gain a deep understanding of the complex isomerization reaction mechanism.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 19574018, 19925416 and 10174026).
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Studies on the isomerization of acetone CH3COCH3 X.S. Xu*, Z. Hu, M.X. Jin, H. Liu, D. Ding1 Institute of Atomic and Molecular Physics, Jilin University, Qianwei Road, Number 10, Changchun 130012, People’s Republic of China Received 2 April 2003; revised 28 July 2003; accepted 28 July 2003
Abstract The isomerization reaction processes of acetone molecule CH3COCH3 has been calculated using the DFT-B3LYP method combined with the standard 6-31G(d,p) basis set. In order to obtain more accurate single-point energy the QCISD(T)/cc-pVDZ method is used. Altogether five isomerization reaction channels are confirmed using the IRC method and the corresponding isomerization products are methyl vinyl ether (cis- and trans-), allyl alcohol, and propen-2-ol, respectively. Acetone isomerizes to trans-methyl vinyl ether in one channel, and in the other four channels it isomerizes through the intermediate propylene oxide. In two of the four channels, acetone isomerizes to cis-methyl vinyl ether. Obviously, propylene oxide is an important intermediate in the isomerization process of acetone molecule. From the number of transition states and intermediates and the heights of barriers we infer that the most possible product is allyl alcohol. Based on the isomerization products and intermediates the possible dissociation channels and product fragments are discussed. q 2003 Elsevier B.V. All rights reserved. Keywords: Acetone; Isomerization; Fragment; DFT method
1. Introduction Acetone as the simplest aliphatic ketone plays an important role in photochemistry and environmental science and has been considered for a long time as an excellent model for the ketone chemistry [1 – 4]. To understand its role in photochemical processes in atmosphere, acetone has been extensively investigated experimentally by interacting with photons or electrons. These processes have also been studied by * Corresponding author. Tel.: þ86-431-5168751; fax: þ 86-4315168816. E-mail addresses: [email protected] (X.S. Xu), [email protected] (D. Ding). 1 Tel.: þ86-431-5168819; fax: þ86-431-5168816.
different theoretic calculation. Generally, the interaction with photons or electrons produces some þ fragments such as CHþ 3 , CH3CO . However, several recent experiments show that some fragments of acetone cannot be obtained through a simple dissociation of acetone, implying that an isomerization from excited acetone occurs prior to decomposition. For example, in an electron bombardment experiment, Zhang et al. showed that a small fraction of A state of acetone radical cation may rearrange to propen-2-ol, the enol form of acetone, through a scheme of acetoneþ $ 1,2-epoxypropaneþ $ propen-2-olþ [5]. Multiphoton ionization and fragmentation studies at 355 nm have been performed for acetone molecule using time-of-flight mass spectrometry, showing a peak at m=e ¼ 29 (COHþ) in the case of CH3COCH3
0166-1280/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0166-1280(03)00585-2
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and at m=e ¼ 30 (CODþ) for CD3COCD3, which provided another evidence for partial isomerisation of the keto form to the enol form Ref. [6]. Further observation in an experiment using 355 nm pulsed laser has presented some fragments such as C2Hþ 3, COHþ, CH3Oþ that could not be produced directly from acetone decomposition, suggesting that the isomerization to propen-2-ol, allyl alcohol or methyl vinyl ether should take place before the dissociation of acetone molecules [7]. All these experiments show that acetone isomerization can be resulted from the incomplete randomization of the excited energy in the molecules, which is in agreement with the early suggestion of non-ergodic process by Lifshitz et al. [8]. For carbonyl compounds such as acetone, UV photo absorption leads to a p p ˆ n transition and the nonradiative decay of the excited state (108 – 109 s21) is three orders of magnitude faster than the radiative process (105 – 106 s21) in the case of acetone [9]. The major process in the nonradiative decay is an intersystem crossing from the excited singlet states to the first triplet state that finally opens a dissociation channel from the first triplet state T1 [10,11]. Another possibility for the nonradiative decay is an internal conversion of the photo excitation energy which will result in a vibrational excitation of the molecule in the singlet ground state. Several studies have indicated that the process from this vibrationally excited molecule can become increasingly competitive with intersystem crossing and finally various excitation energies can be converted into an ‘effective temperature’ of the molecule in the ground state of the singlet manifold [12]. Similar temperature effect has been established on large system of organic molecules (i.e. C60) which shows the electron can be ‘boil-off’ from the vibrational ‘hot’ C60 (namely, thermionic emission of this molecular system) [13,14]. In the aspect of theoretical studies, to our best knowledge, there is no research concerning the unimolecular isomerization of acetone molecule, except some studies about relevant organic molecules. For example, the ring-opening reaction behavior of ethylene oxide was calculated and the results indicated that C –O cleavage is the easiest reactive path [15]; this kind of three-membered rings system was also studied by Yamaguchi et al. [16], they investigated the mechanism of the ring-opening reaction mainly concerning the C – C cleavage. Liu
et al. have calculated the photoionization processes with the aid of Gaussian-2 method for ethylene oxide and propylene oxide systems [17,18], which mainly involve ring opening and H atom migration and the dissociation channels proposed for the formation of fragments include simple bond cleavage reactions. Therefore, it is required in the present stage to do some theoretical studies for explanation of acetone isomerization. In this work, we investigate the isomerization process of acetone molecule from its electronic ground state using density function theory (DFT). The equilibrium geometries were optimized at B3LYP/6-31G(d,p) level and the energies corresponding to these structures were calculated at QCISD(T)/cc-pVDZ level. The calculated results show that acetone molecule can isomerize to methyl vinyl ether, allyl alcohol and propen-2-ol via thermal activation, though this process requires a high excited energy in the freedom of vibronic degree in the electronic ground state, which can be accumulated from photoexcitation energy through various ways of the internal conversion [10]. From these theoretical calculations, a general view for understanding the mechanism of acetone isomerization reaction is given and the possible reaction pathways to produce different fragments are also assigned.
2. Computational methods DFT has emerged as a useful tool for calculating molecular structures of larger systems [19,20]. One of its attractive features is that DFT method offers savings in computational time over conventional HF plus MP3 calculations [21], particularly when the local density approximation (LDA) for exchange and/ or correlation is used. Progress in recent years in the development of non-local corrections to the LDA [22 –24] made DFT method to predict molecular geometries more reliably. The structures and frequencies of the reactant, intermediate isomers, transition states, and products are calculated at the B3LYP/6-31G(d,p) level of the theory while the single-point energy calculations are performed at the QCISD(T)/cc-pVDZ level using the B3LYP/6-31G(d,p) optimized geometries. The zero-point vibration energies (ZPVE) at
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the B3LYP/6-31G(d,p) level are also included and they are scaled by the ZPVE scaling factor of 0.9806 [25]. To confirm the correction of the connection of the transition states with the reactants and products obtained, the intrinsic reaction coordinate (IRC) calculations are also carried out at the B3LYP/631G(d,p) level of the theory.
3. Results and discussion The calculated equilibrium geometric structures and potential surfaces show five possible isomerization pathways, corresponding to the production of trans-methyl vinyl ether, cis-methyl vinyl ether, allyl alcohol, and propen-2-ol, respectively. These isomerization reaction pathways include several transition states (TS) and intermediates (INT), and they are: (1) R(acetone) ! TS1 ! INT1 ! TS2 ! P1(transmethyl vinyl ether); (2)R ! TS3 ! INT2 ! TS4 ! INT3 ! TS5 ! P2(cis-methyl vinyl ether); (3) R ! TS3 ! INT2 ! TS6 ! INT4 ! TS7 ! P2(cismethyl vinyl ether); (4) R ! TS3 ! INT2 ! TS8 ! P3 (allyl alcohol) ! TS9 ! P30 (allyl alcohol); (5) R ! TS3 ! INT2 ! TS10 ! INT5 ! TS11 ! P4(propen-2-ol). There are two pathways that can lead to the product cis-methyl vinyl ether and the differences are the number of H-atom migration and the height of energy barriers. The calculated equilibrium structures and the potential energy surfaces of the five isomerizations are shown in Fig. 1 – 5, respectively. The corresponding total energies, zeropoint vibrational energies, relative energies and imaginary frequencies obtained for all of the species of acetone isomerization are listed in Table 1. From the calculation, all of the predicated isomerizations involve the bond cleavage and formation followed by H-atom and/or hydroxyl migration. The cleavage and formation of bonds and the H-atom and hydroxyl migration determine which isomer is produced. The intermediates and transition states are necessary for all of the acetone isomerization obtained. One important intermediate is propylene oxide (INT2) that was involved in four processes of the five predicted channels. As the one-order saddle point on potential energy surface, transition states also plays an important role in reaction pathway.
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3.1. Acetone to trans-methyl vinyl ether The acetone to trans-methyl vinyl ether (P1) isomerization process is shown in Fig. 1. The calculation shows that the surface contains two transition states (TS1 and TS2), indicating that the isomerization does not proceed via a concerted mechanism. Judged by the atomic movement of the normal mode of the imaginary frequency, the reaction coordinate involves a combination of the bend of C2O3C1 angle and the shortening of the length O3C1 in pathway R ! TS1 ! INT1. The C 2O3C 1 angle changes from 32.78 in the reactant, acetone, and to 116.18 in the intermediate INT1 through a transition state TS1 with an angle of 43.68. Enlargement of the C2O3C1 angle is accompanied by stretching of the ˚ in acetone to 1.816 A ˚ in C1 – C2 distance from 1.520 A ˚ TS1 and finally to 2.340 A in INT1, leading to a bond breaking between C1 and C2 atoms. In addition, the distance between O3 atom and C1 atom also changes ˚ in acetone to 1.711 A ˚ in TS1 and finally from 2.394 A ˚ to 1.442 A in INT1, forming a new bond O3 –C1. By this way, the loss of one bond due to the cleavage of the C1 –C2 bond is compensated by the formation of new bond O3 – C1. Furthermore, the reaction process INT1 ! TS2 ! P1 is a process of hydrogen atom shift. H9 atom migrates from C 4 to C2 atom accompanied by the distance C4 –C2 shortened from ˚ to 1.330 A ˚ , and then the C4 – C2 bond changes 1.508 A from single bond to double one, which leads to the product P1. The energy barriers of the transition state TS1 and TS2 are 117.26 and 95.68 kcal/mol, respectively, and the energy of the product P1 is 31.12 kcal/mol higher than that of the reactant acetone. 3.2. Formation of propylene oxide For the formation of propylene oxide from acetone, a combination of two normal modes, O3C2C1 angle bend and H5 shift, can be considered as the reaction coordinate. In the process, the O3C2C1 angle is reduced from 121.88 in acetone to 59.18 in propylene oxide. This large change needs to pass through a transition state, TS3, in which the angle of O3C2C1 is 111.08. The change of the O 3C 2C 1 angle is accompanied by shortening of the O3 – C1 distance ˚ in acetone to 2.287 A ˚ in transition state from 2.394 A
218 X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228 Fig. 1. Potential energy profile of the R ! P1 isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
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Fig. 2. Potential energy profile of the R ! P2 (path 2) isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
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220 X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228 Fig. 3. Potential energy profile of the R ! P2 (path 3) isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228 Fig. 4. Potential energy profile of the R ! P3 ! P30 isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
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Fig. 5. Potential energy profile of the R ! P4 isomerization. Relative energies (in kcal/mol) are calculated at the QCISD(T)/cc-pVDZ level of theory. Bond lengths are in angstroms and angles in degrees.
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Table 1 Total energies ET, zero-point vibrational energies ZPVE with zero-point correction, relative energies DE and imaginary frequencies n for all of the species of acetone isomerization calculated at QCISD(T)/cc-pVDZ//B3LYP/6-31G(d,p) level Species
B3LYP/6-31G(d,p)
QCISD(T)/cc-pVDZ
ET (hartree)
ZPVE (kcal/mol)
Reactant and products R(acetone) P1(CH2yCH–O–CH3) (trans) P2(CH2yCH–O–CH3) (cis) P3(CH2yCH–CH2OH) P30 (CH2yCH –CH2OH) P4(CH3 –COHyCH2) (CH3C þ OCH3) (CH2CH þ OCH3) (CH2CH þ CH2OH) (CH3C þ CH2OH)
2193.164213 2193.119296 2193.122851 2193.175398 2193.172830 2193.138009 2192.884371 2192.960903 2192.967096 2192.890564
51.51 51.96 52.61 52.52 52.30 51.94 44.45 45.28 45.62 44.80
Transition states and intermediates TS1 INT1 TS2 TS3 INT2 TS4 INT3 TS5 TS6 INT4 TS7 TS8 TS9 TS10 INT5 TS11
2192.451166 2193.057130 2193.014931 2193.005213 2193.162917 2193.011112 2193.047924 2193.027268 2192.992492 2193.094913 2193.007969 2193.017560 2193.119081 2193.002609 2193.010887 2192.981764
49.07 50.98 48.87 48.70 52.70 49.90 50.54 49.57 48.73 50.62 48.98 48.87 52.02 48.88 50.61 49.95
˚ in propylene oxide, (TS3) and finally to 1.434 A which causes a bonding formation between O3 and C1 atoms and a change of the C2 –O3 bond from double ˚ ) to single (1.435 A ˚ ). On the other hand, (1.215 A while the angle is changing, the hydrogen H5 is moving away from the carbon C1. The distance of H5 ˚ , respectively, in to C1 is 1.096, 1.920 and 2.197 A acetone, transition state TS3, and propylene oxide. With this movement, H5 shifts from C1 to C2, and form another new bond, H5 – C2. A reverse process of this process has been discussed in Ref. [26]. Comparing our results with theirs, the obtained transition state (TS3) has the same structure but different energy (110.17 kcal/mol, instead of 86.93 kcal/mol from Ref. [26]). This difference results from the methods adapted in the calculation. Although
21
n (cm )
2878 21438 2948 2549 21226 2847 21453 22104 2135 21193 2895
ET (hartree)
DE (kcal/mol)
2192.638035 2192.588438 2192.592778 2192.598019 2192.595528 2192.610805 2192.356198 2192.430288 2192.439654 2192.365564
0 31.12 28.40 25.11 26.67 17.09 176.86 130.36 121.86 170.98
2192.451166 2192.531765 2192.485562 2192.462466 2192.587467 2192.490383 2192.517008 2192.496349 2192.466182 2192.518981 2192.478732 2192.486471 2192.593582 2192.477582 2192.486095 2192.454058
117.26 66.69 95.68 110.17 31.73 92.65 75.95 88.91 107.84 74.71 99.96 95.11 27.89 100.69 95.34 113.89
we use the same basis set, the method used in the present work is QCISD(T) not CCSD(T) in Ref. [26]. In general, QCISD(T) method is considered to be slightly superior than other methods particularly for the systems close to equilibrium geometries [27] and has been used widely [28 –30]. Propylene oxide is found to be important as the intermediate product in acetone isomerization since the different transformations of propylene oxide determine which reaction channel can be produced. 3.3. Acetone to cis-methyl vinyl ether There are two reaction pathways for the isomerization from acetone to the product P2, cis-methyl vinyl ether, i.e. R ! TS3 ! INT2 ! TS4 ! INT3 !
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TS5 ! P2 and R ! TS3 ! INT2 ! TS6 ! INT4 ! TS7 ! P2. The difference of these two pathways is that the isormerization takes place through the different intermediates and transition states after the process of acetone to propylene oxide. Two potential energy surfaces are shown in Figs. 2 and 3, respectively. The potential energy surface of the INT2 ! TS4 ! INT3 ! TS5 ! P2 isomerization involves two transition states. The reaction coordinate of the first stage is a C2O3C1 angle bend combined with conrotatory double rotation of the H6C1H7 methylene group with respect to the other two carbon atoms and their hydrogen atoms. The bend of the angle and rotation lead to the formation of the intermediate INT3 via a transition state, TS4. After rearranging, the C2O3C1 angle changes from 61.68 in propylene oxide to 128.98 in INT3 through the transition state with the angle of 101.88. In the meantime, the distance of C2 – ˚ in propylene oxide to C1 changes from 1.470 A ˚ 2.366 A in the intermediate INT3 through a value of ˚ in the transition state. This change of the C2 – 2.090 A C1 distance causes the break of C2 –C1 bond in the transition state. In the intermediate INT3, it is believed that the C2 – C1 bond is completely broken. The calculation also indicates that the bond length of C2 – O3 and O3 –C1 become shortened slightly in the process INT2 ! TS4 ! INT3 comparing with the values of C2 – O3 and O3 –C1 in the case of propylene oxide. The process from INT3 to P2 is a process of hydrogen H10 atom migration. The shift of H10 from C4 to C1 atom produces an isomer with a cis-form of P2 via a transition state TS5. Owing to the shift of hydrogen atom the structure of this transition state resembles a five-membered ring composed of the atoms C4, C2, O3, C1 and H10. In reaction pathway of INT3 ! TS5 ! P2, the distance of C4 –H10 changes ˚ and to 2.822 A ˚ , whereas the from 1.100 to 1.290 A ˚ distance of H10 – C1 changes from 2.880 to 1.655 A ˚ and to 1.098 A, and thus, the H10 atom has shifted from C4 to C1 atom. It is noted that the C4 –C2 bond in INT3, which is a single bond with a distance of ˚ , is shortened in the transition state TS5 1.470 A ˚ ) toward the formation of a C4yC2 double (1.410 A ˚ . Thus the new bond in P2 with a distance of 1.336 A product, cis-methyl vinyl ether (P2) is formed finally. Dubnikova had also calculated the process of INT2 ! P2 [26]. Although the reactant and product
in this stage is identical, there are one less intermediate and one less transition state in our work. Fig. 3 gives another reaction pathway leading to the same product, P2 (cis-methyl vinyl ether). This process is also through two intermediates, INT2 (propylene oxide) and INT4, as discussed above. It takes place through two transition states, TS6 and TS7 in order to reach the stage INT4 and the final stage P2 starting from INT2, propylene oxide. The reaction coordinate of the first stage is a C1O3C2 angle bend combined with H5 shift from C2 to C1. As a result of the angle bend, the C2 – C1 bond extends from a ˚ in the transition state distance of 1.470 to 1.850 A TS6. The C2 –C1 bond is completely broken with a ˚ in INT4, and there is no distance of 2.478 A interaction at all between C1 and C2 atom. The second stage INT4 ! TS7 ! P2 of the process is a H9 atom migration. The intermediate INT4, which still has a cis structure, undergoes a H9 shift from C4 to C2 to produce the cis isomer of P2 via the transition state TS7. The distance of C2 – H9 changes from ˚ in INT4 to 1.290 A ˚ in TS7 and to 1.086 A ˚ 2.089 A finally in P2. Following the same procedure as in the case above, the single bond between C4 and C2 in ˚ , is shortened in the INT4 with a length of 1.512 A ˚ transition state TS7 (1.400 A) and then toward the formation of a C4yC2 double bond in P2 with a length ˚. of 1.336 A It should be pointed out that all of these two pathways from acetone to cis-methyl vinyl ether include three transition states and two intermediates and only the first intermediate is identical (INT2, propylene oxide). The processes from INT2 to P2 is different: in the path (2) the bond C2 –C1 breaks firstly and then the H10 atom shifts from the C4 to the C1 atom, whereas in the path (3) the cleavage of the bond C2 – C1 is accompanied by the H5 migration from the C2 to the C1 atom and then the H9 atom shifts from the C4 to the C2 atom. Therefore the number of the shifted hydrogen atoms is different in these two reaction pathways. The energy barriers from INT2 to P2 in the path (2) are 92.65 and 88.91 kcal/mol and in the path (3) they are 107.84 and 99.96 kcal/mol, respectively. As a result, the energy barriers for the path (3) are higher than those for the path (2). Compared with the proposed processes in Ref. [26], these two processes obtained here do not include the stage of a cis – trans isomerization with respect to the O3 – C2 bond.
X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228
Therefore, even with almost the same heights of the energy barriers, the process in our case might be more practical because the number of energy barriers decrease. The isomerization processes from acetone to cismethyl vinyl ether and to trans-methyl vinyl ether are different and the formation of the latter does not relate to the important intermediate propylene oxide. At the first stage, the same is the formation of C – O bond, and the difference is that there need H atom migration in the former and C – C bond cleavage in the latter. After the first step the formation process of cisproduct require two steps to realize the C – C bond cleavage (adding H atom movement from C2 to C1 atom in path (3)) and H atom migration, and that of trans-product requires only one step to complete the H atom migration. Therefore the isomerization process from acetone to methyl vinyl ether is so complex that it cannot be completed with one step and need to pass through more and higher energy barriers. 3.4. Acetone to allyl alcohol The isomerization of acetone to allyl alcohol with different structures (P3 and P30 ) is shown in Fig. 4. The calculation shows that this process takes place via a stepwise mechanism, i.e. the reaction pathway go through the intermediate product, propylene oxide (INT2), then to P3 through a transition state TS8 with a higher energy barrier (95.11 kcal/mol). P3 can transform to P30 through another transition state with a very low barrier (2.78 kcal/mol, relative to the energy of P3). The isomerization process INT2 ! TS8 ! P3 includes the bond cleavage of C2 – O3 with the migration of H10 atom from C4 to O3. As indicated by the atom’s movement in the normal mode of the imaginary frequency, the reaction coordinate is a combination of two movements. The main movement is a H10 migration, with a rotation of the remaining methylene group H8C4H9. The distance of C2 –O3 ˚ in propylene oxide to 1.940 A ˚ increases from 1.435 A ˚ in the transition state, TS8, and finally to 2.432 A in P3. At the same time the C2C1O3 angle increases gradually from 59.28 in propylene oxide to 84.08 in TS8 and to 112.28 in P3. Thus, after the movement the C2 – O3 bond is broken completely. On the other hand the hydrogen atom H10, which moves from the methyl
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group toward the oxygen atom in the transition state, is somewhere in the middle between the oxygen and carbon atom (C 4), with the distances of C4 – ˚ and H10 –O3 ¼ 1.398 A ˚ . Whereas the H10 ¼ 1.364 A ˚ in P3, a new bond is distance of H10 – O3 is 0.967 A formed between H10 atom and O3 atom. Note that the C4 – C2 bond in propylene oxide is a single bond with a ˚ and is shortened in the transition length of 1.507 A ˚ , which form a state, TS8, with a length of 1.412 A ˚. C4yC2 double bond in P3 with a length of 1.333 A This reaction pathway of INT2 ! TS8 ! P3 obtained in the present work is the same as described in Ref. [26]. The transformation between different types of allyl alcohol (P3 to P30 ) only requires small amount of the energy to overcome the very low energy barriers (2.78 kcal/mol) via a transition state TS9 since they have little difference only in the structures, i.e. in P3 hydroxyl O3 – H10 lies in front of C4 – C2 – C1 plane whereas behind of C4 –C2 – C1 plane in P30 . Additionally, hydroxyl O3 –H10 in P30 is parallel to C4 –C2 – C1 plane. We believe that, due to the small energy barriers, P3 and P30 can exist simultaneously and transform to each other easily. 3.5. Acetone to propen-2-ol So far we have discussed the isomerization reaction processes that the products aremethyl vinyl ether and allyl alcohol. Besides, another channel leading to product propen-2-ol (P4) is calculated. This process goes also through INT2 as the first step and then takes place via an intermediate (INT5) and two transition states (TS10 and TS11). The optimized geometries of the molecule in different stages and the energy barriers in this isomerization process are shown in Fig. 5. In the process, the hydrogen atom H5 shifts from the C2 to the O3 atom firstly, through a transition state TS10, to produce an intermediate, INT5. The next step of this pathway is a migration of hydroxyl from the C1 to the C2 atom via a transition state TS11. The energy barriers of INT2 ! TS10 ! INT5 and INT5 ! TS11 ! P4 processes are 100.69 and 113.89 kcal/mol. Although P4 is the most stable product among all of products discussed above, it is difficult to perform this pathway due to its higher energy barriers. There are some results [31,32] shown
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that this kind of keto – enol direct isomerization is difficult and the realization of the process need the aid of the reaction molecule as an intermediate in order to complete the proton transfer. 3.6. The most possible isomerization channel and the dissociation of isomers Generally, one can judge the feasibility of an isomerization process according to its relative energies of the transition states in the process and the number of high barriers involved in the pathway [28, 29,33]. More clearly, the pathway that involves higher-energy transition state and more high barriers may be less competitive. From our calculation, it can be seen that the isomerization processes to form the isomer P2 involves three energetically high transition states and two intermediates through the path (2) or the path (3). Furthermore, path (2) is more competitive than path (3). The process to form P3 (the path (4)) involves only two transition states and one intermediate, and product allyl alcohol (P3 and P30 ) can isomerize itself easily. The highest barriers in path (4) and in path (2) are almost equivalent. In addition, the transition state TS1 (117.26 kcal/mol) linking acetone to P1 has the highest energy in all the transition states. Therefore, from the consideration of the energy relation for all isomers and the number of transition states, the results demonstrates that the path (4) may be the most feasible pathway whereas the path (1) may be the most difficult one to perform among all obtained reaction pathways for acetone isomerization. The isomerization of acetone will result in very different fragments during interaction with photons or electrons, as observed in the experiments [5,6]. In general, acetone molecule can dissociate to CH3, CH3CO, CO fragments. After the internal conversion of photoexcitation energy, vibrationally excited acetone molecule can isomerizes to its various isomers before the dissociation takes place. Therefore, this will lead to different bond cleavage processes. The observation of the different fragments can be taken as the evidence of the isomerization of the molecules. The dissociation energies for different possible isomers of acetone have been calculated in the same methods described in Section 2. From the products P1,
P2, P3, P30 , and the intermediates INT1, INT3, INT4, INT5, the dissociation can occurs as follows. For example, there are eight dissociation channels in which the fragments C2H3 and CH3O can be produced, which are INT1 ðCH3 – C – O – CH3 Þ
110:17 kcal=mol
!
CH3 C
þ OCH3
ð1Þ
INT3 ðCH3 – CH – O – CH3 Þ þ OCH3 INT4 ðCH3 – C – O – CH3 Þ
100:91 kcal=mol
!
CH3 C ð2Þ
102:15 kcal=mol
!
CH3 C
þ OCH3
ð3Þ
P1 ðCH2 yCH – O – CH3 Þ ðtransÞ
99:24 kcal=mol
!
CH 2 CH þ OCH3
ð4Þ
P2ðCH2 yCH – O – CH3 Þ ðcisÞ
101:96 kcal=mol
!
CH2 CH
þ OCH3 P3 ðCH2yCH – CH2OHÞ
ð5Þ 96:75 kcal=mol
!
CH2 CH
þ CH2 OH
ð6Þ
P30 ðCH2yCH – CH2OHÞ þ CH2 OH
95:19 kcal=mol
INT5 ðCH3 – C – CH2 OHÞ
75:64 kcal=mol
þ CH2 OH
!
CH2 CH ð7Þ
!
CH3 C ð8Þ
From the energy consideration, the possibility of performing the channels is in the order (8), (7), (6), (4), (2), (5), (3) and (1) from high to low. Ion fragments may come from the ionization of those neutral fragments. Thus, the possible dissociation fragments C2H3 may be from CH3C and CH2CH, and CH3O may be from CH3O and CH2OH. Besides, if P4 dissociates one can find the fragments COH, CH3, CH2, etc. The dissociation of intermediate INT2 can surely produce the fragments CH3 and C2H3O, etc. The fragments with the highest probability may be CH2OH and CH2CH due to the most competitive pathway (4).
X.S. Xu et al. / Journal of Molecular Structure (Theochem) 638 (2003) 215–228
4. Conclusions We have investigated the isomerization reaction process of acetone molecule using the B3LYP and QCISD(T) methods. Five energetically accessible reaction pathways from the electronic ground state have been obtained. The products are methyl vinyl ether (cis- and trans-), allyl alcohol and propen-2-ol. In five pathways there are two channels that the isomerization product is cis-methyl vinyl ether and four channels pass through the isomerization reaction process from acetone to propylene oxide except the isomerization process from acetone to trans-methyl vinyl ether. Obviously, propylene oxide is an important intermediate in isomerization process of acetone molecule. The isomerization process from acetone to transmethyl vinyl ether was proposed via two transition states and one intermediate. Because of the two higher barriers the realization of this process may be difficult. Although the two reaction channels connecting acetone with cis-methyl vinyl ether all include three transition states and two intermediates with the same process from acetone to propylene oxide, the process from propylene oxide to cis-methyl vinyl ether are different with one process including one C – C bond cleavage and one H-atom migration while the other process including one C – C bond cleavage and two Hatoms migration. Because the energies of transition states in path (2) are lower than that in path (3), path (2) is more competitive. The forming process from acetone to allyl alcohol (P3) proposed in this paper is the simplest one which need to pass through only one transition state after the process from acetone to propylene oxide, and the product P3 can isomerize itself easily with a very low energy barrier (2.78 kcal/mol). Another isomerization reaction process calculated is the process from acetone to propen-2-ol. Also it needs to pass through three transition states and two intermediates, which the first step is still the process from acetone to propylene oxide and the next two steps are C – C bond cleavage with H-atom migration and hydroxyl migration, respectively. Due to the three higher energy barriers this isomerization reaction process may be difficult to complete. Judged by the number of transition states and the height of energy barriers the reaction channel from acetone to allyl alcohol is the most feasible one. Based on
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the isomerization reaction channels calculated, we infer that the possible fragments are CH3C, CH2CH, CH3O, CH2OH, COH, CH3, CH2, CH3, C2H3O, etc. Surely, the fragments with the highest probability may be CH2OH and CH2CH because allyl alcohol is the most possible isomerization product. It is hoped that our results may stimulate future studies on the acetone molecule and gain a deep understanding of the complex isomerization reaction mechanism.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 19574018, 19925416 and 10174026).
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