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Experimental and Theoretical Study of the Kinetics and Mechanism of the Reaction of Cl Atoms with CHCHClCH and CDCDClCD 3

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Dariusz Sarzynski, #ukasz Fojcik, and Zdzislaw Latajka J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10031 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry

Experimental and Theoretical Study of the Kinetics and Mechanism of the Reaction of Cl Atoms with CH3CHClCH3 and CD3CDClCD3

Dariusz Sarzyński*,a, Łukasz Fojcikb, Zdzisław Latajkab a

Faculty of Pharmacy, Wroclaw Medical University, ul. Borowska 211A, 50-556 Wrocław, Poland

b

Faculty of Chemistry, University of Wroclaw, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland

Corresponding author – Dariusz Sarzyński, DSc, email: [email protected] Phone: +48 717840642 Author – Łukasz Fojcik, MSc, email: [email protected] Author – Zdzisław Latajka, Prof., email: [email protected]

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ABSTRACT The overall rate constants for H-abstraction (kH) from CH3CHClCH3 and D-abstraction (kD) from CD3CDClCD3 by chlorine atoms in the temperature range of 298-528.5 K were determined and are described by the expressions: kH = (3.52 ±0.21)×10-11 exp(-184 ±19/T) cm3molecule-1s-1 and kD = (1.91 ±0.16)×10-11 exp(-185 ±31/T) cm3molecule-1s-1 respectively. The results of the experiment show that the value of the kinetic isotope effect (kH/kD) for the overall rate constants is temperature independent and is equal to 1.85 ±0.17. A theoretical examination of these reaction mechanisms revealed some unusual properties, such as negative values of the activation energy for the H-abstraction reaction from the secondary carbon atom. Moreover, it was proved that in the radical process of H-abstraction from the primary carbon atom of 2-chloropropane the created R-Cl···Cl complex is the most stable structure responsible for the value of the activation energy of this transformation.


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1. INTRODUCTION 2-chloropropane may be released into the environment during manufacture or use of various products, because it is used as a solvent and an intermediate in the production of chemicals. For this reason, the results of the experiment at 298 K should be useful in atmospheric chemistry. High temperature results may be useful for combustion chemistry. The kinetics of the reaction of 2-chloropropane with chlorine atoms: CH3CHClCH2 + HCl CH3CHClCH3 + Cl →

(1) CH3CClCH3 + HCl

was the subject of three experimental studies.1-3 The overall rate constant for H-atom abstraction is designated as kH. The data acquired in the experiments are consistent, but the kH values were determined only at ambient temperature. Donaghy et al.1 determined it at 298 K. Tyndall et al.2 and Le Crane et al.3 determined kH at the temperature of 296 K. In this paper, we present measurements of the overall rate constants for the hydrogen abstraction reaction CH3CHClCH3 + Cl, using the relative rate method. The experiment was performed in the temperature range of 298-528.5 K to determine the temperature dependence of the rate constants and the value of the activation energy. The reaction of the entirely deuterated 2-chloropropane with chlorine atoms: CD3CDClCD2 + DCl CD3CDClCD3 + Cl →

(2)

CD3CClCD3 + DCl was also studied in the same temperature range. The overall rate constant for the reactions (2) is referred to as kD. Independent measurements of the relative rate constant ratios for the reactions (1) and (2) at the same temperatures enable determination of the kinetic isotope effect (KIE) defined by the kH/kD ratio. KIE values can provide useful information on the stable isotope composition of organic compounds in the atmosphere.4,5



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We also present the theoretical investigations of hydrogen and deuterium atom abstraction by the chlorine atom from the molecule of 2-chloropropane. This type of reaction is more complex than indicated in the expressions (1) and (2). Our previous calculations6 regarding chloroethane and 1,2-dichloroethane did not confirm the intricate mechanism, but theoretical studies of the reaction of chloromethane7 of the same type clearly indicate that in these processes the complex with the noncovalent R-Cl…Cl interaction can be formed as the first step, followed by a transitional state and, finally, the obtaining of the products. The intermediate structure is probably very stable and has a significant impact on the course of the reaction.

2. EXPERIMENTAL and COMPUTATIONAL DETAILS The gas phase reactions of chlorine atoms with 2-chloropropane and fully deuterated 2-chloropropane were investigated, using a reaction with ethane as a reference at 298-528.5 K. The system and procedure used in the experiments were the same as those applied previously.8 Hence, only a brief description is given here. The concentration ratio of CH3CHClCH3 and CD3CDClCD3 to C2H6 was around 1:1 in every experiment. All experiments were carried out at a total pressure of 100 Torr, with a partial pressure of Cl2 varying from 4.7 to 5.2 Torr (0.8×1017-1.7×1017 molecule cm-3). The partial pressure of CH3CHClCH3, CD3CDClCD3 and C2H6 was in the range of 1.9 to 2.1 Torr (0.3×1017-0.7×1017 molecule cm-3). To obtain the highest possible Cl atom concentration, Cl2 photolysis at 330 nm was used as a source of Cl atoms. At this wavelength, the molar extinction coefficient for Cl2 is the highest.9 The irradiation time (2 s to 15 min) and band width (10, 15 or 20 nm) were varied, depending on the reaction temperature, to achieve appropriate conversion of the reactants. The reactant conversion varied from 3% to almost 55% for the reaction CH3CHClCH3 + Cl, 3%-50% for CD3CDClCD3 + Cl and 5%-96% for C2H6 + Cl. The calibrations before and after the experiments of the peak areas of the reactants (CH3CHClCH3, CD3CDClCD3 and C2H6) vs. their partial pressure were determined by GC analysis. The minimal stated purity of the reactants used: CH3CHClCH3 (>99.5%) Fluka, CD3CDClCD3 (>99.4%) CDN Isotopes, C2H6 (>99.97%) Fluka, Cl2 (>99.5%) Aldrich, N2 (>99.999%) BOC.


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Theoretical calculations at the DFT level of theory with the ωB97X10 exchange correlation functional and MP211-15 method were used in a modelling of the H-abstraction reaction from 2-chloropropane by atomic chlorine and for studies of the kinetics of this chemical process. All results were produced by means of the GAUSSIAN 0916 program package. In every case, irrespective of the method, the aug-cc-pVDZ17 basis set was applied. The investigations included several steps. Firstly, the dissociation energies of the C-H bond in the above-mentioned chloroalkane model were compared. The hydrogen atom was eliminated both from the CHCl- and -CH3 group. Based on these results, it was decided to perform a scan of potential energy18 and thus to find the transition state points. During the next step, the height of the potential energy barriers obtained from the Intrinsic Reaction Coordinates (IRC) procedure were compared.19-21 Finally, using the calculated thermochemical data at temperatures 298.15 and 528.5 K, the rate constant of the studied reactions was determined. Every geometrical structure of the studied systems was fully optimized in a vacuum. The nature of the stationary points on the potential energy surface was confirmed by the values of the vibrational frequencies. As in the case of the experiment, the entirely deuterated systems were also studied theoretically.

3. RESULTS and DISCUSSION 3.1. Experimental kinetic studies of the reaction of CH3CHClCH3/CD3CDClCD3 with Cl atoms The kinetics of the reactions under investigation has been analyzed using the relative rate method. The experimental approach is based on the competition between two reactants reacting with the same reactive compound or radical. H-abstraction from ethane by chlorine atoms can be described by the reaction:

C2H6 + Cl → C2H5 + HCl

(3)

and has been used as the reference reaction. If two reactants CH3CHClCH3 (or CD3CDClCD3) and C2H6 react with a chlorine atom, and assuming that the reaction with chlorine atoms is the only significant loss for both reactants, the following equation can be derived: 5 ACS Paragon Plus Environment


ln

[CX 3 CXClCX 3 ] 0 k X [C H ] ⋅ ln 2 6 0 = [CX 3CXClCX 3 ] t k3 [C 2 H 6 ] t

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(4)

where k3 denotes the rate constant for the reaction (3), kX is the overall rate constant describing the decay of the reactant CX3CXClCX3 (X = H or D), and the subscripts 0 or t indicate the concentration of the reactants at the beginning of the experiment and at a given time t, respectively. The kX/k3 relative rate constant ratio can be derived from the slope of the plot ln([CX3CXClCX3]0/[ CX3CXClCX3]t) vs. ln([C2H6]0/[C2H6]t), and the value of the rate constant kX can be calculated when the temperature dependence of k3 is known. The reference reaction C2H6 + Cl has been a subject of many reviews, theoretical and experimental studies22,23, but only three experimental studies cover the temperature range used in the present work. In 1980, Lewis et al.24 studied this reaction in the temperature range of 220-604 K using the DF-RF method. In 1997, Pilgrim et al.25 studied this reaction in the temperature range of 292-600 K using the PLP method. In 2003, Bryukov et al.26 studied this reaction in the temperature range of 299-1002 K using the DF-RF method. In our calculations, we used the temperature dependence of the rate constant of the reaction C2H6 + Cl determined by Bryukov et al.26 Therefore, we used the expression derived in that study:

k3 = 4.91 × 10-12 T0.47 exp(-82/T) cm3molecule-1s-1

(5)

The tests for possible photolysis, thermal decomposition and wall losses of the organic reactants were conducted in exactly the same way as in ours previous study.8 No photolysis, thermal decomposition or wall losses were recorded. Only at the temperature of 450 K (for times >5 min), were some amounts of photolysis products observed, but the Cl atom concentration was not high enough to perform an accurate quantitative analysis. At 528.5 K, thermal decomposition of Cl2 was discovered. At that temperature, in some experiments with a low conversion of the reactants, thermal decomposition of Cl2 was used as a source of Cl atoms.


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Experiments were performed to establish the correlation between product formation and light intensity. Radiation intensity was decreased by changing the slit width and introducing a mesh. In all cases, the experimental points fulfil the linear correlation (see Figure 1), even for the highest conversion of the reactants. It can be concluded that the reaction of Cl atoms with CH3CHClCH3, CD3CDClCD3 and C2H6 is the only source of the disappearance of the reactants. The kinetics of the reactions CH3CHClCH3 + Cl and CD3CDClCD3 + Cl has been investigated at eight temperatures within the range of 298-528.5 K, and at the constant total pressure of 100 Torr. To the best of our knowledge, it is the first determination of the temperature dependence of the rate constant of the reaction CH3CHClCH3 + Cl. Figure 1 shows the kinetic data obtained from the experiments performed at 450.5 K, plotted according to Eq. (4). The results of the measurements have been analyzed using the least squares procedure, which allows a zero point offset. The results of the kinetic measurements performed at 298 K, 301 K, 316.5 K, 339 K, 383 K, 450.5 K, 486.5 K and 528.5 K are collected in Table 1. The Arrhenius plot for the reaction CH3CHClCH3 + Cl is shown in Figure 2. The temperature dependence of the overall rate constant kH in the temperature range of 298-528.5 K can be expressed as:

kH = (3.52 ±0.21) × 10-11exp(-184 ±19/T) cm3molecule-1s-1

(6)

with 2σ error limits. Our ambient temperature value of kH = (1.97 ± 0.06) × 10-11 cm3molecule-1s-1 for the measurements at a total pressure of 100 Torr (see Table 1) closely agrees with the value of kH = (2.01 ±0.30) × 10-11 cm3molecule-1s-1 as measured by Tyndall et al.2 and the values previously determined by Donaghy et al.1 at 298 K (2.01 ±0.60) × 10-11 cm3molecule-1s-1. The value of the rate constant kH = (2.19 ±0.31) × 10-11 cm3molecule-1s-1 at 296 K determined by Le Crane et al.3 is 10% higher than that determined by us at 298 K. Figure 2 also compares our experiment-based determinations of kH in the temperature range of 298528.5 K with the previous measurements by Donaghy et al.1 at 298 K, by Tyndall et al.2 at 296 K and by Le Crane et al.3 at the temperature of 296 K. Unfortunately, there are no other temperature dependences of kH



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which can be compared with our results. The Arrhenius’ analysis of our results give the activation energy of 1.53 kJ·mol-1 for reaction (1). The temperature dependence of the kD rate constant for the reaction of deuterated 2-chloropropane with chlorine atoms has also been included in our experimental investigation. All experimental data are presented in Table 1. A kinetic analysis of the experimental data of entirely deuterated 2-chloropropane with chlorine atoms leads to the Arrhenius expression:

kD = (1.91 ±0.16) × 10-11 exp(-185 ±31/T) cm3molecule-1s-1

(7)

with 2σ error limits, in the temperature range of 298-528.5 K. The values of kD determined experimentally and the Arrhenius line plotted according to Eq. (7) are shown in Figure 2. The calculation of activation energy on the basis of our Arrhenius expression (7) gives the value of 1.54 kJ mol-1 for reaction (2). The measurements of the ratios kH/k3 and kD/k3 were done at the same temperatures. The experiment allowed the KIE values to be calculated directly on the basis of the results of the experiments. All these values are also presented in Table 1. The highest KIE value, at 2.00, has been obtained at 450.5 K. The lowest KIE value, equal to 1.70, has been obtained at 528.5 K. The value of KIE changes within a very narrow range (see Table 1). The difference between the highest and the lowest values of KIE equals to 0.30 and is comparable with 2σ statistical error with the determined values of KIE. The values of 2σ statistical error varies within the range of 0.14-0.24 (Table 1). As it follows from Figure 2, it is also clear that the Arrhenius relationships for the reactions CH3CHClCH3 + Cl and CD3CDClCD3 + Cl are parallel. According to these considerations, it is postulated that the value of KIE is independent of temperature in the range of 298-528.5 K. The value of KIE is equal to 1.85 ±0.17 and was calculated as the average of all our KIE determinations within the temperature range of 298-528.5 K. In our previous study6,8,27-33 of all the investigated reactions, a stronger or a weaker temperature dependency of KIE was observed. But in a study of the reaction kinetics of the reactions Cl + CH3CHClCH3/CD3CDClCD3 no temperature dependency of KIE was found. In order to better emphasise 8 ACS Paragon Plus Environment

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the differences, Table 2 presents all of the experimentally determined KIE values for the reactions studied in our laboratory. Table 2 presents the EH activation energies for the reactions of halogenated hydrocarbons with chlorine atoms and their fully deuterated substitutes ED. The last column of Table 2 contains the calculated values of ED – EH. The ED – EH difference is a very good indicator of the temperature dependence of KIE. For higher values of ED – EH, the KIE relationship is more temperature dependent. If the ED – EH is equal to zero, the value of KIE is constant within the studied temperature range. 3.2. Two possibilities regarding hydrogen atom abstraction As might be expected, the typical radical reaction of hydrogen atom abstraction from halogenoalkane should have the course that can be expressed by the following equations: CH3CHClCH3 + Cl → CH2CHClCH3 + HCl

(8)

(CD3CDClCD3 + Cl → CD2CDClCD3 + DCl) CH3CHClCH3 + Cl → CH3CClCH3 + HCl

(9)

(CD3CDClCD3 + Cl → CD3CClCD3 + DCl) In this approach, the elementary step is related to the energy barrier depending on the geometrical and energetic parameters of every reaction component of the transition state. However, as part of the study, also a three-step process was considered both for H-atom abstraction from the primary and secondary carbon atom of 2-chloropropane. The first and the most important step of the reaction is a complex creation in which the chlorine atom bonded to the secondary carbon atom is connected via noncovalent interaction with the chlorine atom approaching the molecule. Hence the equations (8) and (9) can be written as: CH3CHClCH3 + Cl → CH3CH(Cl···Cl)CH3 → CH2CHClCH3···HCl → CH2CHClCH3+HCl

(10)

(CD3CDClCD3 + Cl → CD3CD(Cl···Cl)CD3 → CD2CDClCD3···DCl → CD2CDClCD3+DCl) CH3CHClCH3 + Cl → CH3CH(Cl···Cl)CH3 → CH3CClCH3···HCl → CH3CClCH3+HCl

(11)

(CD3CDClCD3 + Cl → CD3CDCl···Cl)CD3 → CD3CClCD3···DCl → CD3CClCD3+DCl)

where the second component of the equations is a complex which goes into the transition state of every reaction. This approach was also proposed in7, where some reactions of halogenomethanes were studied. 3.3. Density functional theory problem 9 ACS Paragon Plus Environment


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The calculations were carried out using two computational techniques. The first of them is a method based on the DFT theory – the ωB97X exchange-correlation functional that previously gave very reliable results, comparable with the experiment performed for the reaction of H-abstraction from chloroethane and 1,2-dichloroethane.6 It must be emphasized that for the reaction (9) examined at the ωB97X level, it was not possible to obtain a correct transition state structure. Figure 3 illustrates the problem. For a potential energy curve generated for a scan at the ωB97X level, as well as for the M0634 functional it is not possible to find the transition states. Because of these problems, no further attempts regarding the use of the method based on the density functional theory were made. All the results presented below were obtained using the MP2 method.

3.4. Geometrical parameters of the investigated systems The following designations were used for individual energetic states. The MIN 1 corresponds to a system in which the hydrogen atom is bonded with the carbon atom from the group -CH3 or -CHCl- and the chlorine radical is moved away. The second state, MIN 2, shows the products of a 2-chloropropyl radical and an HCl molecule. The symbol TS means the structure of the transition state. The schematic molecular systems obtained in the molecular model are presented in Figure 4, whereas Table 3 contains their geometrical parameters of the greatest importance to the investigated reactions. Figure 5 refers to the optimized geometrical structure of the CH3CH(Cl···Cl)CH3 complex. The structural and energetic data for the minima and the transition states were obtained by optimizing every system of endpoints on the IRC plots. As follows from Table 3, the values of reaction (9) and its deuterated analogue are the same as in the case of the reaction (11) including the R-Cl···Cl complex formation. On the other hand, the parameters for the process (8) are different than those for the reaction (10). As it turned out, the values of the bond distances and the bond angle of the MIN1 structure obtained in the IRC procedure for the reaction (10) are the same as in the case of the CH3CH(Cl···Cl)CH3 complex. It can be postulated that the complex with interatomic interactions is the most stable system on the potential energy curves of the reaction (10) and can exist in the experimental system. Finally, two separate reactions can occur: one without creating the R-Cl…Cl complex 10 ACS Paragon Plus Environment

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and one with the creation of the complex. The geometrical parameters of H-abstraction from the secondary carbon atom by a chlorine atom indicate that CH3CH(Cl…Cl)CH3 is not involved in the process. Thus, the reaction (11) could have the form of the reaction (9) and the above-mentioned intermediate molecule can occur as a separate point on the potential energy surface. The calculations indicate interesting changes to the R-Cl bond. Depending on which hydrogen atom was removed, the length of the R-Cl bond can increase or decrease. Thus, in the case of the reaction (9) the bond is 1.823 Å or 1.749 Å long for MIN 1 and for MIN 2 respectively. This value in TS is 1.800 Å. The same tendency can be observed for the reaction (11). The reverse trend was observed for the reaction (8). Here the distance R-Cl is 1.825 Å in the MIN 1 state, increasing up to 1.851 Å in MIN 2. Moreover, the bond in TS is 0.033 Å longer than in the previously mentioned process. For the reaction (10), the calculated values had a length equal to 1.835 Å in MIN 1 (it also refers to the structure of the R-Cl···Cl complex), 1.878 Å in MIN 2 and only 1.812 Å in TS. Table 3 also indicates that the transition states appear at a very short distance C…H, which is about 1.3 Å and in the case of the reaction(9) even 1.15 Å. The above figures were confirmed by a PES scan. 3.5. Theoretical studies into energy and the reaction pathway The first energy parameter under consideration was the value of dissociation energy. Table 4 presents the bonds and dissociation energies related to H/D-abstraction. The data show that the difference of Ediss (∆Ediss) between the reactions (8) and (9) is equal to 3.46 kcal/mol in the case of the nondeuterated systems. This is almost the same as for reactions with isotopic substitution (3.49 kcal/mol). These dissociation energies suggest a slightly greater stability of the C-H/D bond in the -CH3 or -CD3 group and can indicate the highest potential energy barrier in the reaction (8). Unfortunately, based on these results, it is impossible to predict the properties helpful in illustrating the complete mechanisms of the reaction. One of the most important steps aimed at explaining the results of the experiments and determining the correct values of the reaction rate constant is to a perform potential energy scan – illustrating H-abstraction from the -CH3 and -CHCl- groups of 2-chloropropane. The scan curves for the reactions (8) and (9) are presented in Figure 6, whereas for the CH3CH(Cl···Cl)CH3 complex – in Figure 7. For every plot, the



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energy values were converted in respect of the lowest energetic point designated as zero and at every step of the scan the geometrical structures of the complexes were fully optimized. The scan calculations for the processes (8) and (9) were carried out for the MIN 2 structure as the starting geometric structure, because the optimized system in this energetic form is easier to obtain than MIN 1, in which it is difficult to predict the position of the chlorine atom attacking the 2-chloropropane molecule. For this reason, the plots in Figure 6 show the change in system energy as a function of the C···H distance. The situation is different in the case of a simulation of the reactions (10) and (11). Here, the starting geometric structure corresponds to the CH3CH(Cl···Cl)CH3 complex. Thus, the change in the distance between the chlorine atom (interacting with the 2-chloropropane molecule) and the proper hydrogen atom which is going to be removed was studied. Because of this, the shapes of the curves related to the removal of the same hydrogen atom, for example in the reactions (8) and (10) as well (9) and (11), are not similar. Another important phenomenon is a certain disruption on the plots in Figure 7, especially in the case of the reaction (11). An additional maximum of energy between the distances of 2.2 Å and 2.4 Å is closely related to a sudden shift of the chlorine atom on the other side of the system, near the removed hydrogen atom. This forces rapid rupturing of the interaction R-Cl···Cl and the creation of a noncovalent interaction Cl…H-R. On the basis of the curve for the process (11), it can be assumed that the starting complex structure may be any point outside the main reaction path. Analysis of the curve for the terminal H-atom abstraction presented in Figure 6 and 7 indicates that the reactions (8) and (10) cannot be the same and that the CH3CH(Cl…Cl)CH3 complex may play an important role. It is worth noting that the maximum of energy is about two times higher for the process (10) than for (8). The most important observation resulting from the scan-based calculations is that it is possible to find the transition states of the investigated processes. The transition states were found and the correctness of the obtained structures was confirmed by one imaginary vibrational frequency denoted as a negative number, calculated for every system after the TS searching procedure. For the reaction (8), this value was equal to 950.17 cm˗1 and for the deuterated complexes was equal to -719.79 cm˗1. While in the case of the structure of a complex corresponding to the reaction (9), the imaginary frequency value is -150.83 cm˗1, in the case of the process with isotopic substitution it is ˗148.02 cm˗1. On the other hand, this value for TS of the process 12 ACS Paragon Plus Environment

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(10) is -1,005.85 cm˗1 (-767.06 cm˗1 for D-substituted) and for the process (11) is identical to that for (9). Another confirmation of the identity of the transition states of the last two transformations, with the exception of the frequencies, are the same geometrical parameters collected in Table 3 and the same total energies of TS. As described above, the TS geometric structure of the reaction (10) is completely different from that obtained from the process (8) and the total energy is about 2.1 kcal/mol higher for the TS from (10). A search was conducted for the connection between every minimum on PES with TS. This procedure allows the reaction course to be understood and its entire mechanism to be described. Thus, the next step was to carry out the IRC (Intrinsic Reaction Coordinate) procedure for all component reactions of total Habstraction from 2-chloropropane. The pathway searching results are presented in Figures 8 and 9. The calculations were performed in both directions, starting from the point corresponding to the optimized system of the TS state. The plots drafted for all of the processes allow one to assume that the concerned radical reactions consist of two stages. The first stage is the formation of TS structures. The second is a decay of the transition complexes and a conversion to the MIN 2 energetic state that includes hydrogen chloride and a proper 2-chloroalkyl radical. In the case of deuterated modelled systems the curves for every reaction are identical. Considering the IRC curves, it can be seen that in the case of the reactions (8) and (10), the shapes of the plots are very similar, but as previously presented by the scan procedure, the barrier of potential energy with respect to each of the minima is much bigger in the process including CH3CH(Cl···Cl)CH3 complex formation. Figure 9 confirms that the H/D-abstraction from the middle carbon atom is independent of RCl…Cl system creation. Most of all, the IRC procedure allows accurate MIN 1 and MIN 2 with structural and energetic parameters to be obtained. For every endpoint of the plots just described, the structure was additionally optimized, because the IRC method may not terminate in a real energetic minimum of the molecular systems. In order to understand the reaction mechanisms, it is necessary to analyse the height of the potential energy barrier responsible for the activation processes. Table 5 shows the calculated values of the barriers for MIN 1 and MIN 2 for each individual 2-chloropropane H/D-abstraction reaction. 13 ACS Paragon Plus Environment


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Table 5 presents these quantities without appropriate VZPE amendments to the total energy of the systems. Hence, the values of ∆E are the same for both isotopic substituted systems and for molecules without the deuterium isotope with respect to the defined process. Taking into account the reactions (8) and (10), the IRC plots describe the activation energies quite well, whereas the comparison of ∆EMIN 2 with the values on the plots indicates that this procedure understated the barrier heights. The correct energies were only obtained by the optimization after the IRC procedure. A clear situation exists in the case of the reactions (9) and (11). The identical values of activation energy and ∆EMIN 2 confirm decisively that the two processes are essentially the same and entirely exclude the CH3CH(Cl···Cl)CH3 complex from the reaction pathway. This structure can obviously exist, but only as a separate point on the potential energy surface. The difference between ∆EMIN 2 and ∆EMIN 1 with respect to the chemical transformations (8) and (10) is equal to -2.04 kcal/mol and -3.00 kcal/mol respectively, and for the reactions (9) and (11) this value is 3.89 kcal/mol for both cases. Without taking into account the VZPE correction for individual energetic states, it can be concluded that the reactions (8) and (10) are endoenergetic. On the other hand, the reactions (9) and (11) are exoenergetic. Experiments are always based on real compounds. Theoretical considerations of reaction mechanisms and all computations leading to acquiring appropriate rate constant values require that all additional energetic and thermochemical parameters be included. This allows experimental and theoretical data to be compared and modelled equivalents of chemical systems to be described. Therefore, the data in Table 5 refer to the same barriers as those in Table 4, but this time the ∆VZPE correction is taken into account. First of all, considerable differences with regard to the same reaction were observed. A good example illustrating this is the process (8). In the case of a deuterated system, ∆EMIN 1 is over twice as large as in the case of a nondeuterated compound. It is also worth noting that this reaction, independent of the isotope, is not endoenergetic, as previously believed, whereas for the approach including R-Cl…Cl complex formation (process (10)) the difference between ∆EMIN

1

is also greater than in the case of D-abstraction (1.41

kcal/mol), and the position of MIN 2 is lower than that of MIN 1(∆EMIN 2˗∆EMIN 1=˗0.14 kcal/mol). Without isotopic substitution, the reaction (10) has MIN 1 greater than MIN 1 (∆EMIN 2˗∆EMIN 1=0.80 kcal/mol). 14 ACS Paragon Plus Environment

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Despite the fact that MIN 1 and MIN 2 are close to each other for either process of the reaction (10), a significant effect caused by isotopic substitution will be noticed. An interesting phenomenon occurs in the case of the reaction (9) = (11). The activation energy value for H-abstraction is negative. The main observable consequence of this state of affairs is a decrease in the reaction rate constant, with a simultaneous increase in temperature. As discussed further on, the rate constant for this (nondeuterated) process is lower at higher temperatures. A theoretical analysis of the problems related to negative activation energy of the reaction was described previously35, where the authors attempt to explain these properties, while an example of the reaction type involving this phenomenon was also presented in the papers.36,37 Table 6 collects the calculated differences of the total energy values between the transition states and the MIN 1 values for every investigated process. The results in Tables 6 and 7 take account of ∆VZPE. On the basis of an analysis of the data in Table 6, it can be concluded that the zero values for the processes (9) and (11) constitute another proof of the identity of these reactions, and the different values for the processes (8) and (10) indicate that they are completely separate. Table 7 decidedly illustrates two important things. Firstly, the value of ECH3CH(Cl···Cl)CH3˗EMIN 1 is equal to zero for the process (10), which means that these states are indeed the same, as mentioned above while discussing the scans and the IRC curves. Secondly, in other cases the values are negative – hence the certainty that the stability of a complex with Cl…Cl interaction is greater than MIN 1. This suggests that in some reaction types the complex in question is one of the major factors in discovering their mechanism. It is indeed true in the case of the chloroalkane covered by this study, especially 1-chloroalkane. However, it is not a general rule, as shown on the example of the processes (9) and (11). 3.6. Discussion of the reaction rate constants. Comparison of the theoretical results of the experiments The rate constants were calculated based on the thermochemical data from the vibration frequency output files. An accurate method of obtaining rate constant values was described previously.38 However, for the processes analyzed as part of this study, one change was introduced: proper thermochemical corrections were not made to the total energies of the simple radicals and atoms to designate the free energy of activation (∆‡G°) in the expression: 15 ACS Paragon Plus Environment


, =

∆‡° e °

Page 16 of 38

(12)

but instead were taken account of in the case of whole energetic states, such as TS and MIN 1. The calculated values of the rate constants (for every approach of the Cl atom) at a temperature of 298.15 K and 528.5 K are presented in Table 8. Table 8 also provides the theoretical values of the overall rate constants (it is the sum of the rate constants for every approach of the Cl atom) and the values determined experimentally at the same temperature.

The decrease in the value of the rate constant for hydrogen abstraction from the secondary carbon atom when the temperature is changed from 298 K to 528.5 K is conspicuous. This is a result of a negative value of activation energy, which is equal to -0.36 kcal/mol, after the zero point energy amendment. Despite the fact that the same barrier heights for the reaction with isotopic substitution amount to 0.19 kcal/mol and almost disappear, it is still positive, and hence the resulting tendency for k to increase. Generally, in each case, the reaction rate constant is much lower for D-systems; this is particularly well visible for theoretical results, where the k values are smaller by one, two or even three orders of magnitude than for the corresponding non-substituted systems. It is also interesting that the k quantity increases in the case of the reactions (8) and (10) (for the processes (10), H and D, even by about two orders of magnitude) with an increase in temperature, while the values of the rate constant for the transformation of (9) = (11) remain the same. A comparison of Table 5 and Table 8 enables the relationship between the calculated rate constants and potential energy barriers to be found, but this is not a simple relationship, because these are secondorder reactions. In general, the greater the barrier, the lower the rate constant. For example, the process (9) = (11) is faster, every deuterated process is slower than the corresponding reaction with an H isotope and the transformation of (8) is faster than (10), and so on.

4. CONCLUSIONS The kinetics of H-atom or D-atom abstraction from CH3CHClCH3 or CD3CDClCD3 by Cl atoms was investigated in the gas phase within the temperature range of 298-528.5 K, at a total pressure of 100 Torr. The rate constant for hydrogen or deuterium abstraction by chlorine atoms is described by the expressions: kH = (3.52 ±0.21)×10-11 exp(-184 ±19/T) cm3molecule-1s-1 and kD = (1.91 ±0.16)×10-11 exp(-185 ±31/T) 16 ACS Paragon Plus Environment

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cm3molecule-1s-1 respectively. To the best of our knowledge, these are the first determinations of the temperature dependences of the rate constant of the reactions CH3CHClCH3 + Cl and CD3CDClCD3 + Cl. The ambient temperature value of kH = (1.97 ±0.06) cm3molecule-1s-1 established in this study closely corresponds to the value of kH = (2.01 ±0.30) × 10-11 cm3molecule-1s-1 measured by Tyndall et al.2 and the values previously determined by Donaghy et al.1 at 298 K (2.01 ±0.60) × 10-11 cm3molecule-1s-1. The value of the rate constant kH = (2.19 ±0.31) × 10-11 cm3molecule-1s-1 at 296 K determined by Le Crane et al.3 is only 10% higher than that recorded by us at 298 K. It was also shown that the value of KIE = 1.85 ±0.17 is temperature independent within the temperature range of 298-528.5 K. A theoretical study of radical H/D-abstraction from the molecule of 2-chloropropane was conducted using the quantum chemistry computational method at the DFT and MP2 levels. It was assumed that the abstraction of the hydrogen atom could have a different course depending on which carbon atom it would be removed from. Based on the suppositions7, it was taken into account that this type of reaction might lead to the creation of a complex, where the attacking chlorine atom would interact with the chlorine atom bonded to the secondary carbon atom of 2-chloropropane. Such creation was confirmed by scans, IRC curves, and the other geometrical and energetic parameters. The complex structure was also the starting point for the reaction designed as MIN 1. This only applied to H/D-abstraction from the primary carbon atom. As it turned out, there exist two possibilities regarding this type of abstraction: with and without R-Cl…Cl complex creation, depending on which side of the molecule of 2-chloropropane is attacked by the Cl radical. This kind of H/D abstraction is not possible in the case of the secondary carbon atom. Here, a complex structure can exist, but only as a separate point on the potential energy surface (PES) of the reaction course. Therefore, the reaction (9) is the same as the reaction (11). For H/D abstraction from the secondary carbon, a negative value of the activation energy of H-abstraction and a positive one for D-abstraction were recorded.

Acknowledgments The authors would like to extend their thanks to the Wroclaw Centre for Networking and Supercomputing for providing them with computational time.



Page 18 of 38

References

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(8) Sarzyński, D.S.; Gola, A.A.; Brudnik, K.; Berkowski, R.; Jodkowski, J.T. Kinetic study of the reactions of chlorine atoms with fluoroethane and D-fluoroethane in the gas phase. Chem. Phys. Lett. 2013. 581, 30–35. (9) Seery, D.J.; Britton, D. Continuous Absorption Spectra of Chlorine Bromine Bromine Chloride Iodine Chloride + Iodine Bromines. J. Chem. Phys., 1964, 68, 2263-&. (10) Chai, J.D.; Head-Gordon, M. Systematic optimization of long-range corrected hybrid density functional. J. Chem. Phys., 2008, 128, 084106-084106-15. (11) Head-Gordon, M.; Pople, J.A.; Frisch, M.J. MP2 energy evaluation by direct methods. Chem. Phys. Lett., 1988, 153, 503–506. (12) Saebø, S.; Almlöf, J. Avoiding the integral storage bottleneck in LCAO calculations of electron correlation. Chem. Phys. Lett., 1989, 154, 83–89. (13) Frisch, M.J; Head-Gordon, M.; Pople, J.A. Direct MP2 gradient method. Chem. Phys. Lett., 1990, 166, 275–280. (14) Frisch, M.J; Head-Gordon, M.; Pople, J.A. Semi-direct algorithms for the MP2 energy and gradient. Chem. Phys. Lett., 1990, 166, 281–289. (15) Head-Gordon, M.; Pople, J.A.; Frisch, M.J. Analytic MP2 frequencies without fifth order storage: Theory and application to bifurcated hydrogen bonds in the water hexamer. Chem. Phys. Lett., 1994, 220, 122–128. (16) Frisch, M.J.; et al., Gaussian 09, Revision D.01, Gaussian Inc, Wallingford CT, 2009. (17) Dunning, T.H. Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys., 1989, 90, 1007–1023. (18) Foresman J.B. Frisch, Æ. Exploring Chemistry with Electronic Structure Methods, 2nd ed. (Gaussian, Inc., Pittsburgh, PA, 1996). (19) Hratchian, H.P.; Schlegel, H.B. Accurate reaction paths using a Hessian based predictor-corrector integrator. J. Chem. Phys., 2004, 120, 9918–9924.



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(20) Hratchian, H.P.; Schlegel, H.B. Theory and Applications of Computational Chemistry: The First 40 Years, Ed. Dykstra C.E.; Frenking, G.; Kim, K.S.; Scuseria, G. (Elsevier, Amsterdam, 2005) 195– 249. (21) Hratchian, H.P.; Schlegel, H.B. Using Hessian Updating to Increase the Efficiency of a Hessian Based Predictor-corrector Reaction Path Following Method. J. Chem. Theory and Comput., 2005, 1, 61–69. (22) Atkinson, R. et al., Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume IV - Gas Phase Reactions of Organic Halogen Species. Atmos. Chem. Phys. 2008, 9, 4141– 4496. (23) Sander S.P. et al., JPL Publication 10-6 Evaluation 17, Pasadena, CA, 2011. (24) Lewis,R.S.; Sander, S.P.; Wagner, S.; Watson, R.T Temperature-Dependent Rate Constants for the Reaction of Ground-State Chlorine with Simple Alkanes. J. Phys. Chem., 1980, 84, 2009–2015. (25) Pilgrim, J.S.; McIlroy, A.; Taatjes, C.A. Kinetics of Cl Atom Reactions with Methane, Ethane, and Propane from 292 to 800 K. J. Phys. Chem. A, 1997, 101, 1873–1880. (26) Bryukov, M.G.; Slagle, I.R.; Knyazev, V.D. Kinetics of Reactions of Cl Atoms with Ethane, Chloroethane, and 1,1-Dichloroethane. J. Phys. Chem. A, 2003, 107, 6565–6573. (27) Sarzyński, D.S.; Gola, A.A.; Brudnik, K.; Jodkowski, J.T. Kinetic Study of the Reactions of Chlorine Atoms with Fluoromethane and D-Fluoromethane in the Gas Phase. Chem. Phys. Lett. 2012, 525–526, 32–36. (28) Sarzyński, D.S.; Gola, A.A.; Dryś, A.; Jodkowski, J.T. Kinetic Study of the Reaction of Chlorine Atoms with Chloromethane in the Gas Phase. Chem. Phys. Lett. 2009, 476, 138–142. (29) Gola, A.A.; Sarzyński, D.S.; Dryś, A.; Jodkowski, J.T. Kinetic Study of the Reaction of Chlorine Atoms with Bromomethane and D-bromomethane in the Gas Phase. Chem. Phys. Lett. 2010, 486, 7– 11. (30) Sarzyński, D.S.; Gola, A.A.; Brudnik, K.; Jodkowski, J.T. Kinetic Study of the Reaction of Chlorine Atoms with Dichloromethane and D-dichloromethane in the Gas Phase. Chem. Phys. Lett. 2011, 514, 220–225. 20 ACS Paragon Plus Environment

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(31) Gola, A.A.; Sarzyński, D.; Dryś, A.; Jodkowski, J.T. Kinetic Study of the Reaction of Chlorine Atoms with Chloroform in the Gas Phase. Chem. Phys. Lett. 2009, 469, 250–255. (32) Sarzyński, D.S.; Gola, A.A.; Brudnik, K.; Berkowski, R,; Jodkowski, J.T. Temperature Dependence of the Kinetic Isotopic Effect of the Reaction of Cl Atoms with C2H5Cl between 298 and 550 K. Chem. Phys. Lett. 2012, 554, 20–26. (33) Sarzyński, D.S; Gola, A.A.; Dryś, A.; Brudnik, K.; Jodkowski, J.T. Kinetic Study of the Reaction of Chlorine Atoms with Bromoethane and D-bromoethane in the Gas Phase. Chem. Phys. Lett. 2011, 509, 114–118. (34) Zhao, Y.; Truhlar, D.G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functional. Theor. Chem. Acc., 2008, 120, 215–241. (35) Revell, L.E.; Williamson, B.E. Why Are Some Reactions Slower at Higher Temperatures? J. Chem.Educ., 2013, 90, 1024–1027. (36) Silverstein, T.P. Falling Enzyme Activity as Temperature Rises: Negative Activation Energy or Denaturation? J. Chem.Educ., 2012, 89, 1097–1099. (37) Alvarez-Idaboy, J.R.; Mora-Diez, N.; Vivier-Bunge, A. A Quantum Chemical and Classical Transition State Theory Explanation of Negative Activation Energies in OH Addition To Substituted Ethenes. J. Am. Chem. Soc., 2000, 122, 3715–3720. (38) http://www.gaussian.com/g_whitepap/thermo.htm



Page 22 of 38

Table 1 Measured rate constant ratios kH/k3 and kD/k3, the absolute values of kH and kD, and derived values of kinetic isotope effect kH/kD.

a b c

T (K)

kH/k3a

Number of experiments

1011 × kHc (cm3 molecule-1s-1)

Number of experiments

1011 × kDc (cm3 molecule-1s-1)

kH/kDb

298.0

0.34 ± 0.01

31

1.97 ± 0.06

301.0

0.34 ± 0.01

27

1.94 ± 0.06

0.18 ± 0.01

29

1.04 ± 0.06

1.89 ± 0.16

0.18 ± 0.01

28

1.04 ± 0.06

1.89 ± 0.16

316.5

0.33 ± 0.01

27

1.94 ± 0.05

0.18 ± 0.01

30

1.08 ± 0.06

1.83 ± 0.16

339.0

0.33 ± 0.01

32

1.97 ± 0.06

0.18 ± 0.01

29

1.11 ± 0.05

1.83 ± 0.16

383.0

0.33 ± 0.01

29

2.08 ± 0.06

0.18 ± 0.01

29

1.14 ± 0.06

1.83 ± 0.16

450.5

0.34 ± 0.02

28

2.31 ± 0.13

0.17 ± 0.01

30

1.17 ± 0.07

2.00 ± 0.24

450.5

0.35 ± 0.01

24

2.39 ± 0.07

0.18 ± 0.01

25

1.20 ± 0.07

1.94 ± 0.16

486.5

0.35 ± 0.01

31

2.47 ± 0.07

0.20 ± 0.01

25

1.37 ± 0.07

1.75 ± 0.14

528.5

0.34 ± 0.02

28

2.49 ± 0.15

0.20 ± 0.01

33

1.44 ± 0.08

1.70 ± 0.19

kD/k3a

With 2σ statistical uncertainties. The 2σ statistical uncertainties include the 2σ error of the kH/k3 and kD/k3 ratios. The error was calculated using the total differential method. The 2σ statistical uncertainties include only the 2σ error of the kH/k3 or kD/k3 ratios.


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Table 2 Experimentally determined values of KIE, together with the values of activation energies (EH) for reactions of halogenated hydrocarbons with chlorine atoms and for their fully deuterated substitutes (ED). Experimental values of KIE

Reaction

298 K

316 K

325 K

327 K

329 K

337 K

338 K

381 K

383 K

EH 385 K

448 K

450 K

486 K

ED

ED-EH

(kJ·mol )

(kJ·mol-1)

3.3±0.2

9.6

13.3

3.7

[27]

2.9±0.2

8.6

12.2

3.6

[28]

528 K

-1

Ref.

(kJ·mol )

527 K

-1

CH3F+Cl CD3F+Cl

6.2±0.4

6.0±0.4

CH3Cl+Cl CD3Cl+Cl

5.4±0.3

5.2±0.5

CH3Br+Cl CD3Br+Cl

6.5±0.4

6.0±0.5

4.8±0.2

3.7±0.1

2.9±0.1

8.6

13.3

4.7

[29]

CH2Cl2+Cl CD2Cl2+Cl

3.8±0.2

3.6±0.3

3.0±0.2

2.6±0.1

2.3±02

7.9

10.7

2.8

[30]

CHCl3+Cl CDCl3+Cl

4.7

2.2

9.6

13.8

4.2

[31]

C2H5F+Cl C2D5F+Cl

2.7±0.2

C2H5Cl+Cl C2D5Cl+Cl

2.6±0.2

C2H5Br+Cl C2D5Br+Cl

3.2±0.8

CH2ClCH2Cl+Cl CD2ClCD2Cl+Cl

4.0±0.2

CH3CHClCH3+Cl CD3CHClCD3+Cl

1.9±0.2

4.2±0.3

4.2±0.2

3.2

3.2

3.4±0.2

2.9

2.3±0.1

2.5±0.2

2.1

2.2±0.1

2.1±0.1

2.0±0.1

2.6

4.3

1.7

[8]

2.4±0.1

2.2±0.1

2.1±0.1

2.6

3.9

1.3

[32]

3.2

6.2

3.0

[33]

2.7±0.1

6.5

9.5

3.0

[6]

1.7±0.2

1.5

1.5

0.0

This study

2.6±0.4

2.2±0.5

3.4±0.2 1.8±0.2

3.8±0.2

1.8±0.2

2.0±0.2

3.1±0.2

2.7±0.1

1.8±0.2

2.0±0.2

2.0±0.4

1.8±0.2

23

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Table 3 The most important geometrical parameters of investigated complexes obtained for two calculation types. CH2CHClCH3…HCl/CD2CDClCD3…DCl

CH3CClCH3…HCl/CD3CClCD3…DCl

Geometrical structure

RC—H

RH—Cl

ΘCHCl

RC—H

RH—Cl

ΘCHCl

[Å]

[Å]

[˚]

[Å]

[Å]

[˚]

1.102 1.928 1.153

2.679 1.315 1.795

125.1 170.6 160.5

1.102 1.928 1.153

2.679 1.315 1.795

125.1 170.6 160.5

MIN 1 MIN 2 TS

1.099 2.140 1.348

3.076 1.302 1.481

MIN 1 MIN 2 TS

1.099 3.230 1.365

3.054 1.300 1.463

without R-Cl…Cl complex 117.9 168.5 176.9 with R-Cl…Cl complex 120.0 106.2 173.4

Table 4 Calculated C-H bond dissociation energies of all compounds taking part in studied reaction ∆VZPE

Ebond

Ediss

Reaction type

[kcal/mol]

[kcal/mol]

[kcal/mol]

CH3CHClCH3/CH2CHClCH3

8.94

-104.85

-95.91

CD3CDClCD3/CD2CDClCD3

6.54

-104.85

-98.31

CH3CHClCH3/CH3CClCH3

8.71

-101.16

-92.45

CD3CDClCD3/CD3CClCH3

6.34

-101.16

-94.82

Table 5 The values of the potential energy barrier heights for studied reactions and calculated in relative to each of minimum. without VZPE

Reaction type

∆EMIN 1 [kcal/mol]

with VZPE




5.58

3.54

0.98 2.32

2.48 2.91

CH3CHCl(Cl)CH3→CH2CHClCH3+HCl CD3CDCl(Cl)CD3→CD2CDClCD3+DCl

10.45

7.45

5.47 6.88

6.27 6.74

CH3CHClCH3+Cl→CH3CClCH3+HCl CD3CDClCD3+Cl→CD3CClCD3+DCl

1.59

5.48

-0.36 0.19

6.77 6.46

CH3CHCl(Cl)CH3→CH3CClCH3+HCl CD3CDCl(Cl)CD3→CD3CClCD3+DCl

1.59

5.48

-0.36 0.19

6.77 6.46

CH3CHClCH3+Cl→CH2CHClCH3+HCl CD3CDClCD3+Cl→CD2CDClCD3+DCl


Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6 The differences in total energy values between the transition states and between MIN 1states for each type of the studied reactions. Reaction type

∆ETS [kcal/mol]


CH3CHClCH3+HCl→CH2CHClCH3+HCl CH3CHCl(Cl)CH3→CH2CHClCH3+HCl

2.07

-2.42

CD3CDClCD3+Cl→CD2CDClCD3+DCl CD3CDCl(Cl)CD3→CD2CDClCD3+DCl

2.12

-2.44

CH3CHClCH3+Cl→CH3CClCH3+HCl CH3CHCl(Cl)CH3→CH3CClCH3+HCl

0.00

0.00

CD3CDClCD3+Cl→CD3CClCD3+DCl CD3CDCl(Cl)CD3→CD3CClCD3+DCl

0.00

0.00

Table 7 … The differences of the total energies values between the CH3CH(Cl Cl)CH3 complex and each of the MIN 1state of the investigated reactions. Reaction type

∆E(CH3CH(Cl…Cl)CH3-MIN 1) [kcal/mol]

CH3CHClCH3+Cl→CH2CHClCH3+HCl CD3CDClCD3+Cl→CD2CDClCD3+DCl

-2.42 -2.44

CH3CHCl(Cl)CH3→CH2CHClCH3+HCl CD3CDCl(Cl)CD3→CD2CDClCD3+DCl

0.00 0.00

CH3CHClCH3+Cl→CH3CClCH3+HCl CD3CDClCD3+Cl→CD3CClCD3+DCl

-2.43 -2.47

CH3CHCl(Cl)CH3→CH3CClCH3+HCl CD3CDCl(Cl)CD3→CD3CClCD3+DCl

-2.43 -2.47



Page 26 of 38

Table 8 The comparison of the reaction rate constants obtained from theoretical calculations and experiment. Theoretical data Reaction type

11

10 × k (cm molecule-1s-1) 298.15 K 528.50 K 3

CH3CHClCH3+Cl→CH2CHClCH3+HCl CH3CHCl(Cl)CH3→CH2CHClCH3+HCl

8.39 0.0383

21.5 2.93

CH3CHClCH3+Cl→CH3CClCH3+HCl CH3CHCl(Cl)CH3→CH3CClCH3+HCl

148 148

118 118

The overall rate constant for H-abstraction

164.9

166.9

CD3CDClCD3+Cl→CD2CDClCD3+DCl CD3CDCl(Cl)CD3→CD2CDClCD3+DCl

0.822 0.00337

6.10 0.795

CD3CDClCD3+Cl→CD3CClCD3+DCl CD3CDCl(Cl)CD3→CD3CClCD3+DCl

59.4 59.4

73.0 73.0

The overall rate constant for D-abstraction

61.0

86.8

Experimental data 1011 × k (cm molecule-1s-1) 298.0 K 528.5 K 3

1.97

2.49

1.04

1.44


Page 27 of 38

0.8 X=H X=D

ln([CX3CXClCX3]0/[CX3CXClCX3]t)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0.0

0

1

2

3

ln([C2H6]0/[C2H6]t)

Figure 1



k H = (3.52 ± 0.21) × 10−11 exp(−184 ± 19 / T ) ln(k/cm3molecule-1s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

-24.6

CH3CHClCH3 + Cl this study CD3CDClCD3 + Cl this study Tyndall et al. 1997 Danaghy et al. 1993 Le Crane et al. 2004

-24.9

-25.2

k D = (1.91 ± 0.16) ×10−11 exp(−185 ± 31 / T ) 2.0

2.5

3.0

3.5

1000/T

Figure 2


Page 29 of 38

0.6

0.5

E [kcal/mol]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.3

ωΒ97Χ Μ06

0.2

0.1

0.0 0.1

0.1

0.1

0.2

0.2

0.2

0.2

R C...H [Å]

Figure 3



Page 30 of 38

Figure 4


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Figure 5


0.6

0.6

0.5

0.5

0.4

0.4

E [kcal/mol]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

E [kcal/mol]


0.3

Page 32 of 38

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0.1

0.1

0.1

0.2

0.2

0.2

0.2

0.1

0.1

R (C...H ) [Å]

0.1

0.2

0.2

0.2

R (C...H ) [Å]

Figure 6

32


Page 33 of 38

1.0

0.4

0.8 E [kcal/mol]

0.3 E [kcal/mol]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


0.6

0.2

0.4 0.1 0.2 0.0

0.0 0.1

0.1

0.2

0.2

0.2

0.2

0.2

0.3

0.3

0.3

0.1

0.2

R (Cl...H ) [Å]

0.2

0.2

0.2

0.2

0.3

0.3

0.3

R (Cl...H) [Å]

Figure 7

33



0.6 1.0 0.5 0.8

E [kcal/mol]

0.4

E [kcal/mol]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 34 of 38

0.3

0.2

0.6

0.4

0.2

0.1

0.0

0.0 -0.9

-0.7

-0.5

-0.3

-0.1

0.1

0.3

0.5

-2.1

-1.6

-1.1

-0.6

-0.1

0.4

0.9

1.4

react.coord.

react.coord

Figure 8

34



0.6

0.6

0.5

0.5

0.4

0.4

E [kcal/mol]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

E [kcal/mol]

Page 35 of 38

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 -0.8

-0.6

-0.4

-0.2 0.0 react.coord

0.2

0.4

-1.1

-0.9

-0.7

-0.5

-0.3

-0.1

0.1

0.3

0.5

react.coord

Figure 9

35


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 36 of 38

Captions under figures Figure 1. Relative rate data obtained at 450.5 K and a pressure of 100 Torr for the reactions of Cl atoms with CH3CHClCH3 (•) and CD3CHClCD3 (■) using C2H6 as the reference compound.

Figure 2. Arrhenius plots and experimental results for the reactions CH3CHClCH3 + Cl and CD3CDClCD3 + Cl. The error limits represent 2σ. The figure shows also the results of relative experiments of Donaghy et al.1 in 298 K, Tyndall et al.2 at 296 K and Le Crane et al.3 at 296 K. Figure 3. Potential energy curve along C‧‧‧H distance performed at the ωB97X and M06 levels for the reaction of CH3CHClCH3 + Cl → CH3CClCH3 + HCl. As seen above, there is no transition state in the case of both curves.

Figure 4. Schematic geometric structures of modelled molecular systems in their three stationary states. The first row pertain to the H-abstraction reaction from the –CH3 group from 2-chloropropane, whereas the second row presents the abstraction of hydrogen atom from the –CHCl – group.

Figure 5. CH3CH(Cl‧‧‧Cl)CH3 complex structure with basic geometrical parameters.

Figure 6. Potential energy curves along C‧‧‧H distance plotted for the reaction CH3CHClCH3 + Cl → CH2CHClCH3 + HCl (on the left) and CH3CHClCH3 + Cl → CH3CClCH3 + HCl (on the right). The same plots refer to the studied deuterated systems.

Figure 7. Potential energy curves along C‧‧‧H distance performed for the reaction CH3CHClCH3 + Cl → CH3CH(Cl‧‧‧Cl)CH3 → CH2CHClCH3‧ ‧ HCl → CH2CHClCH3+HCl (on the left) and CH3CHClCH3 + Cl → CH3CH(Cl‧ ‧ ‧ Cl)CH3 → CH3CClCH3‧ ‧ ‧ HCl → CH3CClCH3+HCl (on the right). The same plots refer to the studied deuterated systems.

Figure 8. IRC plots for the reaction CH3CHClCH3 + Cl → CH2CHClCH3 + HCl and CH3CHClCH3 + Cl → CH3CH(Cl‧ ‧ ‧ Cl)CH3 → CH2CHClCH3‧ ‧ ‧ HCl → CH2CHClCH3+HCl. The points related with a negative part of x-axis connect the MIN 1 with TS, whereas these on the right from a zero refer to the MIN 2. The same plots also pertain to the studied deuterated systems.

36


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Figure 9. IRC plot for the reaction CH3CHClCH3 + Cl → CH3CClCH3 + HCl (on the left) and CH3CHClCH3 + Cl → CH3CH(Cl‧‧‧Cl)CH3 → CH3CClCH3‧ ‧ ‧ HCl → CH3CClCH3+HCl (on the right). The points related with a negative part of x-axis connect the MIN 1 with TS, whereas these on the right from a zero refer to the MIN 2. The same plots also pertain to the studied deuterated systems.

37


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TOC graphic

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