Quantum Chemical Investigation of the Koch Carbonylation of Methyl

18

Ind. Eng. Chem. Res. 2004, 43, 18-29

Quantum Chemical Investigation of the Koch Carbonylation of Methyl tert-Butyl Ether (MTBE) Ned C. Haubein and Linda J. Broadbelt* Department of Chemical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3120

Richard H. Schlosberg ExxonMobil Chemical Company, Annandale, New Jersey 08801

Methyl tert-butyl ether (MTBE) is a widely used additive in oxygenated gasoline that has recently come under environmental pressure. Consequently, the demand for MTBE is likely to decrease in the future, and new uses for MTBE need to be explored. Production of methyl 2,2dimethylpropanoate (methyl pivalate) via Koch carbonylation chemistry is an attractive option because methyl pivalate has been shown to be a fluid with a much lower ozone-forming potential than many other oxygenated solvents. Semiempirical, Hartree-Fock, MP2, and density functional theory calculations, believed to be the first on this system, indicated that the most energetically difficult step in this chemistry is the decomposition of MTBE to tert-butyl cation and methanol. Specific solvation effects were included in these calculations by inclusion of a small number of explicit solvent molecules, and nonspecific solvation effects were taken into account using a dielectric continuum model. 1. Introduction Methyl tert-butyl ether (MTBE) is a widely used additive in oxygenated gasoline that has recently been identified as a potential health threat to the drinking water supply due to leaking underground storage tanks.1 In response to these concerns, removal of MTBE from gasoline in California by December 2003 was decreed by an executive order of the governor.2,3 Because MTBE will continue to be produced as a byproduct of other processes, it is an attractive candidate as a feedstock for other processes, such as the production of carboxylic acids or esters via carbonylation. These compounds are often used both as intermediates in the synthesis of specialty chemicals and as components in end-use products such as lubricants and paint thinners. An ester of particular interest is methyl 2,2-dimethylpropanoate (methyl pivalate), as it has recently been shown to be a fluid with environmental benefits.4 Methyl pivalate has a much lower ozone-forming potential than common oxygenated solvents such as nbutyl ethanoate (butyl acetate) and butanol, as measured by a maximum incremental reactivity scale.5 Replacing a high ozone-forming fluid with this low ozone-forming equivalent could result in mixtures with a significantly lower environmental impact. Currently, methyl pivalate can be produced via esterification of 2,2-dimethylpropanoic acid (pivalic acid) with methanol. Pivalic acid is produced via Koch carbonylation during a catalytic cycle involving isobutene and high-pressure carbon monoxide in an aqueous strong acid. The mechanism that is generally accepted for this process6 is illustrated in Figure 1. The role of the strong acid (I), represented here as a proton, is to protonate the alkene (II) and form a carbocation (III) * Corresponding author. Phone: 847-491-5351. Fax: 847491-3728. E-mail: [email protected].

Figure 1. Pivalic acid can be produced via the Koch carbonylation of isobutene. The steps involved are protonation of isobutene, carbonylation of tert-butyl cation, methoxylation of the tert-butyl acylium cation, and deprotonation of protonated pivalic acid.

to which carbon monoxide (IV) is added to form an acylium cation (V). The solvent (VI) then adds to the acylium cation to form the protonated acid (VII) which can release a proton (I) to form the carboxylic acid product (VIII). The carboxylic acid product can then be esterified in a separate reaction with methanol to form methyl pivalate and water. While this is a technically feasible route, a more attractive option is a one-step synthesis, as processes with fewer steps are normally preferred on both economic and environmental grounds. Figure 2 shows the postulated mechanism for the Koch carbonylation of MTBE, which produces methyl pivalate without the need for esterification. There are two primary differences in the mechanisms for the carbonylation of isobutene and MTBE. In the case of isobutene, the tert-butyl cation is generated directly via the protonation of isobutene, but generation of this cation during the reaction of MTBE requires protonation of the MTBE and elimination of a methanol

10.1021/ie030139t CCC: $27.50 © 2004 American Chemical Society Published on Web 12/11/2003

Ind. Eng. Chem. Res., Vol. 43, No. 1, 2004 19

are combined to provide an overall picture of the important aspects of this chemistry. 2. Computational Methods

Figure 2. Mechanism for production of methyl pivalate is similar to that for the production of pivalic acid shown in Figure 1. The main difference is the protonation of MTBE and subsequent elimination of methanol needed to produce the tert-butyl cation.

molecule before the tert-butyl cation is formed. Second, in the MTBE carbonylation, isobutene is only present to the extent that it is generated in situ via the de-protonation of tert-butyl cation. This is an important distinction because the key side reaction in this chemistry is oligomerization of isobutene to form higher acids or esters. Controlling the competition between carbonylation and oligomerization is critical in controlling the overall selectivity of the reaction, and one important parameter in determining the isobutene concentration is acid strength. Acids with sufficient strength to generate the required carbenium ion intermediates should be able to catalyze Koch carbonylation of either isobutene or MTBE. Some examples of suitable acids are sulfuric acid, phosphoric acid, hydrofluoric acid, and combinations of these materials with Lewis acids such as BF3, AlCl3, or SbF5.7 Catalysts based on combinations of BF3 and H2O are commonly used in industrial practice because of issues related to catalyst recycle.7 A new catalyst based on BF3 and CH3OH has recently been shown to be effective in the carbonylation of MTBE8 and will be considered in this study. Little research has been published on the carbonylation of MTBE,8-10 and little is known about the energetics which control this chemistry. This lack of experimental information, coupled with the practical difficulties involved in studying reactions catalyzed by very strong acids, makes the investigation of this system by computational means an attractive option. Computational chemistry can be used to make significant contributions in understanding the fundamentals of this chemistry by helping to elucidate the mechanism and mapping out the detailed energetics of the reactions involved. In addition, the study of a multistep, liquid-phase reaction network that is catalyzed by a complex acid provides an opportunity to assess the utility of computational methods in the evaluation of such challenging systems. The aim of this research was to understand the fundamentals of the BF3/CH3OH-catalyzed Koch carbonylation of MTBE. Computational chemistry calculations have been carried out using two different models of the catalyst and using a solvation model to study the reaction in the liquid phase. The results of these studies

Quantum chemical calculations were performed at various levels of theory in the study of this system. Semiempirical calculations were carried out at the PM3 level using both Spartan11 and MOPAC,12 and ab initio calculations were carried out using Gaussian 98.13 Optimized geometries of stable species and transition states for the various steps in the mechanism were found and confirmed by frequency calculations to ensure that only positive vibrational frequencies were present for minimum energy species and only a single imaginary frequency was present for transition state species. Constrained optimizations were used to find initial guesses for transition state geometries, and optimized transition states were shown to connect the reactants and products for the reaction in question by using intrinsic reaction coordinate (IRC) calculations. These IRC calculations also helped to determine the geometries for the intermediate species. Finally, it should be noted that Gibbs free energies were calculated for the species described in this study. The complex nature of the species involved nearly ensures that some hindered rotors are present in the system, but test calculations showed that the error introduced by treating these motions as vibrations was approximately constant among the various species and did not significantly affect the reported free energy changes. Initially, each reaction in the mechanism shown in Figure 2 was characterized at the PM3 level. Electronic energies were computed for the PM3 geometries using the second-order Møller-Plesset method (MP2) and the 6-31G** basis set, and these were combined with thermodynamic corrections from the PM3 method to compute free energies (MP2//PM3). Many semiempirical calculations could be carried out in a short amount of time, and these calculations provided the first data on the system and helped to guide subsequent calculations. Additional optimizations were carried out to determine geometries using the unrestricted Hartree-Fock (HF) method. These geometries were also used to compute MP2/6-31G** electronic energies, which were combined with HF thermodynamic corrections to compute free energies for these species (MP2//HF). Finally, density functional theory (DFT) calculations were performed using the unrestricted BLYP functional and the 6-31G** basis set. These calculations allowed electronic energies that included electron correlation to be evaluated at the same level of theory at which the geometries were determined. The standard ab initio methods detailed above are valid for describing gas-phase reactions but need to be corrected for the presence of a solvent when studying a reaction in the liquid phase. Group additivity methods were considered for this task, but these methods14,15 were not appropriate for the system in question because they are most accurate for stable, neutral species, while the proposed mechanism required solvation energies for transition states and charged species. Instead, a combination of explicit solvent molecules and a continuum solvation method was used to describe the solvation effects. One or two solvent molecules were included in the calculations to capture any specific solvation effects, such as hydrogen bonding, and to allow the solute geometry to respond to the presence of the

20

Ind. Eng. Chem. Res., Vol. 43, No. 1, 2004

Table 1. Overview of Calculations Performed in This Study thermodynamics electronic and energy geometry level level PM3 HF HF HF BLYP a

MP2 MP2 MP2 MP2 BLYP

acid model

solvation method

pathway

CH3OH2+ CH3OH2+ BF3-CH3OH BF3-CH3OH BF3-CH3OH

AM1-SM2 AM1-SM2 PCM PCM PCM

single-sided bi-solvated single-sided bi-solvateda single-sided

Not a good description for tert-butyl; see Figure 17.

solvent. Nonspecific solvation effects, such as cavity formation and dipole interactions, were accounted for by embedding the solute-solvent cluster within a dielectric continuum. Other researchers16 have also implemented a combined method such as this for computing the solvation energy of ions. Readers interested in learning more about continuum solvation methods are referred to an excellent review article on the subject.17 In this study, the type of explicit solvent molecule included depended on the approximation used to model the acid. Two acid models of increasing complexity were used: protonated methanol and BF3-CH3OH. The protonated methanol system was simpler because it contained fewer atoms and did not have a tendency to participate in undesired reactions. The BF3-CH3OH system was more realistic because a negatively charged counterion was present once the proton from the methanol was transferred to the reactants. The AM1-SM218 method was used to compute the nonspecific solvation effects for clusters which did not include a counterion, and the polarizable continuum method (PCM),19 with methanol as the solvent, was used to compute nonspecific solvation effects when a counterion was included. In the case of AM1-SM2, the solvation effect was computed as the difference between the AM1 and AM1SM2 energies at the PM3 geometry. Table 1 provides an overview of the calculations that were performed in this study. An explanation of the pathway column in this table will be provided in the Results section. 3. Results The entire mechanism was studied at several levels of theory using various models for the BF3/CH3OH catalyst, as detailed in Table 1, and these results are presented below. A number of features are common to all of the results and provide insight into the fundamentals of the carbonylation of MTBE. These results are compared with each other and experimental data in the Discussion section. MP2//PM3 (Protonated Methanol). Initial calculations on the carbonylation of MTBE were carried out using semiempirical methods in an effort to learn as much as possible about this system in an efficient manner. Initially, the acid for these calculations was assumed to be a bare proton, but this very simple model led to unrealistic reactions, namely, the elimination of a hydrogen molecule from protonated MTBE, and was deemed unsatisfactory. Instead, the acid was modeled as a protonated methanol molecule, and the entire system carried a net charge of +1. A methanol molecule remained after the protonated methanol transferred a proton to MTBE, and this methanol molecule was retained as a solvent molecule to provide specific solvation interactions. In addition, many of the intermediate species were solvated by a second methanol molecule

Figure 3. Reaction of tert-butyl acylium to protonated methyl pivalate, characterized at the MP2//PM3 level of theory, is shown. Oxygen atoms are shown in black, carbon in gray, and hydrogen in white.

which originated as the methoxy group of MTBE. Each of the reactions shown in Figure 2 was characterized, and the reaction of tert-butyl acylium to protonated methyl pivalate is shown in Figure 3 as an example of a reaction cluster. The reaction proceeds as the proximal methanol molecule in the reactant cluster (Figure 3a) moves closer to the carbonyl carbon, passes through a transition state (Figure 3b), and forms the protonated methyl pivalate product (Figure 3c). The reaction coordinate for this reaction is the distance between the carbonyl carbon on the acylium cation and the oxygen of the methanol molecule, which is 2.85 Å in the reactant cluster. This bond length decreases to 1.65 Å at the transition state, with the second methanol molecule having been drawn closer to maintain its hydrogen bond with the first methanol molecule. Finally, in the product state, the bond length has further decreased to 1.57 Å. This can be compared to a value of 1.37 Å for unprotonated methyl pivalate at the same level of theory. The bond angle between the tertiary tert-butyl carbon, the carbonyl carbon, and the carbonyl oxygen, which begins at 178° in the reactant cluster and moves through a value of 144° at the transition state to a value of 141° for the product, is another measure of the progress of the reaction. The initial value of 178° is close to the expected value of 180° for an acylium cation, and the value of 141° for protonated methyl pivalate can be compared with the value of 128° for unprotonated methyl pivalate


Figure 4. Free energy diagram at 298 K for the reaction of MTBE to methyl pivalate calculated using PM3 geometries and frequencies, MP2 electronic energies, and an AM1-SM2 solvation correction. Neutral species also include protonated methanol, and ionic species also include methanol in the calculations.

at the same level of theory. MP2 single-point calculations, AM1-SM2 solvation calculations, and frequency calculations were carried out on these PM3 geometries to compute the free energies in solution. The other steps in the mechanism were characterized in a similar fashion, and the resulting free energy surface is shown in Figure 4. While each reaction was found to have clearly identified reactants, transition states, and products using PM3 energetics, the shape of the surface changed when MP2 electronic energies were used, and the positions of some transition states and minima were reordered. The most notable change when MP2 was used to calculate the electronic energies was the location of the tert-butyl cation. With use of PM3 energetics, tert-butyl is found to be a minimum energy species, but it shifts upward in energy and becomes a peak on the free energy surface when electron correlation is included. The shift in the location of the tert-butyl species is caused by different values of the potential energy for the two different model chemistries at that position in phase space. The resulting barrier of 32.9 kcal/mol is the largest on the surface and indicates that the decomposition of protonated MTBE is the most energetically difficult step. The second prominent peak on the free energy surface is an intramolecular proton transfer from the alkyl oxygen to the acyl oxygen in methyl pivalate. While protonation on the acyl oxygen of methyl pivalate is more thermodynamically stable, the proton must start out associated with the alkyl oxygen because it is initially part of the methanol molecule that adds to the tert-butyl acylium cation. The forward and reverse barrier heights of 11.9 and 31.6 kcal/mol for the alkyl to acyl proton transfer can be compared to forward and reverse values of 6.2 and 21.9 kcal/mol for the intramolecular proton transfer between the nitrogen and the oxygen in formamide, respectively. Before including the counterion in the calculations, the system was studied with the protonated methanol catalyst at the HF level to determine if the same reactions characterized the chemistry at a higher level of theory. It should be noted that the overall size and nonbonded nature of the system, along with the disk space, memory, and time requirements that they demand, made it impractical to carry out extremely high

level calculations such as coupled cluster and composite methods such as G2 or G3. MP2//HF (Protonated Methanol). In the process of upgrading the PM3 geometries to Hartree-Fock geometries, a slightly different pathway emerged. The primary difference between this pathway and the PM3 pathway is that the CO molecule and solvating methanol molecules are on the same side of the tert-butyl group in the PM3 pathway, while the tert-butyl group is solvated by methanol molecules on both sides in the HF pathway. The PM3 pathway is referred to as singlesided solvation, and the HF pathway is referred to as bi-solvated. The tert-butyl species from both pathways are shown in Figure 5 for comparison. In both cases the tert-butyl group is essentially planar, which is a good indication that it is indeed a tert-butyl cation, but the bi-solvated configuration allows the tert-butyl group to bear less charge density. The tert-butyl group in the single-sided pathway carries a net charge of +0.8, as calculated using Mulliken charges, and in the bisolvated pathway, it bears a net charge of +0.6. Another difference between the single-sided and bi-solvated tertbutyl species is the orientation of the CO molecule. The orientation in both cases emerged naturally from IRC calculations, and no barrier was observed when CO rotated in the bi-solvated case. The free energy surface for the bi-solvated pathway is shown in Figure 6. This energy landscape has a number of features in common with the MP2//PM3 surface. The overall shapes of the two surfaces are the same, with the decomposition of MTBE being the dominant peak on the surface. Also, the addition of methanol to the tert-butyl acylium cation is a reaction with only a small barrier on both surfaces. In addition, the tert-butyl group is again shifted to a position above the transition state for the MTBE decomposition reaction when correlation is included with the MP2 method. This suggests, when coupled with the fact that the tertbutyl species also shifted to a position above the transition state for the MTBE decomposition barrier on the MP2//PM3 surface when correlation was included, that electron correlation is important in accurately describing this species. In addition to these similarities, the MP2//PM3 and MP2//HF surfaces also have important differences. One important difference is that the proton-transfer reaction between oxygen atoms in protonated methyl pivalate could not be characterized on the HF surface. Significant effort was put forth to find a pathway for this reaction, such as investigating direct and methanol-mediated proton transfers, but all attempts were unsuccessful in characterizing a transition state. The shaded portion of the curve in Figure 6 indicates this difficulty. The alkyland acyl oxygen-protonated methyl pivalate geometries are optimized at the HF level, but a range of transition state energies was estimated for the forward and reverse proton-transfer reactions from the MP2//PM3 surface. The lower bound was estimated by applying the forward activation energy of 11.9 kcal/mol from the MP2//PM3 surface to the alkyl-protonated methyl pivalate species on this surface, and the upper bound was estimated by applying the reverse activation energy of 31.9kcal/mol from the MP2//PM3 surface to the acylprotonated methyl pivalate species on this surface. Another difference between the MP2//HF and MP2// PM3 surfaces is that the highest energy species on the MP2//HF surface is the transition state between the tert-

22


Figure 5. Different geometries for the tert-butyl cation show that the essential difference between the (a) PM3 and (b) Hartree-Fock pathways is that the tert-butyl group is solvated on both sides in the Hartree-Fock pathway so that the tert-butyl group has less charge density to bear.

Figure 6. Free energy diagram at 298 K for the reaction of MTBE to methyl pivalate calculated using HF geometries and frequencies, MP2 electronic energies, and an AM1-SM2 solvation correction. The final transition state could not be located at this level and is approximated using both the forward and reverse free energies of activation from the MP2//PM3 surface, as shown by the shaded region. Neutral species also include protonated methanol, and ionic species also include methanol in the calculations.

butyl and tert-butyl acylium cations. On the MP2//PM3 surface, the highest energy species is the tert-butyl cation itself. Also, the height of the first barrier has decreased from 32.9 kcal/mol on the MP2//PM3 surface to 16.7 kcal/mol on the MP2//HF surface, with the MTBE decomposition to tert-butyl being 12.9 kcal/mol endergonic. The decrease in the height of the largest barrier cannot be conclusively attributed to the change in the pathway because the geometries for the two surfaces were determined at different levels of theory. The HF pathway could not be characterized using PM3, and the difficulties in finding the proton-transfer reaction on the HF surface prompted studies that included the counterion in the acid model rather than continued study of the system without the counterion. MP2//HF (BF3-CH3OH). In these calculations, the acid model was no longer protonated methanol, but BF3-CH3OH. This means that the entire system was neutral rather than positively charged. It was hypothesized that the difficulties encountered when studying the proton-transfer reaction might be eliminated if the counterion were included because the counterion would likely be directly involved in protonation reactions. As can be seen in Figure 7, the counterion does play a significant role in the reaction of tert-butyl acylium to methyl pivalate. As the C-O bond length decreases

Figure 7. Reaction of tert-butyl acylium to methyl pivalate, characterized at the MP2//HF level of theory, involves a concerted proton transfer from the methanol to the counterion as the methanol adds to tert-butyl acylium. Oxygen atoms are shown in black, fluorine in dark gray, carbon in gray, boron in light gray, and hydrogen in white.

from 2.60 Å in tert-butyl acylium to 1.94 Å at the transition state and 1.32 Å in methyl pivalate, the proton associated with the methanol taking part in this


Figure 9. Free energy diagram at 298 K for the reaction of MTBE to methyl pivalate calculated using HF geometries and frequencies, MP2 electronic energies, and a PCM solvation correction. The tertbutyl group is only solvated on one side in this pathway. Neutral species also include BF3-CH3OH, and charged species also include BF3-CH3O-.

Figure 8. Reaction of MTBE to tert-butyl, characterized at the MP2//HF level of theory, involves a concerted proton transfer from the counterion to MTBE as the methoxy group is eliminated from MTBE. Oxygen atoms are shown in black, fluorine in dark gray, carbon in gray, boron in light gray, and hydrogen in white.

Figure 10. Geometry for a tert-butyl acylium molecule solvated on both sides of the tert-butyl group. This geometry can be compared to that shown in Figure 7a, which is only solvated on one side of the tert-butyl group. This tert-butyl acylium molecule is 8.3 kcal/mol higher in free energy than that in Figure 7a. Oxygen atoms are shown in black, fluorine in dark gray, carbon in gray, boron in light gray, and hydrogen in white.

reaction is transferred to the anion in a concerted fashion. This simultaneous transfer means that an alkyl oxygen-protonated methyl pivalate is never formed, and no proton-transfer reaction is needed to form the more thermodynamically stable state in which the acyl oxygen interacts with the acid. The counterion is also involved in the protonation of MTBE, which affects the barrier for MTBE decomposition. This can clearly be seen in Figure 8 which shows that the proton from the methanol in BF3-CH3OH is transferred to the methoxy group as it is eliminated from MTBE to form the tert-butyl cation. It should be noted that the transition state in this reaction had two imaginary frequencies. The reaction coordinate was characterized as the vibration of the C-O bond and had an imaginary frequency of -234 cm-1, but a second imaginary mode of -4 cm-1 was present and could not be eliminated. This mode primarily involves torsional motion of the CO and anion and does not involve the reaction coordinate. Attempts were made to eliminate the imaginary mode by tightening the convergence criteria on the optimization and by optimizing with frequencies computed at each step. In the end, this problem is of little consequence because the tert-butyl

species was shifted to a free energy above this transition state when correlation was included via the MP2 method. The free energy diagram for this entire surface is shown in Figure 9. The initial barrier for MTBE decomposition has increased from 16.7 kcal/mol on the MP2//HF surface without a counterion to 44.9 kcal/mol on the current surface. It should be noted, however, that the results shown in Figure 9 are based on a singlesided pathway, which had an MTBE decomposition barrier of 32.9 kcal/mol using MP2//PM3. Another difference between the surface including the counterion and the previous results is that the small barrier for adding methanol to the tert-butyl acylium cation has disappeared. Despite the differences in the magnitude of the MTBE decomposition barrier, all of the surfaces studied thus far have indicated that this is the most energetically difficult step in the mechanism. The MTBE decomposition barrier is also found to be the most difficult step when the bi-solvated pathway is investigated in the presence of the counterion. Figure 10 shows the bi-solvated tert-butyl acylium cation, which can be compared to the structure solvated on one side in Figure 7a. The bi-solvated tert-butyl acylium is 8.23 kcal/mol higher in energy than that solvated on one side, which is explained by the fact that only one solvating

24


Figure 11. Free energy diagram at 298 K for the reaction of MTBE to methyl pivalate calculated using HF geometries and frequencies, MP2 electronic energies, and a PCM solvation correction. The tert-butyl group is solvated on both sides in this pathway. Neutral species also include BF3-CH3OH, and charged species also include BF3-CH3O-.

Figure 12. Free energy diagram for the reaction of MTBE to methyl pivalate calculated using DFT geometries, frequencies, and energetics and a PCM solvation correction. The tert-butyl group is solvated on one side in this pathway. Neutral species also include BF3-CH3OH, and charged species also include BF3CH3O-.

species is present near the charge center, and the counterion interacts only weakly with the nonpolar methyl groups. However, the MTBE decomposition barrier decreased by 12.4 kcal/mol to 32.5 kcal/mol with the new placement of the counterion, as can be seen in Figure 11. This surface has exactly the same features as that shown in Figure 9, with only small shifts in the magnitudes of the barriers. This comparison between the bi-solvated and single-solvated pathways, unlike the comparison with and without the counterion, is made between structures determined at the same level of theory, and the results indicate that the bi-solvated pathway has a smaller overall barrier. One complication that did arise upon inclusion of the counterion was the migration of one of the fluorine atoms from the boron trifluoride to the center of positive charge on the tert-butyl and tert-butyl acylium ions. An experimental 13C NMR study8 of this system did not detect any fluorinated products, so the presence of this species in the ab initio calculations was suspect. It was postulated that interactions with other molecules in the liquid phase might affect the strength of the B-F bonds, so the tert-butyl acylium ion was optimized using the PCM model, in contrast to using the PCM model as a post-optimization correction. While this calculation did not converge to a minimum energy species, the intermediate calculations clearly indicated that the fluorine atom did not migrate to the tert-butyl acylium charge center when optimized within a solvent cavity. Optimization using the PCM method proved difficult, so the B-F bond lengths were frozen in calculations where the fluorine migrated to prevent this spurious reaction from occurring. The calculations presented thus far have all relied on MP2 single-point calculations to include electron correlation, using either PM3 or HF geometries. While this approach has the benefit of reducing the computational demand, it also has the drawback that critical points on the surface may shift as the level of theory is changed. This was seen when studying the tert-butyl cation as it shifted to a higher energy on all of the surfaces studied. This limitation has been overcome by studying the system using DFT because electron correlation is included at the same level of theory at which the geometries were determined.

DFT (BF3-CH3OH). The HF geometries provided good starting points for the DFT optimizations, and the resulting DFT free energy surface has the same overall shape as the MP2//HF surfaces using BF3-CH3OH as the acid model. Analogues of each of the important species for the single-sided pathway on the MP2//HF surface were found at the DFT level, and the corresponding surface is shown in Figure 12. It should be noted that the tert-butyl species determined at the DFT level did not have all positive vibrational frequencies. Three small imaginary frequencies of -50.4, -26.9, and -24.3 cm-1 associated with low-energy, torsional motions of the nonbonded species could not be eliminated. This indicates that the tert-butyl species should be slightly lower in energy, but this fact does not have any serious implications on the overall conclusions about the system. The most important difference between the DFT surface and the MP2//HF surface is that the tert-butyl acylium cation is not a stable species on this surface. An intrinsic reaction coordinate following calculation beginning with the transition state for adding CO to tert-butyl showed that as the tert-butyl acylium cation is formed, the methoxy group of the counterion immediately adds to the tert-butyl acylium species to afford a methyl pivalate species that is solvated by both methanol and BF3. This is consistent with the observation that while the tert-butyl acylium cation could be located as a minimum energy species at the HF level, the MP2 single-point calculations always identified it as an intermediate geometry along the reaction path to methyl pivalate. The initial barrier for MTBE decomposition on the DFT surface is 29.8 kcal/mol, which is close to the PM3 and HF barriers of 32.9 kcal/mol (protonated methanol acid model) and 34.6 kcal/mol (single-sided BF3-CH3OH acid model), respectively. The bi-solvated pathway could not be completely characterized on this surface. The only stable species found on this surface was the tert-butyl acylium species. The difference between this tert-butyl acylium species and the one formed in the single-sided pathway en route to methyl pivalate was that the counterion was located on the opposite side of the tert-butyl group from the carbonyl moiety. A methanol molecule was near the carbonyl carbon, but the high-energy, alkyl oxygenprotonated species would result upon addition because

Ind. Eng. Chem. Res., Vol. 43, No. 1, 2004 25 Table 2. Gibbs Free Energiesa of Activation for Species Involved in Oligomerization Side Reactions for Two Different Acid Models free energy of activation (kcal/mol) reaction tert-butyl h isobutene + H+ forward reaction reverse reaction tert-butyl + isobutene h diisobutyl forward reaction reverse reaction a

protonated methanol

BF3-CH3OH

0.0 9.0

3.3 11.5

1.6 9.1

0.0 13.9

Free energies calculated at MP2//HF level.

the counterion was not in a position where it could accept the proton. This might have been remedied if a second counterion were placed in the cluster, but this calculation was not performed as the large number of heavy atoms required would significantly increase the calculation time. Also, addition of more nonbonded species would have made convergence of the geometry optimization even more difficult. Finally, all reactions attempted with this tert-butyl acylium cation as the reactant led to proton transfer from the tert-butyl group to the counterion, producing isobutene. While this represents an interesting possibility for isobutene formation, the desired reactions could not be characterized. Side Reactions (Protonated Methanol, BF3CH3OH). Reactions for the formation of isobutene and subsequent oligomerization to the diisobutyl cation were investigated for both acid models using the MP2//HF approach. The energetics for all of the side reactions are tabulated in Table 2. The reaction of tert-butyl to isobutene with protonated methanol as the catalyst is shown in Figure 13. The tertbutyl species is shown in Figure 13a, and it should be noted that it is solvated by methanol molecules on both sides. The reaction coordinate for the reaction is the O-H distance which begins at 2.56 Å, closes to 1.21 Å at the transition state in Figure 13b, and ends at a reasonable covalent O-H bond distance of 1.01 Å in the isobutene product. As the positive charge is transferred to the oxygen atom, the methanol solvating on the opposite side is no longer strongly attracted to the tertiary carbon of the tert-butyl species, and it migrates to a position where it weakly interacts with the methyl hydrogens. The reaction of isobutene and tert-butyl cation to diisobutyl cation was also studied in the presence of protonated methanol, and this reaction is characterized in Figure 14. The reactant cluster, shown in Figure 14a, consists of a tert-butyl cation solvated by both a methanol molecule and an isobutene. A CO molecule is also present in this reaction because it was part of an attempt to directly examine the competition between carbonylation and oligomerization. The reaction proceeds as expected, with the methylene group of the isobutene forming a bond with the tertiary carbon of the tert-butyl group and the solvating methanol moving to interact with the methyl hydrogens as the center of charge is shifted away from it. As in the case of the core mechanism, the side reactions were also studied in the presence of the counterion. The presence of the counterion is especially important for the reaction of tert-butyl to isobutene because the counterion is a negatively charged species which accepts the proton from the positively charged

Figure 13. Reaction of tert-butyl cation to isobutene, characterized at the MP2//HF level of theory, with protonated methanol as the catalyst. Oxygen atoms are shown in black, carbon in gray, and hydrogen in white.

tert-butyl group and forms a neutral system. The system remained positively charged throughout the reaction when protonated methanol was the catalyst. The reaction in the presence of the counterion is outlined in Figure 15 with a number of important bond distances indicated. As the proton is transferred from the tertbutyl group in Figure 15a through the transition state in Figure 15b to the counterion in Figure 15c, the B-O bond distance increases from 1.41 to 1.57 Å. It should be noted that the tert-butyl cluster in Figure 15a is the same tert-butyl cluster involved in the core mechanism. This is a marked improvement over the protonated methanol system because the carbonylation and oligomerization reactions now have a structure in common, which allows for an improved comparison of the two reactions. Finally, the oligomerization reaction was studied in the presence of the counterion, as shown in Figure 16. The counterion does not play a significant role in this reaction, other than solvation of the tert-butyl group in the reactant cluster in Figure 16a. An important distinction between this reaction and the same reaction studied in the presence of protonated methanol is that an additional methanol molecule was added to stabilize

26


Figure 14. Reaction of tert-butyl cation and isobutene to diisobutyl cation, characterized at the MP2//HF level of theory, with protonated methanol as the catalyst. Oxygen atoms are shown in black, carbon in gray, and hydrogen in white.

the diisobutyl cation product, as this product included an unsolvated charge center in the protonated methanol case. 4. Discussion One of the motivating factors in studying the carbonylation of MTBE was the fact that little information was known about the reaction. The information put forth in this work is the first detailed examination of this system to our knowledge and also represents a starting point for subsequent experimental or theoretical investigations. Unfortunately, the lack of information in the literature makes it difficult to find experimental data against which these results can be compared. Other researchers have studied MTBE synthesis,21 metal-catalyzed MTBE decomposition,22 and acidcatalyzed, gas-phase MTBE decomposition.23 These studies provide some information against which the results in this study can be compared. In addition, the kinetics of the Koch carbonylation of isobutanol have been studied and provide additional data for comparison. These data are compared below against the computational results from this study24 for the decomposition of MTBE, carbonylation of tert-butyl cation, and methoxylation of tert-butyl acylium cation. In addition, the computational results for the oligomerization side reactions are compared with relevant experimental data. Decomposition of MTBE. All of the energy surfaces presented in this study indicated that the most energetically difficult step was the decomposition of MTBE.

Figure 15. Reaction of tert-butyl cation to isobutene, characterized at the MP2//HF level of theory, with BF3-CH3OH as the catalyst. Oxygen atoms are shown in black, carbon in gray, and hydrogen in white.

This is consistent with a 13C NMR study8 of this system which found that MTBE did not begin to react in BF32.19CH3OH until the temperature reached nearly 70 °C. These results indicate that protonated or complexed MTBE is stable and that it is energetically difficult to eliminate a methanol molecule. This behavior is different than that for MTBE in a very strong acid, such as HSO3F-SbF5 (1 mol:1 mol), where MTBE decomposes to tert-butyl cation and protonated methanol even at -70 °C. This difference can be explained by the extreme differences in the acidity of the systems, as BF3-CH3OH has a Hammett acidity value near -4, and HSO3FSbF5 has a Hammett value near -27. A better comparison is the Koch carbonylation of isobutanol, which was studied in sulfuric acid solutions having Hammett values in the -6 to -10 range. The dehydration of the protonated alcohol was found to be the rate-limiting step in this system. Free energies calculated for the decomposition of MTBE to tert-butyl in the liquid phase are compared with those for isobutanol in solution and MTBE in the gas phase in Table 3. These are not direct comparisons because a primary carbocation is formed in one case and the other takes place in the gas phase, but this is the closest experimental data that was found.


Figure 17. Geometry for the bi-solvated tert-butyl molecule calculated at the MP2//HF level is shown, and it can be seen that the methanol molecule has migrated toward the plane of the tertbutyl group. This geometry is less bi-solvated than the tert-butyl acylium species shown in Figure 10. Oxygen atoms are shown in black, fluorine in dark gray, carbon in gray, boron in light gray, and hydrogen in white. Table 4. Free Energies of Activation for Carbonylation and Decarbonylation of tert-Butyl Cation free energy (kcal/mol)

Figure 16. Reaction of tert-butyl cation and isobutene to diisobutyl cation, characterized at the MP2//HF level of theory, with BF3-CH3OH as the catalyst. Oxygen atoms are shown in black, carbon in gray, and hydrogen in white. Table 3. Barrier Heights for the Decomposition of MTBE to tert-Butyl source

acid

solvation

MP2//PM3 MP2//HF MP2//HF MP2//HF DFT (BLYP) isobutanolb MTBE(g)c

CH3OH2+ CH3OH2+ BF3-CH3OH BF3-CH3OH BF3-CH3OH 85-96% H2SO4


free energy (kcal/mol) 32.9 12.9 34.6 29.7 29.8 28.7 23.9

a tert-Butyl group behaves more like single-sided; see Figure 17. b Brilman et al. c Audier et al.

The results in Table 3 show an interesting trend. With the exception of the bi-solvated, protonated methanolcatalyzed system, all of the MTBE decomposition barriers were near 30 kcal/mol. Both surfaces which use the bi-solvated pathway were expected to have similar barriers, but a discrepancy exists between the values of 12.9 kcal/mol for the bi-solvated system with protonated methanol for the catalyst and 29.7 kcal/mol for the bi-solvated pathway that included the counterion. This discrepancy can be explained by noting that the low barrier in the protonated methanol case is partly due to the fact that the methanol molecule that is solvating the protonated MTBE molecule is located on the opposite side of the tert-butyl group from the methoxy group. This configuration led to incomplete solvation at this site and raised the free energy of MTBE, which in turn decreased the MTBE decomposition barrier height. It should also be noted that the tertbutyl group in the bi-solvated pathway with BF3CH3OH as the catalyst, shown in Figure 17, is only strongly solvated on one side, and it is likely that the barrier of 29.7 kcal/mol would be lower if this species were well-solvated on both sides. Comparing the computational results to experimental results, one would expect that the decomposition barrier

source

acid

solvation

MP2//PM3 MP2//HF MP2//HF MP2//HF DFT literaturec

CH3OH2+ CH3OH2+ BF3-CH3OH BF3-CH3OH BF3-CH3OH FHSO3/SbF5


carbonylation decarbonylation 0.0 3.8 10.2 2.81 4.17 11.2

28.3 18.2 23.5 14.4 b 15.1

a tert-Butyl group behaves more like single-sided; see Figure 17. b tert-Butyl acylium is not a stable structure at this level. c Hogeveen et al.

for isobutanol of 28.7 kcal/mol would be higher than the MTBE barriers, but it is possible that complexation with the sulfuric acid catalyst in this system has altered the mechanism and makes the comparison more tenuous.24 Finally, the decomposition of protonated MTBE in the gas phase has been studied, and a barrier of 23.9 kcal/ mol was found for this reaction.23 The gas-phase value should be larger than the solution value as ions are normally stabilized in solution, so it is likely that the liquid-phase barrier is in the 10-20 kcal/mol range. The only value that falls within this range is the bi-solvated pathway determined for the protonated methanol catalyst, which suggests that two-sided solvation is important. The difficulty in finding this pathway in the presence of the counterion may indicate that additional counterions are needed to accurately describe this pathway. Carbonylation of tert-Butyl Cation. While MTBE decomposition is the most energetically difficult step in the mechanism, the carbonylation step has significant implications for controlling the selectivity to carbonylated products. Experimental data was found for the carbonylation and decarbonylation of tert-butyl cation in FSO3H/SbF5.26 This is a much stronger acid than BF3-CH3OH, but the data still provide a benchmark for the values calculated here. Table 4 lists values for the forward and reverse free energies of activation both calculated in this study and found in the literature. The trend in the free energies of activation for decarbonylation is that the bi-solvated pathways have smaller barriers, which is due to the fact that the bisolvated tert-butyl acylium structures are higher in energy than the single-sided tert-butyl acylium structures, as indicated in the MP2//HF (BF3-CH3OH)

28


section. It should be noted that the bi-solvated tert-butyl acylium structure that includes the counterion is solvated in a manner similar to the bi-solvated tert-butyl acylium cation with protonated methanol as the catalyst and does not suffer from the poor solvation that was seen in Figure 17 for the bi-solvated tert-butyl species that included that counterion. This explains why the decarbonylation barriers for both bi-solvated surfaces agree with one another. The free energies of activation for carbonylation calculated in this study are all lower than the experimental value. One possible explanation for this trend is that the experiment was conducted in a much stronger acid. Because the charge is more localized in the tert-butyl species than the transition state, it is likely that the counterion of a stronger acid would interact more strongly with the tert-butyl species than the transition state, and thus increase the free energy of activation. All of the calculated results indicate that the free energy of activation for the carbonylation of the tert-butyl cation is more energetically facile than the decomposition of MTBE. This does not, however, mean that the MTBE decomposition is necessarily the ratelimiting step, as low concentrations of tert-butyl cation or dissolved CO may affect the rate of carbonylation significantly. Methoxylation of tert-Butyl Acylium. No experimental information was found for this reaction. This was not surprising, as Brilman et al.24 found no information on the corresponding hydroxylation reaction in the isobutanol system, and this is a reaction which is more likely to have been studied. The only relevant experimental data that was found discussed the protonation sites and likely modes of cleavage for esters.27 The results of the present study agree with their conclusions that protonation at the acyl oxygen is more favorable. In addition, the concerted movement of the proton to the counterion from the methanol which methoxylates the tert-butyl acylium cation is consistent with their conclusion that the alkyl oxygen-protonated species is energetically unfavorable. Side Reactions. The results for the formation of isobutene from tert-butyl cation are similar for both acid models in that they both indicate that there is little or no free energy barrier in the forward direction, and the reverse reaction is endergonic by approximately 10 kcal/ mol. The enthalpy of reaction for the protonated methanol-catalyzed reaction of tert-butyl to isobutene is -8.8 kcal/mol and is -6.7 kcal/mol for the BF3-CH3OH catalyzed reaction. A good measure of the enthalpy of reaction for this proton transfer is the difference in the proton affinities of isobutene and methanol. Unfortunately, very few data are available in the literature for proton affinities of species in solution. The reported gasphase proton affinities for methanol (181.9 kcal/mol28) and isobutene (191.7 kcal/mol29) indicate that the enthalpy of reaction for transferring a proton from tertbutyl to isobutene is 9.8 kcal/mol. Enthalpies of reaction calculated in the gas phase at the MP2//HF level using isolated tert-butyl, methanol, isobutene, and protonated methanol species produced an enthalpy of reaction of 9.6 kcal/mol, which is in excellent agreement with the experimental number. A value of -16.0 kcal/mol was found when calculations were carried out on these same species in solution using the PCM method with methanol as the solvent. This result indicates that, contrary to the gas-phase results, the proton affinity for methanol

is greater than that for isobutene in solution. This is consistent with the negative enthalpies of reaction found in the current study. The forward and reverse free energies of activation in Table 2 for the oligomerization reaction also show good agreement between the two acid models, with both indicating that the forward reaction is very facile and the reverse reaction is endergonic by approximately 10 kcal/mol. The enthalpy of activation for the forward reaction with protonated methanol as the catalyst is -3.5 kcal/mol, and the reaction is enthalpically unactivated when BF3-CH3OH is the catalyst. The second oligomerization step, isobutene adding to the diisobutyl cation, has been studied, and it was found that the reaction is nearly independent of temperature, which indicates that the enthalpy of activation should be close to zero. It is expected that the reaction of tert-butyl with isobutene should exhibit similar behavior, which is in agreement with the results of this study. No activation energies for the cationic degradation of polyisobutene could be located in the literature for comparison. 5. Conclusions Current environmental pressures make MTBE an attractive feedstock material for the synthesis of methyl pivalate, which has been shown to have potential environmental benefits as a low ozone-forming solvent. Little is known about the energetics of the acidcatalyzed carbonylation of MTBE, and a computational study was carried out to study the fundamentals of this chemistry. Calculations using two acid models, namely, protonated methanol and BF3-CH3OH, were performed using semiempirical, Hartree-Fock, MP2, and density functional methods. All of the free energy surfaces generated showed that the decomposition of MTBE is the most energetically difficult step in the mechanism. Reactions studied using the BF3-CH3OH acid model, unlike those using the protonated methanol acid model, did not require the high-energy, alkyl oxygen-protonated ester as an intermediate. Study of single-sided and bisolvated pathways indicated that the bi-solvated pathway resulted in a lower energy barrier for the decomposition of MTBE that was in better agreement with experimental observations. The information put forth in this work is the first detailed examination of this system to our knowledge. The results provide an understanding of the controlling reactions in this system and represent a starting point for subsequent experimental or theoretical investigations. Acknowledgment Financial support for this project was provided by ExxonMobil Chemical Company and is gratefully acknowledged. Literature Cited (1) Sissell, K. Health Concerns Dim Prospects for MTBE. Chem. Week 1998, April, 41. (2) Davis, G. Executive Order D-5-99 by the Governor of the State of California, 1999. (3) Davis, G. Executive Order D-52-02 by the Governor of the State of California, 2002. (4) Yezrielev, A. I.; Knudsen, G. A.; Schlosberg, R. H.; Larson, T. M. Environmentally Preferred Fluids and Fluid Blends. U.S. Patent 6,280,159, Exxon Chemical Patents Inc., 1999. (5) Carter, W. P. L. Documentation of the SAPRC-99 Chemical Mechanism for VOC Reactivity Assessment. Report to the Cali-

Ind. Eng. Chem. Res., Vol. 43, No. 1, 2004 29 fornia Air Resources Board 92-329, 95-208, California Air Resources Board, 2000. (6) Falbe, J. Carbon Monoxide in Organic Synthesis; SpringerVerlag: New York, 1970. (7) Bahrmann, H. Koch Reactions. In New Syntheses with Carbon Monoxide, Vol. 11 of Reactivity and Structure Concepts in Organic Chemistry; Falbe, J., Ed.; Springer-Verlag: New York, 1980; pp 372-413. (8) Haubein, N. C.; Broadbelt, L. J.; Mozeleski, E. J.; Schlosberg, R. H.; Cook, R. A.; Mehnert, C. P.; Farcasiu, D. 13C NMR Study of the Acid-Catalyzed Carbonylation of Methyl tert-Butyl Ether (MTBE). Catal. Lett. 2002, 80 (3-4), 139-145. (9) Stouthamer, B.; Kwantes, A. Preparation of Esters by Carbonylation of Ethers. U.S. Patent 3,607,914, Shell Oil Company, 1969. (10) De Benedictis, A.; Furman, K. E. Preparation of Carboxylic Acids. U.S. Patent 2,913,489, Shell Oil Company, 1959. (11) Wavefunction, Inc. MacSpartan Plus 1.2.1, 1998. (12) Stewart, J. J. P. MOPAC 6.00, 1990; QCPE # 464. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A. 7; Gaussian, Inc.: Pittsburgh, PA, 1998. (14) Cabani, S.; Gianni, P.; Mollica, V.; Lepori, L. Group Contributions to the Thermodynamic Properties of Non-Ionic Organic Solutes in Dilute Aqueous Solution. J. Solution Chem. 1981, 10 (8), 563-595. (15) Guthrie, J. P. A Group Equivalents Scheme for Free Energies of Formation of Organic Compounds in Aqueous Solution. Can. J. Chem. 1992, 70, 1042-1054. (16) Pliego, J. R.; Riveros, J. M. The Cluster-Continuum Model for the Calculation of the Solvation Free Energy of Ionic Species. J. Phys. Chem. A 2001, 105 (30), 7241-7247. (17) Tomasi, J.; Persico, M. Molecular Interactions in Solution: An Overview of Methods Based on Continuous Distributions of the Solvent. Chem. Rev. 1994, 94 (7), 2027-2094. (18) Cramer, C. J.; Truhlar, D. G. An SCF Solvation Model for the Hydrophobic Effect and Absolute Free Energies of Aqueous Solvation. Science 1992, 256, 213-217.

(19) Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic Interaction of a Solute with a Continuum. A Direct Utilization of Ab Initio Molecular Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55, 117-129. (20) Rodriquez, C. F.; Conje, A.; Shoeib, T.; Chu, I. K.; Hopkinson, A. C.; Siu, K. W. M. Solvent-Assisted Rearrangements between Tautomers of Protonated Peptides. J. Phys. Chem. A 2000, 104 (21), 5023-5028. (21) Rehfinger, A. Kinetics of Methyl Tertiary Butyl Ether Liquid-Phase Synthesis Catalyzed by Ion Exchange Resin-I. Intrinsic Rate Expression in Liquid-Phase Activities. Chem. Eng. Sci. 1990, 45 (6), 1605-1617. (22) Fields, D. L.; Lim, P. K.; Roberts, G. W. Catalytic Destruction of Methyl Tertiary Butyl Ether (MTBE) with a Pt/Rh Monolithic Automotive Exhaust Catalyst. Appl. Catal., B 1998. (23) Audier, H. E.; Berthomieu, D.; Morton, T. H. Gas-Phase Decomposition of Conjugate Acid Ions of Simple tert-Butyl Alkyl Ethers. J. Org. Chem. 1995, 60 (22), 7198-7208. (24) Brilman, D. W. F.; van Swaaij, W. P. M.; Versteeg, G. F. Gas-Liquid-Liquid Reaction Engineering: The Koch Synthesis of Pivalic Acid from iso- and tert-Butanol; Reaction Kinetics and the Effect of a Dispersed Second-Liquid Phase. Chem. Eng. Sci. 1999, 54, 4801-4809. (25) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; John Wiley and Sons: New York, 1985. (26) Hogeveen, H.; Roobeek, C. F. Chemistry and Spectroscopy in Strongly Acidic Solutions. Part XXXIV Study of the Carbonylation of Carbonium Ions by NMR Spectroscopic Measurements. Recl. Trav. Chim. Pays-Bas 1970, 89, 1121-1132. (27) Hopkinson, A. C.; Mackay, G. I.; Bohme, D. K. Acid Catalysis in the Gas Phase: Dissociative Proton Transfer to Formate and Acetate Esters. Can. J. Chem. 1979, 57, 2996-3004. (28) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard,W. G. Gas-Phase Ion and Neutral Thermochemistry. J. Phys. Chem. Ref. Data 1988, i7 (S1), 1-872. (29) Traeger, J. C. The Absolute Proton Affinity for Isobutene. Rapid Commun. Mass Spectrom. 1996, 10 (1), 119-122. (30) Mayr, H.; Roth, M.; Lang, G. Kinetics of Carbocationic Polymerizations: Initiation, Propagation, and Transfer Steps. In Cationic Polymerization; Faust, R., Shaffer, T. D., Eds.; ACS Symposium Series 665; American Chemical Society: Washington, DC, 1997; pp 25-40.

Received for review February 13, 2003 Revised manuscript received September 19, 2003 Accepted October 6, 2003 IE030139T

Quantum Chemical Investigation of the Koch Carbonylation of Methyl

Recommend Documents