Article pubs.acs.org/JPCA
Dimethoxymethane−Hydrogen Chloride Interaction: Gas Phase versus Low-Temperature Behavior Studied Using Matrix Isolation Infrared and Density Functional Theory Methods K. Sundararajan* and N. Ramanathan Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India S Supporting Information *
ABSTRACT: Premixing of dimethoxy methane (DMM) and hydrogen chloride (HCl) with Ar/N2 in the gas phase resulted in a nucleophilic substitution reaction and yielded products, cis-chloromethyl methyl ether (cis-CMME) and methanol. On the contrary, when DMM and HCl were separately codeposited in a low-temperature Ar matrix produced hydrogen-bonded alkoxy adduct, probably the intermediate in the gas phase nucleophilic substitution reaction. The formation of the alkoxy adduct was evidenced by the shifts in the vibrational frequencies of the DMM and HCl submolecules. The structure and energy of the alkoxy adduct were computed at the B3LYP/ 6-311++G** level of theory. The computations indicated only one minimum for the DMM−HCl adduct. The nucleophilic substitution reaction between DMM and HCl is prevented in the low-temperature matrix probably due to the cage effect in the matrix.
1. INTRODUCTION Dimethoxymethane (DMM), a dimethylacetal of formaldehyde is used in the manufacture of perfumes, resins, adhesives, paint strippers, and protective coatings. It is also utilized both as a solvent and as a protecting group for alcohols in organic synthesis. In automobiles, it is used as one of the oxygenated additives in diesel fuel to reduce the diesel emissions of particulate matter.1,2 The reactions of DMM in the gas phase proved to be intriguing as these reactions serve as model systems for understanding many industrial processes. The free radical reaction of DMM with ethylene was found to yield acetals by the radical attack on the methylene carbon.3 Hunter et al. studied the free radical reaction of DMM with ethylene by trapping and identifying the low-molecular-weight products through an alternate route.4 Shapovalov and Bell theoretically studied the gas phase carbonylation of DMM.5 The gas phase carbonylation of DMM to form methyl methoxyacetate was found to be catalyzed by acid zeolites. This reaction is a critical step in the synthesis of monoethylene glycol (MEG), a highvolume commodity chemical widely used as antifreeze and as an intermediate in the manufacturing of synthetic fibres, foams, and plastics.6 It is interesting to study the intermediates formed during the gas phase reactions using matrix isolation infrared technique. Frequently, the hydrogen-bonded adducts have been encountered to be the initial intermediates in the gas phase reactions. The studies of hydrogen bonds have been realized to play an important role in understanding the molecular structure, molecular assembly, and crystal packing. Understanding the © 2013 American Chemical Society
hydrogen bonding both experimentally and theoretically is of considerable interest in chemistry and biology.7,8 Conformations of DMM were studied by both experimental and theoretical methods by many groups.9−15 Earlier, using matrix isolation infrared technique and ab initio computations, we studied the conformations of DMM and identified both the ground state DMM (GG; “G” refers to gauche orientation of methoxy group) and first higher energy conformers of DMM (TG; “T” refers to trans orientation of methoxy group).16 Recently, we have studied the interaction of trimethyl phosphite (TMPhite) with hydrogen chloride (HCl).17 When TMPhite and HCl were separately codeposited in a N2 matrix, they gave hydrogen-bonded alkoxy adduct, whereas when the reactants were premixed prior to deposition, they yielded methyl chloride as one of the products. The gas phase reaction at room temperature proceeded similarly to the Arbuzov mechanism where the lone pair of electron on phosphorus acts as a nucleophile and attacks the hydrogen of HCl to give methyl chloride as one of the products. There is yet another possibility where the chlorine of HCl can attack the methyl group to yield the same product methyl chloride. This mechanism was not considered because of the presence of a strong nucleophilic center (lone pair on phosphorus), which essentially drives the gas phase reaction. In the present work it was thought interesting to study the gas phase premixing of DMM with HCl Received: January 11, 2013 Revised: February 22, 2013 Published: February 22, 2013 2347
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Figure 1. Matrix isolation infrared spectra where DMM and HCl were premixed prior to deposition in an Ar matrix in the region 600−700 cm−1 (A) [(a) DMM/Ar (0.5/1000); (b) HCl/Ar (2/1000); (c) DMM + HCl mixture/Ar (1/3/1000)] and in a N2 matrix, in the region 600−700 cm−1 (B) [(a) DMM/N2 (1/1000); (b) HCl/N2 (2/1000); (c) DMM + HCl mixture/N2 (1/3/1000)]. Spectra shown here are those recorded at 12 K. In the figure the * represent the product features.
was trapped in a low-temperature bath, which was maintained at a temperature of ∼−100 °C. We performed experiments, in which DMM, HCl, and the matrix gas were premixed in a 1 L glass bulb at room temperature (∼298 K). This gas mixture was then streamed out of a single jet nozzle and deposited onto a KBr substrate maintained at 12 K. We also performed experiments in which cis-CMME and the matrix gas was premixed using proper manometric ratios in the mixing chamber and this gas mixture was then deposited onto the cold KBr substrate. Deposition was carried out at the rate of ∼3 mmol/h and a typical deposition lasted for 1 h. The spectra were recorded using a BOMEM MB 100 FTIR spectrometer, operated at a spectral resolution of 1 cm−1. After a spectrum was recorded, the matrix was warmed to ∼35 K, maintained at this temperature for 15 min, and recooled to 12 K. The spectra of the matrix thus annealed were again recorded. We also performed experiments where DMM and HCl were codeposited in the matrix by streaming them separately through
where the reaction could proceed through alkoxy attack and compare the results with the codeposition experiments of DMM and HCl. The matrix isolation technique was used as a probe to understand the DMM−HCl interaction.
2. EXPERIMENTAL SECTION Matrix isolation experiments were carried out using a Leybold AG He-compressor-cooled closed cycle cryostat. The details of the vacuum system and experimental setup are described elsewhere.18−21 Dimethoxymethane (DMM, 98%, MERCK) and cis-chloromethyl methyl ether (cis-CMME, 90%, Himedia) were used without any further purification. However, the samples were subjected to several freeze−pump−thaw cycles before use. HCl gas was prepared by mixing analytical grade H2SO4 and HCl solutions, in a vacuum bulb attached to a burette. Prior to mixing of the reagents, it was ensured that the vacuum bulb was evacuated to ∼5 × 10−6 mbar. The HCl gas that was produced 2348
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Figure 2. Matrix isolation infrared spectra in an Ar matrix, in the region 850−1350 cm−1 (A) and 2750−3050 cm−1 (B), where DMM and HCl were premixed prior to deposition: (a) DMM/Ar (0.5/1000); (b) HCl/Ar (2/1000); (c) DMM + HCl mixture/Ar (1/3/1000). Spectra shown here are those recorded at 12 K. The feature marked with “M” is due to CH3OH. In the figure the * represents the product, + represents the DMM features, and # represents HCl features.
were then performed to ensure that the computed structures did correspond to minima on the potential surface and also to help us in the assignments of the various vibrational features observed in our experiments. The computed frequencies were then scaled to bring them in agreement with the experimental results. To arrive at the scaling factor, we chose the strongest feature in our experiment (i.e., 1140.8 cm−1 feature) that could be unambiguously assigned to the ground state conformer (GG) of DMM and correlated it with that strongest computed feature for the ground state conformer in this region. The factor that would bring this computed frequency in agreement with the experimental feature was then calculated and used to scale all other vibrational frequencies. It turned out that the scaling factor for the frequencies calculated at the B3LYP/6-311+ +G** level was 0.9906. Similarly, the scaling factor for the frequencies calculated in the HCl region was 0.9840. The B3LYP frequencies corroborates well with our experimental data.
a twin-jet nozzle system. In this experiment, DMM was mixed with the matrix gas, in the required ratio in the mixing chamber and the resultant mixture was allowed to stream through one nozzle and deposited onto the matrix, with the flow being adjusted by a fine needle valve. Through a second nozzle and a needle valve, HCl gas was deposited. In these experiments, HCl in the reservoir was maintained at a temperature of ∼−80 °C to control its concentration in the matrix. High-purity Ar and N2 (Inox, 99.9995%) were used as matrix gases.
3. COMPUTATIONAL DETAILS Computations were performed using a GAUSSIAN 94W package.22 Geometries of the monomers were first optimized at the B3LYP/6-311++G** level of theory. Starting from the optimized monomer geometries, the geometry of the adduct was then optimized. All geometry parameters were left free in the optimization process. Vibrational frequency calculations 2349
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Figure 3. Matrix isolation infrared spectra in a N2 matrix, in the region 900−1350 cm−1 (A) and 2750−3050 cm−1 (B), where DMM and HCl were premixed prior to deposition: (a) DMM/N2 (1/1000); (b) HCl/N2 (2/1000); (c) DMM + HCl mixture/N2 (1/3/1000). Spectra shown here are those recorded at 12 K. The feature marked with “M” is due to CH3OH. In the figure the * represent the product features, + represent the DMM features, and # represent HCl features.
1123.9, 1125.4, 1157.2, 1193.4, 1283.1, 1326.9 (Figure 3A, trace c), and 2965.3 and 3016.9 cm−1 (Figure 3B, trace c). These new features were marked as asterisk in Figures 1, 2 and 3. The feature observed at 667.8 cm−1 is due to CO2 in Ar and N2 matrixes (Figures 1A,B). In the premixing experiments in a N2 matrix, the peak observed at 667.8 cm−1 (Figure 1B, trace c) broadens and increases in intensity and will be discussed in the later section. In Figures 2 and 3, we also observed features at 1033.8 cm−1 in Ar and 1034.3 cm−1 in N2, which are marked as “M” and will be discussed in the later section. The group of features observed at 2869.4, 2854.0, 2816.8, and 2786.9 cm−1 in an Ar matrix and at 2814.9 and 2815.4 cm−1 in a N2 matrix are assigned to the vibrational modes of multimers of HCl.26 When the matrix was annealed at ∼35 K/30 K, no perceptible changes in the spectra were observed. To assign the intense product bands obtained in the gas phase mixing experiments, various possible reaction products were considered, such as cis and trans methyl formate, chlorodimethoxymethane, dichlorodimethoxymethane, 1,1-dichloromethyl
For comparison, the scaled computed vibrational frequencies using standard scaling factor of 0.967323 are given in the Supporting Information. Stabilization energies were computed for the adduct, which were corrected separately, for zero point energies (ZPE) and basis set superposition errors (BSSE) using the method outlined by Boys and Bernardi.24 We did not apply both the corrections together, as it has been shown that a simultaneous application of both BSSE and ZPE correction tends to underestimate the binding energies.25
4. RESULTS AND DISCUSSION 4.1. Results of DMM and HCl Gas Phase Premixing Experiments. When DMM and HCl were premixed in the ratio 1:3:1000, and the mixture was then deposited, we observed intense product bands at 662.5, 665.4 (Figure 1A, trace c), 928.7, 995.7, 1126.4, 1157.2, 1193.9, 1279.2, 1319.7 (Figure 2A, trace c), and 3018.5, 3023.7 cm−1 (Figure 2B, trace c) in an Ar matrix. In a N2 matrix, these product bands occurred at 658.6, 661.1, 667.8, 674.6 (Figure 1B, trace c), 926.5, 1001.0, 2350
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Table 1. Selected Structural Parameters for cis-Chloromethyl Methyl Ether (cis-CMME) and trans-Chloromethyl Methyl Ether (trans-CMME) Calculated at the B3LYP/6-311++G** Level of Theory
a
parameter
cis-CMME
trans-CMME
Cl4−C1 H3−C1 O5−C1 O5−C6 C6−H9 ∠Cl4−C1−O5 ∠H3−C1−O5 ∠C1−O5−C6 ∠Cl4−C1−O5−C6 ∠H3−C1−O5−C6 ∠H2−C1−O5−C6
1.862a 1.085 1.366 1.429 1.096 113.3b 108.5 115.3 72.8 −171.0b −45.4
1.795 1.095 1.393 1.422 1.098 108.6 111.9 112.1 180.0 62.3 −62.3
Bond lengths in angstroms. bAngles in degrees.
respectively. Vibrational frequency analysis ensured that both conformers were indeed minima in the potential energy surface. The raw and ZPE corrected energies for the cis and trans conformers are given in Table 2. From Table 2, it is clear that Table 2. Energy (hartree), ZPE Energy (hartree), and Relative Energies (kcal/mol) (Raw and ZPE Corrected) for cis- and trans- Chloromethyl Methyl Ether (CMME) Calculated at the B3LYP/6-311++G** Level of Theory conformer
energy
ZPE
ΔE (kcal/mol)
cis-CMME trans-CMME
−614.7076115 −614.6997045
0.071260 0.070583
0.0 −4.96/−4.54
trans-CMME is higher in energy than the cis-CMME conformer by 4.96 kcal/mol. Further, on the basis of the Maxwell− Boltzmann population distribution, at room temperature, it can be shown that only the cis-CMME conformer is the preferred conformer in the gas phase. The trans-CMME isomer has negligible population to be of any experimental significance. Table 3 lists the calculated (scaled and unscaled) frequencies of cis-CMME at B3LYP/6-311++G** level of theory.
Figure 4. Structure of the dimethoxymethane (DMM GG), cischloromethyl methyl ether (cis-CMME), and trans-chloromethyl methyl ether (trans-CMME) computed at the B3LYP/6-311++G** level of theory.
methyl ether, dimethoxychloromethane, dimethoxydichloromethane, chloromethyl methyl ether, and methyl chloride. Computations were performed on these compounds and the calculated scaled vibrational frequencies were compared with the experimental spectrum and among these compounds mentioned above we found out that the cis-chloromethyl methyl ether (cisCMME) vibrational frequencies agreed well, a surprise product. Further, we have also carried out matrix isolation infrared experiments of cis-CMME in an Ar and N2 matrixes. First, we will discuss the computations on CMME and later on the experimental part. 4.1.1. Computations on Chloromethyl Methyl Ether (CMME). To our knowledge, there is no detailed computational work on CMME exist in the literature, so we have carried out computations on the CMME using B3LYP/6-311++G** level of theory. Computations indicated two conformers corresponding to the cis and trans form for the CMME. The structures of the two conformers are shown in Figure 4. In the same figure, the structure of ground state GG conformer of DMM is also shown for comparison. The geometrical parameters of the two conformers are listed in the Table 1. It can be seen from Table 1 that the torsional angle (Cl4−C1−O5−C6) between the cis and trans conformer of CMME are 72.8° and 180.0°,
Table 3. Experimental and Calculated (Scaled and Unscaled) Frequencies of cis-Chloromethyl Methyl Ether (cis-CMME) at the B3LYP/6-311++G** Level of Theory experimental
a
calculated
Ar matrix
N2 matrix
unscaleda
scaledb
665.4/662.5 928.7 995.7 1126.4 1157.2 1193.9 1279.2 1319.2 3018.5 3023.7
658.6/661.5/667.8/674.6 926.5 1001.0 1123.9/1125.4 1157.2 1193.4 1283.1 1326.9 2965.3 3016.9
633.9(162) 934.0(38) 1003.6(14) 1146.6(137) 1174.2(12) 1216.0(143) 1301.2(10) 1348.0(32) 3007.3(43) 3050.1(38)
622.4 917.0 985.5 1125.5 1153.0 1194.0 1277.5 1323.4 2982.3 2994.6
Intensity in km/mol. bScaling factor 0.9818.
4.1.2. Matrix Isolation Experiments on cis-CMME. To confirm whether the product formed in the gas phase mixing of DMM and HCl experiments is indeed due to cis-CMME, 2351
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Figure 5. Matrix isolation infrared spectra in the region 900−1350 cm−1: (a) computed spectra of cis-CMME; (b) DMM + HCl mixture/Ar (1/3/ 1000); (c) CMME/Ar (0.5/1000). Spectra recorded at 12 K. In the figure the * represent the product feature and + represent the DMM features.
cis-CMME spectrum in a N2 matrix shows multiple site split features in the 658.6, 661.5, 666.8, 669.3, and 674.6 cm−1 region and the scaled computed frequencies for the cis-CMME has a strong feature at 622.4 cm−1 with appreciable intensity (Table 3). Further, we also observed features due to the C−O stretch of CH3OH at 1033.8 cm−1 in Ar and 1034.3 cm−1 in N2, which is the other reaction product, along with cis-CMME, formed during the gas phase reaction of DMM with HCl. We could not discern any features corresponding to other modes of methanol in our experiments. The intensity of both CMME and CH3OH features in the matrix increased as the concentration of HCl was increased in the premixed gas mixture. We observed peaks at 935.1, 1051.1 and 1119.6 cm−1 in Ar and at 933.0, 1048.7, 1139.4 cm−1 in N2, which are due to unreacted DMM in the gas phase premixing experiments with HCl (Figures 2 and 3 trace c and in Figures 5 and 6 trace b). The above-mentioned features were also observed in the cisCMME spectra in Ar and N2 matrixes (Figures 5 and 6 trace c) probably due to the presence of DMM impurity. The peaks observed at 914.7, 1109.0, and 1110.9 cm−1 (Figure 2A trace c and Figure 5 trace b), 913.2 and 1113.8 cm−1 (Figure 3A trace c and Figure 6 trace b) could not be assigned to either DMM or cis-CMME.
we have carried out matrix isolation infrared experiments of cis-CMME in both Ar and N2 matrixes and compared with the results of the gas phase mixing experiments and computed spectra of cis-CMME. Figures 5c, 6c, and 7c (A, B) show the cis-CMME spectra recorded in Ar and N2 matrixes. In the same figures the computed spectra (Figures 5a, 6a, and 7a (A, B)) and the gas phase mixing experiments (Figures 5b, 6b, and 7b (A, B)) were also shown for comparison. It is clear from the figures, in both Ar and N2 matrixes, the cis-CMME spectra match well with the computed cis-CMME spectra. Further, there is excellent agreement between the infrared spectra of cis-CMME and the product formed in the gas phase premixing experiment of DMM and HCl. Table 3 compares the calculated vibrational frequencies with the experimental value, and there is a good agreement between the two, which further ascertains that the product formed in the gas phase reaction is cis-CMME. As mentioned earlier, we observed small amounts of CO2 impurity peak at 667.8 cm−1 in Ar and N2 matrixes (Figure 1A,B trace b). In the DMM and HCl premixing experiments in a N2 matrix (Figure 1B trace c and Figure 7B trace b) the feature at 667.8 cm−1 broadens and increases in intensity, clearly showing that the product feature (cis-CMME) probably overlaps with the CO2 feature. This is further supported by the fact that the 2352
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Figure 6. Matrix isolation infrared spectra in the region 900−1350 cm−1: (a) computed spectra of cis-CMME; (b) DMM + HCl mixture/N2 (1/3/ 1000); (c) CMME/N2 (0.25/1000). Spectra recorded at 12 K. In the figure the * represent the product features and + represent the DMM features.
4.2. Codeposition Experiments of DMM and HCl Using a Twin-Jet Nozzle. The twin-jet codeposition experiments were performed to trap the initial intermediate, viz., the hydrogen-bonded alkoxy adduct formed in the gas phase reaction. 4.2.1. Experimental Procedures. Figures 8 (850−1250 cm−1) and 9 (2550−2400 cm−1) show the infrared spectra obtained when DMM and HCl codeposited in an Ar matrix. The spectrum shown in Figure 8 corresponds to the C−O stretch and CH3 deformation mixed vibrational modes of the DMM region, whereas those shown in Figure 9 correspond to the HCl stretching region. It can be seen from the above spectra that product bands of the DMM−HCl adduct were observed at 882.9, 900.2, 913.2, 932.5, 1037.2, 1042.5, 1116.2, 1137.9, 1192.4, and 2447.5 cm−1 in an Ar matrix; assignments of these product bands will be discussed in a later section. The features of the adduct were clearly resolved from those of the precursor molecules and are marked with arrows in Figures 8 and 9. All the spectra shown here are those recorded after annealing the matrix. The intensities of the product features increased when the concentration of either of the two precursors was increased, confirming that these features must be due to DMM−HCl adducts. 4.2.2. Computations. In an earlier work16 using HF and B3LYP/6-31++G** levels of theory, the lowest energy con-
former of DMM was shown to have a GG structure, with a population of ∼96%. Three other conformers with TG, G+G− (“G+” refers to clockwise and “G−” refers to anticlockwise gauche orientations of methoxy group), and TT (“T” refers to trans orientation of methoxy group) structures, were also found to be minima, with progressively higher energies, and which together had a population of ∼4%. Consequently, only the GG ground state conformer of DMM was considered in this study, on the DMM−HCl adducts. Only one minimum was located on the DMM−HCl potential surface, corresponding to one proton acceptor site on the DMM molecule. Due to the C2 symmetry of the ground state conformer of DMM, the two oxygens are equivalent and, hence, this result in only one possible adduct at oxygen. The structure of the alkoxy adduct along with the raw energy, ZPE corrected energy, and BSSE corrected energy is shown in Figure 10. The selected structural parameters of the alkoxy adduct are given in Table 4. In the DMM−HCl adduct, the alkoxy oxygen on DMM served as the proton acceptor and the hydrogen of HCl served as a proton donor. The vibrational frequencies of the alkoxy adduct are presented in Table 5. It should be pointed out that in the alkoxy adduct the hydrogen-bonded distance between the H14 of HCl and O4 of DMM is 1.756 Ǻ as a result of this interaction, the 2353
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Figure 7. Matrix isolation infrared spectra in the region 600−700 cm−1 in an Ar matrix (A) and in a N2 matrix (B): (a) computed spectra of cisCMME; (b) DMM + HCl mixture/Ar or N2 (1/3/1000); (c) CMME/Ar (0.5/1000) and CMME/N2 (0.25/1000). Spectra recorded at 12 K. In the figure the * represents the product features.
infrared spectra of HCl trapped in an Ar matrix. The feature obtained at 2887.7 cm−1 (not shown in figure) is assigned to the HCl monomer in an Ar matrix. When HCl was codeposited with DMM and the matrix then annealed, a broad and strong feature appeared at 2447.5 cm−1 (Figure 9c,d), which amounts to a red shift of ∼440.0 cm−1 from the HCl monomer feature. This value agrees well with the scaled computed value of 2473.4 cm−1 for the alkoxy adduct, a red shift of ∼414.0 cm−1 (Table 5). The agreement between the experimental and computed frequencies supports the computationally derived hydrogen-bonded structure of the alkoxy adduct, with HCl being the proton donor. 4.3. Correlation between Gas Phase and LowTemperature Matrix Experiments. It can be reiterated that our recent work on the TMPhite−HCl system yielded CH3Cl as one of the products in the gas phase premixing experiments whereas in the twin-jet deposition experiments, the hydrogen-bonded alkoxy adduct was produced.17 The gas phase reaction proceeds similarly to the Arbuzov mechanism, where the lone pair on phosphorus served as a nucleophilic center, and the reaction is prevented in the matrix due to the cage effect.
corresponding H−Cl bond length is marginally increased (by 2.4%) and the vibrational frequency is red-shifted by about ∼420.0 cm−1 from their values in free HCl. 4.2.3. Vibrational Assignments of DMM−HCl Adduct. 4.2.3.1. Features of the DMM Submolecule in the DMM− HCl Adduct: C−O and CH3 Deformation Modes. The spectral features at 934.9, 1051.1, 1119.6, 1140.8, 1190.0 cm−1 (Figure 8a) are assigned to both the C−O stretch and CH3 deformation modes of the ground state GG conformer of DMM.16 When DMM and HCl were codeposited and annealed (Figure 8b,c), new spectral features were observed at 882.9, 900.2, 913.2, 932.5, 1037.2, 1042.5, 1116.2, 1137.9, and 1192.4 cm−1, which are due to the adduct of DMM and HCl. Vibrational frequencies are presented in Table 5. The features observed at 882.9/900.2/913.2 and 1037.2/1042.5 cm−1 are likely to be site split multiplets. The theoretical frequencies of the alkoxy adduct (Table 5) in the DMM region, after appropriate scaling, were computed to occur at 888.4, 922.1, 1043.8, 1033.8, 1114.3, 1142.0, and 1199.6 cm−1. It can be seen that there is excellent agreement between the experimental and computed frequencies of the alkoxy adduct. 4.2.3.2. Features of the HCl Submolecule in the DMM−HCl Adduct: H−Cl Stretch. Figure 9a shows the matrix isolation 2354
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Figure 8. Matrix isolation infrared spectra in an Ar matrix, in the region 850−1250 cm−1, where DMM and HCl were separately codeposited: (a) DMM/Ar (0.5/1000); (b) DMM/HCl/Ar (0.5/0.5/ 1000); (c) DMM/HCl/Ar (0.5/1/1000). Spectra shown here are those recorded at 12 K after annealing the matrix at 35 K.
Figure 9. Matrix isolation infrared spectra in an Ar matrix, in the region 2550−2400 cm−1, where DMM and HCl were separately codeposited: (a) HCl/Ar (1/1000); (b) DMM/Ar (0.5/1000); (c) DMM/HCl/Ar (0.5/0.5/1000); (d) DMM/HCl/Ar (0.5/1/1000). Spectra shown here are those recorded at 12 K after annealing the matrix at 35 K.
In our present work on the DMM and HCl interaction, as DMM does not have a strong nucleophilic center like phosphorus in TMPhite, the occurrence of reaction in the gas phase is surprising. Unlike TMPhite, DMM possesses methyl and methylene carbons and the attack of chlorine of HCl on these carbons would yield two different products. Figure 11 shows the intermediate and the probable mechanistic pathways “I” and “II” of the gas phase reaction between DMM and HCl. On the basis of the gas phase premixing experiments, cisCMME spectra in Ar and N2 matrixes, and computational results, we believe that the product formed follows the reaction pathway “I” to yield cis-CMME and methanol. The product formation could be a two-step process. In the first step the hydrogen of HCl forms a hydrogen-bonded adduct with the alkoxy oxygen, thus forming an intermediate. It should be pointed out that this intermediate is the one that could be trapped in the twin-jet deposition experiments of DMM and HCl. Subsequently, the nucleophilic chlorine of HCl attacks the electropositive methylene carbon, forming cis-CMME and methanol. The second step is prevented in the twin-jet deposition experiments because of the matrix cage effect. The gas phase premixing experiments of DMM and HCl revealed the occurrence of a new reaction channel and yielded only cis-
Figure 10. Structure of the DMM−HCl alkoxy adduct computed at the B3LYP/6-311++G** level of theory. Raw/ZPE/BSSE corrected stabilization energies in kcal/mol are given against the structure.
CMME and methanol as products. In this work, we have reported for the first time that HCl acts as a nucleophile at room temperature, which supports the attack of chlorine of HCl on to the methylene carbon. 4.4. Computations for the Gas Phase Reaction Channel. To rationalize the reaction between DMM and 2355
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Figure 11. Two different pathways for the reaction between DMM and HCl in the gas phase to produce (I) chloromethyl methyl ether (CMME) and CH3OH and (II) methoxymethanol (GG) and CH3Cl.
Table 4. Selected Structural Parametersa for the Alkoxy Adduct and the DMM Monomer Computed at the B3LYP/ 6-311++G** Level parameter
DMM
alkoxy adduct
C1−H2 C1−O5 C1−O4 O5−C10 O4−C6 H14−O4 Cl15−H14 ∠C1−O4−C6 ∠C1−O5−C10 ∠Cl15−H14−O4 ∠C10−O5−C1−O4 ∠C6−O4−C1−O5 ∠Cl15−H14−O4−C1
1.096 1.405 1.405 1.424 1.424
1.096 1.393 1.421 1.429 1.432 1.756 1.318 113.3 114.4 178.2 72.8 66.9 86.3
113.9 113.9 69.0 69.0
Table 5. Experimental and Computed Vibrational Frequencies (Unscaled and Scaled) for DMM, HCl, and DMM−HCl Alkoxy Adduct Calculated at the B3LYP/ 6-311++G** Level of Theory calculated frequencies (cm−1) experimental frequencies (cm−1) DMM
unscaleda
scaled
931.5(85) 1053.7(220) 1126.5(84) 1151.6(156) 1207.9(26)
922.8 1043.8 1115.9 1140.8 1196.6
b
934.9 1051.1 1119.6 1140.8 1190.0 HClc 2887.7
2934.7(32) DMM−HCl Alkoxy Adduct 882.9/900.2/913.2 896.8(104)b 932.5 930.8(52)b 1037.2/1042.5 1043.5(229)b 1116.2 1124.8(74)b 1137.9 1152.8(134)b 1192.4 1211.0(28)b 2447.5 2513.7(1182)c
a
Bond lengths in angstroms. Bond angles and dihedral angles in degrees.
HCl in the gas phase, we performed free energy calculation for both reactions, as shown in Figure 11. Free energy values were calculated for both the reactants and products using the B3LYP/6-311++G** level of theory. For DMM, the most stable GG geometry was taken into account. Similarly, only the ground state conformation of product, cis-CMME and CH3OH for the reaction pathway “I” and methoxy methanol (GG) and methyl chloride for the reaction pathway “II”, was considered. The calculated free energy change for this substitution reaction pathway “I” and “II” was found to be ∼0.22 and −5.05 kcal/mol, respectively. Table 6 gives the internal energy, enthalpy, entropy, and free energy changes for both reaction pathways “I” and “II”. Both reactions “I” and “II” proceed through the same hydrogen-bonded alkoxy adduct intermediate. By comparing the free energy values for both reactions, one can be surmise that reaction pathway “II” is thermodynamically more favorable than “I”. On the contrary, experimentally, we observed the products following only pathway “I”. It is likely that pathway “I” is kinetically favored over pathway “II”. It can therefore be concluded that, in the gas phase, where the reaction is not frustrated, the nucleophile directs the kinetic products through pathway “I”.
a
Intensities in km/mol are given in parentheses. 0.9906. cScaling factor 0.9840.
2887.7 888.4 922.1 1033.8 1114.3 1142.0 1199.6 2473.4 b
Scaling factor
Table 6. Thermodynamic Parameters Such as Change in Internal Energies (ΔE), Enthalpies (ΔH), Entropies (ΔS), and Free Energies (ΔG) for Reaction Pathways I and II Calculated at the B3LYP/6-311++G** Level of Theory thermodynamic parameters reaction pathways
ΔE (kcal/mol)
ΔH (kcal/mol)
ΔS [kcal/(mol/K)]
ΔG (kcal/mol)
I II
1.9157 −3.5065
1.9164 −3.5058
0.0057 0.0052
+0.22 −5.05
5. CONCLUSIONS Matrix isolation infrared spectroscopy was used to study the interaction between DMM and HCl both in the gas phase and in the low-temperature matrix. In the gas phase, DMM and HCl probably proceed through a two-step process where the initial step is the hydrogen-bonded alkoxy intermediate 2356
dx.doi.org/10.1021/jp400332v | J. Phys. Chem. A 2013, 117, 2347−2357
The Journal of Physical Chemistry A
Article
G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, revision B.3; Gaussian, Inc.: Pittsburgh, PA, 1995. (23) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111, 11683−11700. (24) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553−556. (25) Turi, L.; Dannenberg, J. J. J. Phys. Chem. 1993, 97, 7899−7909. (26) Perchard, J. P.; Maillard, D.; Schriver, A.; Girardet, C. J. Raman Spectrosc. 1981, 11, 406−415.
formation followed by the nucleophilic attack of chlorine of HCl on the electropositive methylene carbon to yield cis-CMME and CH3OH. When DMM and HCl are separately codeposited using the twin jet technique in an Ar matrix, hydrogen-bonded alkoxy adduct is produced which is corroborated using the B3LYP/6-311++G** level of theory. In the codeposition experiments, the reaction is further frustrated due to the cage effect of the matrix. Interestingly, the hydrogen-bonded alkoxy adduct intermediate formed in the gas phase reaction is the one trapped in the low-temperature matrix and the results obtained are in contrast with the TMPhite−HCl studies.
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ASSOCIATED CONTENT
* Supporting Information S
Cartesian coordinates, absolute energies, frequencies (unscaled and scaled) of all the structures, and thermodynamic parameters for the reaction pathways calculated at B3LYP/ 6-311++G** levels of theory. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] Notes
The authors declare no competing financial interest.
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REFERENCES
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