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Argon matrix isolation study of the thermal and photochemical reaction of OVCl3 with (CH3)2CO
Chemical Physics 290 (2003) 211–221 www.elsevier.com/locate/chemphys
Argon matrix isolation study of the thermal and photochemical reaction of OVCl3 with ðCH3Þ2CO David A. Kayser, Bruce S. Ault * Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA Received 10 January 2003; in final form 28 February 2003
Abstract The matrix isolation technique has been combined with infrared spectroscopy and theoretical calculations to identify and characterize the initial and secondary products in the thermal and photochemical reactions of OVCl3 with ðCH3 Þ2 CO. Initial deposition into argon matrices at 14 K led to the formation, in high yield, of the 1:1 molecular complex. This species appears to be strongly bound, leading to large shifts to certain vibrational modes of both the acid and base subunits. Bands due to the complex were destroyed by near-UV irradiation (k > 300 nm), and led to the formation of intense product bands. In contrast to previous studies, HCl elimination from an initial complex was not observed. Many possible products were considered, including isomerization, decomposition, addition, and addition followed by fragmentation pathways. The products were identified by the use of isotopic labeling and comparison to theoretical calculations. The primary product was determined to be Cl3 VðCH3 ÞOCðOÞCH3 , formed through rupture of a C–C bond in ðCH3 Þ2 CO and addition of the two fragments to OVCl3 . Possible evidence for a second isomer, slightly higher in energy ðCl3 VðOCH3 ÞðCðOÞCH3 ÞÞ was also found. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction High valent transition metal oxo compounds, including OVCl3 and CrCl2 O2 , are very strong oxidizing agents and are known to oxidize a wide range of organic substrates. This oxidizing power has applications in catalysis as well as in organic synthesis [1,2]. Despite the utility of these reagents, the mechanism of oxidation is not well under-
*
Corresponding author. Tel.: +1-513-556-9200; fax: +1-513556-9239. E-mail address: [email protected] (B.S. Ault).
stood. Theoretical calculations have also sought to explore the reaction surface, but only for a limited number of organic substrates [3–6]. The matrix isolation technique [7–9] was developed to facilitate the isolation and spectroscopic characterization of reactive intermediates. This approach has been applied to the study of a wide range of species, including radicals, weakly bound molecular complexes, and molecular ions. A recent series [10–15] of matrix isolation studies has examined the reactions of both OVCl3 and CrCl2 O2 with small molecules, including H2 O, CH3 OH, CH3 SH and NH3 . For each system, a sequence of intermediate species was observed,
0301-0104/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-0104(03)00140-X
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from the initially formed 1:1 molecular complex to the thermally or photochemically induced HCl elimination product. The reactions of OVCl3 and CrCl2 O2 with small molecules that do not contain an acidic hydrogen have not been examined extensively, due to the reduced reactivity in these systems [16]. Acetone has an electronic absorption in the near UV, increasing the possibility of photochemical reaction with OVCl3 (this species also absorbs in the near UV as well as in the visible). Consequently, a study was undertaken to explore the photochemical reaction of acetone with OVCl3 in argon matrices, using theoretical calculations to complement the experimental data.
2. Experimental section All of the experiments in this study were carried out on a conventional matrix isolation apparatus that has been described [17]. Oxyvanadium trichloride OVCl3 (Aldrich), was introduced into the vacuum system as the vapor above the room temperature liquid, after purification by freeze– pump–thaw cycles at 77 K. Acetone (Fisher), ðCD3 Þ2 CO and ðCH3 Þ2 13 CO (both Cambridge Isotope Laboratories, 99%) were introduced in a similar manner into a separate vacuum manifold and were purified by repeated freeze–pump–thaw cycles at 77 K. Argon and nitrogen (Wright Brothers) were used as the matrix gas in different experiments, and were used without further purification. Matrix samples were deposited in both the twin jet and merged jet modes. In the former, the two gas samples were deposited from separate nozzles onto the 14 K cold window, allowing for only a very brief mixing time prior to matrix deposition. Several of these matrices were subsequently warmed to 33–35 K to permit limited diffusion and then recooled to 14 K and additional spectra recorded. In addition, most of these matrices were irradiated with the H2 O/Pyrex filtered output of a 200 W medium pressure Hg arc lamp, after which additional spectra were recorded. Irradiation times as short as 1 min and as long as 3 h were used. Additional experiments were conducted using a range of wavelength filters. In several experiments,
the matrix was irradiated during the 20 h of the deposition process. A few experiments were conducted in the merged jet mode [18], in which the two deposition lines were joined with an Ultratorr tee at a distance from the cryogenic surface, and the flowing gas samples were permitted to mix and react during passage through the merged region. The length of this region was variable; typically, a 90 cm length was employed. In both twin and merged jet, matrices were deposited at the rate of 2 mmol/h from each sample manifold onto the cold window. Final spectra were recorded on a Perkin Elmer Spectrum One Fourier transform infrared spectrometer at 1 cm1 resolution. Theoretical calculations were carried out on a number of possible intermediates in this study, using the Gaussian 98W suite of programs [19]. Several different model chemistries were employed, including Hartree Fock and density functional calculations. These calculations were used to locate energy minima, determine structures and calculate vibrational spectra. Final calculations with full geometry optimization employed the 6311G+(d,2p) basis set, after initial calculations with smaller basis sets were run to approximately locate energy minima.
3. Results Prior to any codeposition experiments, blank experiments were run on each of the reagents used in this study. In each case, the blanks were in good agreement with literature spectra [20,21] and with blanks run previously in this laboratory. Each blank experiment was then irradiated by the H2 O/ Pyrex filtered output of a 200 W Hg arc lamp for 1.5 h. No changes were observed in any of the blank spectra as a result of irradiation. Weak bands due to HCl impurity were noted in all of the OVCl3 blank experiments [22]. 3.1. OVCl3 þ ðCH3 Þ2 CO In an initial experiment, a sample of Ar= ðCH3 Þ2 CO ¼ 500 was codeposited with a sample of Ar=OVCl3 ¼ 200 in the twin jet mode. Upon
D.A. Kayser, B.S. Ault / Chemical Physics 290 (2003) 211–221
initial deposition, new bands of moderate intensity were noted near 430, 453, 568, 1025, 1250, 1421, and 1678 cm1 (hereafter referred to as set A). The band near 1678 cm1 was the most intense and distinctive of this set of bands. When this matrix was subsequently irradiated for 1.0 h with the H2 O/Pyrex-filtered output of a medium pressure Hg arc lamp, all of these bands were completely destroyed. New bands were noted at 419, 478, 679, 891, 988, 1011, 1124 and 1170 cm1 (hereafter referred to as set B), several of which were quite intense, as shown in Fig. 1. No other new bands were noted in the spectrum; the region near 2800 cm1 was examined closely and no changes were apparent in this region. This pair of reactants was studied in a series of 15–20 additional experiments, using a wide range of reactant concentrations. In every experiment, all of the set A bands were observed after initial sample deposition. Moreover, the relative intensities of these bands remained constant over all of these experiments, while the absolute intensities varied directly with the concentrations of the reactants. When several of the matrices were annealed to approximately 25 K and then recooled to 12 K, all of the set A bands grew slightly, and at the same rate. All of these matrices were then irradiated with the H2 O/Pyrex filtered output of a medium pressure Hg arc. All of the set A bands were destroyed in each experiment, and concurrently the set B bands appeared. In all of these experiments, all of the bands in set B maintained a constant intensity ratio with respect to one another. In addition, in several experiments weak bands were observed after irradiation, at 798, 2808, 3450 and 3598 cm1 . These have been seen previously and have been assigned [23] to the photoproduct of OVCl3 and impurity H2 O, namely the cage-paired species Cl2 VðOÞOH and HCl. Several experiments were conducted to better characterize the processes leading to product formation. When merged jet experiments were employed, the results were essentially identical to those obtained with twin jet deposition. In another experiment, a sample of Ar=ðCH3 Þ2 CO ¼ 200 was codeposited with a sample of Ar=OVCl3 ¼ 200. This sample was irradiated as before, but for just 1 min. The resultant spectrum showed nearly com-
213
plete destruction of the set A bands and nearly complete formation of the set B bands. No other new bands were observed. Further irradiation of this sample led to complete destruction of the set A bands, and slight additional growth of the set B bands. Also, a similar pair of samples were codeposited, and irradiated continuously during the deposition process. The resulting spectrum showed strong bands due to set B, while the bands of set A were absent. In addition, new weak bands were noted near 1304 and 2138 cm1 . Several experiments were conducted using filtered irradiation in different wavelength regions. When a similar matrix containing OVCl3 and ðCH3 Þ2 CO was irradiated with light of k > 595 nm for 1.5 h, bands due to set A were reduced by 10–15%, while weak bands grew nearby, at 423, 466, 562, 1032 and 1686 cm1 . When this matrix was then irradiated with light of k > 530 nm for 1.5 h, bands due to set A were completely destroyed, while the bands at 423, 466, 562, 1032 and 1686 cm1 were reduced. New bands, again nearby, were noted at 469, 1008 and 1702 cm1 . Irradiation of this matrix with light of k > 510 nm did not lead to any changes. Finally, when this matrix was irradiated with light of k > 300 nm (H2 O/Pyrex filter), the bands at 469, 1008, 1420 and 1702 cm1 were completely destroyed, and the bands of set B grew in with characteristic intensity. 3.2. Isotope studies Samples of Ar=ðCD3 Þ2 CO were deposited with samples of Ar=OVCl3 in several twin jet experiments. Initial product bands, analogous to the set A band, above, were observed in all of these experiments, at 423, 452, 1024, 1267 and 1667 cm1 . In these experiments, bands in this set maintained a constant intensity ratio with respect to one another. Each matrix was then irradiated for 1.5 h with the H2 O/Pyrex filtered output of a medium pressure Hg arc (k > 300 nm). All of the above bands were destroyed, and a new set analogous to set B was observed, with bands at 530, 614, 768, 928 (br), 986, 1068 and 1142 cm1 . These bands were reproducible and maintained a constant intensity ratio with respect to one another in all experiments.
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Fig. 1. Infrared spectra, over selected spectral regions, of a matrix prepared by the twin jet codeposition of a sample of Ar=OVCl3 ¼ 200 with a sample of Ar=ðCH3 Þ2 CO ¼ 200. The lower trace is before irradiation while the upper trace is after 1.5 h irradiation. Bands marked with a ‘‘+’’ grew in upon irradiation, while bands marked with a ‘‘)’’ were destroyed by irradiation.
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A similar series of experiments was conducted using acetone with a 13 C label in the 2 position [CH3 13 CðOÞCH3 ], and very similar results were obtained. Bands of set A were observed at 427, 453, 476, 567, 1027, 1223, 1366 and 1642 cm1 . These were destroyed by irradiation, and set B bands grew in at 418, 666, 884, 982, 1112, 1154 and 1372 cm1 . The positions and intensities of these bands mirrored closely those observed in the normal isotopic experiments. Again, the relative intensities of the bands within each set were constant through this series of experiments. All product bands are compiled in Tables 1 and 2, while Fig. 2 shows comparative spectra from 12 C and 13 C experiments.
4. Discussion Upon initial deposition, either twin jet or merged jet, a number of product bands were de-
215
tected, many with substantial intensity (set A bands, above). The observation of these bands following twin jet deposition strongly suggests that there are the initial intermediate in the reaction between OVCl3 and ðCH3 Þ2 CO, since the time available for reaction during twin jet deposition is very limited. Further, these bands grew when the initially deposited matrices were annealed to 30 K and recooled, indicating that the barrier to reaction is very low, approaching zero. In addition, each of the bands in set A was observed relatively near (within 50 cm1 ) of a parent band of either OVCl3 or ðCH3 Þ2 CO, and with similar relative intensities (i.e., set A bands near intense parent bands were relatively intense, while set A bands near weaker parent bands were relatively weaker). Finally, the isotopic dependence (deuterium or 13 C) of each set A band was nearly identical to the isotopic dependence of the nearby parent mode; for example, the 1678 cm1 band of set A showed an 11 cm1 red shift when ðCD3 Þ2 CO was
Table 1 Product band positions before irradiation of matrices containing OVCl3 and ðCH3 Þ2 CO and its isotopomers OVCl3 þ ðCH3 Þ2 CO
OVCl3 þ ð13 CH3 Þ2 CO
430 453 568 1025 1250 1421 1678
427 453, 476 567 1027 1223
1024 1280
1642
1667
OVCl3 þ ðCD3 Þ2 CO 423 452
Assignment –VCl3 str. –VCl3 str. C–C@O bend V@O stretch CH3 rock CH3 bend C@O stretch
Table 2 Product band positions after irradiation of matrices containing OVCl3 and ðCH3 Þ2 CO and its isotopomers OVCl3 þ ðCH3 Þ2 CO
OVCl3 þ ð13 CH3 Þ2 CO
419 478
418
679 891 988 1011 1124 1170
666 884 982
a b
1112 1154 1372
Approximate descriptions, modes strongly mixed. Strongly mixed with –CH3 rock.
OVCl3 þ ðCD3 Þ2 CO
Assignmenta VCl3 sym. st. VCl3 asym. st.
530 614 768 928 986 1068 1142
V–O–C bend V–O–C st. CH3 rock C–C st.b C–C st.b
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Fig. 2. Infrared spectra, over selected spectral regions, of a matrix prepared by the twin jet codeposition of a sample of Ar=OVCl3 ¼ 200 with a sample of Ar=ðCH3 Þ2 CO ¼ 200 after 1.5 h irradiation (lower trace) compared to a similar sample employing ðCH3 Þ2 13 CO (upper trace). Bands marked with a ‘‘+’’ grew in upon irradiation.
employed, which is the same shift observed for the nearby C@O stretching mode of parent acetone. These observations are indicative of the formation of a molecular complex between the two reacting species. In a molecular complex, each subunit is perturbed upon complex formation, and vibrational modes are shifted, yet each subunit in the complex maintains its molecular identity [24]. The shifting of key vibrational modes, with identical isotopic behavior, supports this conclusion. In addition, complex formation has very little or no activation barrier, and is likely to occur upon annealing the matrix to 30 K. The bands of set A maintained constant relative intensities in the many experiments over a wide range of concentrations that were performed in this study, indicating that they must all be assigned to a single complex. Given the low concentrations that were employed in a number of the experiments, this complex very likely has 1:1 stoichiometry. This conclusion is consistent with several earlier studies
[10–15] of the reactions of Lewis bases with OVCl3 and CrCl2 O2 , and is consistent with the ability of ðCH3 Þ2 CO to coordinate to transition metal centers [25]. Therefore, the product bands formed upon initial matrix deposition (set A bands) are assigned to the 1:1 molecular complex between OVCl3 and ðCH3 Þ2 CO and its isotopomers. In the large majority of the previous studies in this series, the initially formed 1:1 complex was photoreactive (and for some, thermally reactive as well), eliminating HCl and forming a novel transition metal compound. The remainder of the previously studied complexes were not photoreactive; each of these was a complex involving a Lewis base without an active hydrogen (e.g. ðCH3 Þ3 N). The present 1:1 complex between OVCl3 and ðCH3 Þ2 CO does not fit into either of these categories. The complex was clearly photoreactive, with bands of the complex being nearly completely destroyed in 1 min of irradiation with light of k > 300 nm. At the same time, HCl was
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clearly not a product of this photochemically induced reaction. Careful examination of the H–Cl stretching region (both free [26] and weakly hydrogen bonded) found no trace of HCl formation, nor was any DCl observed when ðCD3 Þ2 CO was employed. This is consistent with the fact that ðCH3 Þ2 CO does not contain an ‘‘active’’ hydrogen. The identity of the photoproduct remains to be determined. The photoproduct(s) arising from irradiation of the 1:1 complex are characterized by the set B bands. The intensities of these bands were measured carefully in all of the experiments, over a wide range of sample concentrations. In every experiment, they maintained a constant intensity ratio with respect to one another, within experimental error. Further, when the 1:1 complex was subjected to wavelength-selective irradiation, from red through near-UV, all of the set B bands grew at the same rate. In addition, the same product bands were observed in the same intensity ratios whether the irradiation was for 1.0 min over several hours. These results indicate that either a single species is formed upon irradiation in all of these experiments, or that multiple species are formed, but always in exactly the same ratio. A single product species could arise either from the addition of the ðCH3 Þ2 CO subunit to the OVCl3 subunit (photo-induced combination reaction) or by a photo-induced isomerization reaction, leading to another isomer in the C3 H6 O potential energy surface, plus OVCl3 . Multiple species could form through a photo-induced decomposition reaction in which either the ðCH3 Þ2 CO or the OVCl3 subunits, or both, undergo decomposition upon irradiation of the complex. Also, multiple species might arise from a direct reaction between the two, such as oxygen atom transfer from OVCl3 to ðCH3 Þ2 CO. All of these possibilities must be considered. Photochemically induced isomerization would produce new species on the C3 H6 O potential energy surface. Many such species are known, including oxetane, dimethyl dioxirane, methyl oxirane (propylene oxide), methyl vinyl ether, cyclopropanol, and the enol form of acetone (H3 C–CðOHÞ@CH2 ). All of these potential products are known compounds, and infrared spectra
217
are available in the literature [27–32] (at least for the normal isotope and the fully deuterated species), for comparison. Even taking into account the possibility of weak complexation through the oxygen to OVCl3 in the same matrix cage, the spectra of these compounds do not match the spectra obtained here. As such, the photochemically induced isomerization products must be eliminated as candidates for the product formed here. The possibility of photochemically induced decomposition of acetone must be considered as well. The most likely channels include formation of C2 H6 þ CO, and C2 H4 þ H2 CO. All four of these species are well known, and have distinctive infrared spectra. Again, the spectra of these compounds [33–36] do not match the spectra obtained here, and must be eliminated as candidates. A similar mechanism is direct oxidation of acetone by OVCl3 , to form either a single product of the formula C3 H6 O2 , or by oxidation followed by decomposition. In the former case, formation of methyl acetate is most likely, while in the latter case, formation of C2 H6 þ CO2 is most likely (in either case, VCl3 would be an additional product). While direct oxidation has been observed for the photochemical reaction of OVCl3 with PH3 [16], the spectra of methyl acetate, C2 H6 and CO2 again do not match the observed spectra [33,37]. Thus the mechanism of direct oxidation through O atom transfer can be ruled out. Photochemical addition of acetone to OVCl3 is a viable mechanism as well. This might occur either through formation of a metalocycle, or through addition and expanded coordination at the vanadium center. There are two possibilities for the former case (2 + 2 and 2 + 4 cycloaddition, to form 4-member and 6-member metalocycles, respectively), while in the latter case, several possibilities exist. All of these involve transfer of one fragment from ðCH3 Þ2 CO to the oxo group, and addition of the second fragment to the V center. While such reactions have not been observed experimentally, Zeigler and coworkers [4–6] have calculated that the reaction of CH3 OH with OVCl3 could occur by H atom transfer to the oxo group, and addition of the methoxy fragment to the vanadium to form Cl3 VðOHÞOCH3 . While experimental evidence
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[12] shows that under matrix isolation conditions this reaction does not occur (Cl2 VðOÞOCH3 þ HCl are formed), Cl3 VðOHÞ OCH3 was shown to be a local minimum on the potential energy surface, and hence transfer reactions of this type are feasible. Finally, there is the possibility of addition followed by elimination, such as forming Cl2 VðOÞ CðOÞCH3 þ CH3 Cl. However, this last possibility can be ruled out by the lack of observation [38] of bands due to CH3 Cl. All of the potential products of photochemical addition are as-yet unknown chemical species, so that direct comparison of infrared spectra is not possible. Therefore, the best alternative is theoretical calculation. Calculations can determine whether different possible structures represent local energy minima (i.e., stable species), and can predict infrared spectra, including isotopic shifts. Previous studies of similar compounds (e.g., Cl2 VðOÞOCH3 and Cl2 VðOÞNH2 ) have shown that good agreement is generally obtained using B3LYP/6-311G+(d,2p), within 1–3% on band positions and within a few cm1 on isotopic shifts. Calculations were carried out on both possible metalocycles, as well as on the three addition products ðCl3 VðOCH3 ÞðCðOÞCH3 Þ, Cl3 VðOCðOÞCH3 ÞðCH3 Þ and Cl3 VðOCH3 ÞðOCH2 CH2 ÞÞ. All five candidates optimized to stable minima at all levels of theory. Then, infrared spectra were calculated for all five species, for the normal isotopic species, the perdeuterated species, and the 13 C labeled species arising from acetone labeled with 13 C in the 2 position. Of these 5, two can be eliminated quickly, the 6-membered metalocycle and Cl3 V ðOCH3 ÞðOCH2 CH2 Þ. For both, the agreement of calculated and experimental spectra is very poor. In addition, both products required extensive hydrogen migration, which is itself unlikely, and would probably have led to HCl formation within the matrix cage. Thus, these two candidates are eliminated. Of the remaining three, the 4-membered metalocycle can be eliminated as well, by comparison of experimental and theoretical spectra. The calculations predict that the most intense band in the spectrum would occur at 763 cm1 , whereas no band at all was seen near this position (the closest, at 679 cm1 , was one of the weaker bands). In addition, the calculated spectrum does not account for
two intense bands between 1100 and 1200 cm1 . Thus, this species is eliminated from consideration. The two remaining addition products arise from the photochemical cleavage of a C–C bond in ðCH3 Þ2 CO, leaving a –CH3 fragment and a –CðOÞCH3 fragment (this corresponds to the primary mode of photochemical decomposition of ðCH3 Þ2 CO). One of these adds to the oxo group, and the other to the V center, yielding the two remaining candidates. Both of these novel species calculated to stable energy minima at all levels of theory, suggesting both are viable candidates. Infrared spectra and isotopic shifts were calculated as well. Since each species has a large number of normal vibrations, and since relatively fewer product bands were observed, attention was focused on those calculated product bands with calculated intensities of 20 km/mol or greater. With this in mind, and the spectrometer cut-off of 400 cm1 , the bands listed in Table 2 were considered, and compared to calculated for each species. The fit of experimental and theoretical spectra was much better for Cl3 VðOCðOÞCH3 ÞðCH3 Þ, in which the –CH3 fragment attached to the V center, and the –CðOÞCH3 fragment attaches to the oxo group. For example, two very intense bands were calculated between 1100 and 1200 cm1 , in agreement with experiment (none were calculated for Cl3 VðOCH3 ÞðCðOÞCH3 ÞÞ. Likewise, two intense bands were calculated between 900 and 1000 cm1 , matching the experimental bands at 891 and 988 cm1 (only one intense band was calculated in this region for Cl3 VðOCH3 ÞðCðOÞCH3 ÞÞ. Calculated isotopic shifts were also in reasonable agreement with experiment, given the high degree of mode mixing that occurs in this region. It should be noted that no C@O stretch was observed for the product species. However, as seen in Table 3, this mode should come within a few wavenumbers of parent ðCH3 Þ2 CO, and would be hidden underneath the intense parent band in each experiment. Table 3 summarizes the calculated and experimental band positions and isotopic shifts. Overall, assignment to Cl3 VðOCðOÞCH3 ÞðCH3 Þ is warranted, and is made. Fig. 3 shows the optimized structure of this compound, based on a trigonal bipyramidal geometry around the V center, with two chlorines and the methyl group in
)65 )123 )60 )56 )28 )9 )27 )38 )112 )66 )97 )10 )366 )4
equatorial positions. Table 4 gives the key calculated geometric parameters. One product band remains, at 1011 cm1 . This might be assigned to a site splitting of the 988 cm1 , although the magnitude of the splitting is larger than normal. Alternatively, by far the most intense band calculated for Cl3 VðOCH3 Þ ðCðOÞCH3 Þ is at 1070 cm1 (calculated intensity of 332 km/mol). With a scaling factor of 0.97 (typical for B3LYP/6-311G+(d,2p)) [39], this is predicted at 1038 cm1 , in reasonable agreement with the 1011 cm1 product band. While this is only a single band, is does suggest the possibility of the formation of the second isomer. It should be noted
)13 )7 )6 )12 )16
)1
Expt. shift Comp. shift
Fig. 3. Optimized structure of Cl3 VðCH3 ÞOCðOÞCH3 at the B3LYP/6-611G+(d,2p) level of theory.
0.44 0.22 0.22 0.61 0.74 0.78 1
0 0 )4 )3 )7 )21 )4 0 )43
Comp. shift
Expt. shift
Assignment
419 478 679 891 988 1124 1170
Table 4 Key geometric parameters of Cl3 VðCH3 ÞOCðOÞCH3 calculated at the B3LYP/6-611G+(d,2p) level of theory
185 145 116 355 417 470 323 45 255
Parent acetone: 1785 cm1 . Intensity in km/mol. b Relative intensity.
0.39 0.31 0.25 0.76 0.89 1 0.69 Intensity
409 451 561 951 996 1171 1192 1400 1794
a
Comp. freq.
a
Cl3 VðCD3 ÞOCðOÞCD3 Cl3 VðCH3 ÞO13 CðOÞCH3 Expt. I, rel.b Expt. freq. Calc. I, rel.b Cl3 VðCH3 ÞOCðOÞCH3
Table 3 Calculated versus experimental frequencies, B3LYP/G-311G+(d,2p), frequencies > 400 cm1 , with I > 20
219
VCl3 sym. stretch VCl3 antisym. stretch V–O–C bend V–O–C stretch CH3 rock C–C stretch/CH3 rock C–C stretch/CH3 rock CH3 bend C@O stretch
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Parameter
Value
R (V–Cl) R (V–O) R (V–C) R (C–H) R (C@O) R (C–C) a ðCl1 –V–Cl2 Þ a ðCl1 –V–Cl3 Þ a ðCl2 –V–Cl3 Þ a (C–V–O) a (V–O–C) a (O–C@O) a (O–C–C) a (H–C–H)
2.18–2.23 A 1.78 A 2.08 A 1.09 A 1.20 A 1.50 A 92.5° 125.1° 92.4° 86.0° 163.1° 121.2° 112.0° 110.0°
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that the calculations predict a relatively small energy difference between these two species, approximately 7 kJ/mol, with Cl3 VðOCðOÞCH3 Þ ðCH3 Þ being the more stable species, in agreement with the results here. Nonetheless, the possibility of formation of a small amount of the higher energy isomer Cl3 VðOCH3 ÞðCðOÞCH3 Þ is possible, and may account for the product band at 1011 cm1 . Nonetheless, this identification must be taken as tentative. When irradiation was conducted continuously during deposition, the initial complex was not observed, and the set B bands due to the photoproduct(s) were seen with high intensity. In addition, weak product bands were detected at 1304 and 2138 cm1 . These come exactly at the positions [34,40] for CH4 (most intense infrared absorption) and CO. These are very likely photoproducts from a small amount of decomposition of the primary photoproduct(s). Since irradiation occurred while the species still had mobility (i.e., before being rigidly trapped in the argon matrix), these product could separate and be isolated. Even so, the initial photoproducts were not particularly photosensitive, and only a small amount of decomposition occurred.
5. Conclusions The twin jet or merged jet codeposition of OVCl3 and ðCH3 Þ2 CO into argon matrices leads initially to formation of a rather strongly bound 1:1 complex. This species is photochemically sensitive, and is destroyed by light of k > 300 nm. Intense product bands grow in upon irradiation, and do not differ with either the length of irradiation time, or the wavelength of irradiation (with an onset of k < 500 nm). These bands are assigned to Cl3 VðOCðOÞCH3 ÞðCH3 Þ, formed through photochemical cleavage of a C–C bond in ðCH3 Þ2 CO, and addition of the two fragments to OVCl3 . This species is itself stable with respect to irradiation, to wavelengths as short as 300 nm, although when the irradiation was conducted continuously during deposition (rather than after matrix deposition) the growth of a small amount of CH4 and CO was observed. The formation of a
small amount of the (slightly) higher energy isomer Cl3 VðOCH3 ÞðCðOÞCH3 Þ could not be ruled out, on the basis of a product band near 1011 cm1 . This photochemical reaction represents a less common pathway for the reaction of acetone and related compounds. C–H bond activation is the more common pathway, while C–C bond activation occurs here. Acknowledgements The authors gratefully acknowledge support of this research by the National Science Foundation through Grant CHE 9877076. D.E. Lewis is gratefully acknowledged for helpful suggestions, while A.D. Gudmundsdottir is gratefully acknowledged for computer time. References [1] K.B. Sharpless, A.Y. Teranishi, J.E. Backvall, J. Am. Chem. Soc. 99 (1977) 3120. [2] G.K. Cook, J.M. Mayer, J. Am. Chem. Soc. 116 (1994) 1855. [3] A.K. Rappe, W.A. Goddard III, J. Am. Chem. Soc. 104 (1982) 3287. [4] T. Ziegler, J. Li, Organometallics 14 (1995) 214. [5] L. Deng, T. Ziegler, Organometallics 15 (1996) 3011. [6] L. Deng, T. Ziegler, Organometallics 16 (1997) 716. [7] S. Craddock, A.J. Hinchliffe, Matrix Isolation, Cambridge University Press, Cambridge, 1975. [8] L. Andrews, Annu. Rev. Phys. Chem. 2 (1971) 109. [9] H.E. Hallam, Vibrational Spectroscopy of Trapped Species, Wiley, New York, 1973. [10] B.S. Ault, J. Am. Chem. Soc. 120 (1998) 6105. [11] B.S. Ault, J. Mol. Struct. 526 (2000) 227. [12] B.S. Ault, J. Phys. Chem. A 103 (1999) 11474. [13] B.S. Ault, J. Phys. Chem. A 105 (2001) 4758. [14] S.R. Anderson, B.S. Ault, J. Phys. Chem. A 106 (2002) 1419. [15] B.S. Ault, J. Phys. Chem. A 104 (2000) 11796. [16] D.A. Kayser, B. S. Ault, J. Phys. Chem. A, submitted. [17] B.S. Ault, J. Am. Chem. Soc. 100 (1978) 2426. [18] J.D. Carpenter, B.S. Ault, J. Phys. Chem. 95 (1991) 3502. [19] M.J. Frisch et al. Gaussian 98, Gaussian, Inc., Pittsburgh, PA, 1998. [20] R.R. Filgueira, L.L. Fournier, E.L. Varetti, Spectrochim. Acta Part A 38 (1982) 965. [21] A. Marcou, D. Roche, Compt. Rend. Acad. Sci. Ser. B 276 (1973) 599. [22] B.S. Ault, J. Mol. Struct. 526 (2000) 97.
D.A. Kayser, B.S. Ault / Chemical Physics 290 (2003) 211–221 [23] B.L. Subel, D.A. Kayser, B.S. Ault, J. Phys. Chem. A 106 (2002) 4998. [24] B.S. Ault, Rev. Chem. Intermed. 9 (1988) 233. [25] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds Part B, fifth ed., Wiley Interscience, New York, 1997. [26] D. Maillard, A. Schriver, J.P. Perchard, J. Chem. Phys. 71 (1979) 505. [27] G. Banhegyi, P. Pulay, F. Fogarashi, Spectrochim. Acta A 39 (1983) 761. [28] B. Cadioli, E. Gallinella, U. Pincelli, J. Mol. Struct. 78 (1982) 215. [29] X.K. Zhang, J.M. Parnis, E.G. Lewars, R.E. March, Can. J. Chem. 75 (1997) 276. [30] G.A. Williams, J.N. MacDonald, J.E. Boggs, J. Chem. Soc. Faraday Trans. 86 (1990) 2805.
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[31] R.A. Kydd, H. Weiser, W. Kiefer, Spectrochim. Acta 39A (1983) 173. [32] M.C. Tobin, J. Chem. Soc. (1960) 1108. [33] R.H. Hauge, S. Gransden, J.L.-F. Wang, J.L. Margrave, J. Am. Chem. Soc. 101 (1979) 6950. [34] P. Wermer, B.S. Ault, M. Orchin, J. Organomet. Chem. 162 (1978) 189. [35] B.S. Ault, J. Phys. Chem. 91 (1987) 4723. [36] S.B.H. Bach, B.S. Ault, J. Phys. Chem. 88 (1984) 3600. [37] K.O. Patten Jr., L. Andrews, J. Phys. Chem. 90 (1986) 1073. [38] A.J. Barnes, H.E. Hallam, J.D.R. Howells, G.F. Scrimshaw, J. Chem. Soc. Faraday Trans. 2 (69) (1973) 738. [39] I. Bytheway, M.W. Wong, Chem. Phys. Lett. 282 (1998) 219. [40] M.G. Govender, T.A. Ford, J. Mol. Struct. 550 (2000) 445.
Argon matrix isolation study of the thermal and photochemical reaction of OVCl3 with ðCH3Þ2CO David A. Kayser, Bruce S. Ault * Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, USA Received 10 January 2003; in final form 28 February 2003
Abstract The matrix isolation technique has been combined with infrared spectroscopy and theoretical calculations to identify and characterize the initial and secondary products in the thermal and photochemical reactions of OVCl3 with ðCH3 Þ2 CO. Initial deposition into argon matrices at 14 K led to the formation, in high yield, of the 1:1 molecular complex. This species appears to be strongly bound, leading to large shifts to certain vibrational modes of both the acid and base subunits. Bands due to the complex were destroyed by near-UV irradiation (k > 300 nm), and led to the formation of intense product bands. In contrast to previous studies, HCl elimination from an initial complex was not observed. Many possible products were considered, including isomerization, decomposition, addition, and addition followed by fragmentation pathways. The products were identified by the use of isotopic labeling and comparison to theoretical calculations. The primary product was determined to be Cl3 VðCH3 ÞOCðOÞCH3 , formed through rupture of a C–C bond in ðCH3 Þ2 CO and addition of the two fragments to OVCl3 . Possible evidence for a second isomer, slightly higher in energy ðCl3 VðOCH3 ÞðCðOÞCH3 ÞÞ was also found. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction High valent transition metal oxo compounds, including OVCl3 and CrCl2 O2 , are very strong oxidizing agents and are known to oxidize a wide range of organic substrates. This oxidizing power has applications in catalysis as well as in organic synthesis [1,2]. Despite the utility of these reagents, the mechanism of oxidation is not well under-
*
Corresponding author. Tel.: +1-513-556-9200; fax: +1-513556-9239. E-mail address: [email protected] (B.S. Ault).
stood. Theoretical calculations have also sought to explore the reaction surface, but only for a limited number of organic substrates [3–6]. The matrix isolation technique [7–9] was developed to facilitate the isolation and spectroscopic characterization of reactive intermediates. This approach has been applied to the study of a wide range of species, including radicals, weakly bound molecular complexes, and molecular ions. A recent series [10–15] of matrix isolation studies has examined the reactions of both OVCl3 and CrCl2 O2 with small molecules, including H2 O, CH3 OH, CH3 SH and NH3 . For each system, a sequence of intermediate species was observed,
0301-0104/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-0104(03)00140-X
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from the initially formed 1:1 molecular complex to the thermally or photochemically induced HCl elimination product. The reactions of OVCl3 and CrCl2 O2 with small molecules that do not contain an acidic hydrogen have not been examined extensively, due to the reduced reactivity in these systems [16]. Acetone has an electronic absorption in the near UV, increasing the possibility of photochemical reaction with OVCl3 (this species also absorbs in the near UV as well as in the visible). Consequently, a study was undertaken to explore the photochemical reaction of acetone with OVCl3 in argon matrices, using theoretical calculations to complement the experimental data.
2. Experimental section All of the experiments in this study were carried out on a conventional matrix isolation apparatus that has been described [17]. Oxyvanadium trichloride OVCl3 (Aldrich), was introduced into the vacuum system as the vapor above the room temperature liquid, after purification by freeze– pump–thaw cycles at 77 K. Acetone (Fisher), ðCD3 Þ2 CO and ðCH3 Þ2 13 CO (both Cambridge Isotope Laboratories, 99%) were introduced in a similar manner into a separate vacuum manifold and were purified by repeated freeze–pump–thaw cycles at 77 K. Argon and nitrogen (Wright Brothers) were used as the matrix gas in different experiments, and were used without further purification. Matrix samples were deposited in both the twin jet and merged jet modes. In the former, the two gas samples were deposited from separate nozzles onto the 14 K cold window, allowing for only a very brief mixing time prior to matrix deposition. Several of these matrices were subsequently warmed to 33–35 K to permit limited diffusion and then recooled to 14 K and additional spectra recorded. In addition, most of these matrices were irradiated with the H2 O/Pyrex filtered output of a 200 W medium pressure Hg arc lamp, after which additional spectra were recorded. Irradiation times as short as 1 min and as long as 3 h were used. Additional experiments were conducted using a range of wavelength filters. In several experiments,
the matrix was irradiated during the 20 h of the deposition process. A few experiments were conducted in the merged jet mode [18], in which the two deposition lines were joined with an Ultratorr tee at a distance from the cryogenic surface, and the flowing gas samples were permitted to mix and react during passage through the merged region. The length of this region was variable; typically, a 90 cm length was employed. In both twin and merged jet, matrices were deposited at the rate of 2 mmol/h from each sample manifold onto the cold window. Final spectra were recorded on a Perkin Elmer Spectrum One Fourier transform infrared spectrometer at 1 cm1 resolution. Theoretical calculations were carried out on a number of possible intermediates in this study, using the Gaussian 98W suite of programs [19]. Several different model chemistries were employed, including Hartree Fock and density functional calculations. These calculations were used to locate energy minima, determine structures and calculate vibrational spectra. Final calculations with full geometry optimization employed the 6311G+(d,2p) basis set, after initial calculations with smaller basis sets were run to approximately locate energy minima.
3. Results Prior to any codeposition experiments, blank experiments were run on each of the reagents used in this study. In each case, the blanks were in good agreement with literature spectra [20,21] and with blanks run previously in this laboratory. Each blank experiment was then irradiated by the H2 O/ Pyrex filtered output of a 200 W Hg arc lamp for 1.5 h. No changes were observed in any of the blank spectra as a result of irradiation. Weak bands due to HCl impurity were noted in all of the OVCl3 blank experiments [22]. 3.1. OVCl3 þ ðCH3 Þ2 CO In an initial experiment, a sample of Ar= ðCH3 Þ2 CO ¼ 500 was codeposited with a sample of Ar=OVCl3 ¼ 200 in the twin jet mode. Upon
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initial deposition, new bands of moderate intensity were noted near 430, 453, 568, 1025, 1250, 1421, and 1678 cm1 (hereafter referred to as set A). The band near 1678 cm1 was the most intense and distinctive of this set of bands. When this matrix was subsequently irradiated for 1.0 h with the H2 O/Pyrex-filtered output of a medium pressure Hg arc lamp, all of these bands were completely destroyed. New bands were noted at 419, 478, 679, 891, 988, 1011, 1124 and 1170 cm1 (hereafter referred to as set B), several of which were quite intense, as shown in Fig. 1. No other new bands were noted in the spectrum; the region near 2800 cm1 was examined closely and no changes were apparent in this region. This pair of reactants was studied in a series of 15–20 additional experiments, using a wide range of reactant concentrations. In every experiment, all of the set A bands were observed after initial sample deposition. Moreover, the relative intensities of these bands remained constant over all of these experiments, while the absolute intensities varied directly with the concentrations of the reactants. When several of the matrices were annealed to approximately 25 K and then recooled to 12 K, all of the set A bands grew slightly, and at the same rate. All of these matrices were then irradiated with the H2 O/Pyrex filtered output of a medium pressure Hg arc. All of the set A bands were destroyed in each experiment, and concurrently the set B bands appeared. In all of these experiments, all of the bands in set B maintained a constant intensity ratio with respect to one another. In addition, in several experiments weak bands were observed after irradiation, at 798, 2808, 3450 and 3598 cm1 . These have been seen previously and have been assigned [23] to the photoproduct of OVCl3 and impurity H2 O, namely the cage-paired species Cl2 VðOÞOH and HCl. Several experiments were conducted to better characterize the processes leading to product formation. When merged jet experiments were employed, the results were essentially identical to those obtained with twin jet deposition. In another experiment, a sample of Ar=ðCH3 Þ2 CO ¼ 200 was codeposited with a sample of Ar=OVCl3 ¼ 200. This sample was irradiated as before, but for just 1 min. The resultant spectrum showed nearly com-
213
plete destruction of the set A bands and nearly complete formation of the set B bands. No other new bands were observed. Further irradiation of this sample led to complete destruction of the set A bands, and slight additional growth of the set B bands. Also, a similar pair of samples were codeposited, and irradiated continuously during the deposition process. The resulting spectrum showed strong bands due to set B, while the bands of set A were absent. In addition, new weak bands were noted near 1304 and 2138 cm1 . Several experiments were conducted using filtered irradiation in different wavelength regions. When a similar matrix containing OVCl3 and ðCH3 Þ2 CO was irradiated with light of k > 595 nm for 1.5 h, bands due to set A were reduced by 10–15%, while weak bands grew nearby, at 423, 466, 562, 1032 and 1686 cm1 . When this matrix was then irradiated with light of k > 530 nm for 1.5 h, bands due to set A were completely destroyed, while the bands at 423, 466, 562, 1032 and 1686 cm1 were reduced. New bands, again nearby, were noted at 469, 1008 and 1702 cm1 . Irradiation of this matrix with light of k > 510 nm did not lead to any changes. Finally, when this matrix was irradiated with light of k > 300 nm (H2 O/Pyrex filter), the bands at 469, 1008, 1420 and 1702 cm1 were completely destroyed, and the bands of set B grew in with characteristic intensity. 3.2. Isotope studies Samples of Ar=ðCD3 Þ2 CO were deposited with samples of Ar=OVCl3 in several twin jet experiments. Initial product bands, analogous to the set A band, above, were observed in all of these experiments, at 423, 452, 1024, 1267 and 1667 cm1 . In these experiments, bands in this set maintained a constant intensity ratio with respect to one another. Each matrix was then irradiated for 1.5 h with the H2 O/Pyrex filtered output of a medium pressure Hg arc (k > 300 nm). All of the above bands were destroyed, and a new set analogous to set B was observed, with bands at 530, 614, 768, 928 (br), 986, 1068 and 1142 cm1 . These bands were reproducible and maintained a constant intensity ratio with respect to one another in all experiments.
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Fig. 1. Infrared spectra, over selected spectral regions, of a matrix prepared by the twin jet codeposition of a sample of Ar=OVCl3 ¼ 200 with a sample of Ar=ðCH3 Þ2 CO ¼ 200. The lower trace is before irradiation while the upper trace is after 1.5 h irradiation. Bands marked with a ‘‘+’’ grew in upon irradiation, while bands marked with a ‘‘)’’ were destroyed by irradiation.
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A similar series of experiments was conducted using acetone with a 13 C label in the 2 position [CH3 13 CðOÞCH3 ], and very similar results were obtained. Bands of set A were observed at 427, 453, 476, 567, 1027, 1223, 1366 and 1642 cm1 . These were destroyed by irradiation, and set B bands grew in at 418, 666, 884, 982, 1112, 1154 and 1372 cm1 . The positions and intensities of these bands mirrored closely those observed in the normal isotopic experiments. Again, the relative intensities of the bands within each set were constant through this series of experiments. All product bands are compiled in Tables 1 and 2, while Fig. 2 shows comparative spectra from 12 C and 13 C experiments.
4. Discussion Upon initial deposition, either twin jet or merged jet, a number of product bands were de-
215
tected, many with substantial intensity (set A bands, above). The observation of these bands following twin jet deposition strongly suggests that there are the initial intermediate in the reaction between OVCl3 and ðCH3 Þ2 CO, since the time available for reaction during twin jet deposition is very limited. Further, these bands grew when the initially deposited matrices were annealed to 30 K and recooled, indicating that the barrier to reaction is very low, approaching zero. In addition, each of the bands in set A was observed relatively near (within 50 cm1 ) of a parent band of either OVCl3 or ðCH3 Þ2 CO, and with similar relative intensities (i.e., set A bands near intense parent bands were relatively intense, while set A bands near weaker parent bands were relatively weaker). Finally, the isotopic dependence (deuterium or 13 C) of each set A band was nearly identical to the isotopic dependence of the nearby parent mode; for example, the 1678 cm1 band of set A showed an 11 cm1 red shift when ðCD3 Þ2 CO was
Table 1 Product band positions before irradiation of matrices containing OVCl3 and ðCH3 Þ2 CO and its isotopomers OVCl3 þ ðCH3 Þ2 CO
OVCl3 þ ð13 CH3 Þ2 CO
430 453 568 1025 1250 1421 1678
427 453, 476 567 1027 1223
1024 1280
1642
1667
OVCl3 þ ðCD3 Þ2 CO 423 452
Assignment –VCl3 str. –VCl3 str. C–C@O bend V@O stretch CH3 rock CH3 bend C@O stretch
Table 2 Product band positions after irradiation of matrices containing OVCl3 and ðCH3 Þ2 CO and its isotopomers OVCl3 þ ðCH3 Þ2 CO
OVCl3 þ ð13 CH3 Þ2 CO
419 478
418
679 891 988 1011 1124 1170
666 884 982
a b
1112 1154 1372
Approximate descriptions, modes strongly mixed. Strongly mixed with –CH3 rock.
OVCl3 þ ðCD3 Þ2 CO
Assignmenta VCl3 sym. st. VCl3 asym. st.
530 614 768 928 986 1068 1142
V–O–C bend V–O–C st. CH3 rock C–C st.b C–C st.b
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Fig. 2. Infrared spectra, over selected spectral regions, of a matrix prepared by the twin jet codeposition of a sample of Ar=OVCl3 ¼ 200 with a sample of Ar=ðCH3 Þ2 CO ¼ 200 after 1.5 h irradiation (lower trace) compared to a similar sample employing ðCH3 Þ2 13 CO (upper trace). Bands marked with a ‘‘+’’ grew in upon irradiation.
employed, which is the same shift observed for the nearby C@O stretching mode of parent acetone. These observations are indicative of the formation of a molecular complex between the two reacting species. In a molecular complex, each subunit is perturbed upon complex formation, and vibrational modes are shifted, yet each subunit in the complex maintains its molecular identity [24]. The shifting of key vibrational modes, with identical isotopic behavior, supports this conclusion. In addition, complex formation has very little or no activation barrier, and is likely to occur upon annealing the matrix to 30 K. The bands of set A maintained constant relative intensities in the many experiments over a wide range of concentrations that were performed in this study, indicating that they must all be assigned to a single complex. Given the low concentrations that were employed in a number of the experiments, this complex very likely has 1:1 stoichiometry. This conclusion is consistent with several earlier studies
[10–15] of the reactions of Lewis bases with OVCl3 and CrCl2 O2 , and is consistent with the ability of ðCH3 Þ2 CO to coordinate to transition metal centers [25]. Therefore, the product bands formed upon initial matrix deposition (set A bands) are assigned to the 1:1 molecular complex between OVCl3 and ðCH3 Þ2 CO and its isotopomers. In the large majority of the previous studies in this series, the initially formed 1:1 complex was photoreactive (and for some, thermally reactive as well), eliminating HCl and forming a novel transition metal compound. The remainder of the previously studied complexes were not photoreactive; each of these was a complex involving a Lewis base without an active hydrogen (e.g. ðCH3 Þ3 N). The present 1:1 complex between OVCl3 and ðCH3 Þ2 CO does not fit into either of these categories. The complex was clearly photoreactive, with bands of the complex being nearly completely destroyed in 1 min of irradiation with light of k > 300 nm. At the same time, HCl was
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clearly not a product of this photochemically induced reaction. Careful examination of the H–Cl stretching region (both free [26] and weakly hydrogen bonded) found no trace of HCl formation, nor was any DCl observed when ðCD3 Þ2 CO was employed. This is consistent with the fact that ðCH3 Þ2 CO does not contain an ‘‘active’’ hydrogen. The identity of the photoproduct remains to be determined. The photoproduct(s) arising from irradiation of the 1:1 complex are characterized by the set B bands. The intensities of these bands were measured carefully in all of the experiments, over a wide range of sample concentrations. In every experiment, they maintained a constant intensity ratio with respect to one another, within experimental error. Further, when the 1:1 complex was subjected to wavelength-selective irradiation, from red through near-UV, all of the set B bands grew at the same rate. In addition, the same product bands were observed in the same intensity ratios whether the irradiation was for 1.0 min over several hours. These results indicate that either a single species is formed upon irradiation in all of these experiments, or that multiple species are formed, but always in exactly the same ratio. A single product species could arise either from the addition of the ðCH3 Þ2 CO subunit to the OVCl3 subunit (photo-induced combination reaction) or by a photo-induced isomerization reaction, leading to another isomer in the C3 H6 O potential energy surface, plus OVCl3 . Multiple species could form through a photo-induced decomposition reaction in which either the ðCH3 Þ2 CO or the OVCl3 subunits, or both, undergo decomposition upon irradiation of the complex. Also, multiple species might arise from a direct reaction between the two, such as oxygen atom transfer from OVCl3 to ðCH3 Þ2 CO. All of these possibilities must be considered. Photochemically induced isomerization would produce new species on the C3 H6 O potential energy surface. Many such species are known, including oxetane, dimethyl dioxirane, methyl oxirane (propylene oxide), methyl vinyl ether, cyclopropanol, and the enol form of acetone (H3 C–CðOHÞ@CH2 ). All of these potential products are known compounds, and infrared spectra
217
are available in the literature [27–32] (at least for the normal isotope and the fully deuterated species), for comparison. Even taking into account the possibility of weak complexation through the oxygen to OVCl3 in the same matrix cage, the spectra of these compounds do not match the spectra obtained here. As such, the photochemically induced isomerization products must be eliminated as candidates for the product formed here. The possibility of photochemically induced decomposition of acetone must be considered as well. The most likely channels include formation of C2 H6 þ CO, and C2 H4 þ H2 CO. All four of these species are well known, and have distinctive infrared spectra. Again, the spectra of these compounds [33–36] do not match the spectra obtained here, and must be eliminated as candidates. A similar mechanism is direct oxidation of acetone by OVCl3 , to form either a single product of the formula C3 H6 O2 , or by oxidation followed by decomposition. In the former case, formation of methyl acetate is most likely, while in the latter case, formation of C2 H6 þ CO2 is most likely (in either case, VCl3 would be an additional product). While direct oxidation has been observed for the photochemical reaction of OVCl3 with PH3 [16], the spectra of methyl acetate, C2 H6 and CO2 again do not match the observed spectra [33,37]. Thus the mechanism of direct oxidation through O atom transfer can be ruled out. Photochemical addition of acetone to OVCl3 is a viable mechanism as well. This might occur either through formation of a metalocycle, or through addition and expanded coordination at the vanadium center. There are two possibilities for the former case (2 + 2 and 2 + 4 cycloaddition, to form 4-member and 6-member metalocycles, respectively), while in the latter case, several possibilities exist. All of these involve transfer of one fragment from ðCH3 Þ2 CO to the oxo group, and addition of the second fragment to the V center. While such reactions have not been observed experimentally, Zeigler and coworkers [4–6] have calculated that the reaction of CH3 OH with OVCl3 could occur by H atom transfer to the oxo group, and addition of the methoxy fragment to the vanadium to form Cl3 VðOHÞOCH3 . While experimental evidence
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[12] shows that under matrix isolation conditions this reaction does not occur (Cl2 VðOÞOCH3 þ HCl are formed), Cl3 VðOHÞ OCH3 was shown to be a local minimum on the potential energy surface, and hence transfer reactions of this type are feasible. Finally, there is the possibility of addition followed by elimination, such as forming Cl2 VðOÞ CðOÞCH3 þ CH3 Cl. However, this last possibility can be ruled out by the lack of observation [38] of bands due to CH3 Cl. All of the potential products of photochemical addition are as-yet unknown chemical species, so that direct comparison of infrared spectra is not possible. Therefore, the best alternative is theoretical calculation. Calculations can determine whether different possible structures represent local energy minima (i.e., stable species), and can predict infrared spectra, including isotopic shifts. Previous studies of similar compounds (e.g., Cl2 VðOÞOCH3 and Cl2 VðOÞNH2 ) have shown that good agreement is generally obtained using B3LYP/6-311G+(d,2p), within 1–3% on band positions and within a few cm1 on isotopic shifts. Calculations were carried out on both possible metalocycles, as well as on the three addition products ðCl3 VðOCH3 ÞðCðOÞCH3 Þ, Cl3 VðOCðOÞCH3 ÞðCH3 Þ and Cl3 VðOCH3 ÞðOCH2 CH2 ÞÞ. All five candidates optimized to stable minima at all levels of theory. Then, infrared spectra were calculated for all five species, for the normal isotopic species, the perdeuterated species, and the 13 C labeled species arising from acetone labeled with 13 C in the 2 position. Of these 5, two can be eliminated quickly, the 6-membered metalocycle and Cl3 V ðOCH3 ÞðOCH2 CH2 Þ. For both, the agreement of calculated and experimental spectra is very poor. In addition, both products required extensive hydrogen migration, which is itself unlikely, and would probably have led to HCl formation within the matrix cage. Thus, these two candidates are eliminated. Of the remaining three, the 4-membered metalocycle can be eliminated as well, by comparison of experimental and theoretical spectra. The calculations predict that the most intense band in the spectrum would occur at 763 cm1 , whereas no band at all was seen near this position (the closest, at 679 cm1 , was one of the weaker bands). In addition, the calculated spectrum does not account for
two intense bands between 1100 and 1200 cm1 . Thus, this species is eliminated from consideration. The two remaining addition products arise from the photochemical cleavage of a C–C bond in ðCH3 Þ2 CO, leaving a –CH3 fragment and a –CðOÞCH3 fragment (this corresponds to the primary mode of photochemical decomposition of ðCH3 Þ2 CO). One of these adds to the oxo group, and the other to the V center, yielding the two remaining candidates. Both of these novel species calculated to stable energy minima at all levels of theory, suggesting both are viable candidates. Infrared spectra and isotopic shifts were calculated as well. Since each species has a large number of normal vibrations, and since relatively fewer product bands were observed, attention was focused on those calculated product bands with calculated intensities of 20 km/mol or greater. With this in mind, and the spectrometer cut-off of 400 cm1 , the bands listed in Table 2 were considered, and compared to calculated for each species. The fit of experimental and theoretical spectra was much better for Cl3 VðOCðOÞCH3 ÞðCH3 Þ, in which the –CH3 fragment attached to the V center, and the –CðOÞCH3 fragment attaches to the oxo group. For example, two very intense bands were calculated between 1100 and 1200 cm1 , in agreement with experiment (none were calculated for Cl3 VðOCH3 ÞðCðOÞCH3 ÞÞ. Likewise, two intense bands were calculated between 900 and 1000 cm1 , matching the experimental bands at 891 and 988 cm1 (only one intense band was calculated in this region for Cl3 VðOCH3 ÞðCðOÞCH3 ÞÞ. Calculated isotopic shifts were also in reasonable agreement with experiment, given the high degree of mode mixing that occurs in this region. It should be noted that no C@O stretch was observed for the product species. However, as seen in Table 3, this mode should come within a few wavenumbers of parent ðCH3 Þ2 CO, and would be hidden underneath the intense parent band in each experiment. Table 3 summarizes the calculated and experimental band positions and isotopic shifts. Overall, assignment to Cl3 VðOCðOÞCH3 ÞðCH3 Þ is warranted, and is made. Fig. 3 shows the optimized structure of this compound, based on a trigonal bipyramidal geometry around the V center, with two chlorines and the methyl group in
)65 )123 )60 )56 )28 )9 )27 )38 )112 )66 )97 )10 )366 )4
equatorial positions. Table 4 gives the key calculated geometric parameters. One product band remains, at 1011 cm1 . This might be assigned to a site splitting of the 988 cm1 , although the magnitude of the splitting is larger than normal. Alternatively, by far the most intense band calculated for Cl3 VðOCH3 Þ ðCðOÞCH3 Þ is at 1070 cm1 (calculated intensity of 332 km/mol). With a scaling factor of 0.97 (typical for B3LYP/6-311G+(d,2p)) [39], this is predicted at 1038 cm1 , in reasonable agreement with the 1011 cm1 product band. While this is only a single band, is does suggest the possibility of the formation of the second isomer. It should be noted
)13 )7 )6 )12 )16
)1
Expt. shift Comp. shift
Fig. 3. Optimized structure of Cl3 VðCH3 ÞOCðOÞCH3 at the B3LYP/6-611G+(d,2p) level of theory.
0.44 0.22 0.22 0.61 0.74 0.78 1
0 0 )4 )3 )7 )21 )4 0 )43
Comp. shift
Expt. shift
Assignment
419 478 679 891 988 1124 1170
Table 4 Key geometric parameters of Cl3 VðCH3 ÞOCðOÞCH3 calculated at the B3LYP/6-611G+(d,2p) level of theory
185 145 116 355 417 470 323 45 255
Parent acetone: 1785 cm1 . Intensity in km/mol. b Relative intensity.
0.39 0.31 0.25 0.76 0.89 1 0.69 Intensity
409 451 561 951 996 1171 1192 1400 1794
a
Comp. freq.
a
Cl3 VðCD3 ÞOCðOÞCD3 Cl3 VðCH3 ÞO13 CðOÞCH3 Expt. I, rel.b Expt. freq. Calc. I, rel.b Cl3 VðCH3 ÞOCðOÞCH3
Table 3 Calculated versus experimental frequencies, B3LYP/G-311G+(d,2p), frequencies > 400 cm1 , with I > 20
219
VCl3 sym. stretch VCl3 antisym. stretch V–O–C bend V–O–C stretch CH3 rock C–C stretch/CH3 rock C–C stretch/CH3 rock CH3 bend C@O stretch
D.A. Kayser, B.S. Ault / Chemical Physics 290 (2003) 211–221
Parameter
Value
R (V–Cl) R (V–O) R (V–C) R (C–H) R (C@O) R (C–C) a ðCl1 –V–Cl2 Þ a ðCl1 –V–Cl3 Þ a ðCl2 –V–Cl3 Þ a (C–V–O) a (V–O–C) a (O–C@O) a (O–C–C) a (H–C–H)
2.18–2.23 A 1.78 A 2.08 A 1.09 A 1.20 A 1.50 A 92.5° 125.1° 92.4° 86.0° 163.1° 121.2° 112.0° 110.0°
220
D.A. Kayser, B.S. Ault / Chemical Physics 290 (2003) 211–221
that the calculations predict a relatively small energy difference between these two species, approximately 7 kJ/mol, with Cl3 VðOCðOÞCH3 Þ ðCH3 Þ being the more stable species, in agreement with the results here. Nonetheless, the possibility of formation of a small amount of the higher energy isomer Cl3 VðOCH3 ÞðCðOÞCH3 Þ is possible, and may account for the product band at 1011 cm1 . Nonetheless, this identification must be taken as tentative. When irradiation was conducted continuously during deposition, the initial complex was not observed, and the set B bands due to the photoproduct(s) were seen with high intensity. In addition, weak product bands were detected at 1304 and 2138 cm1 . These come exactly at the positions [34,40] for CH4 (most intense infrared absorption) and CO. These are very likely photoproducts from a small amount of decomposition of the primary photoproduct(s). Since irradiation occurred while the species still had mobility (i.e., before being rigidly trapped in the argon matrix), these product could separate and be isolated. Even so, the initial photoproducts were not particularly photosensitive, and only a small amount of decomposition occurred.
5. Conclusions The twin jet or merged jet codeposition of OVCl3 and ðCH3 Þ2 CO into argon matrices leads initially to formation of a rather strongly bound 1:1 complex. This species is photochemically sensitive, and is destroyed by light of k > 300 nm. Intense product bands grow in upon irradiation, and do not differ with either the length of irradiation time, or the wavelength of irradiation (with an onset of k < 500 nm). These bands are assigned to Cl3 VðOCðOÞCH3 ÞðCH3 Þ, formed through photochemical cleavage of a C–C bond in ðCH3 Þ2 CO, and addition of the two fragments to OVCl3 . This species is itself stable with respect to irradiation, to wavelengths as short as 300 nm, although when the irradiation was conducted continuously during deposition (rather than after matrix deposition) the growth of a small amount of CH4 and CO was observed. The formation of a
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