Article pubs.acs.org/JPCA
Infrared Matrix Isolation Study of the Thermal and Photochemical Reactions of Ozone with Dimethylcadmium Devin McNally and Bruce S. Ault* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States S Supporting Information *
ABSTRACT: The matrix isolation technique has been combined with infrared spectroscopy and theoretical calculations to explore the reaction of (CH3)2Cd with O3 over a range of time scales and upon irradiation. During twin jet deposition, multiple novel product species were observed along with several stable “late” products. Following annealing of these matrices to 35 K, absorptions due to two novel product species increased in intensity. In addition, new bands appeared, indicating the formation of an additional product. Subsequent UV irradiation destroyed several of the initial products and produced a new photoproduct. On the basis of 18O and 16,18O spectroscopic data and theoretical calculations, the novel intermediates H3COCdCH3, H3CCdCH2OH, H3COCdOOCH3, and H3CCdCHO were identified. Merged jet deposition led to a number of stable “late” products, including H2CO, CH3OH, and C2H6, identifications that were confirmed by 18 O substitution. Mechanistic inferences for this reaction are discussed.
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INTRODUCTION The application of semiconducting thin films in the microelectronics industry has completely revolutionized society, through the development of computers and a wide range of electronic devices. The importance of these films has led scientists and engineers to develop a variety of methods to produce these films.1−3 These include electrolytic deposition from solution, plasma deposition, chemical vapor deposition (CVD), and atomic layer deposition (ALD).4 Metal oxide thin films have a range of important properties and see wide use in the solar energy conversion and electronics industries. Cadmium oxide thin films function as a transparent conducting oxide with excellent carrier mobility5−8 as high as 105 cm2/(V s) and resistivities in the region of 2 × 10−4 Ω cm−1. These films also show optical transparency between 70 and 85% throughout the visible region of the spectrum.9,10 They have found use in solar cells, photovoltaic electrodes, and window deicers.8 For CVD and ALD processes leading to metal thin film formation, O2, H2O, and H2O2 have been used as oxidants, all of which have drawbacks. In recent years, the search for a better oxygen source has led researchers11 to ozone, O3. It has many attractive features, including high oxidation potential (e.g., higher reactivity than O2), cleanliness, and volatility, leading to rapid and effective metal oxide thin film formation.12,13 With these characteristics in mind, the use of O3 as the oxidant in ALD and CVD processes for metal oxide thin film formation is increasing at a rapid rate. However, very little is known about the mechanism of reaction of ozone with organometallic compounds, including (CH3)2Cd. The matrix isolation technique14−17 was developed to facilitate the isolation and spectroscopic characterization of reactive intermediates and has been applied to the study of a wide range of novel species, including radicals, ions, and weakly © 2012 American Chemical Society
bound molecular complexes. The matrix isolation approach, combined with theoretical calculations, was used to study18 initial intermediates in the reaction of Me2Zn with O3 and O atoms as well as to shed light on the reaction mechanism. The present study reports a study of twin jet and merged jet (thermal) and twin jet (photochemical) reactions of (CH3)2Cd and O3 with trapping in cryogenic matrices. Although Zn and Cd occupy adjacent (vertical) positions in the periodic table, a substantially greater and more complex thermal reactivity was observed with (CH3)2Cd, as described below.
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EXPERIMENTAL SECTION All of the experiments in this study were carried out on a conventional matrix isolation apparatus that has been described.19 Dimethylcadmium (Strem Chemical) was introduced into the vacuum system from a lecture bottle into the vacuum system and was purified by freeze−pump−thaw cycles at 77 K. O3 was produced by Tesla coil discharge of O2 (Wright Brothers) and trapping at 77 K to remove residual O2 and trace gases. 18O3 was produced in the same manner from 18O labeled O2 (94%, Cambridge Isotope Laboratories). Argon (Wright Brothers) was used as the matrix gas without further purification. Matrix samples were deposited in three different modes: twin jet, merged jet, and concentric jet mode. In the first, the two gas samples were deposited from separate nozzles onto the 14 K window, allowing for only a brief mixing time prior to matrix deposition. Several of these matrices were subsequently warmed to 33−35 K to permit limited diffusion and then Received: December 28, 2011 Revised: February 1, 2012 Published: February 1, 2012 1914
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spectra,21−23 and with blanks run previously in this laboratory. A blank experiment of each reagent was then irradiated by the H2O/Pyrex-filtered output of a 200 W Hg arc lamp for 1.0 h and no changes were noted. (CH3)2Cd + O3, Twin Jet. In an initial experiment, a sample of Ar/(CH3)2Cd = 250 was codeposited with a sample of Ar/ O3 = 250 using twin jet deposition. Approximately more than 30 product bands were observed upon initial deposition of this sample, demonstrating substantial reactivity in this system. When this matrix was then annealed to 35 K, recooled to 14 K and another scan recorded, many changes were noted. Eight of the initial product bands increased substantially in intensity whereas the rest were unaffected. In addition, twelve new bands grew in as a result of annealing. Clearly, reactions occurred under these low temperature conditions. Finally, this matrix was irradiated with a medium pressure Hg arc for 3 h, leading to the growth in some product bands, a reduction in some product bands and no change for yet other product bands. All of these results are tabulated in Table 1 and spectral regions of interest are shown in Figures 1 and 2. This twin jet experiment was repeated multiple times, including annealing and irradiation, while varying initial sample concentration and irradiation time. The results were consistent throughout these experiments, with different sets of bands exhibiting different behaviors with respect to initial deposition, annealing and irradiation. A similar set of twin jet experiments were conducted with samples of Ar/(CH3)2Cd and Ar/18O3 made from 18O2 containing approximately 94% 18O. Upon initial sample deposition, a large number of product bands were observed, similar to the bands with 16O3, although shifted somewhat from the positions with 16O in a number of cases. For example, the initial 980 cm−1 band shifted to 957 cm−1, whereas the 654 cm−1 band did not shift at all. In some cases, the product bands could not be observed due to overlap with the intense parent peaks. Each of these matrices were subsequently annealed to 35 K, recooled, scanned, and then irradiated for between 1 and 3 h and scanned again. Similar results were obtained, with some bands increasing on annealing, some unchanged and additional new bands growing in. As before, irradiation decreased some product bands, increased others and left a third group unchanged. In a related experiment, Ar/(CH3)2Cd = 250 was deposited with a sample of Ar/16,18O3 = 250, made from 50% 18O and containing a completely scrambled set of all six isotopic combinations. All of the 16O product bands were observed as were all of the 18O product bands listed above. In addition, some product bands showed intermediate bands between the pure 16O and pure 18O bands, whereas others did not. For example, the 848 cm−1 16O product band with an 18O counterpart at 802 cm−1 appeared as a distinct sextet of bands. In contrast, the 16O product bands at 980, 1076, and 1708 cm−1 all appeared at distinct doublets, with a pure 16O band, 18O band, and no intermediate bands. This experiment was repeated once, leading to nearly identical results. Product band positions in these isotopic experiments are also listed in Table 1. (CH3)2Cd + O3, Merged Jet. In an initial experiment, a sample of Ar/(CH3)2Cd = 250 was codeposited with a sample of Ar/O3 = 250 using merged jet deposition with a 55 cm merged (reaction) region. Numerous product bands were observed upon deposition, as shown in Figure 3. A substantial number of these matched certain of the product bands
recooled to 14 K and additional spectra recorded. In addition, many of these matrices were irradiated for 1.0 h or more hours with either the H2O/Pyrex-filtered or the H2O/quartz-filtered output of a 200 W medium-pressure Hg arc lamp, after which additional spectra were recorded. A number of experiments were conducted in the merged jet mode, 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. To more effectively model the gas phase, the merged region was constructed of Teflon FEP, either 1/4 or 1/8 in. o.d. in a number of experiments. Copper tubing (1/4 in. o.d.) was used in other additional experiments. The length of the merged region (or reaction zone) was varied from as short as 6 cm to as long as 70 cm. Twin jet and merged jet deposition probe different time scales for reaction, very short for twin jet and somewhat longer for merged jet. As will be reported below, quite different results were obtained with these two modes. To probe the intermediate time scale between twin and merged jet, a concentric jet device was employed.18 In this approach, an 1/8 in. o.d. Teflon FEP tube was inserted inside of a larger, 1/4 in. o.d. tube, also Teflon FEP. The length of the 1/8 in. tube could be adjusted to be shorter, longer, or the same as the outer tube. The distance between the outlet ends of the two tubes is referred to as Δd = (position of inner tube − position of outer tube). d > 0 indicates that the inner tube extends beyond the outer tube, and d < 0 indicates that the inner tube is shorter than the outer tube (more like merged jet). d = 0 indicates that the ends of the two tubes are at the same distance from the cold window. Mixing of the two samples begins at the outlet of the inner tube and continues until deposition onto the cold window (typically 2−3 cm). In this manner, the time scale available for reaction could be adjusted from nearly that of merged jet to nearly that of twin jet. In all three deposition modes, matrices were deposited at the rate of 2 mmol/h from each sample manifold onto the cold window for approximately 24 h. Final spectra were recorded on a Perkin-Elmer Spectrum 2000 Fourier transform infrared spectrometer at 1 cm−1 resolution. Theoretical calculations were carried out on likely intermediates in this study, using Gaussian 03 and 03W suite of programs.20 Density functional calculations using the hybrid B3LYP functional were used to locate energy minima, determine structures, and calculate vibrational spectra. Final calculations with full geometry optimization employed the lanl2dz and dgdzvp basis sets, after initial calculations with smaller basis sets were run to approximately locate energy minima. Thermodynamic functions for the reactants and potential intermediates were also calculated with accuracies limited by the available basis sets.
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RESULTS Prior to any codeposition experiments, blank experiments were run on each of the reagents used in this study. In blank experiments employing (CH3)2Cd, CH4 was always observed at 1304 cm−1, likely due to the reaction of (CH3)2Cd with adventitious water in the vacuum manifold. The amount of CH4 that was observed varied from one experiment to another. A blank of C2H6 in argon was run to provide an authentic spectrum for comparison to spectra recorded in this study; some C2H6 was noted in blank experiments of (CH3)2Cd. In each case, the blanks were in good agreement with literature 1915
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between twin and merged jet with no additional product bands observed. Though these results demonstrate the high reactivity of the system, no additional product bands were seen.
Table 1. Product Bands Observed in the Twin Jet Codeposition of (CH3)2Cd and O3 into Argon Matrices
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product band position (cm−1) 16
O
469 478 524 559 568 613 654 820 848 980 1067 1076 1134 1154 1176 1212 1249 1374 1407 1432 1466 1499 1708 1742 2719 2799 2799 2834 2846 2884 2891 2901 2922 2949 2986
18
O
452 504
613 820 802 957 1037 1047 1133 1154 1166 1206 1242 1374 1405 1432 1466 1486 1675 1708 2708 2793 2799 2834 2846 2884 2891 2901 2922 2949 2986
present initially?
anneal behavior
irradiation behavior
16
N Y N N Y Y N Y N Y N Y Y Y Y N Y Y Y N Y Y N Y Y Y Y Y Y Y Y N Y Y Y
↑ nc ↑ ↑ nc nc ↑ nc ↑ ↑ ↑ ↑ ↑ nc ↑ ↑ ↑ nc ↑ ↑ nc nc ↑ nc nc ↑ nc nc nc nc nc ↑ nc nc nc
↓ nc ↓ ↓ nc nc ↓ nc ↓ ↓ ↓ ↑ ↓ nc ↓ ↑ ↓ nc ↓ ↓ nc nc ↑ nc nc ↑ nc nc nc nc nc ↓ nc nc nc
a a a
RESULTS OF CALCULATIONS Likely radical and closed shell products arising from the reaction of (CH3)2Cd with O3 were considered. Many possible products are known species for which experimental spectra are available for comparison. For possible products that are not well-known and characterized spectroscopically, theoretical calculations using density functional methods were carried out to locate potential energy minima and compute infrared spectra, including 18 O isotopic shifts. These included H3COCdCH3, H3COCdOOCH3, H3COOCdCH3, H3CCdCHO, and similar molecules. All of the species under consideration optimized to energy minima on their respective potential energy surfaces, with all positive vibrational frequencies. Of particular interest was H3COCdCH3, because the analogous Zn product was identified in an earlier study18 of the reaction of (CH3)2Zn with O3. Also, H3COOCdCH3 has been reported in early solution phase studies,24,25 whereas a hybrid of the two, H3COCdOOCH3, is also a viable candidate. Computed structures of species of interested (to be identified as species B, C, D, and E, below) are shown in Figure 4.
O, 18O behavior
a
a a a a sextet doublet doublet doublet a a doublet a a a a a a doublet doublet doublet a doublet a a a a a a a a a
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DISCUSSION
Twin jet deposition of (CH3)2Cd and O3 into argon matrices led to the formation of a large number of product bands. Sets of these bands showed distinctly different behavior as a function of initial deposition, annealing, irradiation, and isotopic labeling. As a result, the observed bands (including those formed only after annealing and/or irradiation) can be sorted into five groups, A−E. The bands in group A appeared in initial deposition and were unaffected by annealing and irradiation. Most of these bands had also been observed in the earlier study18 of the reaction of (CH3)2Zn with O3, and were assigned to two stable “late” reaction products. These products were identified as CH2O and C2H6 on the basis of isotopic shifts, literature spectra,26,27 and comparison to authentic spectra of these species and so assigned here. It should be noted that some ethane is observed in (CH3)2Cd blanks, so the level of production by reaction could not be readily quantified. One additional band in group A was consistently observed at 613 cm−1. This band did not shift with 18O labeling and matches exactly the position of matrix-isolated methyl radical CH3. On the basis of previous studies,28 isotopic behavior, and the likely mechanism proposed below, this band is assigned to CH3 produced in these twin jet studies. Finally, it is possible that CH4 is formed in these reactions. However, because CH4 was always present in samples of (CH3)2Cd, no firm conclusion could be reached on this point. Groups B and C were both formed on initial twin jet deposition where the available reaction time is very short, marking them as early intermediates in the reaction. Group B, dominated by the strong absorption at 980 cm−1, grew very strongly on annealing and then decreased significantly on irradiation. Bands in this group exhibited a sharp, distinct doublet when 50% 18O scrambled 16,18O3 was employed. This doublet structure indicates the presence of one O atom in the absorbing species. In contrast, the bands of group C, which were dominated by the strong absorption at 1076 cm−1, grew slightly on annealing and then grew greatly on irradiation.
a
Could not be determined, due to small shift, low intensity and/or parent band overlap.
observed in the twin jet experiments, almost entirely those product bands that were not affected by either annealing or irradiation. Additional product bands were observed that were not seen in the twin jet experiments. As shown below, most of these can be assigned to known, stable reaction products. These product bands are listed in Table 2. (CH3)2Cd + O3, Concentric Jet. A concentric jet apparatus was employed to probe the range of reaction times available to the reactants from short times (twin jet-like) to longer time (merged jet-like) by varying the distance between the tip of the inner deposition line and the outer tubing or deposition line. Four experiments were run, with d ranging from +1 to −2 cm in 1 cm steps. Even over this small range, the resulting spectra varied dramatically. With d = 1 (inner tube extending beyond the outer tube), the spectral results were very similar to those obtained using twin jet deposition. With d = −2, the spectral results were very similar to those obtained using merged jet deposition. With d = 0 and −1, the results were intermediate 1916
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Figure 1. Infrared spectra between 840 and 1100 cm−1 following twin jet codeposition of samples of Ar/(CH3)2Cd and Ar/O3: initial deposition (red, lower on left), after annealing to 35 K (green, middle on left), and after irradiation with λ > 220 nm (blue, top on left). Species B, C, and E are labeled per text.
Figure 2. Infrared spectra between 1640 and 1780 cm−1 following twin jet codeposition of samples of Ar/(CH3)2Cd and Ar/O3: initial deposition (red, bottom), after annealing to 35 K (green, middle), and after irradiation with λ > 220 nm (blue, top). The behavior of species D at 1708 cm−1 is shown.
very similar to the band observed here at 1076 cm−1 with a 29 cm−1 isotopic shift. These observations suggest the formation of a similar O-atom insertion product, namely H3COCdCH3. An O atom insertion product is also consistent with the very short time available for reaction in twin jet deposition. To complement this experimental data, DFT calculations were carried out for H3COCdCH3, to determine the optimized structure, and calculate the infrared spectrum of this species.
These bands also appeared as doublets in the scramble 18O isotopic experiments, indicating the presence of one oxygen atom in the species responsible for group C. It is noteworthy that the bands of group C had characteristics that were very similar to the bands observed for H3COZnCH3 in the earlier (CH3)2Zn + O3 study.18 In that study, the dominant band was observed at 1090 cm−1, with a 30 cm−1 18O shift, as was assigned to the C−O stretching mode of H3COZnCH3. This is 1917
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Figure 3. Infrared spectra of the spectral region from 1640 to 1900 cm−1 following merged jet deposition of samples of Ar/(CH3)2Cd and Ar/O3 (green trace, top) compared to blank spectra of Ar/(CH3)2Cd (red trace, bottom) and Ar/O3 (blue trace, middle).
Table 2. Band Positionsa and Assignmentsb for Products Formed in Merged Jet Deposition Experiments (CH3)2Cd + 16O3 612 m 768 vw 821 w 1034 m 1163 vw 1169 vw 1206 w 1245 vw 1375 w 1466 m 1474 w 1499 w 1729 w 1743 m 1747 w 1768 w 2719 vw 2798 mw 2848 w 2865 w 2956 2962 2981 3006 3465 3667 a
w w w w w mw
(CH3)2Cd + 18O3
assignment
612
CH3
821 1008
C2H6 CH3OH H2CO H2CO
1167 1186 1240? 1374 1466 1489 1699 1708 1735 2708 2798 2847 2892 2955 2962 2981 3004
H2CO C2H6 C2H6 CH3OH H2CO H3CCHO H2CO
Figure 4. Computed structures of intermediates observed in cryogenic matrices from the twin twin jet codeposition of samples of Ar/ (CH3)2Cd and Ar/O3: (a) H3CCdCH2OH; (b) H3CCdOCH3; (c) H3CCdCHO; (e) H3COCdOOCH3.
between experimental and calculation is excellent. The most intense (235 km/mol) band is calculated (unscaled) to come at 1109 with a 31 cm−1 18O shift using the dgdzvp basis set (1091 cm−1 using the lanl2dz basis set). The most intense band of set C came at 1076 cm−1, with a 29 cm−1 isotopic shift. The second and third most intense bands, two C−H stretches, were computed at 3005 and 2953 cm−1 (harmonic frequencies) with intensities of 108 and 178 km/mol, respectively. With scaling due to anharmonicity, these calculations match well the two additional observed bands of group C at 2883 and 2799 cm−1. No other bands of H3COCdCH3 were calculated to have high intensity (>100 km/mol). Overall, the agreement between the calculated spectrum, including isotopic shifts and relative intensities, and the experimental results is excellent. Consequently, the group C bands are assigned to the novel product H3COCdCH3. Group B bands were also formed on initial twin jet deposition and are due to a species with a single O atom. Group B bands grew strongly on annealing, indicating a
HCOOH H2CO H2CO CH3OH H2CO C2H6 C2H6 C2H6, CH3OH CH3OH H2CO (2ν2) CH3OH
Band positions in cm−1. bAssignments based on literature spectra.
Using B3LYP with both the lanl2dz and dgdzvp basis sets, an energy minimum was located, and the vibrational spectra of both the normal isotopic species and the 18O-substituted species were calculated. As shown in Table 3, the agreement 1918
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this molecule does not match at all the 1708 cm−1 band position of species D, or its 33 cm−1 18O shift. Thus, this addition pathway can be ruled out. As noted above, species B (H3CCdCH2OH) was destroyed by irradiation. Because the O atom addition is not a likely source of species D, the growth of species D may be connected to the destruction of B. The band at 1708 cm−1 and its 18O shift are strongly suggestive of the formation of a carbonylcontaining species. No other photoproducts are detected, suggesting that any additional photoproduct(s) are infrared silent. The photoelimination of H2 would meet these requirements, leading to either H3CdCHO + H2 or HCdC(O)CH3 + H2. Formyl derivatives of transition metals are known intermediates,29 so there is some precedent for such a species. In addition, photoelimination of H2 from matrix isolated species has also been reported.30−32 Support for either of the possibilities must come through theoretical calculations. Calculations at the B3LYP/dgdzvp level for H3CCdCHO predict the most intense absorption to come at 1708 cm−1 with a −41 cm−1 18O shift. This compares to the position and shift given above. The second most intense band was calculated to be the C−H stretch of the −CHO unit at 2771 cm−1 (2808 cm−1 at the B3LYP/lanl2dz level). A product band at 2799 cm−1 grew strongly on irradiation. Although this was associated, above, with species C, it is certainly possible that two species (C and D) contribute to the band at 2799 cm−1. The remaining calculated product bands were all a factor of 3 or more less in intensity and could have escaped detection. Also, though calculations for HCdC(O)CH3 indicate that this species is a stable energy minimum, the computed infrared spectrum is not consistent with the spectra obtained here and may be ruled out. Thus, although the identification of a product species based on a single band is always somewhat tentative, species D is likely identified as the formyl derivative, H3CCdCHO. Bands due to species E were not present on initial deposition, grew in upon annealing, and were reduced significantly by irradiation. Although species B also grew on annealing and decreased on irradiation, B and E are clearly different species based on intensities and on isotopic behavior. Specifically, although species B clearly contains one O atom, a distinct sextet was observed for the 848 cm−1 band (16O) of species E when 50% 18O scrambled 16,18O3 was employed. A sextet is most readily derived from three O atoms in a species, two of which are equivalent and one of which is unique (a doublet of triplets). The presence of three O atoms is perhaps not surprising in that each parent ozone provides three O atoms to the reaction in the matrix. One structure that meets these requirements is H3COCdOOCH3. Peroxo linkages such as these are known in transition metal chemistry, including for cadmium,24,25 and could occur through the insertion in the second Cd−C bond of the remaining O2 molecule left cagepaired with species C when the first O atom inserts into the first Cd−C bond. Theoretical calculations for this species demonstrate it is an energy minimum on the potential energy surface. Further, one vibrational mode of this species is calculated to appear near 800 cm−1, with medium intensity, a 38 cm−1 18O shift, and a nearly sextet structure (nearly, due to partial overlap of bands of different isotopomers). A second mode that showed strong 18O dependence was observed experimentally at 1067 cm−1 with a 30 cm−1 18O shift. Calculations predict a band with reasonably intensity at 1065 cm−1 with a 32 cm−1 18O shift, in good agreement with observation. Experimentally, this mode appears as a doublet
Table 3. Comparison of Experimental and Computed Frequencies of Novel Product Species calc 18O
calc expt freqa 469 524 559 654 848 980 1067 1076 1134 1176 1212 1249 1407 1432 1708 2799 2799 2901
freqa,b
freqa,c
461 569
491 588
789 913 1065 1091 1124 1170 1196 1259
843 992 1089 1105 1159 1224 1195 1322
1476 1577 2963 2808 2981
1452 1708 2953 2771 2971
18 O a
expt shift
−17 −20 d d −46 −23 −30 −29 −1 −10 −6 −7 −2 0 −33 0 0 0
shifta,b shifta,c −17 −25
−18 −27
−38 −23 −35 −30 −1 0 −4
−44 −25 −32 −31 −1 −4 0 −6
0 −38 0 0 0
0 −41 0 0 0
assignment H3COCdOOCH3 H3COCdOOCH3 ? ? H3COCdOOCH3 H3CCdCH2OH H3COCdOOCH3 H3COCdCH3 H3CCdCH2OH H3CCdCH2OH H3CCdCHO H3CCdCH2OH ? H3COCdOOCH3 H3CCdCHO H3COCdCH3 H3CCdCHO H3COCdOOCH3
Frequencies and shifts in cm−1. bModel 1, B3LYP/lanl2dz, unscaled. Model 2, B3LYP/dgdzvp, unscaled. dNot observed.
a c
reaction is occurring with a very low activation barrier. These observations suggest a second O-atom insertion product, the only candidate formed through insertion into a C−H bond to form H3CCdCH2OH. Consequently, DFT calculations were carried out for this potential product species. Using the dgdzvp basis set, the most intense band (88 km/mol) calculated for this species came at 992 cm−1 with a 25 cm−1 shift with 18O substitution. This compares very well with the most intense band in group B, at 980 cm−1 with a 23 cm−1 18O shift. Other weaker bands of group B also match reasonably well. One expects an O−H stretching mode for this species, a band that was not observed. However, calculations predict this band to be quite weak, between 1 and 16 km/mol depending on the basis set, so it is understandable that this mode was not observed. Consequently, the bands of group B are assigned to the novel species H3CCdCH2OH formed through the insertion of an O atom from ozone into a C−H bond of parent (CH3)2Cd. As noted above, this species was substantially destroyed by irradiation; the possible product(s) of this photochemistry will be discussed below. Species D is clearly associated with one product band, at 1708 cm−1, and possibly a very weak band at 1212 cm−1. The 1708 cm−1 band was not present on initial deposition, grew in slightly upon annealing and then grew strongly upon irradiation. This 16O band had an 18O counterpart at 1675 cm−1 and showed doublet structure when 50% 18O scrambled 16,18 O3 was employed. Thus, species D contains a single O atom and is produced photolytically. Either it may form by O atom addition to (CH3)2Cd through photodissociation of ozone, or it may form as a photoproduct of the destruction of a precursor species containing one O atom. With respect to the former, two O atom addition/insertion products have already been identified (species B and C). The only other possibility is (CH3)2CdO formed through the addition of an oxygen atom to the central Cd. Although DFT calculations indicate that (CH3)2CdO is a stable species, the calculated spectrum for 1919
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when 50% 18O scrambled 16,18O3 was employed. Calculations show that although six components should be observed for this mode, they cluster with three bands calculated within 1 cm−1 of 1065 cm−1 and three within 1 cm−1 of 1033 cm−1, thus appearing as a doublet. This is understood by looking at the nature of the vibration, which is the C−O stretch of the C−O− Cd linkage which is essentially uncoupled to the Cd−O−O− CH3 linkage. Hence, only one O atom contributes to the mode and a doublet is observed. The remaining bands of species E are either attributable to the CH3 groups and do not show measurable 18O shifts or are sufficiently weak that intermediate components could be observed. Thus, species E is tentatively identified as the HC3OCdOOCH3 species. Table 3 summarizes the experimental product bands for species B−E and the corresponding calculated values, using the B3LYP/lanl2dz and B3LYP/dgdzvp models. Merged jet experiments were carried out as well for this system, leading to the observation of a large number of product bands as listed in Table 2. The more intense of these bands matched exactly the bands in set A and have been assigned above to CH2O and C2H6. Additional weaker new bands were observed. Several of these can be assigned to weaker fundamentals and combinations of CH2O and C2H6, which were too weak to be observed in the twin jet experiments. The remaining new bands can all be assigned to additional known stable products, including CH3OH, CH3CHO, and tentatively HCOOH.33−35 These would all represent “late” products in the overall reaction of (CH3)2Cd with O3 in contrast the earlier intermediates identified in the twin jet experiments (species, B, C, D, and E). Finally, one weak band was seen in the merged jet experiments that matched the most intense band of species C, suggesting that a trace amount of H3COCdCH3 was formed and trapped. This is not surprising in view of the fact that some parent (CH3)2Cd and O3 survived passage through the merged region and were deposited into the argon matrix.
Formation of H 3 COCdCH 3 was greatly enhanced by irradiation, suggesting that O atoms produced photolytically from ozone may diffuse and find a nearby parent (CH3)2Cd and insert to form H3COCdCH3. H3CCdCH2OH on the other hand is substantially destroyed by irradiation, whereas H3CCdCHO is produced. This suggests the follow reaction: H3CCdCH2OH + hν → H3CCdCHO + H2
The major products formed in the merged jet experiments are all stable, known species that must be accounted for in the reaction mechanism. Again by analogy to the Zn system, the following reaction may account for two of the primary stable products: H3CO + H3CO → CH3OH + H2CO*
(CH3)2 Cd + O3 → H3CO + Cd + CH3 + O2
(1b)
O atom insertion into a C−H appears to occur in parallel, and to be enhanced by annealing: (1c)
For steps 1a or 1b in the matrix or in the relatively dense region immediately in front of the condensing matrix, these species recombine to form the observed H3COCdCH3 product, whereas in the gas phase they separate and react further. Reaction 1c appears to have a very low barrier so that this reaction occurs upon annealing to 35 K. Also formed on annealing is H3COCdOOCH3, presumably from the insertion of cage-paired O2: H3COCdCH3 + O2 → H3COCdOOCH3
(2b)
H3CO + CdCH3 → H3COH + CdCH2
(2c)
In (2b), CH4 is a second product which might be observed. However, the ubiquitous presence of CH4 in (CH3)2Cd experiments makes it difficult to determine whether CH4 is formed in the reaction scheme. Certainly, reaction 2b cannot be eliminated. There are no experimental reports in the literature concerning the potential product CdCH2. Theoretical calculations performed here indicate that the strongest absorption of this species should occur at 1724 cm−1 with a very high intensity (dgdzvp basis set) and no 18O shift. No such band was experimentally observed in this region, suggesting that reaction 2c is less likely. These potential steps (2a)−(2c) may account for the formation of two of the major observed products, methanol and formaldehyde. The observation of CH3 as an early intermediate in this reaction is consistent with step 1b. Further reaction of two CH3 radicals could then lead to formation of C2H6 as observed, the third “late” product formed upon twin jet deposition. Although C2H6 would be formed with substantial excess energy in this reaction, no spectral evidence of C2H4 or C2H2 formed via H2 elimination from excited C2H6 was observed. This may be a consequence of the condensing matrix in twin jet deposition quenching the excitation, leading to the stabilization of C2H6. In addition, alternative pathways to the formation of C2H6 as a secondary product in this reaction may take place as well. A firm conclusion on this point is not readily made. Finally, though Cd is clearly present in the twin jet experiments in several of the intermediates that were formed, its fate is not as apparent in the merged jet experiments. No Cd-containing products were observed in these experiments. Many of these would be very weak infrared absorbers, including CdOx species, and might have escaped detection. Of course, Cd and Cd2 are infrared inactive. It is noteworthy that Zare et al.36 observed Zn atom emission, as well as ZnH emission, from the reaction of O3 with (C2H5)2Zn, so that Cd atom formation is reasonable here. Finally, CdO, CdO2, and HCdCH3 are also reasonable products in this system. The fundamental of CdO has been reported at 719 cm−1 in N2 matrices.37 The argon matrix positions are likely to shift slightly but should be in the
or
(CH3)2 Cd + O3 → H3CCdCH2OH
H3CO + CH3 → H2CO + CH 4
or
REACTION MECHANISM The “snapshots” of the reaction at different times probed by twin and merged jet deposition provide some insight into the mechanism of the reaction. Analogous to the proposal by Zare et al.36 for the reaction of (CH3)2Zn with O3, one initial step appears to be oxidation of the Cd−C bond, forming H3CO and (presumably) CdCH3 or Cd + CH3: (1a)
(2a)
However, this step requires two H3CO species, which are present in low concentration, to collide during the matrix condensation process, which has low probability. Alternatively, other H atom acceptors and donors (e.g., CH3 and CdCH3, respectively) could also react with H3CO,
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(CH3)2 Cd + O3 → H3CO* + CdCH3 + O2
(1e)
(1d) 1920
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The Journal of Physical Chemistry A same region. No absorptions were observed that could be attributed to CdO. CdO2 has been observed at 626 cm−1 in argon matrices, with a shift to 598 cm−1 with full 18O substitution.38 These bands were not observed in the present study. Likewise, HCdCH3 has also been observed in argon matrices.39 The major bands of this species were not observed in the spectra obtained here, leading to the conclusion that HCdCH3 was not observed in the present study. The most likely explanation for the fate of cadmium under the present reaction conditions is the formation of Cd atoms or dimers that are not observable spectroscopically and/or may have deposited on the walls of the merged jet reaction region. The relative reaction rates of (CH3)2Cd and (CH3)2Zn with O3 are also of interest. The reaction of (CH3)2Zn with O3 led to a very small amount of product upon twin jet deposition, and only slight growth on annealing. In contrast, the initial twin jet reaction of (CH3)2Cd with O3 led to the formation of novel intermediates in substantial amounts, along with several stable species that must require multiple reaction steps to form. Annealing led to substantial growth of one product and the formation of a second. Thus, (CH3)2Cd demonstrates substantially increased reactivity toward O3 than does (CH3)2Zn. This result is counter to the relative oxidation potentials of the metals (which may or may not be consistent with the oxidation potentials of the corresponding organometallic compounds). Of course, an argument using relative oxidation potentials is based on thermodynamic considerations, whereas the reaction may be kinetically controlled. It is noteworthy that (CH3)3Al is much more reactive toward O3 than is (CH3)3In under twin jet deposition conditions,40,41 consistent with their relative oxidation potentials.
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REFERENCES
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CONCLUSIONS Twin jet deposition of (CH3)2Cd and O3 into argon matrices led to the initial formation of at least five different species, three of which are known from previous studies and two of which were novel Cd-containing compounds. These were identified as the O-atom insertion products H 3 COCdCH 3 and H 3 CCdCH 2 OH. Annealing led to the growth of H3CCdCH2OH and the formation of the peroxide product H3COCdOOCH3 via O2 insertion. UV irradiation led to the growth of H3COCdCH3, reduction of H3CCdCH2OH and H3COCdOOCH3, and the formation of a novel formyl derivative, H3CCdCHO. In contrast, merged jet deposition led to the formation of a series of “late” reaction products, including CH2O, C2H6, CH3OH, CH3CHO, and tentatively HCOOH. A potential mechanism for the early steps in this reaction was discussed. ASSOCIATED CONTENT
S Supporting Information *
Additional twin jet spectra before and after annealing and compared to a blank spectrum of (CH3)2Cd over the spectral range 400−1800 and 2600−3200 cm−1 are provided. Complete ref 20. This information is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
The National Science Foundation is gratefully acknowledged for their support of this research through grant CHE-1110026. Hairong Guan is acknowledged for helpful discussions.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest. 1921
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The Journal of Physical Chemistry A
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