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J. Phys. Chem. 1994,98, 4-7
Site Propensities for HCl and DCI Formation in the Reaction of Cl with Selectively-Deuterated Propanes Yu-Fong Yen, Zhongrui Wang, Bing Xue, and Brent Koplitz' Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received: October 14, I993@
We report on the gas-phase reaction of C1 with CH3CD2CH3 and C D J C H ~ C D to~form HCl or DCl. C1 atoms are generated by the 351-nm photolysis of Cl2, and both the HCl and DCl products are detected via 2 + 1 multiphoton ionization at -240 nm. The use of labeled compounds allows positive identification of individual reactive sites within the propane molecule. With respect to the reaction products over the quantum states probed, DCl is formed with nearly equal probability independent of whether CH3CD2CH3 or CD3CH2CD3 is the target molecule. For HCl formation, C1 reacting with CH3CD2CH3 generates 1.5 times more HCl than does C1 reacting with C D ~ C H ~ C D On J . a per site basis, we conclude that the middle carbon site is more reactive than an end site, and the overall results are discussed in terms of possible reaction mechanisms.
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Introduction
Understanding and ultimately controlling chemical reactions has been a goal in physical chemistry for decades. Over the years, improved laser technology has enabled the photoinitiated chemistry of certain isolated molecules to be controlled with respect to bond cleavage. Examples where lasers have been used to effect preferential bond cleavage include experiments on haloalkanes1 as well as more recent studies involving HOD.24 Remarkably,control over the bimolecularselectivityof H reacting with HOD was pioneered recently by Crim and co-w~rkers,~ and subsequent efforts by Zare and co-workers on the H plus HOD system have further explored its controllable nature.6 Sinha et al. have also succeeded in influencing HOD chemistry when the reactive partner is a photolytically-generatedC1 atom instead of an H atom? With larger systems, routes to controlling chemistry are often more elusive. As a first step toward site-selective chemistry, it can be instructive to explore the site-specific chemistry of a particular system. Here, the term site-specificis used to describe the propensity for bond cleavage at a particular site that one simply observes in a given experiment. Significant, understandable control has not yet been exerted over the relevant bond chemistry. With regard to isolated molecules, recent work in our own laboratory has focused on the photolysis of haloalkanes and their associated alkyl radicals, where the use of selectivelydeuterated compoundsenables one to explore the sites and routes by which atomic hydrogen is generated.*-I2 These studies are examplesof what we term site-specific photochemistry. However, such studies are not limited to isolated molecules. In the present Letter, we report on experiments that fall into thecategoryof site-specific bimolecular chemistry. Recent efforts by Matsumiet al. have focused on O(lD) reacting with deuteriumlabeled alkanes to produce H or D atoms.13 In our experiments, selectivelydeuteratedpropanes (CH3CD2CH3and CD3CH2CD3) are reacted with photolytically generated C1 atoms having a welldefined kinetic energy (0.53eV). Subsequent HCI and DCl formation is monitored using 2 + 1 resonant ionization methods.14J5 On a per site basis, one observes a greater tendency for hydrogen abstraction to occur at the middle carbon rather than at an end carbon atom. Thecurrent results as well as the potential for future studies on systems of this type are discussed below. Experimental Section
As shown in Figure 1, the experiments are performed within .Abstract published in Advance ACS Abstracts, December IS, 1993.
0022-3654/94/2098-0004$04.50/0
To Ion Detector
-Probe Laser
Figure 1. Diagram depicting the ionization/interaction region of the time-of-flight mass spectrometer.
the ionization region of a time-of-flight mass spectrometer (TOFMS). Sample introduction is achieved using a commercially-available pulsed nozzle (General Valve) modified with a Teflon extender that protrudes into the ionization region. The Teflon extender facilitates the introduction of a relatively large number of molecules into the ionizationregion, yet the extraction fields of the TOFMS are not unduly perturbed. To initiate the photolysis of C12, the output of an excimer laser (Questek 2520, XeF, 35 1 nm, -50 mJ) is mildly focused with a lens (1 -m focal length) and propagatesaxially with respect to thenozzleextender, finally impinging on the throat of the nozzle itself. Probe laser radiation for HCl and DC1 detection is generated by frequencydoubling with a BBO crystal the output of an excimer-pumped dye laser (Lambda Physik LPX 105,FL 3002)to produce 1 mJ of tunable radiation at -240 nm. This probe laser output is focused with a lens (75-mm focal length) and passes perpendicular to the photolysis beam approximately 8 mm downstream from the tip of the nozzle extender. With regard to procedure, HCl, DCl, or a mixture of Clz and C H J C D ~ C Hor~CD3CH2CD3 is introduced through the pulsed nozzle. Deuterium-labeled samples were obtained from MSD with a stated D-atom enrichment of 298%. Calibration spectra for neat HCl and DCl are obtained using 2 + 1 multiphoton ionization methods as described by Wallace,14 Gordon,1Sand their respective co-workers. For HC1, mass 36 (representative of H X 1 ) is monitored as a function of wavelength, while for DCl, mass 37 is measured. In the bimolecular studies, C11 and the specific propane reactant were coexpanded into the ionization
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0 1994 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 98, No. 1. 1994 5
Neat HCI (cooled) 0 Q'
1
2
3
I
I
1
....
". CI + CD3CH,CD3
"...*.I......
h
.-v) .L
E
240.70
240.74
240.70
(nm) Figure 2. Expansion-cooled 2
mass 36 was monitored.
+ 1 ionization spectrum of HCI.
Here,
region using a nozzle backing pressure of approximately 2 atm. Typical propane/Cl2 mixtures were 1:1. Photolysis of C12 produces C1 atoms with a well-defined kinetic energy, and a nascent C1 atom can subsequently reactant with a propane molecule to form HCl or DCl via hydrogen abstraction. The particular experimental arrangement shown in Figure 1 enables a variety of nozzle/photolysis/probe timing delays to be employed. However, the bimolecular reaction studies presented in the current work were all conducted downstream in the expansion and involved a photolysis/probedelay of 120 ns. Here, sufficient time is availablefor the onset of the abstraction reaction, but there is relatively little time for additional collisions that might relax the nascent HCl or DCl product. We submit that the spectra observed are, in fact, representative of nascent HC1 and DCI products. With respect to potential experimental complications,two issues must be addressed. One concerns possible contamination due to the formation of HCl (or DCl) prior to expansion. In the bimolecular experiments, care was taken to record spectra only under conditions where the observed HC1 and/or DC1 product signal could be positively correlated with the photolysis laser. In other words, the observed spectra are not due to HCl or DCI already present in the expansion mixture. A second issueconcerns the possible role of van der Waals complexes. Under the current conditions, we see no evidence for direct involvementof complexes in the reaction. There exists a significant rise time to the signal-on the order of tens of nanoseconds. If reactions within a complex were contributing to the signal, one would be surprised to see a significant rise time. In these experiments, the laser beams are adjusted spatially and temporally to maximize the signal, and inevitably there is a significant rise time when these adjustments are optimized. Note, however, that complexes still may be present, and their presence may produce secondaryeffects such as altering the CI atom kinetic energy or providing a complexed propane molecule as a reaction target. Further experiments are needed to address these concerns.
Results In Figure 2, a 2 1 resonance ionization spectrum of HCl is shown involving the ground electronic state, XlZ+(O+), and the glZ-(O+) state as assigned by Wallace and co-workers.14 In this particular experiment, neat HCI was expanded through the nozzle using a backing pressure of 1 atm. Since the observed rotational structure can be described reasonably well by a Boltzmann distribution corresponding to -70 K, this spectrum,serves as a temperature calibrant for the overall experiment. In the current work, the focus is on bimolecular reactions of C1atoms with selectively-deuterated propanes. In order to collect data efficiently, minimize systematic effects, and conserve the selectively-deuterated compounds, a wavelength region (240.7-
+
240.75
240:94
241.13
2, 2.3
Figure 3. A portion of the 2 + 1 ionization spectrum for DCI under several conditions. Part a shows the result of a photolytically-generated C1 atom reacting with two different selectively-deuteratedpropanes. Part b arises from neat, expansion-cooledDCI, while part c was simply taken with neat DCI under room temperature conditions.
241.4 nm) was selected whereby DCI and HCl formation could be monitored simultaneously. Unfortunately, in this wavelength region we are not certain of the rotational assignments. However, this situation can be addressed by using several neat HCl and DCI spectra as references, where the individual experimental conditions can be adequately characterized. The results of bimolecular as well as calibration experiments in the wavelength region from 240.7 to 241.4 nm are shown in Figures 3 and 4. Figure 3a presents the results for DCI formation arising from the reaction of a C1 atom (0.53 eV of kinetic energy) with CD3CH2CD3and CHJCD~CHS.Over the wuuelengfhregion probed, one can see that DCl production from C1 reacting with CHJCD~CHJ is almost equivalent to DCl formation arising from CI reacting with C D J C H ~ C DNote ~ . that this roughly equivalent production occurs despite the fact that CDjCH2CDj has twice the potentially reactive carbon sites and 3 times the number of D atoms available when compared with C H ~ C D Z C H ~ . In Figure 3b,c, spectra for neat samples of DCl under two different experimental conditions are provided for reference as a measure of the extent of rotational excitation present in the DCl product. Figure 3b was taken under nozzle-extender expansion conditionssimilar to Figure 2 for HC1, which produced a rotational temperature of -70 K. Clearly, the bimolecular reaction results for Figure 3a a display comparable amounts of rotational excitation. The similarity of Figure 3a,b is in contrast to Figure 3c, which was taken under room temperature conditions involving sample introduction via molecular leak. In Figure 4a, we again present the results for photolyticallygenerated C1 reacting with CD3CH2CD3 and CHsCD2CH3, but here the reaction product is HCI. Note that each spectrum for HCl in Figure 4a was taken simultaneously with its DCl counterpart in Figure 3a. In the case of HCl, there is a slightly higher propensity for hydrogen abstraction from the two end carbon positionscombined as opposed to the single middle position (approximately 1.5:1), However, on a per site basis, the middle position is still preferred. With regard to rotational excitation
6 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
(a)
. . . . . . . . . . . . I
-
h
.-Cv) 3
2
i?
em CI
v
240.75
240.94
241.13
241.33
Figure 4. A portion of the 2 ’ + 1 ionization spectrum for HC1 under several conditions. Part a shows the result of a photolytically-generated C1atom reacting with two different selectively-deuteratedpropanes. Part b arises from neat, expansion-cooledHCl, while part c was simply taken with neat HCl under room temperature conditions.
within u = 0, Figure 4b,c provides a semiquantitative means of comparison. Figure 4b displays the expansion-cooled spectrum for HCl taken under the same conditions as those for Figure 2 in which a 70K rotational temperature was produced. In contrast, Figure 4c contains the room temperature spectrum for HCl. Clearly, the reaction-formed HCl or Figure 4a more closely resembles the expansion-cooled spectrum, although the data indicate that the reaction product is slightly warmer. In terms of an approximate temperature, we estimate the rotational temperature to be -70-100 K with the HCl product arising from the middle carbon being slightly warmer than that from an end carbon, but not to any significant degree.
Discussion Hydrogen abstraction reactions involving chlorine atoms and hydrocarbons have been the focus of much research effort. (See ref 15 and references contained therein.) Recently, Flynn and co-workersutilized infrared absorption spectroscopycoupled with a diode laser to measure DCl products resulting from the reaction of C1 atoms with cyclohexane-dlz.16 Similar to our work, they observed a relatively cold rotational temperature (135 K) for u = 0 in the DCl product state. Results from state-resolved abstraction reactions between C1 atoms and CH, have also just been reported by Simpson et a1.17 Here, a relativelycold rotational temperaturewas alsoobserved,although the monitoredvibrational state in this case was u = 1. When discussing these systems, it is necessary to identify the relevant energetics. Using a bond energy of 2.48 eV for the chorine molecule,1sphotolysis of Clz at 351 nm produces predominantly chlorine atoms with 0.53 eV of kinetic energy. Although the photolytic production of spin-orbit excited C1 (defined as C1*) can occur, at 351 nm the yield for C1+ from Clz is
