Product State Resolved Study of the Cl + (CH3)3CD Reaction

J. Phys. Chem. 1996, 100, 4365-4374

4365

Product State Resolved Study of the Cl + (CH3)3CD Reaction: Comparison of the Dynamics of Abstraction of Primary versus Tertiary Hydrogens David F. Varley† and Paul J. Dagdigian* Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218-2685 ReceiVed: October 24, 1995; In Final Form: December 13, 1995X

The reaction of Cl atoms with the selectively deuterated isobutane, 2-methylpropane-2-d1, has been investigated under single-collision conditions using state-selective detection of the products by resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The reaction was initiated in a crossed, pulsed flow of the reagents by 355 nm photolysis of Cl2 precursor. The internal state distributions of HCl and DCl products, formed by abstraction of primary and tertiary hydrogen atoms, respectively, from the hydrocarbon reagent are reported. The degree of rotational excitation of both products was found to be very low. By comparison of the intensities of the HCl and DCl REMPI signals, the ratio of the yield of HCl to DCl product was found to be 3.3 ( 0.4. With the known speed and angular distribution of the Cl reagent, it was possible to obtain information on the product center-of-mass angular distributions from measurement of the masses 37 and 38 time of arrival profiles. The DCl product is found to be mainly backward scattered with respect to the incoming Cl atom, while the HCl product is sideways peaked. Inferences on the dynamics of the two possible abstraction pathways from these results are discussed.

1. Introduction Recent experiments from several laboratories have begun to provide detailed information on the dynamics of the reactions of chlorine atoms with hydrocarbons. Park et al.1 have employed high-resolution infrared absorption spectroscopy to interrogate the internal and translational excitation of the DCl product from the reaction of Cl atoms with c-C6D12. Simpson et al.2 have utilized resonance enhanced multiphoton ionization (REMPI) detection in a time-of-flight mass spectrometer (TOFMS) to observe HCl product in its V ) 1 vibrational level from the reaction of Cl atoms with vibrationally excited CH4(ν3)1). An especially novel aspect of this study was the extraction3 of the product center-of-mass (c.m.) angular distribution from analysis of the projection of the HCl product laboratory velocity distribution along the TOFMS axis. This reaction is one of a growing number of examples of photoinitiated reactions and energy-transfer processes for which vector correlations have been derived.4-11 Yen et al.12, 13 have also employed REMPI detection in order to investigate site specificity in the reaction of Cl atoms with selectively deuterated propane and butane isotopomers. In all these experiments, the reaction was initiated by photolysis of a Cl atom precursor. We have recently carried out a study14 of the photoinitiated reaction of Cl atoms with methane, propane, and isobutane, all in their ground vibrational states. In that investigation, we reported product HCl ro-vibrational state distributions and a preliminary product c.m. angular distribution for the Cl + isobutane reaction. The angular distribution was derived from analysis of the projection of the HCl(V)0) product laboratory velocity distribution along the TOFMS axis with a procedure similar to those employed by Zare, Hall, and co-workers.2, 3, 9 A notable feature of our experiment was the use of crossed pulsed flows of Cl2, the Cl atom photolytic precursor, and the hydrocarbon reagent, in order to avoid prereaction between these reagents in premixed flows. † Present address: Stanford Research Systems, 1290-D Reamwood Ave., Sunnyvale, CA 94089. X Abstract published in AdVance ACS Abstracts, February 15, 1996.

0022-3654/96/20100-4365$12.00/0

As in the Cl + hydrocarbon reactions studied by Park et al.1 and Simpson et al.,2 we found little rotational excitation of the HCl product in the Cl + CH4, C3H8, and i-C4H10 reactions.14 These observations are consistent with our initial expectations for the dynamics of these reactions since the geometry of the transition state for the Cl + CH4 reaction has been found in ab initio calculations15-17 to have a linear Cl-H-CH3 structure. With this transition state geometry, there will be a bias toward linear Cl-H‚‚‚R recoil, and hence little torque will be exerted on the departing HCl product. Park et al.18 were able to rationalize the low product DCl vibrational excitation in the Cl + c-C6D12 reaction with classical trajectory calculations on an empirical three-body (Cl-D-C6D11) potential energy surface, consistent with a direct abstraction mechanism through a collinear Cl-D‚‚‚R recoil geometry. We have also found that the HCl(V)0) product from the Cl + isobutane reaction was primarily backward scattered with respect to the incoming Cl atom.14 Simpson et al.2 find the particularly surprising result that the HCl(V)1) product from Cl + CH4(ν3)1) was scattered mainly in the forward direction with respect to the incoming Cl reagent. This suggests that the dynamics of reactions of Cl atoms with hydrocarbons, particularly in vibrationally excited states, is richer than we might have anticipated just from knowledge of the transition state geometry. In contrast to the prototype Cl + CH4 reaction, the reaction of Cl atoms with larger hydrocarbons is complicated by the availability of several reaction pathways, involving the abstraction of a primary, secondary, or tertiary hydrogen. Hence, the HCl product detected in a our previous study14 of the reactions of Cl atoms with propane and isobutane could have been produced by abstraction of several types of hydrogen atoms from the hydrocarbon. This ambiguity significantly hampered our analysis of the dynamics of these reactions. To gain more insight into the dynamics of these two reaction pathways, we have carried out a study of the reaction of chlorine atoms with selectively deuterated isobutane reagent in order to distinguish these two pathways isotopically: © 1996 American Chemical Society

4366 J. Phys. Chem., Vol. 100, No. 11, 1996

Varley and Dagdigian

Cl + (CH3)3CD f HCl + (CH3)2CHCH2, ∆H° ) -11 kJ mol-1 (1a) f DCl + (CH3)3C, ∆H° ) -26 kJ mol-1 (1b) The results of these experiments are reported in the present paper. The use of isotopically labeled reagent allows us not only to probe the dynamics of the two pathways separately but also to derive a branching ratio for the abstraction of the tertiary vs primary hydrogen atoms. The exothermicity (enthalpy change at 0 K) of pathway (1b) was calculated using the (CH3)3C-H bond energy at 0 K reported in a recent review of three methods to determine bond energies19 and was corrected for differences in the zero-point energies of the reagent and product isotopomers. The exothermicity for pathway 1b is slightly less than that of the corresponding Cl + (CH3)3CH pathway. We were not able to find in the literature a value for the (CH3)2CHCH2-H bond energy for the calculation of the exothermicity of pathway 1a. We have assumed that this bond energy, involving the removal of a primary hydrogen, is the same as the CH3CH2CH2-H bond energy. We have hence assumed that the exothermicity of pathway 1a can be equated with the exothermicity (corrected to 0 K) of the Cl + C3H8 f HCl + (CH3)2CHCH2 reaction, reported in a recent compilation20 of rate constants of atmospherically relevant reactions. The Cl + isobutane reaction was found to have an approximately temperature-independent rate constant21 and hence only a very small activation energy. This suggests that the slight changes in the energetics from isotopic substitution should have only a small effect on the reaction dynamics. It should also be noted that the energy available to the products in both pathways 1a and 1b is significantly less than the energy required for the butyl radicals to fragment to a butene + H atom. Hence, there is no possibility that a butyl radical product formed in this reaction could subsequently decompose in a unimolecular fashion. While the crossed-beam geometry for introducing the reagents into the reaction zone allowed us to suppress completely prereaction of the reagents, this arrangement had the unfortunate consequence of yielding a large spread in the reagent relative translational energy distribution because of the vector addition of the reagent beam velocities with the Cl photofragment velocity, as discussed in detail in our previous paper.14 For the present experiments, we have modified the apparatus so as to reduce the beam intersection angle from 90° to 20°. As we discuss below, this has significantly reduced the spread in the collision energy distribution. In the present study, we report internal state distributions for the HCl and DCl products of pathways 1a and 1b for the reaction of Cl atoms with selectively deuterated isobutane. As in the experiment by Simpson et al.2 and in our previous study,14 we have recorded mass peak profiles in order to determine the distribution of the product laboratory velocity component along the TOFMS axis. From these distributions, the product c.m. angular distributions can be characterized. We have also remeasured the product HCl(V)0) rotational state distribution for the Cl + i-C4H10 reaction and have compared this distribution with the one measured previously,14 in order to determine the effect of the large reagent collision energy spread in the earlier study on the product state distribution. 2. Experimental Section The present set of experiments was carried out in a recently constructed time-of-flight mass spectrometer (TOFMS), which

Figure 1. Schematic drawing of the two beam sources mounted within the ionization-extraction region of the time-of-flight mass spectrometer. Here, we have reduced the intersection angle of the beams from 90° in our earlier study to an angle of 20°.

has been described in detail previously.14 We present here a brief description of the apparatus, with emphasis on the modifications we have made for this study. Hydrocarbon reagents were allowed to react with chlorine atoms, produced by 355 nm photolysis of Cl2, in the ionization-extraction region of the TOFMS. The HCl and DCl reaction products were detected after a short delay in specific vibration-rotation states by 2 + 1 REMPI detection. The ionization-extraction region and the flight tube of the TOFMS were separately evacuated by trapped oil diffusion pumps. The hydrocarbon reagent and Cl2 photolytic precursor were injected into the ionization-extraction region through separate pulsed valves (orifice diameters 0.5 cm). The intersection angle between these beams was reduced from 90° in the previous experiments to 20° here. This reagent injection system is illustrated in Figure 1. In order to minimize the distortion of the electric field in the ionization region from the source assembly, a set of four guard electrodes was installed around the sources, as discussed in detail previously.14 The hydrocarbon reagent 2-methylpropane-2-d1, (CH3)3CD, was obtained from Isotec, Inc. The stated isotopic purity was 98 atom % D. This reagent was used without further purification. As discussed in our previous study,14 molecular chlorine reagent (Matheson, UHP grade) was fractionally distilled to remove noncondensable species and HCl impurity. Purified samples were stored in glass bulbs shielded from room lights. In most experiments, the hydrocarbon reagent was injected in a neat expansion at a pressure of 400 Torr, while the Cl2 reagent was employed as a 7-10% mixture in He at 900 Torr total pressure. It is possible that clusters of reagents could be formed under our expansion conditions. Naaman and co-workers22,23 have studied the clustering of propane under similar conditions through mass spectrometric detection. With neat expansion of 300 Torr of propane (0.5 cm diameter nozzle), they find little clustering, while addition of several atmospheres of rare gas caused extensive clustering. On this basis, we assume that the hydrocarbon reagent in our experiment is mainly monomeric. The photolysis laser (frequency-tripled Nd:YAG laser radiation) and probe laser (frequency-doubled dye laser) were combined with a dichroic mirror and focused into the TOFMS with a 40 cm focal length fused silica lens. The delay between the laser pulses (typically 100-120 ns), as well as the opening of the pulsed valves, was controlled with a digital delay generator. For the determination of product internal state distributions, the ion signal on a given mass channel was monitored with a gated integrator as the probe laser wavelength was scanned. In order to compensate for the background signal, particularly in the detection of HCl products, the photolysis laser Q switch was fired on alternate shots, and the signal was

Study of the Cl + (CH3)3CD Reaction

J. Phys. Chem., Vol. 100, No. 11, 1996 4367

Figure 3. Schematic diagram of our two-beam arrangement. The velocities of the Cl2 photolytic precursor and the hydrocarbon reagent are denoted vAX and vBC, respectively, while the c.m. velocity of the photofragment A is denoted vA. The direction of the electric vector of the photolysis laser is denoted Eˆ .

Figure 2. Calculated reagent relative translational energy distribution for the reaction of photolytically prepared Cl atoms with (a) CH4, (b) C3H8, and (c) (CH3)3CD in our present experimental geometry.

recorded as the difference with the photolysis laser on vs. off. For the determination of mass peak profiles, the laser was tuned to a given molecular line, and the ion signal was monitored with a digital oscilloscope (LeCroy Model 9360). The mass peak profile was obtained by accumulating the waveform over many laser shots (typically 2000-3000). If there was a significant background signal on the transition, the mass peak profile was taken as the difference of the wave forms with the photolysis laser on vs off. 3. Results 3.1. Reagent Relative Translational Energy Distribution. Figure 2 presents the calculated reagent relative translational energy distribution for reaction 1, as well as for the previously studied Cl + CH4 and C3H8 reactions,14 with our present experimental geometry. These distributions were calculated by Monte Carlo integration of eq A2 of ref 14. The distributions displayed in Figure 2 have significantly smaller spreads than the distributions appropriate to our original geometry with a 90° beam intersection angle. (See the translational energy distributions shown in Figure 11 of ref 14.) We see from Figure 2 that the average translational energy, as well as the spread of the distribution, increases with increasing mass of the hydrocarbon reagent. The former is readily explained by kinematic effects, in which the larger mass of the hydrocarbon reagent allows the laboratory translational energy of the Cl reagent to be more efficiently converted into relative translational energy. The narrow width of the distribution for Cl + CH4 results from the fact that the laboratory speeds of the reagent Cl2 and CH4 beams are approximately equal. In this case, the component of the vector sum of the reagent velocities along the laser beam direction (see Figure 3) is approximately zero. This leads to a smaller spread in the initial translational energy from the convolution of the velocity of the Cl photolysis fragment and the reagent beam velocities. The form of the distributions shown in Figure 2 represents a subtle interplay between the anisotropy of the Cl fragment velocity distribution and the vector sum of the reagent velocities. If we take the average (34 kJ mol-1) of the translational energy distribution displayed in Figure 2c, then the total energy

available to the products Etot is 45 and 60 kJ mol-1 for pathways 1a and 1b, respectively. Thus, the highest energetically accessible HCl and DCl vibrational levels in the two pathways are V ) 1 and 2, respectively. Within the ground vibrational states of these products, the highest accessible rotational level J is 18 and 30, respectively. 3.2. Product Internal State Distributions. As in our previous study of the reaction of Cl atoms with small hydrocarbons,14 REMPI detection was employed to determine the relative populations of the various internal states of the products. Considerable information on 2 + 1 REMPI transitions in the HCl molecule is available from extensive studies by Gordon,24-26 Wallace,27,28 Chandler,29,30 and co-workers. The most convenient transitions for REMPI detection of HCl are the strong E1Σ+-X1Σ+ and F1∆-X1Σ+ band systems, which were first observed by one-photon vacuum-UV absorption spectroscopy.31 For the detection of HCl(V)0) rotational levels, we have employed the E-X (0,0) and F-X (0,0) bands, for which calibration factors relating REMPI intensities to relative populations have been determined.25,29 Because of the extensive perturbations between HCl excited electronic states, it is more appropriate to determine these intensity factors experimentally, rather than through calculations. A recent two-color experiment has provided evidence for predissociation of the HCl(F1∆) state through an indirect mechanism.26 We have employed the previously reported25,29 calibration factors in calculating relative populations from our measured HCl REMPI signal strengths. In contrast to the considerable spectroscopic information available on these transitions in HCl, relatively little work has been reported on the corresponding REMPI transitions in the DCl isotopomer. Green et al.27 have reported transition wavenumbers for rotational lines in the D35Cl E1Σ+-X1Σ+ (0,0) band. In collaboration with Gordon and co-workers, we have carried out a REMPI spectroscopic study of several DCl E-X and F-X bands to determine the intensity factors. These results will be reported in a separate publication.32 Among the quantities determined in this work are calibration factors for the conversion of DCl REMPI signals to relative rotational populations and the relative detection sensitivity for DCl vs HCl. These parameters were employed here to determine the product DCl(V)0) rotational state distributions and the branching ratio for pathways 1a and 1b. Figure 4 displays a typical REMPI spectrum for the detection of the reaction products HCl(V)0) and DCl(V)0) from pathways 1a and 1b, respectively. Since the average translational energy of the Cl + isobutane reaction has been reduced from that in our previous study14 by a decrease in the reagent beam intersection angle, we have measured the rotational state distribution in the HCl(V)0) vibrational level for the reaction of Cl atoms with i-C4H10, in order to compare with the product rotational state distribution which we previously measured for

4368 J. Phys. Chem., Vol. 100, No. 11, 1996

Figure 4. REMPI spectra of the (a) H35Cl and (b) D35Cl products from the reaction of Cl atoms with (CH3)3CD at a photolysis-probe delay of 140 ns. Plotted are the m/e ) 36 and 2 signals as a function of the two-photon laser wavenumber. Assignments for the Q-branch lines of the E1Σ+ - X1Σ+ (0,0) band for the HCl and DCl isotopomers are given. There is a small gap in the spectrum displayed in panel a because a strong Cl atomic transition occurs in this region and causes interference with the mass 36 signal. In panel b, transitions of both D35Cl and D37Cl isotopomers (the latter separately indicated) are observed since D+ ions were monitored.

Figure 5. Measured rotational state distributions for HCl(V)0) products from the reaction of Cl atoms with isobutane: (a) 90° reagent beam intersection angle (distribution taken from ref 14) and (b) 20° beam intersection angle. The distributions have been normalized so that the sum of the rotational populations is unity.

this reaction.14 Figure 5 compares our measured distributions from the two studies. The most probable rotational level for both reagent beam intersection angles is very low (Jmp ) 2 or 3). However, the distribution measured with a 90° beam intersection angle extends to much higher values of J than does the distribution obtained with a 20° intersection angle. The average rotational excitation is calculated to equal 2.1 and 0.8 kJ mol-1 for 90° and 20° intersection angles, respectively. Thus, we see that the higher average translational energy for the former beam geometry is reflected in a somewhat higher rotational excitation of the HCl(V)0) products.

Varley and Dagdigian

Figure 6. Measured rotational state distributions for (a) HCl(V)0) and (b) DCl(V)0) products from the reaction of Cl atoms with (CH3)3CD. The distributions have been normalized so that the sum of the rotational populations is unity.

Figure 6 compares the rotational state distributions for the HCl(V)0) and DCl(V)0) products from pathways (1a) and (1b), respectively, for the reaction of Cl atoms with (CH3)3CD. As expected from the rotational state distribution presented in Figure 3b for the reaction of the fully hydrogenated reagent, the degrees of rotational excitation for both pathways 1a and 1b are low. The distribution for DCl(V)0) extends to higher J than does the HCl(V)0) distribution, but this comparison is somewhat misleading because of the different rotational constants for these isotopomers.33 The average rotational excitation is calculated to be 0.6 and 1.2 kJ mol-1 for the HCl and DCl product, respectively. Hence, the rotational excitation of the product is found to be slightly higher for the extraction of the tertiary vs primary hydrogen atom from the hydrocarbon reagent. In our previous study of the Cl + isobutane reaction,14 we observed a small formation of HCl product in the V ) 1 vibrational level, relative to that for V ) 0. Comparison of REMPI signals for the F-X (0,0) and (1,1) bands yielded a V ) 1 to V ) 0 population ratio of 0.10 ( 0.02. In calculating this ratio, we assumed that these bands have the same strengths since no information on the intensity factors for these bands is presently available. In the present study, we also have detected V ) 1 product from Cl + i-C4H10 with a 20° beam intersection geometry and find a considerably smaller V ) 1 to V ) 0 population ratio, approximately 0.01. Since the average relative translational energy is considerably smaller in the present study, this indicates that the degree of vibrational excitation is decreased with decreasing collision energy. In view of the small V ) 1 product observed for the Cl + i-C4H10 reaction and the expense of the selectively deuterated reagent, we have not searched for V ) 1 products in pathways 1a and 1b. As mentioned in the Introduction, there is considerable interest in the measurement of vector properties, for example the rotational alignment, of products from chemical reactions.34 We have employed the Q branch of the E-X transition for the determination of rotational state distributions in the HCl and DCl V ) 0 products. The intensities of these lines are not significantly affected by possible rotational alignment, and no information on this vector correlation can thus be obtained from these data. We have also employed the F-X transition for some

Study of the Cl + (CH3)3CD Reaction

J. Phys. Chem., Vol. 100, No. 11, 1996 4369 mass peak profiles with REMPI detection in a TOFMS. To simplify the analysis,3 the velocities of the photolytic precursor and the molecular reagent should be negligible, as for example in the case of premixed reagents in a supersonic expansion.2 In this case, Shafer et al.3 showed that the 3-dimensional laboratory velocity distribution for the reaction product AB from the A + BC f AB + C reaction can be written as

f(vAB) ) (2uuABVAB)-1 [1 + βP2(vˆ AB‚uˆ ) P2(vˆ AB‚Eˆ )](1/σ)(dσ/dΩr), for |u - uAB|