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J. Phys. Chem. B 2005, 109, 8388-8392
Vibrationally Controlled Chemistry: Mode- and Bond-Selected Reaction of CH3D with Cl† Sangwoon Yoon,‡ Robert J. Holiday, and F. Fleming Crim* Department of Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706 ReceiVed: August 12, 2004; In Final Form: January 3, 2005
Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH3D with Cl, which forms either HCl + CH2D or DCl + CH3. The reaction of CH3D molecules with the first overtone of the C-D stretch (2ν2) excited selectively breaks the C-D bond, producing CH3 exclusively. In contrast, excitation of either the symmetric C-H stretch (ν1), the antisymmetric C-H stretch (ν4), or a combination of antisymmetric stretch and CH3 umbrella bend (ν4 + ν3) causes the reaction to cleave only a C-H bond to produce solely CH2D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (ν1) of CH3D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (ν4) even though the two lie within 80 cm-1 of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.
I. Introduction Control of chemical reactions is a central aspect of chemistry. Chemists use a variety of approaches, such as changing external conditions and introducing catalysts, to control the outcome or rate of a reaction. One conceptually appealing, but currently less practical, approach is using lasers to alter the course of a reaction. However, its greatest advantage may be the singular understanding of the details of reaction at the quantum state resolved level that it provides. Among approaches using lasers, vibrational excitation of molecules is an intuitively appealing means of controlling chemistry1-3 since reaction occurs through rearrangement of nuclear positions. Motion that carries the system over a barrier along the reaction coordinate can be as simple as stretching a bond to transfer an atom or as complex as a concerted motion of many atoms to make and break several bonds at once. Exciting vibrations that resemble the motion along the desired reaction coordinate can potentially control the course of a reaction. Experiments on vibrationally excited water have demonstrated that vibrational excitation of O-H stretch or O-D stretch in HOD leads to selective cleavage of the excited bond in reactions with H atoms4-7 or Cl atoms.8,9 The key to these bond-selected reactions is preparation of vibrational eigenstates that have a large component of motion along the desired reaction coordinate. Another approach to vibrational control is preparation of nonstationary vibrational states of the system either with shaped pulses or via interfering excitation pathways,10,11 an approach that has proven particularly effective for unimolecular processes such as photodissociation and photoionization. The challenge for bimolecular reactions is still greater because of the random phase and timing of collisions.12-16 Extending the conceptually simple idea of vibrational eigenstate-controlled bimolecular reaction to ever more complex reactants is clearly a challenge. In this context it is useful to †
Part of the special issue “George W. Flynn Festschrift”. Present address: Department of Chemistry, University of California, Berkeley. * To whom correspondence should be addressed. E-mail:
[email protected] ‡
describe vibrational eigenstates as linear combinations of zeroorder states that correspond to simple vibrational motions such as stretches and bends about which one has good chemical intuition.17 For sufficiently large and strongly coupled molecules, the individual eigenstates may well be mixtures of many different zero-order states such that no one eigenstate has a large component of motion along the reaction coordinate. In particular, the reactive zero-order state might be diluted among so many eigenstates that none of them contains enough excitation of one bond over another to induce selective reaction. Experiments that explore the influence of vibrational excitation on reactivity in larger molecules promise to enhance our understanding of the vibrational couplings within the reactant and the dynamics of the reactive encounter. In this report we describe the use of vibrational excitation to control the rate and outcome of the reaction of CH3D with Cl. The reaction of vibrationally excited CH4 with Cl is potentially a benchmark for polyatomic reactions. Experimental18-24 and theoretical25-29 studies show that excitation of C-H stretching vibration of CH4 enhances the reaction rate. Closely related experiments on the CHD3 and CH2D2 isotopologues of methane demonstrate preferential cleavage of a vibrationally excited C-H bond and retention of the energy initially deposited in a nonreacting bond during the reaction.21 Monodeuterated methane, CH3D, has both a C-H and a C-D bond for vibrational excitation and potentially selective cleavage, and it also has several vibrational modes involving the CH3 group that might accelerate the reaction differently. Both H- and D-atom abstraction reactions of CH3D with Cl are slightly endothermic and have energy barriers of 1800 and 2200 cm-1, respectively.30 In the experiments described here we excite four different vibrations, one of which has C-D stretching character and the rest of which involve C-H stretching motions, to demonstrate the preferential cleavage of the excited bond.31 We also compare the rate of H-atom abstraction in reactions of CH3D having symmetric and antisymmetric C-H stretching vibrations excited, demonstrating a 7-fold difference in the reactivity of two states lying within 80 cm-1 of each other. Ab initio calculations help explain the different reactivities of molecules having these two seemingly similar stretching vibrations excited.32 Thus, the
10.1021/jp0463565 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/25/2005
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reaction of initially prepared vibrational eigenstates of CH3D with Cl demonstrates both bond- and mode-selected chemistry, and we discuss them together here to emphasize the complexity of the roles of different vibrations in a relatively simple H-atom abstraction reaction. It is not only possible to promote cleavage of one bond over another by exciting very different vibrational motions (C-D or C-H stretches), but it is also possible to produce substantially different reaction rates by preparing two apparently similar vibrational states (the symmetric and antisymmetric C-H stretching vibrations) that differ in subtle but important ways. II. Experimental Approach The experimental approach closely resembles one we have described previously.23,31 A mixture of CH3D (98%), Cl2 (99.9%), and He (99.999%) prepared in a ratio of 1:1:4 at a total pressure of 660 Torr expands through a pulsed valve into a high-vacuum reaction chamber where the supersonic expansion cools the molecules to a rotational temperature of 70 ( 5 K.31 Infrared absorption prepares the CH3D molecules in the various vibrational eigenstates. We produce tunable infrared pulses by generating the difference frequency between dye laser light and 1064-nm Nd:YAG laser light in a LiNbO3 crystal. Subsequent optical parametric amplification in a second LiNbO3 crystal produces 5 mJ of light near 3000 cm-1 and 15 mJ of light near 4300 cm-1 in a 7-ns pulse. We use a second Nd:YAG laser to generate 25-mJ pulses of the third harmonic (355 nm) for photodissociation of molecular chlorine to produce Cl (2P3/2) atoms.33 These Cl atoms collide and react with the vibrationally excited CH3D molecules, forming either CH3 + DCl or CH2D + HCl products. We detect the CH3 and CH2D products by (2 + 1) resonantly enhanced multiphoton ionization (REMPI).34,35 Frequency-doubling light from a Nd:YAG-pumped dye laser in a β-barium borate (BBO) crystal gives 15-mJ pulses of 333nm probe light that ionizes the methyl radicals, which then enter a time-of-flight (TOF) mass spectrometer. The different REMPI transition wavelengths and arrival times in the mass spectrometer discriminate the fragments. We digitize the amplified and integrated signal from the multichannel plate detector in the mass spectrometer and transfer it to a computer for analysis. To distinguish between the signal from the thermal reaction, which does not depend on the presence of infrared light, and that from the vibrationally driven reaction, we operate the infrared laser at 10 Hz and the photolysis and probe lasers at 20 Hz and perform active background subtraction between alternating laser pulses. III. Results and Discussion A. Bond-Selected Reaction. Driving a reaction to form one product over another requires excitation of a vibration that preferentially couples to motion along the desired reaction coordinate. We excite four different vibrational eigenstates of CH3D and detect the products of the vibrationally driven CH3D(V) + Cl reaction. Figure 1 shows the energy levels of the vibrational eigenstates we explore. These states are nominally the first overtone of C-D stretch (2ν2, 4343.7 cm-1), the combination of antisymmetric C-H stretch and CH3 umbrella bend (ν4 + ν3, 4313.9 cm-1), the antisymmetric C-H stretch (ν4, 3017.2 cm-1), and the symmetric C-H stretch (ν1, 2969.4 cm-1). As discussed below, the nominal zero-order state is the predominant component of each of the eigenstates except for the eigenstate labeled as ν1, in which there is a Fermi resonance with the bending overtone, 2ν5. The molecules drawn on the diagram illustrate the vibrational motions corresponding to the
Figure 1. Schematic energetics of CH3D + Cl reaction and massselected (2 + 1) REMPI excitation spectra for four vibrational excitations. Direct infrared absorption prepares CH3D in four different vibrational eigenstates: the first overtone of C-D stretch (2ν2), the combination of antisymmetric C-H stretch and CH3 umbrella bend (ν4 + ν3), the antisymmetric C-H stretch (ν4), and the symmetric C-H stretch (ν1). Schematic vibrational motions of the major zero-order state component in each eigenstate are shown in the figure. The CH3 and CH2D products of vibrationally excited CH3D with Cl atoms are detected by (2 + 1) REMPI of the vibrational ground state. We obtain the REMPI excitation spectra (a-d) by scanning the probe laser wavelength while detecting either CH3+ (m/z ) 15) or CH2D+ (m/z ) 16) ions in the TOF mass spectrometer for each initially excited vibration. The small feature on the right of the scan in b comes from vibrationally excited CH2D fragments.
dominant zero-order state in each eigenstate. We obtain the mass-resolved (2 + 1) REMPI product excitation spectra shown on the right of the figure by scanning the probe laser wavelength while monitoring the ion signals from either CH3 or CH2D. The 000 electronic transition of CH3 appears at a slightly different wavelength from that of CH2D, as observed previously.34,35 The unknown Franck-Condon factors for the CH3 and CH2D radicals make an absolute determination of the extent of bond selectivity impossible. The results of the CH3D (V ) 0) + Cl (2P3/2) reaction show a 6-fold increase in the signal for the CH2D product over that for the CH3 product.31 This is consistent with the number of hydrogens and the kinetic isotope effect, suggesting that the Franck-Condon factors do not differ by more than a factor of 2. Given the signal-to-noise ratio observed in Figure 1, we conservatively estimate that the enhancement of the observed channel is at least an order of magnitude greater than that for the undetected channel for all the different vibrations we excited. The REMPI spectra for different initially excited vibrational states show that vibrational excitation of the first overtone of the C-D stretch (2ν2) preferentially promotes cleavage of the C-D bond to yield CH3 products but no CH2D products. In contrast, excitation of the other three vibrational eigenstates that have C-H stretching character selectively breaks the excited C-H bond to produce only CH2D products in the reaction. There is an interesting comparison between the 2ν2 and ν4 + ν3 vibrational states, which differ by only 30 cm-1 in total
8390 J. Phys. Chem. B, Vol. 109, No. 17, 2005 energy but lead to completely different reaction products. Preferential cleavage of the excited bond indicates that the C-D stretching vibration is uncoupled from the C-H stretching vibration. Indeed, vibrational action spectra obtained by scanning the vibrational excitation wavelength while detecting the CH3 product show only 2ν2 (A1) rovibrational transitions, and vibrational action spectra for the CH2D product show only ν4 + ν3 (E) rotational transitions in the region of 4300 cm-1 where the two transitions overlap.31 In the 3000 cm-1 region excitation of the antisymmetric C-H stretch (ν4) or the symmetric C-H stretch (ν1) leads to exclusive cleavage of the C-H bond, yielding only CH2D products and suggesting that C-H stretching vibrations preferentially direct the system along the H-atom abstraction pathway. The key to our bond-selected reactions is excitation of a vibration that has a significant component of motion along the desired reaction coordinate. The 2ν2 eigenstate, for example, is 90% C-D stretching vibration with a negligible contribution from CH3 motion36 and, thus, couples strongly to the D-atom abstraction reaction coordinate. Although the nuclear motions of the stretch-bend combination (ν4 + ν3) are complex, 96% of its eigenstate consists solely of C-H motions and, thus, only drives the H-atom abstraction reaction. The mixing of C-H stretch with C-D stretch is also negligible within the ν4 and ν1 eigenstates.37 Exciting the vibrational eigenstates that have dominant C-D stretch or C-H stretch zero-order vibrations controls the course of the abstraction reaction of CH3D with Cl. B. Mode-Selected Reaction. The variable coupling of the zero-order vibrations to the reaction coordinate determines the rate of vibrationally enhanced reactions, and proper choice of vibrational excitation permits control of the reaction rate. As shown in Figure 1, vibrational excitation of both the symmetric (ν1) and antisymmetric (ν4) C-H stretches promotes cleavage of the C-H bond. In the simplest picture, exciting either vibration should have similar effects since they both have about 3000 cm-1 of vibrational energy and contain C-H stretching motions. Our experiments and analysis show that the behavior is actually more subtle. A detailed comparison of the reaction rates for CH3D molecules with excited symmetric C-H and antisymmetric C-H stretches reveals significant differences in their dynamics during a reactive encounter. We measure the yield of the product from the two vibrations by comparing the vibrational absorption spectrum with the vibrational action spectrum. Figure 2a shows the simulated absorption spectrum of CH3D at the 70 K rotational temperature of our experiment. We use the high-resolution transmission (HITRAN) molecular absorption database of CH3D lines38 and convolute them with a Gaussian line width of the vibrational excitation laser (0.6 cm-1) to simulate the spectrum. The simulated absorption spectrum shows the transitions to the symmetric C-H stretching eigenstates near 2970 cm-1 (ν1) and the antisymmetric C-H stretching vibrational eigenstates near 3020 cm-1 (ν4). Figure 2b shows the experimental action spectrum obtained by scanning the vibrational excitation laser wavelength while detecting the CH2D (V ) 0) reaction products.39 The intensities in the vibrational action spectrum reflect both the probability of absorption and the probability of reaction to form the detected product. Therefore, the ratio of the action spectrum to the absorption spectrum gives the relative reactivity (or yield) of the vibrational eigenstate. Comparison between the simulated absorption spectrum (Figure 2a) and the experimental action spectrum (Figure 2b) shows immediately that eigenstate ν1 is more reactive than eigenstate ν4. The relative
Yoon et al.
Figure 2. (a) Simulated vibrational absorption spectrum of CH3D at 70 K. The assignments ν1 and ν4 designate symmetric C-H stretching vibrational eigenstate and antisymmetric C-H stretching vibrational eigenstate, respectively. (b) Vibrational action spectrum. We obtain the action spectrum by scanning the vibrational excitation laser wavelength while detecting the CH2D (V ) 0) product by (2+1) REMPI. (c-e) Scaled absorption spectra. Scaling the intensities of ν1 transitions in the absorption spectrum by a factor of 4.4 and those of ν4 transitions by a factor of 1 produces the best agreement between the experimental action spectrum (b) and the simulated action spectrum (c). The asterisks in (e) mark the K′ ) 1 rotational lines of ν4 whose intensities we decrease by one-half (ref 40).
size of the transitions to ν1 compared to those to ν4 in the action spectrum is much larger than in the absorption spectrum, reflecting the greater reactivity of molecules with symmetric stretch excitation. Scaling the intensities of the absorption spectrum to simulate the action spectrum gives quantitative information on the relative reactivity of the eigenstates. Figure 2d shows the simulated spectrum of the symmetric C-H stretch (ν1) scaled by a factor of 4.4, and Figure 2e shows the unscaled ν4 absorption except for the K′ ) 1 rotational lines (marked by asterisks) which have only one-half their calculated intensity.40 Adding these two spectra together gives the calculated spectrum in Figure 2c, which agrees well with the experimental action spectrum in Figure 2b. This good agreement suggests that the ν1 eigenstate is four times more reactive than the ν4 eigenstate. One complication of the analysis above is that the ν1 eigenstate prepared by infrared excitation contains only 60% symmetric C-H stretching zero-order vibration because of a strong Fermi resonance interaction with the first overtone of the CH3 bend (2ν5). Our analysis uses the limiting assumption that 2ν5 does not enhance the H-atom abstraction at all. Recent experiments find that excitation of the bending modes in the reactions of CH4 and CD4 with Cl (2P3/2) atoms increases the reaction rate by only about a factor of 3 compared to the thermal
Vibrationally Controlled Chemistry
Figure 3. Calculated evolution of vibrational eigenvectors of the antisymmetric C-H stretch zero-order state (ν4) and symmetric C-H stretch zero-order state (ν1). The curve in the figure is the intrinsic reaction coordinate. The curve and the vibrational eigenstates are from calculations using the MP2/6-311++G(2d,2p) level of theory. The top portion of the figure illustrates vibrational eigenvectors of ν4 and ν1 in the entrance channel and exit channel (divided by a dashed line) at the locations marked by the solid circles. The numbers in parentheses are the distances between Cl and the center of mass of CH3D. The eigenvector of the unbound mode that has an imaginary frequency at the transition state is also depicted to show the motion along the reaction coordinate.
reaction.41 This enhancement is not mode specific, being comparable to that obtained by adding the same amount of translational energy. In contrast to the situation with the symmetric C-H stretch, the antisymmetric C-H stretching zero-order state constitutes 96% of the ν4 eigenstate.37 This dilution of the zero-order state actually makes the reactivity of the zero-order symmetric C-H stretch larger than the simple scaling factor for the ν1 eigenstate predicts. Using the mixing coefficients37 we determine that the zero-order symmetric C-H stretch is 7 times more efficient in promoting the H-atom abstraction reaction than the zero-order antisymmetric C-H stretch.32 Exciting vibrational eigenstates that contain different amounts of zero-order states with different reactivities allows us to control the reaction rate. Different reactivity of the symmetric and antisymmetric C-H stretching vibrations is rather counterintuitive since they have similar energies and vibrational motions that differ primarily by the phase of the C-H bond stretches. Thus, we have performed ab initio calculations to help understand the dynamics. Using second-order Møller-Plesset (MP2) perturbation theory and a 6-311++G(2d,2p) basis set,42 we optimize the geometry of the reactants, products, and transition state and calculate the intrinsic reaction coordinate, a minimum energy path that connects the transition state to reactants and products. These calculations use the limiting case of collinear transition-state geometry in which the Cl atom has a zero impact parameter with the reactive C-H bond. Calculations of vibrational eigenvectors along the reaction coordinate map out the behavior of each vibration during the course of the reaction. Figure 3 shows the evolution of the vibrational eigenvectors of the symmetric and antisymmetric C-H stretches of CH3D along the calculated reaction coordinate. The most interesting features appear in the entrance channel of the reaction. When the chlorine
J. Phys. Chem. B, Vol. 109, No. 17, 2005 8391 atom is far enough from CH3D, the symmetric and antisymmetric C-H stretching vibrations retain their normal mode character. As the chlorine atom approaches along a C-H bond, the two vibrational excitations behave differently. The displacements of atoms in the antisymmetric C-H stretch become quarantined in the C-H bonds pointing away from the approaching Cl atom. In contrast, the symmetric C-H stretching vibration adiabatically changes to a localized vibrational motion of the reactive C-H bond, corresponding to motion along the reaction coordinate as shown in the unbound mode of the transition state in the inset. Therefore, the symmetric C-H stretching vibration more strongly couples to the reaction coordinate and promotes the H-atom abstraction reaction more efficiently than the antisymmetric C-H stretching vibration. Experimental23 and theoretical25,26,29,43-47 studies predict a similar propensity of more reactive symmetric stretching vibration than antisymmetric stretching vibration in other reactions. Schatz43 was one of the first to describe such vibrationally adiabatic behavior in a bimolecular reaction, and Halonen et al.44 and Fair et al.45 predicted the same localization of vibrational excitation that we describe here in surface reactions of methane and in gas-phase reactions of water, respectively. A simple model32 using vibrational symmetries and adiabaticity consistently explains the qualitatiVe difference in reactivity between the symmetric and antisymmetric stretches in generic abstraction reactions, but such a rudimentary picture cannot account for the more subtle features of the reactions. For instance, a purely adiabatic correlation model predicts that the antisymmetric C-H stretch should not enhance the H-atom abstraction reaction and that any CH2D radicals formed in the unenhanced reaction should preserve C-H vibrational excitation. Our simple model, however, describes only the limiting case and neglects vibrationally nonadiabatic dynamics and collisions with nonzero impact parameters. A complete description requires a more detailed calculation including these possibilities, and perhaps others, but the qualitative picture seems clear. IV. Summary Vibrational excitation controls the pathways and rate of the reaction of CH3D with Cl. Exciting a vibrational eigenstate containing large amounts of C-D stretching excitation promotes cleavage of that bond, while excitation of one containing large amounts of C-H stretching excitation drives cleavage of a C-H bond. Because these vibrational eigenstates contain a substantial amount of motion along the reaction coordinate, their excitation produces bond-selected chemistry in which the initially excited bond breaks preferentially. Mode-selected chemistry is possible as well. To probe this more subtle behavior, we excite one of two similar vibrations, the symmetric and antisymmetric C-H stretching vibrations, in CH3D. Abstraction of H atoms by Cl is 7 times more likely from CH3D molecules with the symmetric C-H stretching vibration excited than for ones with the antisymmetric C-H stretch excited, even though the energies of the two states differ by only 80 cm-1. A simple model that accounts for the changes in the distribution of energy among the C-H bonds as the reactive Cl atom approaches explains the observed behavior. The perturbation preferentially converts the symmetric C-H stretch into motion of the H atom pointing toward the incoming Cl, but it converts the antisymmetric stretch into motion of H atoms away from the incoming Cl atom, isolating the energy away from the reaction coordinate.
8392 J. Phys. Chem. B, Vol. 109, No. 17, 2005 References and Notes (1) Crim, F. F. J. Phys. Chem. 1996, 100, 12725. (2) Zare, R. N. Science 1998, 279, 1875. (3) Crim, F. F. Acc. Chem. Res. 1999, 32, 877. (4) Sinha, A.; Hsiao, M. C.; Crim, F. F. J. Chem. Phys. 1990, 92, 6333. (5) Sinha, A.; Hsiao, M. C.; Crim, F. F. J. Chem. Phys. 1991, 94, 4928. (6) Bronikowski, M. J.; Simpson, W. R.; Girard, B.; Zare, R. N. J. Chem. Phys. 1991, 95, 8647. (7) Metz, R. B.; Thoemke, J. D.; Pfeiffer, J. M.; Crim, F. F. J. Chem. Phys. 1993, 99, 1744. (8) Sinha, A.; Thoemke, J. D.; Crim, F. F. J. Chem. Phys. 1992, 96, 372. (9) Thoemke, J. D.; Pfeiffer, J. M.; Metz, R. B.; Crim, F. F. J. Phys. Chem. 1995, 99, 13748. (10) Shapiro, M.; Brumer, P. Int. ReV. Phys. Chem. 1994, 13, 187. (11) Shapiro, M.; Brumer, P. J. Chem. Soc., Faraday Trans. 1997, 93, 1263. (12) Krause, L.; Shapiro, M.; Brumer, P. J. Chem. Phys. 1990, 92, 1126. (13) Shapiro, M.; Brumer, P. Phys. ReV. Lett. 1996, 77, 2574. (14) Holmes, D.; Shapiro, M.; Brumer, P. J. Chem. Phys. 1996, 105, 9162. (15) Abrashkevich, A.; Shapiro, M.; Brumer, P. Phys. ReV. Lett. 1998, 81, 3789. (16) Frischman, E.; Shapiro, M.; Brumer, P. J. Chem. Phys. 1999, 110, 9. (17) Nesbitt, D. J.; Field, R. W. J. Phys. Chem. 1996, 100, 12735. (18) Simpson, W. R.; Orr-Ewing, A. J.; Zare, R. N. Chem. Phys. Lett. 1993, 212, 163. (19) Simpson, W. R.; Rakitzis, T. P.; Kandel, S. A.; Orr-Ewing, A. J.; Zare, R. N. J. Chem. Phys. 1995, 103, 7313. (20) Simpson, W. R.; Rakitzis, T. P.; Kandel, S. A.; Lev-On, T.; Zare, R. N. J. Phys. Chem. 1996, 100, 7938. (21) Kim, Z. H.; Bechtel, H. A.; Zare, R. N. J. Am. Chem. Soc. 2001, 123, 12714. (22) Kim, Z. H.; Bechtel, H. A.; Zare, R. N. J. Chem. Phys. 2002, 117, 3232. (23) Yoon, S.; Henton, S.; Zivkovic, A. N.; Crim, F. F. J. Chem. Phys. 2002, 116, 10744. (24) Bechtel, H. A.; Camden, J. P.; Zare, R. N. J. Chem. Phys. 2004, 120, 4231. (25) Duncan, W. T.; Truong, T. N. J. Chem. Phys. 1995, 103, 9642. (26) Espinosa-Garcia, J.; Corchado, J. C. J. Chem. Phys. 1996, 105, 3517. (27) Yu, H.-G.; Nyman, G. Phys. Chem. Chem. Phys. 1999, 1, 1181. (28) Yu, H.-G.; Nyman, G. J. Chem. Phys. 1999, 110, 7233. (29) Corchado, J. C.; Truhlar, D. G.; Espinosa-Garcia, J. J. Chem. Phys. 2000, 112, 9375. (30) Boone, G. D.; Agyin, F.; Robichaud, D. J.; Tao, F.-M.; Hewitt, S. A. J. Phys. Chem. A 2001, 105, 1456.
Yoon et al. (31) Yoon, S.; Holiday, R. J.; Crim, F. F. J. Chem. Phys. 2003, 119, 4755. (32) Yoon, S.; Holiday, R. J.; Sibert, E. L.; Crim, F. F. J. Chem. Phys. 2003, 119, 9568. (33) Matsumi, Y.; Tonokura, K.; Kawasaki, M. J. Chem. Phys. 1992, 97, 1065. (34) Hudgens, J. W.; DiGiuseppe, T. G.; Lin, M. C. J. Chem. Phys. 1983, 79, 571. (35) Brum, J. L.; Johnson, R. D., III; Hudgens, J. W. J. Chem. Phys. 1993, 98, 3732. (36) Wang, X.-G.; Sibert, E. L., III Private communication. (37) Wang, X.-G.; Sibert, E. L., III J. Chem. Phys. 1999, 111, 4510. (38) Rothman, L. S.; Rinsland, C. P.; Goldman, A.; Massie, S. T.; Edwards, D. P.; Flaud, J.-M.; Perrin, A.; Camy-Peyret, C.; Dana, V.; Mandin, J.-Y.; Schroeder, J.; McCann, A.; Gamache, R. R.; Wattson, R. B.; Yoshino, K.; Chance, K. V.; Jucks, K. W.; Brown, L. R.; Nemtchinov, V.; Varanasi, P. J. Quant. Spectrosc. Radiat. Transfer 1998, 60, 665. (39) We monitored the IR power using a pyroelectric detector and maintained constant IR laser power during the scan. The simulated absorption spectrum at room temperature agrees well with the photoacoustic spectrum taken simultaneously with the action spectrum, demonstrating the reliability of the simulated absorption spectrum. (40) We scaled the intensities of K′ ) 1 transition lines by a factor of 0.5 as described in ref 32. (41) Zhou, J. G.; Lin, J. J.; Zhang, B. L.; Liu, K. P. J. Phys. Chem. A 2004, 108, 7832. (42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98 (Rev A.7); Gaussian, Inc.: Pittsburgh, PA, 1998. (43) Schatz, G. C. J. Chem. Phys. 1979, 71, 542. (44) Halonen, L.; Bernasek, S. L.; Nesbitt, D. J. J. Chem. Phys. 2001, 115, 5611. (45) Fair, J. R.; Shaefer, D.; Kosloff, R.; Nesbitt, D. J. J. Chem. Phys. 2002, 116, 1406. (46) Clary, D. C. Phys. Chem. Chem. Phys. 1999, 1, 1173. (47) Palma, J.; Clary, D. C. Phys. Chem. Chem. Phys. 2000, 2, 4105.
