Infrared Laser Spectroscopy of the CH3OO Radical Formed from the

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Infrared Laser Spectroscopy of the CH3OO Radical Formed from the Reaction of CH3 and O2 within a Helium Nanodroplet Alexander M. Morrison,‡ Jay Agarwal,† Henry F. Schaefer, III,† and Gary E. Douberly*,‡ †

Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States

‡

S Supporting Information *

ABSTRACT: Helium nanodroplet isolation and infrared laser spectroscopy are used to investigate the CH3 + O2 reaction. Helium nanodroplets are doped with methyl radicals that are generated in an effusive pyrolysis source. Downstream from the introduction of CH3, the droplets are doped with O2 from a gas pick-up cell. The CH3 + O2 reaction therefore occurs between sequentially picked-up and presumably cold CH3 and O2 reactants. The reaction is known to lead barrierlessly to the methyl peroxy radical, CH3OO. The ∼30 kcal/mol bond energy is dissipated by helium atom evaporation, and the infrared spectrum in the CH stretch region reveals a large abundance of droplets containing the cold, helium solvated CH3OO radical. The CH3OO infrared spectrum is assigned on the basis of comparisons to high-level ab initio calculations and to the gas phase band origins and rotational constants.

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INTRODUCTION Liquid helium nanodroplets are increasingly being employed to study the outcome of reactive collisions between atomic and molecular reactants at sub-Kelvin temperatures (0.4 K).1−7 The liquid helium droplets are dissipative, allowing for the spectroscopic or mass spectrometric study of the cold and isolated reaction products or intermediates. One notable example is the Ba + N2O → BaO* + N2 reaction, which was the first bimolecular reaction to be studied in helium droplets.1 The BaO* product of this exothermic reaction (∼95 kcal/mol) produces chemiluminescence, enabling its direct detection. Another dramatic demonstration of reactions occurring within helium droplets is the reaction between neutral cesium clusters and water clusters, which was shown to produce a variety of cold cesium hydroxide and cesium hydride products.4 In this report, we sequentially add CH3 and O2 to the droplets and probe for the outcome of the barrierless, bimolecular CH3 + O2 reaction. Furthermore, we address several issues associated with the application of infrared (IR) laser spectroscopy for the detection of the outcome of organic radical−radical reactions that occur within helium nanodroplets. A number of reactions between metal atoms and O2 have also been carried out within helium droplets.5−7 On the basis of electron bombardment mass spectrometry measurements, it was discovered that these barrierless and highly exothermic reactions (∼100−150 kcal/mol) lead to the ejection of the product from the droplet, despite the fact that the droplets used were sufficiently large to dissipate the reaction energy. Given that each evaporating helium atom removes ∼5 cm−1 (0.014 kcal/mol) of thermal energy,8 a droplet containing ∼104 helium © 2012 American Chemical Society

atoms is required to dissipate the bond energy associated with these reactions, assuming a thermal evaporation process. Nevertheless, in many cases, it was observed that droplets much larger than this were required to cage the products of these reactions. Similarly, the Ba + N2O reaction produces a substantial fraction of gas phase BaO* due to product ejection. For this system, however, the ejected fraction can be suppressed by introducing a small cluster of Xe atoms to the droplet prior to the introduction of the reactants. The Xe atoms attract the Ba reactant to the interior of the droplet, indicating that product ejection in this case is dependent on whether the reaction occurs on the surface or in the interior of the droplet. Unfortunately, the mechanistic details associated with these ejection processes are not entirely understood. For low temperature droplet mediated reactions involving organic radicals, it is advantageous to use direct spectroscopic probes to determine product branching ratios, rather than relying on mass spectrometry data alone. When reaction products do not fluoresce or produce chemiluminescence, wellestablished IR beam depletion spectroscopy can be used,9,10 provided the products remain in the droplets and are carried downstream to the detector. Using velocity map imaging techniques, Braun and Drabbels have studied the 266 nm photodissociation of CH3I in helium droplets and found that a fraction of the photofragments are thermalized via collisions with the helium and recombine within the droplet.11 Received: March 19, 2012 Revised: May 4, 2012 Published: May 7, 2012 5299

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Table 1. Harmonic (ω) and Fundamental (ν) Vibrational Frequencies (cm−1) for CH3OO with IR Intensities (km·mol−1) Computed at the CCSD(T)/cc-pVTZ Level of Theorya

a

mode

description

ωtheory

νtheory

Ar matrixb

ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12

CH3 sym stretch CH3 total sym stretch CH3 deformation CH3 umbrella CH3 rock+OO stretch CH3 rock + OO stretch CO stretch COO bend CH3 asym stretch CH3 asym deformation CH2 wag CH3 torsion

3168 (9.0) 3061 (16.8) 1497 (8.2) 1449 (1.4) 1212 (10.5) 1160 (9.5) 949 (14.7) 493 (6.2) 3159 (13.1) 1484 (7.5) 1144 (1.1) 149 (0.2)

3022 (7.7) 2956 (17.4) 1456 (5.9) 1417 (2.8) 1181 (9.2) 1128 (9.4) 917 (13.8) 493 (6.3) 3011 (12.6) 1440 (6.3) 1118 (1.1) 131 (0.1)

3032 2954 1448 (71) 1410 (8) 1180 (37) 1109 (25) 902 (78) 492 (30) 3024 1434 (100)

gas phasec,d 3033 2954 1453 1408 1183 1117

± ± ± ± ± ±

1 1 2 1 1 2

(100) (250) (100) (54) (68) (74)

482 ± 9d 3020 ± 2 (70) 1441 ± 1 (68)

He 3034.7 2955.5

3024.5

Relative intensities for experimental measurements are given in parentheses where available. bReference 20. cReference 23. dReference 19.

near 700 K, DTBP fragments into two CH3 radicals and two (CH3)2CO molecules. The DTBP pressure is kept low to prevent recombination reactions within the source and to optimize for the pick-up of single molecules (either CH3 or (CH3)2CO).29 The droplets subsequently pass through a second differentially pumped chamber that contains O2 at a pressure of 3 × 10−6 Torr. On the basis of the well-known statistics associated with droplet doping,30 under these conditions, approximately 8% of all droplets within the beam will pick up a single CH3 radical and a single O2 molecule. The other 92% of the droplet ensemble will either be devoid of dopants or contain other combinations of CH3, (CH3)2CO, and O2. Following each pick-up event, the droplets cool by evaporation to 0.37 K, and the molecular degrees of freedom of the dopant are assumed to be at equilibrium with the droplet degrees of freedom at this temperature.31 The idler output from a continuous wave, IR, optical parametric oscillator (OPO) is aligned to counter-propagate the helium droplet beam. The tuning and calibration of this IROPO system is described elsewhere.32 The vibrational excitation of helium solvated molecular dopants followed by vibrational energy relaxation leads to a laser induced depletion of the droplet beam intensity. This reduction of the average geometric droplet cross section results in a reduction in the ionization cross section as measured by a quadrupole mass spectrometer equipped with an electron impact ionizer. An IR spectrum is obtained as a depletion signal in a specific mass channel by fixing the quadrupole mass spectrometer to a particular mass to charge ratio while scanning the IR-OPO.

Furthermore, they provide convincing evidence for the trapping of the CH3 + I reaction product (CH3I), provided the droplet has an initial size greater than ∼6000 atoms. This result suggests that, by simply increasing the droplet size, the products of organic radical reactions can indeed be trapped and cooled by the helium droplet and carried downstream for spectroscopic interrogation. However, because helium is conserved in the nozzle expansion, the conditions that produce larger droplets also produce fewer droplets, resulting in fewer chromophores available for spectroscopic detection. For example, the signals associated with the IR laser spectroscopy of helium solvated molecules are typically optimized for nozzle conditions that produce droplets containing between 3000 and 7000 atoms on average.10 Because of these potential issues, it is worthwhile to test the feasibility of the IR laser based beam depletion approach for probing the outcome of an organic radical reaction. One class of reactions that is of particular interest is alkyl radical oxidation reactions, which are important in both combustion environments12,13 and in the atmosphere.14 The extent to which the products of these reactions experience droplet ejection may be dramatically different from the metal atom reactions, because the typical exothermicity of these reactions can be much smaller. As a test case, we have chosen the CH3 + O2 reaction, which leads barrierlessly to the methyl peroxy radical (CH3OO). The associated reaction enthalpy is ∼30 kcal/ mol.15 If we assume that this energy is dissipated by a thermal evaporation process, then this reaction will lead to the evaporation of ∼2000 He atoms. The spectroscopy of this system is well studied,16−23 allowing us to make direct comparisons of our results to previous gas phase measurements.

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THEORETICAL METHODS Reference geometries for CH3OO were optimized by coupled cluster theory incorporating single, double, and perturbative triple excitations [CCSD(T)]33−35 using a valence correlationconsistent triple-ζ basis-set (cc-pVTZ).36 Fundamental frequencies were evaluated using VPT2 theory37,38 via analytic second derivatives.39 Results from these computations are given in Table 1. Relative energies for the torsional barrier were obtained by employing the focal-point method,40−42 a convergent scheme in which both electron correlation and basis set limits are systematically approached. Energies were extrapolated to the complete basis-set limits using a threeparameter exponential function for the Hartree−Fock energy43 and a two-term X−3 form for the correlation energy.44 The barrier for torsional rotation about the C−O bond in the

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EXPERIMENTAL METHODS The helium nanodroplet isolation (HENDI) methodology has been reviewed recently.9,10,24 Helium droplets are formed in a cryogenic nozzle expansion of high purity helium gas (30 bar) into vacuum through a 5 μm diameter nozzle operated at 17 K. These nozzle conditions produce droplets with ∼5000 helium atoms on average.25,26 The droplet expansion is skimmed into a beam, enters a differentially pumped chamber and passes across the opening of an effusive, low pressure pyrolysis source.27,28 The pyrolysis source consists of a heated quartz tube separated from a precursor reservoir by a fine metering valve. The precursor used to generate methyl radicals is di-tert-butyl peroxide (DTBP). When the tip of the quartz tube is heated to 5300

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ground electronic state (2A″) is 250 cm−1 with an accuracy of ±30 cm−1. This result is in good agreement with previous calculations of the barrier height at lower levels of theory.45 All computations utilized the CFOUR46 suite of electronic structure codes. Further details of these computations are provided in the Supporting Information.

downstream to the detector by the droplet. However, the ionization of CH3OO may lead to extensive fragmentation, and it is therefore difficult to draw any conclusions regarding the outcome of the CH3 + O2 reaction by only considering these mass spectrometry results. A section of the survey spectrum of the CH stretch region (2850−3100 cm−1, Figure S2, Supporting Information) is shown in Figure 2, which is obtained by measuring the

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RESULTS AND DISCUSSION The evolution of the mass spectrum upon sequential addition of dopants to the droplet beam is shown in Figure 1. The mass

Figure 2. Survey scan of the depletion in mass channel m/z = 29 upon doping the droplets with the products of DTBP pyrolysis followed by O2 pick-up. The bands assigned to the ν1,2,9 transitions of CH3OO are all within one or two cm−1 of the reported gas phase band origins. The band at 3018 cm−1 is due to acetone, which is the other product of DTBP pyrolysis in addition to CH3.

Figure 1. Evolution of the mass spectrum as the reactants are introduced sequentially to the droplet beam. From bottom to top: neat droplet beam, droplet beam doped with DTBP precursor, doped droplet beam with pyrolysis source heated to ∼700 K, and doped droplet beam with heated pyrolysis source and with O2 added to the downstream gas pick-up cell.

depletion in mass channel m/z = 29 while scanning the IROPO. In addition to bands due to (CH3)2CO, the only other intense features in this region are three bands centered at 2955.2, 3024.5, and 3034.7 cm−1. These bands were initially found in mass channel m/z = 15 (CH3)+, and it was later found that the m/z = 29 channel provided the largest depletion signals for these three bands (vide infra). Each of these three bands goes away entirely either if O2 is removed from the gas pick-up cell or if the pyrolysis source temperature is reduced to room temperature. Moreover, each band has an intensity that depends similarly on the O2 gas pick-up cell pressure, peaking at ∼6 × 10−6 Torr and rapidly dropping off at higher O2 pressures. This optimal pressure is consistent with the pick-up of a single O2 molecule. The band origins of these three transitions are in excellent agreement with both our fundamental vibrational frequency computations47 and previous gas phase measurements23 of the methyl peroxy radical CH stretch vibrations (ν2, ν9, ν1). We therefore assign these three bands to the CH stretch vibrations of CH3OO. Furthermore, the characteristic O2 pressure dependence of the band intensities provides strong support for the formation of CH3OO species within the droplet, following the sequential pick-up of CH3 and O2. Further evidence for the presence of helium solvated CH3OO comes from the expanded view of the 2955.2 cm−1 band, which we assign to the ν2 totally symmetric CH3 stretch vibration (a′) of the methyl peroxy radical. As shown in Figure 3, the ν2 band exhibits partially resolved rotational fine structure. The solid red curve is a simulation of an a,b-hybrid band of a rigid asymmetric top using the rotational constants in

spectrum at the bottom of the figure exhibits the well-known signature of the helium droplet beam.9 Electron bombardment ionization of the droplet beam generates a mass spectrum consisting of Hen+ peaks due to the initial formation of He+ within the droplets followed by the ejection of small helium cluster ions. As the DTBP precursor is bled into the vacuum chamber through the room temperature pyrolysis source, additional peaks appear in the mass spectrum due to the charge transfer ionization (He+ + DTBP → He + DTBP+) of the helium solvated DTBP molecules, which is apparently followed by the extensive fragmentation of DTBP+. The peaks at m/z = 43 (COCH3)+, 57 (C4H9)+, and 73 (C4H9O)+ are signatures of DTBP doped helium droplets. When the pyrolysis source is heated to near 700 K, the peaks at 57 and 73 disappear, signaling the pyrolysis of DTBP. Furthermore, peaks at 15 (CH3)+, 27 (C2H3)+, and 29 (C2H5)+ gain intensity. Along with the peak at m/z = 43 (COCH3)+, these features are consistent with the pyrolytic formation of CH3 and (CH3)2CO and the subsequent droplet pick-up of these DTBP fragments. Upon addition of O2 to the pick-up cell located downstream from the hot pyrolysis source, a peak at m/z = 32 appears along with a small decrease (increase) in the m/z = 15 (m/z = 29) peak relative to m/z = 12 (He3+). It is interesting to note that a peak at m/z = 47 (CH3O2)+ is completely absent from the mass spectrum obtained with both O2 and CH3 present. Without any other evidence, a peak at this mass would be indicative of the sequential pick-up and reaction between CH3 and O2 to form the CH3OO radical, assuming the product of this reaction is completely cooled, remains in the droplet, and is carried 5301

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that result in ejected products if we assume that the detection efficiency of each species in the m/z = 29 mass channel is similar. Furthermore, if we assume the relative intensities of these two bands from the ab initio computations (4:1) and the expected abundance of droplets containing single (CH3)2CO molecules (∼12%) relative to those that have picked-up exactly one CH3 radical and one O2 (∼8%), we estimate that the majority (>80%) of the CH3OO radicals remain in the droplets. Of course, this is necessarily qualitative given the above assumptions and the possibility for methyl radical loss due to reactions within the pyrolysis source. The results from the spectroscopy suggest rather clearly that upon sequential pick-up of CH3 and O2, the barrierless reaction leading to CH3OO occurs within the droplet. Furthermore, we can confidently say that the ∼30 kcal/mol bond energy is effectively dissipated, and that the product molecule is completely cooled, remains solvated, and is carried downstream for spectroscopic interrogation and detection. As discussed above, it would be difficult to arrive at this conclusion without this spectroscopic evidence. Electron impact ionization of the droplets leads to the production of a solvated helium cation that can transfer its charge to the solvated CH3OO. The difference in the ionization potential of He and CH3OO is ∼13 eV,22 which hardly represents a soft ionization mechanism. Indeed, it is common to observe extensive fragmentation of helium solvated organic molecules that are ionized in this way.48 Therefore, we find no reason to suspect a priori that a peak at m/z = 47 should appear upon the formation of CH3OO within the droplets. This is justified by Figure 4, which shows a comparison of the mass spectrum shown at the top of Figure 1 to the difference mass spectrum (laser off-on) obtained under the same conditions. The difference mass spectrum is obtained with the laser fixed to the peak of the CH3OO ν2 band, and it is therefore representative of the mass spectrum of droplets that contain the species in resonance with the IR radiation, namely CH3OO. The difference mass spectrum shows that He+ charge transfer to CH3OO leads to the production of HCO+ (m/z = 29) and H2O, along with other minor channels. It is reasonable to suspect that the mass spectrometric characterization of helium solvated products of organic radical reactions will generally suffer from this ionization induced fragmentation. For this class of low temperature chemical reactions carried out in helium nanodroplets, it is therefore advisable to use the more selective laser based spectroscopic methods rather than mass spectrometry to detect product branching ratios.

Figure 3. Expanded view of the ν2 CH3 (a′) symmetric stretch band of the methyl peroxy radical. The red curve and stick spectrum correspond to a rigid rotor asymmetric top simulation using the constants given in Table 2. The inset shows the optimized geometry of CH3OO at the CCSD(T)/cc-PVTZ level of theory. Bond lengths are shown in angstroms and bond angles in degrees.

Table 2. The relative intensity of a and b type bands is fixed by employing the ab initio inertial components of the dipole derivative vector associated with the CH3OO ν2 transition. The effective rotational constants obtained from rotationally resolved vibrational bands in helium nanodroplet isolation spectra are typically reduced from the gas phase spectra by a factor of 2−3 due to the contribution of the helium solvent to the rotor’s effective moment of inertia.9,10 Consistent with this, the ν2 (B + C)/2 constant is reduced by a factor of 2.2 in helium. In comparison, the larger A constant is reduced by a factor of 1.7, which is somewhat smaller than for (B + C)/2. This is also consistent with established trends.10 Overall, the simulation provides convincing evidence for the assignment of this band to the ν2 transition of CH3OO. Just and co-workers have computed the ground vibronic torsional eigenstates for CH3OO, and the separation between 0A1 and 0E states is 0.092 cm−1 at the B3LYP/6-31+g(d) level of theory.45 At high resolution, the ν2 transition observed here should consist of two bands separated by the difference in this tunneling splitting in the ground and excited vibrational states. At this same level of theory, we determine that the ground and ν2 vibrationally excited torsional barrier heights differ by only 4 cm−1, which represents only 2% of the zero-point energy corrected barrier height (200 cm−1). Therefore, we do not expect the dif ference in the tunneling splitting to be of any significance to the simulation of the ν2 band observed here, especially given the presumably homogeneous 0.3 cm−1 line width associated with this transition. The ratio of the integrated intensity of the CH3OO ν9 band to the (CH3)2CO band at 3018.2 cm−1 is ∼7:1. This allows us to estimate the fraction of droplet mediated CH3 + O2 reactions

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CONCLUSIONS The sequential addition of a methyl radical and molecular oxygen to helium nanodroplets leads to the barrierless reaction, CH3 + O2 → CH3OO. The reaction enthalpy is ∼30 kcal/mol and therefore requires the dissipation of ∼2000 helium atoms to cool the CH3OO to 0.37 K. The product CH3OO radical remains in the droplet and is observed downstream with IR

Table 2. Constants Used for the Simulation of the CH3 (ν2, a′) Symmetric Stretcha

He gas

A

(B + C)/2

(B − C)

v0

Trot (K)

Γ (fwhm)

1.05 1.730b

0.16(1) 0.355b

0.01 0.049b

2955.15(1) 2954 ± 1c

0.35

0.3

The gas phase vibrational ground state rotational constants are from FTMW measurements. All values are reported in cm−1. The ab initio transition moment components are used in the simulations. The band is an a,b-hybrid band with an a-axis transition moment component that is 2.63 times larger than the component along the b-axis. The simulation was carried out with PGOPHER.49 bReference 50. cReference 23. a

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ACKNOWLEDGMENTS

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REFERENCES

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G.E.D. acknowledges support from the National Science Foundation (CHE-1054742) and the donors of the American Chemical Society Petroleum Research Fund (50223-DNI6). H.F.S. acknowledges support from the Department of Energy, Office of Basic Energy Sciences (Grant No. DEFG02-97ER14748).

(1) Lugovoj, E.; Toennies, J. P.; Vilesov, A. Manipulating and Enhancing Chemical Reactions in Helium Droplets. J. Chem. Phys. 2000, 112, 8217. (2) Farnik, M.; Toennies, J. P. Ion−Molecule Reactions in 4He Droplets: Flying Nano-Cryo-Reactors. J. Chem. Phys. 2005, 122, 014307. (3) Toennies, J. P. Molecular Low Energy Collisions: Past, Present and Future. Phys. Scr. 2007, 76, C15. (4) Müller, S.; Krapf, S.; Koslowski, T.; Mudrich, M.; Stienkemeier, F. Cold Reactions of Alkali-Metal and Water Clusters inside Helium Nanodroplets. Phys. Rev. Lett. 2009, 102, 183401. (5) Krasnokutski, S. A.; Huisken, F. Ultra-Low-Temperature Reactions of Mg Atoms with O2 Molecules in Helium Droplets. J. Phys.Chem. A 2010, 114, 7292. (6) Krasnokutski, S. A.; Huisken, F. Oxidative Reactions of Silicon Atoms and Clusters at Ultra Low Temperature in Helium Droplets. J. Phys. Chem. A 2010, 114, 13045. (7) Krasnokutski, S. A.; Huisken, F. Low-Temperature Chemistry in Helium Droplets: Reactions of Aluminum Atoms with O2 and H2O. J. Phys. Chem. A 2011, 115, 7120. (8) Brink, D. M.; Stringari, S. Density of States and Evaporation Rate of Helium Clusters. Z. Phys. D 1990, 15, 257. (9) Toennies, J. P.; Vilesov, A. F. Superfluid Helium Droplets: A Uniquely Cold Nanomatrix for Molecules and Molecular Complexes. Angew. Chem., Int. Ed. 2004, 43, 2622. (10) Choi, M. Y.; Douberly, G. E.; Falconer, T. M.; Lewis, W. K.; Lindsay, C. M.; Merritt, J. M.; Stiles, P. L.; Miller, R. E. Infrared Spectroscopy of Helium Nanodroplets: Novel Methods for Physics and Chemistry. Int. Rev. Phys. Chem. 2006, 25, 15. (11) Braun, A.; Drabbels, M. Photodissociation of Alkyl Iodides in Helium Nanodroplets. III. Recombination. J. Chem. Phys. 2007, 127, 114305. (12) Taatjes, C. A. Uncovering the Fundamental Chemistry of Alkyl + O2 Reactions via Measurements of Product Formation. J. Phys. Chem. A 2006, 110, 4299. (13) Wilke, J. J.; Allen, W. D.; Schaefer, H. F. Establishment of the C2H5+O2 Reaction Mechanism: A Combustion Archetype. J. Chem. Phys. 2008, 128, 074308. (14) Tyndall, G. S.; Cox, R. A.; Granier, C.; Lesclaux, R.; Moortgat, G. K.; Pilling, M. J.; Ravishankara, A. R.; Wallington, T. J. Atmospheric Chemistry of Small Organic Peroxy Radicals. J. Geophys. Res-Atmos 2001, 106, 12157. (15) Zhu, R.; Hsu, C. C.; Lin, M. C. Ab Initio Study of the CH3+O2 Reaction: Kinetics, Mechanism and Product Branching Probabilities. J. Chem. Phys. 2001, 115, 195. (16) Hunziker, H. E.; Wendt, H. R. Electronic Absorption-Spectra of Organic Peroxyl Radicals in Near-Infrared. J. Chem. Phys. 1976, 64, 3488. (17) Ase, P.; Bock, W.; Snelson, A. Alkylperoxy and Alkyl Radicals 0.1. Infrared-Spectra of CH3O2 and CH3O4CH3 and the Ultraviolet Photolysis of CH3O2 in Argon + Oxygen Matrices. J. Phys. Chem. 1986, 90, 2099. (18) Pushkarsky, M. B.; Zalyubovsky, S. J.; Miller, T. A. Detection and Characterization of Alkyl Peroxy Radicals Using Cavity Ringdown Spectroscopy. J. Chem. Phys. 2000, 112, 10695. (19) Blanksby, S. J.; Ramond, T. M.; Davico, G. E.; Nimlos, M. R.; Kato, S.; Bierbaum, V. M.; Lineberger, W. C.; Ellison, G. B.; Okumura, M. Negative-Ion Photoelectron Spectroscopy, Gas-Phase Acidity, and

Figure 4. (a) Electron impact ionization mass spectrum of the doped droplet beam with the DTBP pyrolysis source hot and the O2 pick-up pressure set to ∼3 × 10−6 Torr, which is sufficiently low to suppress the pick-up of multiple O2 molecules. Note the absence of a peak at m/z = 47 corresponding to the intact (CH3O2)+ molecular ion. The mass spectrum in (b) is the difference mass spectrum (laser off-on) with the laser fixed to the peak of the ν2 CH3OO band. The majority of the depletion signal appears in the m/z = 29 mass channel, which indicates that CH3OO fragments into HCO+ and H2O upon charge transfer ionization from He+.

laser beam depletion spectroscopy. All three CH stretch bands of CH3OO are observed, and rotational fine structure is partially resolved for the ν2 totally symmetric CH stretch band, indicating complete internal cooling of the product to the droplet temperature. Electron impact ionization of the droplets containing CH3OO results in the charge transfer reaction He+ + CH3OO → CH3O2+ + He, which is followed by the fragmentation of the CH3O2+ ion. The major fragmentation channel is the production of HCO+ and H2O. The outcome of this work is encouraging and suggests that infrared laser spectroscopy can be employed as a selective probe of the outcome of organic radical−radical reactions in which multiple products may be formed. By integrating the total ion signal arising from the ionization of the droplet beam, the relative IR beam depletion band intensities associated with the various possible products can be used directly to determine product branching ratios.

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ASSOCIATED CONTENT

* Supporting Information S

Optimized ground state structures, survey scan of the CH stretch region, Cartesian coordinates of the optimized CH3OO structures, harmonic frequencies, and focal point table for the computation of the ground state torsional barrier height.This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5303

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