ARTICLE pubs.acs.org/JPCA
Excitation and Emission Spectra of Jet-Cooled Naphthylmethyl Radicals Nahid Chalyavi,† Tyler P. Troy,† Masakazu Nakajima,†,§ Bligh A. Gibson,† Klaas Nauta,† Robert G. Sharp,‡,|| Scott H. Kable,† and Timothy W. Schmidt*,† † ‡
School of Chemistry, The University of Sydney, NSW 2006, Australia Anglo-Australian Observatory, PO Box 296, Epping NSW 1710, Australia ABSTRACT: Gas phase excitation and emission spectra of three naphthylmethyl radical chromophores are presented. These resonance-stabilized species, 1-naphthylmethyl, 2-naphthylmethyl, and R-acenaphthenyl, each possessing an sp2 carbon adjacent to a naphthalene moiety, are studied by resonant two-color two-photon ionization, laser induced fluorescence, and dispersed fluorescence spectroscopy. Identification of the radicals is made through a combination of dispersed fluorescence and density functional theory calculations. All three species possess spectra in the 580 nm region. The possible relevance to unidentified spectroscopic features such as the diffuse interstellar bands and emission from the Red Rectangle nebula is discussed.
’ INTRODUCTION Resonance is a concept which lies at the very heart of chemistry. It was introduced into quantum mechanics by Werner Heisenberg1 and Paul Dirac2 and was championed as a chemical theory by Linus Pauling.3,4 Resonance allows the valence bond picture and molecular orbital picture to be bridged, the various “resonance structures” forming a basis with similar energies. These “resonate” such that the eigenstates become a superposition of these forms, with the ground state significantly stabilized. As such, resonance-stabilized radicals (RSRs) are formed more readily by dissociation of closed shell species and react more slowly compared to non-resonance-stabilized counterparts. Resonance-stabilized radicals are of great importance in fuelrich combustion. Indeed, the resonance-stabilized propargyl radical is held responsible by many for the formation of benzene in fuel-rich combustion environments, its stability allowing it to build up to high concentrations before undergoing bimolecular reaction and subsequent rearrangement to benzene.5,6 Apart from fundamental interest, our study of these species is motivated by the science of planetary atmospheres and the chemistry of the interstellar medium (ISM). During the formation of the solar system, samples of the latter were incorporated into carbonaceous chondrites such as the Orgueil and Murchison meteorites.79 These have been found to contain resonance stabilized “benzylic” radicals, where an sp2 carbon is adjacent to an aromatic moiety. Deuteration enrichment at these sites indicates cycling of hydrogen at this position and underlines their lability—a result of the relative ease with which RSRs are formed.8 Furthermore, amorphous hydrocarbon is believed to be ubiquitous in the ISM.1012 As this erodes due to absorption of the harsh ultraviolet radiation which pervades interstellar space, it will preferentially give rise to RSRs. While the propargyl r 2011 American Chemical Society
radical itself is yet to be identified in interstellar space, due to its small dipole moment,13 its l-C3H2 derivative has been detected, and was recently suggested as the carrier of the λ4881 and λ5450 diffuse interstellar bands.14,15 One of the closed shell precursors of propargyl, propyne, is a well-known interstellar species, and is also found in the atmospheres of Jupiter, Saturn, Uranus, and Neptune.1622 In supersonically cooled plasmas seeded with hydrocarbon precursors, we have recently identified a number of RSRs, including phenylpropargyl (C9H7)23 and vinylpropargyl (C5H5)24 radicals, each with a higher calculated radical stabilization energy compared to propargyl itself.23,2527 These substituted propargyls are found to be general products of an electrical discharge containing hydrocarbons, with both species readily formed from, for example, benzene or 1-hexyne. From a closed shell precursor bearing a hydrogen on one of the radical “positions”, our laboratory and others have found it relatively facile to produce a range of RSRs based on the benzyl motif, such as indanyl (C9H9)28 and tetralyl (C10H11);29 the phenylallyl motif, such as cinnamyl (C9H9), inden-2-ylmethyl, benzylallenyl,25,30 and 1-hydronaphthyl (C10H9);29 and the phenalenyl radical (C13H9).31 Production of these radicals in the gas phase facilitates the collection of the instrinsic excitation (absorption) and emission spectra of these species, which can then be compared to astronomical spectral features such as the diffuse interstellar bands (DIBs)32 and red rectangle bands (RRBs).33 The carriers of the DIBs and RRBs are widely held to be gas phase carbonaceous molecules. Notwithstanding the recent Received: April 19, 2011 Revised: May 31, 2011 Published: June 07, 2011 7959
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Figure 1. Structures of the RSRs studied here: 1-NpMe, 1-naphthylmethyl; 2-NpMe, 2-naphthylmethyl; and R-acNp, acenaphthenyl.
suggestions of l-C3H2 and HC4H+ as DIB carriers,14,34 it is believed that, in general, a molecule must attain a certain size before it can withstand the radiation field of the diffuse ISM.35,36 It is thus of interest to obtain the gas phase spectra of larger carbonaceous species bearing absorptions in the visible and NIR region of relevance to the DIBs and RRBs. In this paper we present the first gas phase spectroscopic study of RSRs based on the naphthylmethyl chromophore, an analogue of the benzyl radical with the benzene ring replaced with naphthalene. Three radicals are presented: 1-naphthylmethyl (1-NpMe), 2-naphthylmethyl (2-NpMe), and R-acenaphthenyl (R-acNp), illustrated in Figure 1. The spectra are discussed in the light of unidentified astronomical spectral features.
’ EXPERIMENTAL METHODS 1-Methylnaphthalene (Fluka, 97%), 2-methylnaphthalene (Aldrich, 97%), and acenaphthene (Aldrich, 99%) were used, without further purification, as precursors for the formation of 1-NpMe, 2-NpMe, and R-acNp radicals, respectively. Resonant two-color two-photon ionization (R2C2PI) spectra were measured in a two-stage differentially pumped vacuum chamber fitted with a pulsed discharge nozzle (PDN) source as used previously.23,24,28 The precursors were stored in a stainless steel container inside the chamber and, in order to form a rich molecular beam, both the sample container and the PDN were heated to about 100 °C using wire heaters. The PDN consisted of a pulsed valve coupled to an electrical discharge, timed to strike during the gas pulse. The emerging beam, containing radicals of interest, was passed into the ionization region through a 2 mm skimmer. Radicals were ionized by two laser pulses. The first laser pulse, provided by a Nd:YAG pumped dye laser, was tuned over the vibrational levels of the excited electronic state while the second, fixed wavelength pulse was used to ionize the excited molecules. The 193 nm photons for the ionization step came from the output of an ArF excimer laser. The cations were extracted vertically and perpendicularly to the laser and molecular beam into the time-of-flight tube. In the extraction region of the WileyMcLaren time-of-flight mass spectrometer,37 the electric field was about 100 V/cm. Ions were detected with a tandem microchannel plate. The signal was viewed on a digital oscilloscope as a function of laser wavelength and processed using in-house software. Dispersed fluorescence (DF) spectra were obtained using another vacuum chamber by fixing the pump laser frequency at the maximum of the origin peak identified for each species from the R2C2PI spectrum. The fluorescence was then collected with a quartz lens mounted in the chamber at 90° to the laser beam. A second lens, external to the vacuum chamber, was used to image the fluorescence onto the entrance slit of a 0.75 m monochromator, while the slit widths were kept at 0.75 mm, corresponding
Figure 2. R2C2PI spectra of the three radicals . Mutual impurities in the 1-NpMe and 2-NpMe are indicated with asterisks.
to a spectral resolution of 15 cm1. Laser induced fluoroescence (LIF) spectra were obtained to confirm that the emission spectra were free from spectral contaminants such as C2.
’ COMPUTATIONAL METHODS For the purpose of confirming the identities of the radicals herein studied, we performed geometry optimization and harmonic frequency calculations using the Gaussian03 electronic structure program.38 We employed density functional theory (DFT) using the 6-311++G(d,p) basis set with the B3-LYP functional. To calculate vertical D1 r D0 electronic excitation energies, we performed complete active space self-consistent field theory (CASSCF) calculations with the GAMESS electronic structure program39 at the B3-LYP/6-311++G(d,p) geometry. The orbitals were separately optimized for the ground and excited states, with the 6-311G(d,p) basis, in turn using the optimized orbitals from a smaller active space for the larger one to ensure convergence. The active spaces were chosen to include only the π-type orbitals of a00 symmetry. The smallest such active space which captures the essence of the excited states is three electrons in three orbitals. The effects of dynamic correlation were accounted for by multireference second-order perturbation theory (MRPT2). Parameters for rotational contour simulations, calculated for 1-NpMe using the Asyrotwin program,40 were obtained by optimizing the geometries of the ground and excited states of 1-NpMe at the [11,11] CASSCF/6-311++G(d,p) level of theory, using the Molpro electronic structure program.41 The transition dipole moment vector was determined from the CASSCF calculation at the equilibrium geometry of the ground state. ’ RESULTS AND DISCUSSION Figure 2 shows the R2C2PI spectra of 1-NpMe and 2-NpMe, monitoring the m/z 141 mass channel (C11H9), and R-acNp, monitoring the m/z 153 mass channel (C12H9) in the 1710018700 cm1 region. They all exhibit a strong origin transition in the 1710017250 cm1 region, and some vibronic structure. With acenaphthene as the precursor, only one resonance-stabilized isomer is formed by loss of one hydrogen and therefore the excitation spectrum recorded is expected to be purely from R-acNp radical. The 2-methylnaphthalene precursor was chosen to favor formation of the 2-NpMe radical isomer, 7960
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Table 1. Vertical Excitation Energies (cm1) and Oscillator Strengths, f, Calculated at the CASSCF/6-311G(d,p) Level with Various Active Spaces of N Electrons in M a00 Orbitals ([N,M])a 1-NpMe
2-NpMe
R-acNp
[3,3]
23191
25316
23439
[5,5]
25529
23723
26594
[7,7]
22400
22212
22876
[9,9]
20602
21365
20370
[11,11]
19871
20325
19863
[3,3]PT2
17973
20815
17722
[5,5]PT2 [7,7]PT2
17866 17636
20606 18379
17602 16507
[9,9]PT2
17775
18308
17541
[11,11]PT2
18220
18231
18025
exp. T0
17240
17134
17175
f-value
0.005
0.004
0.002
method
a
Multireference second-order perturbation theory (PT2) accounts for dynamic correlation. The experimental value is that of the origin band (000).
which can be formed with high efficiency by loss of one hydrogen from the methyl group positioned at carbon 2 of the naphthalene ring, and shows an electronic origin at 17134 cm1. Similarly, the spectrum recorded with 1-methylnaphthalene as precursor was assigned to 1-NpMe. The electronic origin of this isomer is at 17240 cm1, shifted to higher energy than 2-NpMe by just 106 cm1. As 1-NpMe and 2-NpMe are structural isomers with identical mass-to-charge ratios, mutual impurities in the precursor material may cause spectral contamination. In the excitation spectrum of 2-NpMe, bands are observed attributable to 1-NpMe (indicated with asterisks), demonstrating its presence as an impurity. Likewise, bands observed in the 1-NpMe spectrum can also be unambiguously assigned as being carried by 2-NpMe. The observed impurity peaks exceed the expectations based on the stated purities from the chemical suppliers and may indicate isomerization in the electrical discharge. The observed origin transitions are entirely consistent with the observations by Cofino et al., who studied the photolysis products of the same precursors in Shpolskii matrices,42 and Hilinksi et al. and Weir et al., who studied emission spectra originating in the 580 nm region from excitation of transient species resulting from the photolysis of halo-methylnaphthalenes.43,44 The results of ab initio calculations of the vertical excitation energies are given in Table 1. In general, the CASSCF values are higher than those including the effects of configurations arising from excitations into and out of the active space (PT2). The MRPT2 results are satisfactory even with the smallest [3,3] active space, and settle down on going to larger active spaces. The experimental number in Table 1 is not strictly a vertical excitation energy, since the origin transition takes account of the geometry relaxation and difference in zero-point energy between the ground and the excited states. Both of these effects will contribute to the vertical transition being an overestimate of the origin band position. That the observed origin is about 1000 cm1 redder than the best calculated vertical excitation energy is thus not unexpected. The calculated excitation energies are consistent with the molecular carriers, but they cannot be used to make a definite assignment. The oscillator strengths are rather low, on the order of a thousandth of that of a harmonically bound
Figure 3. Dispersed fluorescence spectra of the origin bands of the three radicals, with assignments (Table 2).
electron. The low oscillator strengths are due to a cancellation of transition moments for the HOMOSOMO and SOMO LUMO transitions, as discussed by Longuett-Higgins (see ref 45) and thus there is some variability despite the similar chromophores. Conclusive identification of the radicals is possible by comparing dispersed fluorescence spectra to calculated vibrational frequencies. We first reproduced the mass-selective excitation spectra by laser induced fluorescence, and dispersed the fluorescence from the origin bands to obtain ground state frequencies. These spectra are shown in Figure 3, plotted against displacement from the pump laser frequency. As supported by the DFT calculations 1-NpMe, 2-NpMe, and R-acNp radicals are (heavy atom) planar in their ground electronic state and belong to the Cs point group. Therefore the ground state frequencies are divided into in-plane modes (having a0 symmetry) and out-of-plane modes (having a00 symmetry). Assuming the excited states to have a (heavy atom) planar equilibrium geometry, single quanta of a00 modes will not be observed due to the FranckCondon principle. Therefore, comparison is made only between calculated frequencies of a0 symmetry and the experimentally observed bands. The 1-NpMe and 2-NpMe radicals each exhibit 37 a0 modes, while R-AcNp has 38. Assignment of the lowest frequency a0 modes (below 1000 cm1) provides the surest identification of the spectral carriers. As listed in Table 2, the lowest modes are all well matched between experiment and theory for the three radicals under study. In order to reproduce experimental ground state frequencies, calculated frequencies were scaled by a factor of 0.97. This scaling factor was successfully used by our group in previous studies of resonance stabilized radicals.23,24 While the DF spectra of these radicals are broadly similar, the identification is unmistakable from the vibrational signature. Our experimentally derived numbers are close to the theoretical numbers provided by The NASA Ames Polycyclic Aromatic Hydrocarbon Infrared Spectroscopic Database.46
’ CONNECTION WITH EXTRATERRESTRIAL CHEMISTRY The Red Rectangle Bands. Polycyclic aromatic hydrocarbon (PAH) molecules have been proposed as the carriers of the socalled aromatic infrared bands (AIBs, or unidentified infrared bands, UIRs) observed in star-forming regions as well as the circumstellar shells of carbon-rich post-asymptotic giant branch 7961
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Table 2. Calculated and Experimental Ground State Vibrational Frequencies (cm1) of 1-NpMe, 2-NpMe, and r-acNp Radicals from Assignment of the DF Spectra of Their Origin Bands (Figure 3)a 1-NpMe mode
R-acNp
2-NpMe
exptl
calcd
exptl
00
00
00
calcd
exptl
calcd
00
ν38
a
a
a
408
407
ν37
305
304
269
269
456
456
ν36
437
436
402
400
(498)
494
ν35
466
467
450
448
534
529
ν34
494
493
509
506
647
639
ν33
566
566
615
614
668
664
ν32 ν31
700 779
697 775
(722) 759
711 748
792 (830)
784 830
ν30
(870)
861
886
875
944
930
ν29
942
939
ν28
a
1018
984
923
997
975
1020
a
Frequencies are calculated at the B3LYP/6-311++G(d,p) level, scaled by 0.97. Tentative assignments and weak emission features are given in parentheses.
Figure 4. Cartoon of a small hydrogenated amorphous carbon grain.47,11 Upon fragmentation, the weakest bond will be broken: That which leads to the most stable products. In this picture, the grain is fragmented to yield the resonance-stabilized 1-naphthylmethyl radical. This type of radical, where the unpaired electron is delocalized throughout the π-system of the PAH moiety, is stabilized compared to σ-type radicals by as much as 1 eV.
stars.48,49 The AIBs are a group of emission bands brought about by transient heating of PAHs by ultraviolet radiation. The strongest such bands have wavelengths of 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 μm, characteristic of PAHs, either neutral or
ionized.50 However, no specific PAH has been unambiguously determined to be a spectral carrier. The AIBs, and by implication PAHs, have been detected in the Red Rectangle nebula,52,53 an unusual protoplanetary nebula which exhibits several unidentified emission bands occurring in the visible region.54 The strongest features are observed around 5800 Å, with weaker features to longer wavelength superimposed on a continuum of emission known as the extended red emission.33 On going from the central object to the edge, some bands disappear while others narrow with their centroids shifting to the blue, possibly demonstrating cooling, or chemical evolution of the carriers. The shortest wavelength and most prominent band in the Red Rectangle, peaked near 5799 Å far from the central object, appears to converge toward the narrow DIB at 5797 Å and therefore there has been a suggestion that both bands have an identical carrier.55 The emission bands are speculated to be due to unusual carbon-containing molecules, with Duley having suggested that the carriers are a “product of the dissociation of carbonaceous dust”.56 The structure of the carbonaceous dust has been addressed by several investigators.11,57 All representations of this material feature aromatic islands linked by aliphatic and olefinic linkages. The model presented in Figure 4 illustrates a naphthalene-like moiety and a pyrene-like moiety linked by aliphatic groups, as might exist in a larger structure. If a covalent bond is broken, for instance by excess vibrational energy following absorption of a high energy photon, then it is the weakest bond which will break, that which produces the most stable fragments. Organic radicals are more stable if the unpaired electron can be delocalized over several atoms to produce RSRs. As shown here, the class of resonance-stabilized radical with a chromophore made up of a PAH (naphthalene) unit with the π-system extended to an sp2-hybridized R-carbon exhibit absorption and emission in the 5800 Å region. If these are produced abundantly in the Red Rectangle, they may contribute to the observed emission. The origin band of 1-NpMe coincides with the λ5799 feature far from the central object. In Figure 5 we simulate the observed LIF band profile with a 40 K rotational temperature using the calculated rotational constants given in Table 3. This simulation is then convolved with a 0.8 Å (fwhm) Gaussian and superimposed on the Doppler-corrected λ5799 Red Rectangle emission feature. The line width imposed accords well with the estimates of Glinski et al.33 of between 0.6 and 1.6 Å for velocity dispersion in the Red Rectangle. The match is excellent. However, the RRB is in emission, rather than absorption, so there are several factors which need to be considered in assigning the RRB emission band to the 1-NpMe isomer. A shown in Figure 3, there is emission some 500 cm1 to lower energy than the excitation wavelength, assigned as 3501 and 3301. While we cannot judge the relative intensity of these bands compared to 000 in emission, from the excitation spectrum in Figure 2, we might expect them to be strong. There is no sharp feature in the Red Rectangle emission spectrum immediately attributable to these bands. However, it is unknown if the emitter is excited by starlight or by charged particle impact. If indeed starlight is the excitation source, then the vibronic bands of the emitter will be populated according to the absorption spectrum, modulated by the star’s spectrum. The emitter will then emit from these levels simultaneously, bringing about multiple emission features. In the case that the transition system is origin-dominated, the emission will be predominantly at the wavelength of the origin, differing by the difference in frequencies between the excited vibrational mode and the 7962
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Figure 5. From top: The laser induced fluorescence spectrum of the origin band of 1-NpMe. A 40 K rotational simulation of the 1-NpMe origin band. The λ5799 Red Rectangle band measured ≈1500 from the central object, compared to the 40 K 1-NpMe simulation convolved with a 0.8 Å Gaussian to simulate velocity dispersion (thick line).51 The origin band of 1-NpMe measured by R2C2PI. A 5 K simulation of the 1-NpMe origin band compared to the λ5797 DIB.
Table 3. Rotational Constants (cm1) Used in the Rotational Contour Simulations of 1-NpMe Presented in Figure 5 D0
D1
A B
0.066595 0.037315
0.066297 0.036774
C
0.023915
parameter
intensity ratio (B/A)
0.023654 1.325
corresponding ground state mode. Such a feature would be a superposition of bands designated Ann. Such sequence structure has been invoked by us previously to model the structure of the Red Rectangle bands.51 If the excitation is by charged particle impact, then the molecule is first excited to high-lying electronic states which then undergo internal conversion to yield a statistical population of vibronic levels which will be involved in the emission. In a study of single vibronic level emission spectra of anthracene, it was shown that even high lying levels yielded emission spectra mimicking that of the vibrationless level, albeit red shifted and broadened.58 As such, given the absence of features in the Red Rectangle Band spectrum attributable to the {Ann}3501 and {Ann}3301 features (see ref 59), on the basis of the
presently available data, the astronomical feature is not assignable to the 1-naphthylmethyl radical. It is worth considering whether the RRBs are due to the summation of emission spectra from the family of radicals with the same π-chromophore such as those reported here. There are a great many such structures which are all plausible breakdown products of carbonaceous grains, and they all fluoresce in the region of interest (580590 nm). For instance, the 4-methyl-1naphthylmethyl radical exhibits an origin band at 5866 Å and 5-methyl-1-naphthylmethyl radical strongly absorbs and emits at 5890 Å.60 On the journey into the interstellar medium, the photophysically least stable may become further photodissociated, leaving only certain isomers to carry the RRBs at the edges of the nebula. From such a hypothesis, a natural question which arises is whether benzylic radicals are observed in the Red Rectangle. These exhibit spectra in the 450480 nm region.61,62,23,28,29 Indeed, blue luminescence extending into this spectral region has been reported.63 The Diffuse Interstellar Bands. Figure 5 shows the origin region of the R2C2PI spectrum of 1-NpMe. It is well simulated with a 5 K rotational temperature. In R2C2PI, only the coldest part of the free jet expansion passes into the time-of-flight extraction region and thus the observed spectrum is much colder than LIF where the entire free jet is probed only centimeters from the nozzle. PAHs, and in particular their radicals and cations, are considered to be strong candidate carriers for diffuse interstellar bands (DIBs),64,65 Generally, the strength of any pair of DIBs across different lines of sight is poorly correlated,66 which can be interpreted as indicating that DIBs mostly arise from unique carriers and that the spectra of the carriers are dominated by a single vibronic transition,23 such as the origin. All three radicals under study exhibit an origin band stronger than any other vibronic feature. However, the wavelengths of the origin bands of the 1-NpMe, 2-NpMe, and R-acNp do not exhibit a coincidence with the nearest DIBs. The closest is 1-NpMe, which at 5798.9 Å, misses the λ5797 DIB by more than its width (see Figure 5).67 However, the width of the 5 K band is about 1 Å, similar to the nearby λ5797 DIB (0.91 Å fwhm).68 Moreover, molecules of this size exhibit band profiles consistent with the narrower DIBs. The absence of a match between any hitherto observed RSR and a DIB suggests that these radicals do not contribute strongly to the interstellar extinction curve. The transitions of these species are relatively weak, there being a cancellation of transition moment from the two one-electron transitions which bring about the excited state configuration(s).45 Higher transitions will be stronger, but the spectra will likely not be as sharply defined as for the transitions to the lowest excited states, as seen in radical cations where the D2 r D0 transitions are much broader than most DIBs.69 It has been estimated that many DIB carriers have ionization energies (IEs) above 10 eV, including the carrier of the λ5797 feature.70 Given the relatively low (∼7 eV) IEs of the present species, it is likely that they would be predominantly singly ionized in the ISM. The ionization energy to the dication is in excess of 12 eV, consistent with estimates for the IEs of some DIB carriers. Cations of RSRs are closed shell species, which will exhibit their lowest electronic transitions in the visible region.45 As such, they should have similar band profiles to the neutral radicals. For instance, protonated naphthalene, the cation of 1-hydronaphthyl radical, was recently found to absorb sharply at 503.36 nm,71 where the neutral absorbs at 527.6 nm.29 However, no match to a DIB was observed. By using similar spectroscopic 7963
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’ CONCLUSIONS The excitation spectra of 1-naphthylmethyl, 2-naphthylmethyl, and R-acenaphthenyl radicals in the region 1710018700 cm1 have been measured by R2C2PI and LIF. A DF spectrum observed after pumping the origin band has been recorded for each radical, and assignments of the DF spectra were made with the aid of quantum chemical calculations. All three molecules containing the naphthylmethyl chromophore exhibit transitions in the wavelength range relevant to the DIBs and particularly the RRBs. Although no close match was found between the radicals under study and any DIB, 1-NpMe exhibited an origin band matching the strongest RRB. Whether the RRBs are due to radicals of this type is being investigated further. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
)
IR FEL Research Center, Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda City, Chiba Prefecture, 278-8510, Japan. Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Rd., Weston Creek, Canberra, ACT 2611, Australia.
’ ACKNOWLEDGMENT This research was supported under the Australian Research Council’s Discovery funding scheme (Project Number DP0985767). T.P.T. acknowledges The University of Sydney for a University Postgraduate Award. N.C. acknowledges the Endeavor International Postgraduate Research Scholarship and the University of Sydney International Scholarship. K.N. thanks the Australian Research Council for the award of an Australian Research Fellowship. ’ REFERENCES (1) Heisenberg, W. Z. Phys. 1926, 39, 499–518. (2) Dirac, P. A. M. Proc. R. Soc. London, Ser. A 1926, 112, 661. (3) Pauling, L. Chem. Rev. 1928, 5, 173–213. (4) Pauling, L. Proc. Natl. Acad. Sci. U.S.A. 1928, 14, 359–362. (5) McEnally, C. S.; Pfefferle, L. D.; Atakan, B.; Kohse-H€ oinghaus, K. Prog. Energy Combust. Sci. 2006, 32, 247–294. (6) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565–608. (7) Binet, L.; Gourier, D.; Derenne, S.; Robert, F.; Coifini, I. Geochim. Cosmochim. Acta 2004, 68, 881–891. (8) Gourier, D.; Robert, F.; Delpoux, O.; Binet, L.; Vezin, H.; Moissette, A.; Derenne, S. Geochim. Cosmochim. Acta 2008, 72, 1914– 1923. (9) Delpoux, O.; Gourier, D.; Vezin, H.; Binet, L.; Derenne, S.; Robert, F. Geochim. Cosmochim. Acta 2011, 75, 326–336. (10) Ehrenfreund, P.; Charnley, S. B. Annu. Rev. Astron. Astrophys. 2000, 38, 427–83. (11) Pendleton, Y.; Allamandola, L. Astrophys. J., Supp. Ser. 2002, 138, 75–98. (12) Dartois, E.; Mu~noz-Caro, G. M. Astron. Astrophys. 2007, 476, 1235–1242.
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(13) Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Varner, M. E.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2005, 109, 3812–21. (14) Maier, J. P.; Walker, G. A. H.; Bohlender, D. A.; Mazzotti, F. J.; Raghunandan, R.; Fulara, J.; Garkusha, I.; Nagy, A. Astrophys. J. 2011, 726, 41. (15) Oka, T.; McCall, B. J. Science 2011, 331, 293. (16) Irvine, W. M.; Hoglund, B.; Friberg, P.; Askne, J.; Ellder, J. Astrophys. J. Lett. 1981, 248, L113–L117. (17) Agundez, M.; Fonfra, J. P.; Cernicharo, J.; Pardo, J. R.; Guelin, M. Astron. Astrophys. 2008, 479, 493–501. (18) Fouchet, T.; Lellouch, E.; Bezard, B.; Feuchtgruber, H.; Drossart, P.; Encrenaz, T. Astron. Astrophys. 2000, 355, L13–L17. (19) Guerlet, S.; Fouchet, T.; Bezard, B.; Moses, J. I.; Fletcher, L. N.; Simon-Miller, A. A.; Flasar, F. M. Icarus 2010, 209, 682–695. (20) Burgdorf, M.; Orton, G.; van Cleve, J.; Meadows, V.; Houck, J. Icarus 2006, 184, 634–637. (21) Meadows, V. S.; Orton, G.; Line, M.; Liang, M.-C.; Yung, Y. L.; Van Cleve, J.; Burgdorf, M. J. Icarus 2008, 197, 585–589. (22) Snyder, L. E.; Buhl, D. Nat. Phys. Sci. 1973, 243, 45. (23) Reilly, N. J.; Kokkin, D. L.; Nakajima, M.; Nauta, K.; Kable, S. H.; Schmidt, T. W. J. Am. Chem. Soc. 2008, 130, 3137–3142. (24) Reilly, N. J.; Nakajima, M.; Troy, T. P.; Chalyavi, N.; Duncan, K. A.; Nauta, K.; Kable, S. H.; Schmidt, T. W. J. Am. Chem. Soc. 2009, 131, 13423–13429. (25) Troy, T. P.; Chalyavi, N.; Menon, A. S.; O’Connor, G. D.; F€uckel, B.; Nauta, K.; Radom, L.; Schmidt, T. W. Chem. Sci. 2011in press. (26) Menon, A. S.; Wood, G. P. F.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111, 13638–13644. (27) Menon, A. S.; Henry, D.; Bally, T.; Radom, L. Org. Biomol. Chem. 2011, 9, 3636–3657. (28) Troy, T. P.; Nakajima, M.; Chalyavi, N.; Clady, R. G. C. R.; Nauta, K.; Kable, S. H.; Schmidt, T. W. J. Phys. Chem. A 2009, 113, 10279–10283. (29) Sebree, J. A.; Kislov, V. V.; Mebel, A. M.; Zwier, T. S. J. Phys. Chem. A 2010, 114, 6255–6262. (30) Sebree, J. A.; Kidwell, N.; Buchanan, E. G.; Zgierski, M.; Zwier, T. S. Chem. Sci. 2011in press. (31) O’Connor, G. D.; Troy, T. P.; Roberts, D. A.; Chalyavi, N.; F€uckel, B.; Crossley, M. J.; Nauta, K.; Stanton, J. F.; Schmidt, T. W. In preparation (32) Sarre, P. J. J. Mol. Spectrosc. 2006, 238, 1–10. (33) Glinski, R.; Michaels, P.; Anderson, C.; Schmidt, T.; Sharp, R.; Sitko, M.; Bernstein, L.; Van Winckel, H. Astrophys. Space Sci. 2009, 323, 337–344. (34) Krelowski, J.; Beletsky, Y.; Galazutdinov, G. A.; Kolos, R.; Gronowski, M.; LoCurto, G. Astrophys. J. Lett. 2010, 714, L64–L67. (35) Allain, T.; Leach, S.; Sedlmayr, E. Astron. Astrophys. 1996, 305, 602–615. (36) Allain, T.; Leach, S.; Sedlmayr, E. Astron. Astrophys. 1996, 305, 616–630. (37) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150. (38) Frisch, M. J. et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (39) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. J.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347–1363. (40) Judge, R. H.; Clouthier, D. J. Comput. Phys. Commun. 2001, 135, 293. (41) Werner, H.-J. et al. MOLPRO, version 2006.1, a package of ab initio programs. 2006 (42) Cofino, W. P.; Hoornweg, G. P.; Gooijer, C.; MacLean, C.; Velthorst, N. H. Spectrochim. Acta 1983, 39A, 283–288. (43) Hilinski, E. F.; Huppert, D.; Kelley, D. F.; Milton, S. V.; Rentzepis, P. M. J. Am. Chem. Soc. 1984, 106, 1951–1957. (44) Weir, D.; Johnston, L. J.; Scaiano, J. C. J. Phys. Chem. 1988, 92, 1742–1746. 7964
dx.doi.org/10.1021/jp203638h |J. Phys. Chem. A 2011, 115, 7959–7965
The Journal of Physical Chemistry A
ARTICLE
(45) Longuet-Higgins, H. C.; Pople, J. A. Proc. Phys. Soc. London, Sect. A 1955, 68, 591–600. (46) Bauschlicher, C. W.; Boersma, C.; Ricca, A.; Mattioda, A. L.; Cami, J.; Peeters, E.; Sanchez de Armas, F.; Puerta Saborido, G.; Hudgins, D. M.; Allamandola, L. J. Astrophys. J., Suppl. Ser. 2010, 189, 341–351. (47) Kwok, S. Nature 2004, 430, 985. (48) Leger, A.; Puget, J. L. Astron. Astrophys. 1984, 137, L5–L8. (49) Allamandola, L. J.; Tielens, A. G. G. M.; Barker, J. R. Astrophys. J. 1985, 290, L25–L28. (50) Schmidt, T. W.; Pino, T.; Brechignac, P. J. Phys. Chem. A 2009, 113, 3535–3541. (51) Sharp, R. G.; Reilly, N. J.; Kable, S. H.; Schmidt, T. W. Astrophys. J. 2006, 639, 194. (52) Russell, R. W.; Soifer, B. T.; Merrill, K. M. Astrophys. J. 1977, 213, 66–70. (53) Song, I.-O.; McCombie, J.; Kerr, T. H.; Sarre, P. J. Mon. Not. R. Astron. Soc. 2007, 380, 979–985. (54) Schmidt, G.; Cohen, M.; Margon, B. Astrophys. J. 1980, 239, L133–L138. (55) Sarre, P. J. Nature 1991, 351, 356. (56) Duley, W. W. Mon. Not. Roy. Astro. Soc. 1998, 301, 955–959. (57) Dartois, E.; Mu~noz-Caro, G. M.; Deboffle, D.; Montagnac, G.; d’Hendecourt, L. Astron. Astrophys. 2005, 432, 895–908. (58) Lambert, W. R.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1983, 81, 2209–2216. (59) Van Winckel, H.; Cohen, M.; Gull, T. R. Astron. Astrophys. 2002, 390, 147–154. (60) Chalyavi, N.; Troy, T.; Schmidt, T. W. Unpublished results (61) Fukushima, M.; Obi, K. J. Chem. Phys. 1990, 93, 8488–8497. (62) Lee, G. W.; Lee, S. K. J. Chem. Phys. 2007, 126, 214308. (63) Vijh, U.; Witt, A.; Gordon, K. Astrophys. J. Lett. 2004, 606, L65. (64) Leger, A.; d’Hendecourt, L. Astron. Astrophys. 1985, 146, 81–85. (65) Van der Zwet, G. P.; Allamandola, L. J. Astron. Astrophys. 1985, 146, 76–80. (66) McCall, B. J.; Drosback, M. M.; Thorburn, J. A.; York, D. G.; Friedman, S. D.; Hobbs, L. M.; Rachford, B. L.; Snow, T. P.; Sonnentrucker, P.; Welty, D. E. Astrophys. J. 2010, 708, 1628. (67) Kerr, T. H.; Hibbins, R. E.; Fossey, S. J.; Miles, J. R.; Sarre, P. J. Astrophys. J. 1998, 495, 941–945. (68) Hobbs, L. M.; York, D. G.; Thorburn, J. A.; Snow, T. P.; Bishof, M.; Friedman, S. D.; McCall, B. J.; Oka, T.; Rachford, B.; Sonnentrucker, P.; Welty, D. E. Astrophys. J. 2009, 705, 32. (69) Brechignac, P.; Pino, T. Astron. Astrophys. 1999, 343, L49–L50. (70) Sonnentrucker, P.; Cami, J.; Ehrenfreund, P.; Foing, B. H. Astron. Astrophys. 1997, 327, 1215–1221. (71) Alata, I.; Omidyan, R.; Broquier, M.; Dedonder, C.; Dopfer, O.; Jouvet, C. Phys. Chem. Chem. Phys. 2010, 12, 14456–14458.
7965
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