Article pubs.acs.org/JPCA
Hydroxyl Addition to Aromatic Alkenes: Resonance-Stabilized Radical Intermediates Tyler P. Troy, Masakazu Nakajima,† Nahid Chalyavi, Klaas Nauta, Scott H. Kable, and Timothy W. Schmidt* School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia S Supporting Information *
ABSTRACT: The spectra of 1-indanyl-based resonance-stabilized radicals containing a hydroxyl group are identified in an electrical discharge containing indene and its alkylated derivatives. It is argued that such species form by addition of a dischargenascent hydroxyl radical, formed from trace water, to the π bond on the five-membered ring of the parent molecule. The spectral carriers are identified by analysis of their excitation and emission spectra guided by the results from quantum chemical calculations. All three hydroxylated radicals are found to exhibit origin bands in the 21300 cm−1 region: the 2-hydroxy-indan-1-yl radical at 21364 cm−1, the 2-hydroxy-2-methyl-indan-1-yl radical at 21337 cm−1, and the 2-ethyl-2-hydroxy-indan-1-yl radical exhibiting two origins of similar intensity at 21287 and 21335 cm−1.
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INTRODUCTION Volatile organic compounds play a significant role in atmospheric chemistry. Of particular interest are the reactions involving the oxidation of alkenes, which are understood to be a major source of tropospheric ozone.1 Alkenes are introduced into the atmosphere from a variety of emitters with the most significant contribution coming from vegetation, soils, and other biogenic sources, where the dominant species emitted are isoprene (2-methyl-1,3-butadiene) and its derivatives and aromatic terpenoids.2,3 The oxidation of alkenes in the atmosphere is widely agreed to be initiated by the addition of •OH across the double-bond site according to the reaction
Figure 1. Dominant reaction channels for •OH addition to isoprene.
(2HI1R), the 2-hydroxy-2-methyl-indan-1-yl radical (2H2MI1R), and the 2-ethyl-2-hydroxy-indan-1-yl radical (2E2HI1R), formed after the addition of •OH to indene and its alkylated derivatives, 2-methylindene and 2-ethylindene. These radicals and their precursors are pictured in Figure 2.
̇ −R′ R−CHCH−R′ + •OH → R−CH(OH)−CH
The exact site of attack crucially depends on the substituents R and R′. Most previous studies have focused on •OH addition to isoprene because of its atmospheric profligacy.4−9 These studies report that all of the sites open to attack contribute some component to the complete reaction rate but that the dominant reaction channel is the •OH addition to the 1- and 4terminal carbons of isoprene, with the same result observed for 1,3-butadiene. Addition to these sites produces the substitutedallylic resonance-stabilized radicals (RSRs) which represent the most stable reaction intermediates.10 These reaction channels are depicted in Figure 1. With the rapid increase in anthropogenic alkene emissions from transport and industrial processes, especially in the form of alkylated aromatic hydrocarbons,11 it is of great interest to explore the oxidative process of combustion products such as indene and its alkylated derivatives. In this contribution we identify the RSR intermediates, the 2-hydroxy-indan-1-yl radical © 2012 American Chemical Society
Figure 2. Hydroxyl-containing radicals observed in this work and their discharge precursors. Received: May 20, 2012 Revised: July 3, 2012 Published: July 6, 2012 7906
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Figure 3. R2C2PI spectra recorded while monitoring m/z 133 after the discharge of (a) 2-indanol, (b) 1-indanol, and (c) indene. The signal observed from the 2-indanol discharge is ∼10 times stronger than those of the other precursors. Here, these spectral features are assigned to be carried by the 2-hydroxy-indan-1-yl radical (2HI1R).
slewing the grating of a 0.75 m monochromator operated with slit widths of 200 μm, corresponding to a spectral bandpass of approximately 10 cm−1. Reported emission frequencies are expected to be accurate to within 1 cm−1. The experiment was conducted at a repetition rate of 10 Hz. 2DF allows for the simultaneous mapping of the ground and excited states of a system. In essence, LIF and DF are performed in one experiment.12,13 2DF spectra were recorded using a Princeton Instruments SpectraPro 2300i Acton Series spectrometer (300 mm focal length, f/3.9). The fluorescence emission from 2H2MI1R was dispersed in wavelength across a 1024-pixel intensified charge-coupled device (iCCD) array exposed for 20 laser shots, producing a one-dimensional (1D) spectrum of intensity versus emission wavelength over the wavelength range of 388−660 nm (∼25770−15150 cm−1, respectively) using 25 μm slits. A 2D map of emission versus excitation was produced by stepping the laser between 472 and 440 nm (∼21180 and 22730 cm−1, respectively).
They are identified by the synergy of several spectroscopic techniques and computational analysis. The ground-state vibrational structure and vibronic characteristics of the radicals are presented and discussed, and two-dimensional fluorescence (2DF) spectroscopy12,13 is used to disentangle the various discharge products generated in a 2-methylindene/argon discharge.
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EXPERIMENTAL SECTION A pulsed discharge nozzle (PDN), similar to that described by Endo and co-workers,14 was used to produce the species of interest. For the 2-hydroxy-indan-1-yl radical (2HI1R), a precursor, indene (Fluka, 95%, 1.9 Torr, 298 K), was seeded in a backing gas (typically argon, 4 bar, 298 K) and expanded into the vacuum chamber. A voltage of −1.5 kV for a 30 μs duration was applied to the outer electrode of the PDN through 2.5 kΩ of resistance. For the 2-hydroxy-2-methylindan-1-yl radical (2H2MI1R), 2-methylindene (Aldrich, 98%, 2.0 Torr, 330 K) was seeded into a carrier gas (argon, 3 bar, 298 K) and likewise 2-ethylindene served as a precursor for the 2-ethyl-2-hydroxy-indan-1-yl radical (2E2HI1R). Hydrocarbon radical products were interrogated with the output of a XeCl excimer laser-pumped dye laser circulating coumarin 460. Ionizing radiation for the measurement of the resonant twocolor two-photon ionization (R2C2PI) spectra was provided with the fourth harmonic of a Nd:YAG laser and the 193 nm output of a GAM EX250 excimer laser for the 2-hydroxy-indan1-yl radical and its alkylated derivatives, respectively. The photoionization spectrum of 2H2MI1R was obtained by scanning the frequency-doubled output of a Quantel Nd:YAG-pumped dye laser as the second, ionizing photon. For LIF measurements, the induced fluorescence was imaged by a quartz lens onto the entrance slit of a Spex Minimate 0.25 m monochromator mounted to the vacuum chamber at 90° to the laser beam. The monochromator, equipped with horizontal 5.0 mm entry and exit slits, was tuned to 500 nm and used as a bandpass filter to minimize scattered laser light and discharge afterglow. The dispersed fluorescence (DF) was detected with a photomultiplier tube. The DF spectrum was measured by fixing the pump laser frequency at the radical’s origin band while
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RESULTS AND DISCUSSION Identification of the 2-Hydroxy-indan-1-yl Radical. In our previous work on the 1-indanyl radical15 (1IR), we reported the R2C2PI excitation spectrum of a species carried by m/z 133 that was observed from the discharge of indene (m/z 116). A mass difference of 17 amu heavier than the precursor suggests that the addition of the hydroxyl radical, •OH, to indene may have taken place. Such a species may form in the electrical discharge whereby traces of water provide the •OH source. Significant production of •OH in a corona discharge has been observed for experiments for which water concentrations were as low as 100 ppm.16 Alternatively, indanol may be present as an impurity formed by the hydration of indene exposed to humid air. In order to determine the degree of impurity present in the indene sample, characterization by 1 H NMR was undertaken. Figure S1 (see Supporting Information) shows the 200 MHz 1 H NMR spectrum of the indene sample used for the indene discharge experiment. This revealed no evidence of impurities represented by either 1- or 2-indanol, which implies generation 7907
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Figure 4. Emission observed from an indene/argon discharge. Spectrum (c) shows the dispersed fluorescence from the discharge afterglow. Spectrum (b) shows the dispersed fluorescence after 21364 cm−1 origin band excitation without subtraction of the discharge emission. Spectrum (a) shows the dispersed fluorescence after subtraction of the discharge emission. Here, the laser-induced emission features are determined to be carried by the 2-hydroxy-indan-1-yl radical. Assignments for spectrum (a) are given in Table 1. Large peak labels denote modes assigned to single quanta, and small labels indicate combination bands. See Figure S3 of the Supporting Information for assignment of the discharge emission features.
133 spectrum, whereas a 1-indanol precursor should yield little or no signal. The results of substitution of the indene precursor for 1- and 2-indanol are presented in Figure 3. Discharge of 2indanol results in an emphatic 10-fold m/z 133 signal increase, whereas the 1-indanol discharge results in signal levels similar to that observed for indene. The origin band (000) lifetime was measured by R2C2PI and fluorescence and was found to be 1.38 and 1.44 μs, respectively (see Figure S2 of the Supporting Information), which is consistent with other benzylic radicals.20 It is also worth noting that significant production of the 1phenylpropargyl radical (m/z 115), identified by Reilly et al.17,21 to be a dominant fluorescent aromatic-benzene discharge product and a significant discharge product of an indene discharge,15 was also measured from the 2-indanol discharge. This further underlines the importance of the 1phenylpropargyl radical in energized hydrocarbon environments. The DF spectrum of an indene discharge after excitation of the m/z 133 origin band, recorded with 0.4 mm slits, is presented in Figure 4. Spectrum (c) shows the background fluorescence emanating from the discharge afterglow, hence not induced by the excitation laser. As can be seen in Figure S3 of the Supporting Information, most of these features can be assigned to emission from Ar+ and C2 species. Spectrum (b) in Figure 4 shows fluorescence induced by the excitation laser without subtraction of the discharge emission, and spectrum (a) shows the signal due to laser-induced fluorescence after subtraction of the discharge emission. Several of the laserinduced emission features are possibly coincidental with the discharge emission, causing occlusion of these spectral features. As such, the low-frequency modes lying at 218 and 318 cm−1 are less certain to be due to emission from the m/z 133 carrier. The region containing these features is enlarged. Assignment of the 2-hydroxy-indan-1-yl radical as the m/z 133 carrier should be reflected by a close match between the observed ground-state vibrational frequencies with the scaled (×0.97) density functional theory (DFT)-calculated frequencies, as has previously been shown to be successful for other
of C−O bonds in the discharge environment. This is further evidenced by substitution of the 95% indene sample with ≥99% high-purity indene. This test revealed the presence of the m/z 133 signal in a proportion to the 1-indanyl radical similar to that observed for the 95% indene discharge. Therefore, the source of the OH is likely to be water present in the atmosphere as evidenced by attempts to measure dischargenascent fluorescence under completely dry conditions followed by the introduction of air. The dry conditions showed only minimal fluorescence due to the putative hydroxylated radical signal but a significant gain after air addition. Indeed, using laboratory air (1 atm) as the backing gas produced significant amounts of the radical in question and the phenylpropargyl radical.17 •OH addition to indene could plausibly occur across the aromatic π bonds in the phenyl system or across the π bond in the five-membered ring. Baulch and co-workers report the reaction rate of •OH with indene at 295 K to be about 2 orders of magnitude larger than the reaction rate of the •OH addition to benzene. 18 Furthermore, the •OH/indene reaction exhibited, at 295 K, a rate constant similar to those measured for the •OH addition to styrene and other polyalkenes. These results imply that the dominant addition reaction proceeds according to the •OH addition to the π bond in the fivemembered ring. These results have also been confirmed by more recent studies.19 Just as the •OH addition to isoprene has been found to be dominated by its resonance-stabilized intermediates (Figure 1), it could be expected that indene will behave in the same manner. Addition to carbon 1 (see Figure 2) would result in a structure with an isolated carbon 2 radical site that is not resonance stabilized. Conversely, •OH attack on carbon 2 will generate the benzylic indanyl radical resonance form with the radical site residing on carbon 1. The similarity of this system to the 1-indanyl-radical chromophore is evidenced by the m/z 133 origin band lying within just 1% of the 1-indanyl radical (2HI1R, 21364 cm−1; 1IR, 21159 cm−1). Substitution of the indene precursor for 2-indanol should, therefore, significantly increase the signal of the observed m/z 7908
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Table 1. Comparison of Experimentally Determined and Calculated [Scaleda B3-LYP/6-311++G(d,p)] Ground-State Frequencies of the 2-Hydroxy-indan-1-yl Radicalb
a
exptl
calcd
assignment
exptl
calcd
assignment
exptl
calcd
assignment
0 218 318 505 546 581 656 681 720 757 803
− 218 312 498 540 573 642 675 707 746 783
000 [4901] [4701] 4301 4201 4101 [4001] [3901] 3801 [3701] 3501
1015 1098 1157 1220 1271 1312 1376 1568 2929 2948 3058
1020 1081 1165 1215 1274 1304 1367 1553 2942 2958 3064
2701 2601 2201 2001 1801 1701 [1501] [1001] [901] [801] [601]
3075 3082 1434 1601 1946 2019 2290 2378 2782 3142
3093 3105 1414 1630 1922 2030 2290 2368 2768 3106
[301] [201] [3802] [3502] [38012001] [35012001] [38011001] [35011001] [20011001] [1002]
Frequencies scaled by 0.97. See text. bTentative assignments are enclosed in brackets.
Figure 5. Relaxed scan of the hydroxyl torsion of the 2-hydroxy-indan-1-yl radical calculated with B3-LYP/6-311++G(d,p). The lowest-energy wave functions for the one-dimensional motion are indicated.
frequency of the O−H stretch at 3687 cm−1 lies outside of the scan region. A similar ground-state vibrational structure is observed for the 1-indanyl radical (1IR).15 In the 1IR DF spectrum, the strongest bands lying at 529, 583, 700, 794, 1228, and 1570 cm−1 are found to correspond to the strongest bands in the present spectrum found to lie at 546, 581, 720, 803, 1220, and 1568 cm−1, respectively. A comparison of the vibrational modes assigned to these bands in both 1IR and 2HI1R demonstrates almost identical characteristic displacements represented by inplane distortions of the carbon skeleton. Similarly, there is evidence in both spectra of the C−H stretching vibrations near 3000 cm−1. The present DF spectrum also shows similarities to the emission dispersed from the origin of 2-indanol (the closedshell precursor to 2HI1R) in a helium buffer gas recorded by Das and co-workers.27 They observed strong features at 719, 809, and 1208 cm−1 as well as a strong low-frequency mode at 90 cm−1 assigned to a vibration which involved puckering of the five-membered ring. They calculated this mode to be 93 cm−1 by B3-LYP/6-311G(d). Indeed, a disruption of the 1-indanylradical Cs symmetry by the addition of the hydroxy group allows for coupling of the electronic excitation to the out-ofplane modes for which several low frequencies are calculated,
RSRs.17,22,23 Calculations were carried out using Gaussian 03.24 DFT using the B3-LYP functional25,26 and a 6-311++G(d,p) basis set were used to obtain energies, ground-state geometries, and normal-mode frequencies. We found that the calculated vibrational frequencies demonstrate a mean absolute deviation (MAD) of 10 cm−1 from the experimental values. This close match allows for the assignment of the observed vibrational frequencies, which are presented in Table 1. The DF spectrum shows strong Franck−Condon activity for the features at 720, 803, 1220, and 1568 cm−1 assigned to ν38, ν35, ν20, and ν10, respectively. Despite the low signal to noise of the spectrum, there is evidence of bands that may be assigned to combinations of the these dominant modes. Indeed, we see weak bands represented by two quanta of ν38, ν35, and ν10 as well as combinations of single quanta of these modes for those features assigned to 38011001 and 35011001. There is also evidence of combinations of single quanta of modes ν38 and ν35 with ν20. Since these bands are very weak, their assignments are considered tentative, as are the assignments of modes ν49, ν47, ν40, and ν39. The five high-frequency bands observed near 3000 cm−1 may be assigned to the C−H stretching modes which are observed to be quite strong in this spectrum; the strongest of these, assigned to ν2, represents the C−H stretch of the hydrogen at the principal radical site. The scaled-calculated 7909
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Figure 6. R2C2PI excitation spectra recorded while monitoring m/z 147 or 161 reported here to be carried by the 2-hydroxy-2-methyl-indan-1-yl radical (2H2MI1R) and the 2-ethyl-2-hydroxy-indan-1-yl radical (2E2HI1R). The same excitation spectrum for 2H2MI1R is observed by LIF. Those LIF bands not observed in R2C2PI are indicated with * and ○ and are reported to be carried by the 1-phenylpropargyl radical (1PPR) and the 2methyl-indan-1-yl radical (2MI1R), repectively. The symbol ? indicates a band for which the carrier identity is unknown.
including a five-membered ring puckering mode, characterized by out-of-plane motion of the 2-carbon and the hydroxy group, ν51, at 38 cm−1. The low resolution of the present spectrum prevents clear analysis of this region; however, the observed shoulder structure of the 000 band peaking at 40 cm−1 (see Figure 4, *) is possible evidence for the calculated ν51 mode. The excitation spectrum (Figure 3) shows evidence of one and two quanta of ν51 lying at 39 and 78 cm−1, respectively. However, the excited state also shows several other bands in the first 200 cm−1 from the origin that are not predicted by the ground-state frequency calculations. The next lowest frequencies, ν50 and ν49, are calculated to be 168 and 225 cm−1, respectively, which might plausibly account for the features at 141 and 182 cm−1, but this fails to account for the bands at 51, 63, 95, 132, and 172 cm−1. A comparison of the three spectra in Figure 3, generated from the three precursor molecules, reveals the presence of the 51 and 95 cm−1 bands in (a) and (b), but not (c). H loss from carbon 1 of 1-indanol will yield a benzylic radical (1-hydroxy-indan-1-yl) that likely exhibits a spectrum in this region. This radical is not generated from indene by OH radical addition but requires a further 1,2-hydride shift. It is noted that the potential energy surface for the puckering mode is likely to be complicated, and in 1-indanol, it is calculated to be a double minimum.27 A change in the potential of the excited state would give rise to additional bands. Another
possible explanation for this low-frequency structure in the excited state is the existence of conformational isomers. The presence of conformational isomers in the jet-cooled R2C2PI spectrum of the parent, 2-indanol, in an argon buffer gas, arising from a rotation about the hydroxyl group, has been reported.27,28 A relaxed scan of this coordinate was achieved by freezing the dihedral angle defining the hydroxyl rotational orientation and stepping this angle between 0° and 360° in 10° increments. This resulting potential energy surface generated by this treatment is presented in Figure 5. The two lowestenergy conformers are found to be those at −60° and +50° with the former stabilized by 46 cm−1 and a small 170 cm−1 barrier to isomerization. A third conformer, higher in energy by 320 cm−1, is found at −150°, although this conformer shows only a very small 5 cm−1 barrier to isomerization to the −60° conformer. However, the numerical solution of the Schrödinger equation for this motion reveals that the second vibrational level already transcends the barrier. More detailed experiments, such as spectral hole burning, will be required to clarify the genesis of these unassigned peaks. Despite the difficulty in accounting for this low-frequency structure, the other arguments presented here make a strong case for the assignment of the 2-hydroxy-indan-1-yl radical as the m/z 133 spectrum carrier. The evidence for the related systems, the 2-hydroxy-2methyl-indan-1-yl radical and the 2-ethyl-2-hydroxy-indan-1-yl 7910
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radical, provides further proof for the •OH addition to indene and its substituted analogs. Identification of the 2-Hydroxy-2-methyl-indan-1-yl Radical. Just as the •OH addition was observed in an indene discharge, it could be expected that the same addition process should occur in the discharge of the methyl- and ethylsubstituted indenes, 2-methylindene and 2-ethylindene. Indeed, the R2C2PI experiments probing discharges of these precursors revealed strong excitation spectra carried by mass channels m/z 147 and 161, a difference of 17 amu from the precursors. The 200 MHz 1H NMR spectrum of 2-methylindene showed an absence of a contaminant which would give rise to the observed m/z 147 spectrum. The measured excitation spectra of the m/z 147 species are presented in Figure 6. These spectra were observed from the discharge products of 2-methylindene in the wavelength range 473−442 nm (∼21150−22700 cm−1, respectively). The R2C2PI spectrum shows a dominant origin band at 21337 cm−1 and two strong low-frequency bands at 21 and 49 cm−1. Beyond 200 cm−1, the number of bands increases significantly. This is most apparent in the regions between 350−500 cm−1 and 650−900 cm−1 relative to the origin which show two dense envelopes of bands with typical separations of ∼10 cm−1. The 2H2MI1R spectrum observed by LIF is reflected below the R2C2PI spectrum in Figure 6. It demonstrates the same features, albeit with a different intensity profile, indicating a saturation of the R2C2PI spectrum. The LIF spectrum demonstrates that the features carried by m/z 147 represent the dominant fluorescent species which absorbs in this wavelength region. The LIF spectrum also contains a number of bands not observed in R2C2PI and a broadening of many of the R2C2PI band features caused by an overlap of bands due to the fluorescence from other species. The band features unique to the LIF spectrum indicate emission from species not carried by the m/z 147 mass channel. These are marked ○, *, and ?. The measured excitation spectrum of the m/z 161 species is presented in Figure 6. The added conformational flexibility of the ethyl group complicates the spectrum, with at least two dominant conformers present. It is not analyzed here, beyond noting that the wavelength region is consistent with the assignment to the 2-ethyl-2-hydroxy-indan-1-yl radical. It remains to be seen whether the spectrum obtained is due in part, or whole, to another isomer with m/z 161, but the identification of the 2-hydroxy-indan-1-yl radical and the 2hydroxy-2-methyl-indan-1-yl radical argues for the formation of the 2-ethyl-2-hydroxy-indan-1-yl radical under these conditions. In order to aid in the identification of the carriers of the observed emission features and to disentangle the m/z 147 spectrum from the features not carried by m/z 147, the 2methylindene discharge was probed with 2DF spectroscopy. Figure 7 shows the excitation and emission features observed from the discharge of 2-methylindene in the region 440 nm ≤ λexc ≤ 474 nm (∼21190−22730 cm−1) and 388 nm ≤ λem ≤ 660 nm (∼25770−15150 cm−1) plotted relative to the dominant emission at 21337 cm−1, which is determined to be the origin. High-resolution emission (DF) and excitation (LIF) spectra are inset on the horizontal and vertical axes, respectively. The inset DF spectrum represents emission after excitation of the origin transition and therefore best matches the dominant emission in the 2DF between 0 and 1600 cm−1. Comparison of the high-resolution inset spectra with the 2DF features demonstrates that all of the dominant features are carried by the m/z 147 species. Almost all of the observed
Figure 7. 2DF spectrum observed from the discharge of 2methylindene in the wavelength ranges 440 nm ≤ λexc ≤ 474 nm (∼21190−22730 cm−1) and 388 nm ≤ λem ≤ 660 nm (∼25770− 15150 cm−1). Both emission and excitation values are plotted in cm−1 relative to the vibrationless levels in the destination state. The top and right borders show the emission and excitation spectra recorded by high-resolution DF and LIF, respectively. The red strip indicates transitions for which vertical emission is dominant. A close-up of the region cordoned by the dashed box is presented in Figure 8.
strong features show emission at the origin wavelength (21337 cm−1). This indicates that these emission features are carried by a common absorbing species and indicates little change in the geometry of the ground and excited states. These features lie along the diagonal (red) line superimposed on the spectrum. Some of the emission features do not exhibit a strong band on the diagonal line. These features either represent vibronic transitions of the m/z 147 carrier, for which there is a significant change in the excited-state frequency, or represent emission from another carrier. The most prominent of these features is cordoned within the dashed box superimposed on the 2DF spectrum (see Figure 7). For example, taking the feature corresponding to excitation at ∼600 cm−1 with emission centered at ∼1000 cm−1, we find no according feature with that relative intensity (relative to 000) in the high-resolution inset LIF spectrum (vertical axis). This indicates that this feature is caused by emission from another carrier present in the discharge. Closer inspection of this cordoned region presented in the left panel of Figure 8 reveals a number of features carried by other species present in the discharge. Just as the emission features common to m/z 147 are found to lie on the diagonal line (see Figure 7), emission features common to other carriers present in the discharge are found to exhibit the same behavior, but on different diagonal lines owing to their unique positions of origin. From this, it can be seen that three unique species are found to be emitting in this region indicated by the colored diagonal lines (Figure 8). Considering that the 2-methylsubstituted 1-indanyl radical, the 2-methyl-indan-1-yl radical (2MI1R), was observed to absorb in this region with R2C2PI,29 it can be expected to fluoresce in this region like its 1-indanylradical counterpart. Indeed, the features lying on the blue diagonal strip are found to match the R2C2PI excitation 7911
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Figure 8. The left panel shows a close-up of the cordoned region in Figure 7 within which multiple species are found to fluoresce. The colored diagonal strips indicate the emission from three unique carriers assigned to the 2-methyl-indan-1-yl radical (2MI1R, blue) and the 1-phenylpropargyl radical (1PPR, gray). The colored spectra lying on either axis show the R2C2PI spectra recorded for these species. The right panel shows the 1D emission spectra extracted from the 2DF spectrum averaged (colored) and raw (light gray). Beneath 2H2MI1R and 1PPR21 are the equivalent spectra (black) recorded with high-resolution DF.
Figure 9. DF spectrum of the 000 band of the 2-hydroxy-2-methyl-indan-1-yl radical (2H2MI1R). Assignments are given in Table 2. Large peak labels denote modes assigned to single quanta; small labels indicate combination bands, and small labels in parentheses indicate combination bands built on the 6010 mode.
setup) and the wide slit settings (25 μm), the general structure of the high-resolution m/z 147 origin DF is clearly reproduced as seen by comparing the red spectrum (1D emission) with the black spectrum directly underneath it (2H2MI1R 000 DF). The gray 1D emission spectrum represents an average of six horizontal slices centered at ∼580 cm−1, which correspond to emission from the 21924 cm−1 band of 1PPR. The black spectrum directly beneath it is adapted from Reilly et al.21 It shows the high-resolution DF of the most prominent 1PPR features assigned to 2101 in Fermi resonance with the combination band 39202910. The blue spectrum represents an average of six horizontal slices centered at ∼560 cm−1, which correspond to emission from the 21975 cm−1 band of 2MI1R. The weakness of 2MI1R fluorescence prohibited the measurement of useful DFs for this system. A comparison of the complete 1PPR and 2MI1R spectra with the high-resolution LIF spectrum of a 2-methylindene discharge allows for the assignment of the carriers of the emission features not carried by m/z 147. These features are indicated in Figure 6 by *
features measured for 2MI1R, as demonstrated by its superimposition on the right axis of the left panel of Figure 8, colored blue. Following the strongest bands of the R2C2PI 2MI1R spectrum horizontally demonstrates their concomitance with the emission features in the 2DF spectrum. The features indicated by the gray strip are found to match with the 1phenylpropargyl radical (1PPR) spectrum measured by R2C2PI and inset on the left axis of the left panel of Figure 8, colored gray. The identity of the present assignment for carriers of these features is further evidenced by inspection of the 1D emission spectra extracted from the 2DF data. These respectively colored spectra are shown in the right panel of Figure 8. The emission spectra are produced by averaging horizontal data slices of the emission region, shown in Figure 7, for a given excitation wavelength. For example the red spectrum (Figure 8, right panel) is the average of eight horizontal slices centered at the origin excitation wavelength, 21337 cm−1. Despite significant broadening caused by the limited resolution of the spectrograph (a resolution limited by the experimental 7912
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(1PPR) and ○ (2MI1R). The only feature not accounted for by other species is the weak shoulder structure blue of the origin, marked with a question mark. This structure is probably due to hot bands from m/z 147, as will be discussed. Ground and Excited-State Assignments. Figure 9 shows the DF spectrum of the origin band of the species carrying m/z 147 in R2C2PI. A lifetime of 1.384 ± 0.005 μs was measured for the excited state, consistent with a benzylic carrier.20 A comparison with the calculated ground-state vibrational frequencies shows good agreement with the emission frequencies, as demonstrated in Table 2, supporting the
assignment of the 2-hydroxy-2-methyl-indan-1-yl radical (2H2MI1R) as the carrier of this spectrum. The dominant features in the DF are represented by bands at 22, 586, 637, 778, 1207, 1238, 1265, 1384, and 1569 cm−1. The closest-scaled calculated frequencies are found to be 33, 572, 626, 766, 1183, 1222, 1252, 1360, and 1553 cm−1. The observed frequencies are therefore assigned to single quanta of the vibrational modes associated with those calculated frequencies. Although 2H2MI1R is described by the C1 point group, the geometry of the ring system in which the absorbing chromophore resides favors in-plane vibrational modes coupling to the electronic state. This is reflected in the assignments just described, where all of those dominant emission bands are characterized by vibrational modes which are largely in-plane. This is further exemplified by inspection of the energy region between the strong bands at 637 (ν46) and 778 cm−1 (ν42) which is devoid of any prominent features (except the 659 cm−1 band, which will be analyzed below). The vibrational modes associated with the energies of this region correspond to the distinctly out-ofplane modes ν45−ν43. Many of the weaker bands could also be tentatively assigned to single quanta modes which are mostly in-plane. There is evidence for combination bands of the strongest features at 586, 637, 778, and 1569 cm−1 which correspond to modes ν47, ν46, ν42, and ν12, respectively, adding surety to their assignments. For example, ν42 is found to be built upon modes ν47, ν46, and itself with features lying at 1365, 1415, and 1556 cm−1, respectively. There is also evidence at higher frequencies (see Figure 9 inset) for combinations of these three modes with ν12 and itself, indicated by the bands at 2156, 2206, 2347, and 3140 cm−1 leading to assignments for these bands of 12014701, 12014601, 12014201, and 1202, respectively. The structure between 2300 and 3100 cm−1 could also plausibly be assigned to multiple combinations of these strong bands, with the bands between 1200 and 1400 cm−1. The most intriguing progression-forming mode in this spectrum is the very low-frequency mode at 22 cm−1. This mode corresponds to an out-of-plane puckering of the molecule pivoting on carbons 1 and 3 of the five-membered ring. This is the same mode that was discussed for the 2-hydroxy-indan-1-yl
Table 2. Comparison of Experimental and Calculated [Scaleda B3-LYP/6-311++G(d,p)] Ground-State Frequencies of the 2-Hydroxy-2-methyl-indan-1-yl Radical (2H2MI1R)
a
exptl
calcd
assignment
exptl
calcd
assignmnet
22 262 280 293 329 510 586 637 778 823 881 1011 1090 1107 1155 1207 1238 1265 1315 1335
33 246 277 305 323 487 572 626 766 800 865 1000 1081 1130 1142 1183 1222 1252 1300 1312
6001 5701 5601 5501 5401 5001 4701 4601 4201 4101 3801 3301 3001 2901 2801 2501 2401 2301 2201 2101
1384 1432 1442 1476 1556 1569 435 608 659 904 1026 1036 1286 1365 1407 1415 2156 2206 2347 3140
1360 1413 1433 1454 1534 1553 432 605 659 868 1004 1016 1285 1338 1393 1392 2125 2179 2319 3106
2001 1801 1701 1401 1301 1201 5802 47016001 46016001 5202 4902 4802 23016001 42014701 20016001 42014601 12014701 12014601 12014201 1202
Calculated frequencies scaled by 0.97.
Figure 10. Close-up of the origin region of the excitation spectrum of the 2-hydroxy-2-methyl-indan-1-yl radical demonstrating the progression in ν60. 7913
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Figure 11. Photoionization spectrum recorded while monitoring m/z 147 after excitation of the 21331 cm−1 origin. Here, this spectrum is assigned to be carried by the 2-hydroxy-2-methyl-indan-1-yl radical (2H2MI1R). Illustrated on either side of the onset are the relaxed geometries for the neutral and cation of 2H2MI1R. The radical-site carbon and hydrogen atoms in each model have been removed for clarity.
to estimate the experimental adiabatic ionization energy. As was demonstrated in our previous work,23 this level of theory consistently underestimates the experimental AIE by an average of 0.08 eV. As such, an AIE of 6.82 eV is not unreasonable. Unlike other RSR photoionization spectra,15,23,30 this spectrum demonstrates a significantly slower onset showing a steady rise in signal over approximately 0.06 eV. This is to be compared with those other systems which exhibit a rapid onset of signal, reaching their plateau within ∼0.01 eV. The gradual onset observed for the present spectrum likely indicates a marked change in geometry for the cation compared to the excited state neutral. This is evidenced in the difference between the optimized geometries for both the ground state neutral and the cation of 2H2MI1R . Attempts to optimize the D1 geometry were unsuccessful, but the origin dominance of its spectra indicates a geometry similar to that of the ground state. Illustrations of these relaxed geometries appear on either side of the curve onset in Figure 11 (the radical-site carbon and hydrogen atoms have been omitted for clarity). Conversely, similar treatment of the neutral and cations of the 1-indanyl radical, the 2-methyl-indan-1-yl radical, the inden-2-ylmethyl radical, and the trans-1-phenylallyl radical all showed only minor geometric changes between the neutral and cation geometries. This slow onset was also observed by Sebree et al. for the photoionization threshold measured for the benzylallenyl radical.31 They calculate a significant change in the geometry of the cation compared with that of the neutral for both the D0 and D1 states.
radical which was suggested to be obscured in the lower resolution of that spectrum. In the present spectrum we found that this mode, ν60, built upon all of the strong features (ν47, ν46, ν42, and ν12) and itself, as indicated by the bands at 44, 608, 659, 802, and 1592 cm−1, respectively. There is also evidence for ν60 built upon some of the weaker bands as indicated in Table 2. This vibrational mode is the same mode described by Das et al.27 and He et al.28 to be progression forming in their spectra of 2-indanol.27 This mode is also observed to be progression forming in the excited state as depicted in Figure 10. As many as four quanta of ν60 are seen built upon the origin, leading to the assignments of 6010, 6020, 6030, and 6040 for the bands at 21, 49, 81, and 116 cm−1, respectively, indicating significant (negative) anharmonicity for this mode in the excited state. Only two quanta of this mode are observed in the ground state, likely indicating a saturation in the excited-state spectrum. Hot bands of this progression can be seen starting at −22 cm−1, upon which one to four quanta of ν60 are built. These are indicated in Figure 10 in the top, left close-up region. This assignment would make 6011 coincident with the origin, explaining the emission 22 cm−1 to the blue of the origin seen in Figure 9. Almost buried in the noise, there is possible evidence of the hot bands for 6003, 6004, and 6005 lying at −62, −92, and −120 cm−1, respectively. However, no peak with similar intensity is observed for 6002. Taking the difference of 5 cm−1 between two quanta of ν60 in the ground and excited state makes 6012 coincident with 6001 and puts 6022 at 5 cm−1. There is evidence for 6022 in the very weak shoulder, denoted ?, centered at 5 cm−1 blue of the origin. This might explain the origin shoulder observed to be more prominent in LIF, denoted ?. This is also consistent with the typically hotter molecules found in the unskimmed molecular beam probed in fluorescence. The multitude of higher-frequency features in the excited state are not considered in the present study. A further piece of evidence implicating 2H2MI1R as the carrier of the m/z 147 spectrum is found in its photoionization spectrum presented in Figure 11. This spectrum was recorded while monitoring m/z 147 after excitation of the 21331 cm−1 origin band. The adiabatic and vertical IEs of 2H2MI1R were calculated by B3-LYP/6-311++G(d,p) to be 6.74 and 6.88 eV, respectively. The slow onset of the ionization makes it difficult
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CONCLUSIONS The chemical assignment and spectroscopy of oxygencontaining 1-indanyl-type radicals have been presented. In three separate experiments, a spectrum is observed at 17 amu to higher mass than the precursor molecule. These species have been argued to form from the addition of •OH to indene and its alkylated derivatives, 2-methylindene and 2-ethylindene, across the π-bonded carbons to the 2 site. The experiments underline the ease with which OH radicals can add to aromatic alkenes and demonstrate the power of synergistic spectroscopies in identifying complex radical species. The measured 7914
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(15) 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. (16) Mikoviny, T.; Skalny, J. D.; Orszagh, J.; Mason, N. J. J. Phys. D: Appl. Phys. 2007, 40, 6646−6650. (17) Reilly, N. J.; Kokkin, D. L.; Nakajima, M.; Nauta, K.; Kable, S. H.; Schmidt, T. W. J. Am. Chem. Soc. 2008, 130, 3137−3142. (18) Baulch, D. L.; Campbell, I. M.; Saunders, S. M.; Louie, P. K. K. J. Chem. Soc., Faraday Trans. 2 1989, 85, 1819−1826. (19) Kwok, E. S. C.; Atkinson, R.; Arey, J. Int. J. Chem. Kinet. 1997, 29, 299−309. (20) Fukushima, M.; Obi, K. J. Chem. Phys. 1990, 93, 8488−8497. (21) Reilly, N. J.; Nakajima, M.; Gibson, B. A.; Schmidt, T. W.; Kable, S. H. J. Chem. Phys. 2009, 130, 144313−14324. (22) 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. (23) Troy, T. P.; Chalyavi, N.; Menon, A. S.; O’Connor, G. D.; Fückel, B.; Nauta, K.; Radom, L.; Schmidt, T. W. Chem. Sci. 2011, 1755−1765. (24) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condensed Matter and Materials Physics 1988, 37, 785−789. (27) Das, A.; Mahato, K. K.; Panja, S. S.; Chakrabortya, T. J. Chem. Phys. 2003, 119, 2523−2530. (28) He, Y.; Kon, W. J. Chem. Phys. 2006, 124, 204306−204313. (29) Troy, T. P.; Chalyavi, N.; Schmidt, T. W. In preparation. (30) Newby, J. J.; Liu, C.-P.; Müller, C. W.; James, W. H., III; Buchanan, E. G.; Lee, H. D.; Zwier, T. S. J. Phys. Chem. A 2010, 114, 3190−3198. (31) Sebree, J. A.; Kidwell, N. M.; Buchanan, E. G.; Zgierski, M. Z.; Zwier, T. S. Chem. Sci. 2011, 2, 1746−1754.
origin wavelengths provide the means to detect these species in more detailed experiments of direct relevance to atmospheric chemistry. The formation of the 1-phenylpropargyl radical from the discharge of 2-methylindene further underlines the importance of this radical in hydrocarbon chemistry and its potential role in the sooting process.
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ASSOCIATED CONTENT
S Supporting Information *
Auxiliary spectra of 1-indanol 1H NMR, 1-indanol excited-state lifetime, and 1-indanol discharge emission vs emission from Ar+ and C2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +61 2 93 51 27 81. Fax: +61 2 93 51 33 29. E-mail:
[email protected]. Present Address †
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Discovery funding figure (Grants DP0985767 and DP120102559). 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.
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