Feature Article pubs.acs.org/JPCA
Infrared Laser Spectroscopy of Mass-Selected Carbocations Michael A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States ABSTRACT: Small carbocations are of longstanding interest in mass spectrometry, organic chemistry and astrophysics, but there are few measurements of their spectroscopy in the gas phase. Of existing infrared measurements, few are available across the full range of IR frequencies. In new work described here, a pulsed-discharge supersonicnozzle ion source produces higher densities of carbocations at low temperatures (20−100 K). Mass-selected photodissociation spectroscopy and the method of rare gas “tagging”, together with new broadly tunable infrared OPO lasers, produce IR spectra for a variety of small carbocations including C2H3+, C2H5+, C3H3+, C3H5+, the tert-butyl cation, protonated benzene, and protonated naphthalene. Spectra in the frequency range 600−4500 cm−1 provide new IR data for these ions and evidence for the presence of coexisting isomeric structures (e.g., C3H3+ is present as both cyclopropenyl and propargyl). Protonated naphthalene has bands at 6.2, 7.7, and 8.6 μm matching prominent features in the interstellar unassigned infrared (UIR) emission spectra.
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INTRODUCTION
Unfortunately, the exact structures of carbocations have rarely been measured because spectroscopy on ions is extremely difficult. Infrared and NMR measurements have been reported in so-called “superacid” matrices, designed to stabilize the ion in a suitable solvation environment with a strong counterion.6−8,17 X-ray crystallography has been reported for some systems in these films;18 however, the effects of solvent and counterions are difficult to assess. Therefore, gas-phase experiments which eliminate these complications are more desirable. Unfortunately, ions produced under energetic conditions in plasmas or by electron impact excitation of gases are usually present in very low density in a complex mixture containing neutral precursors and a distribution of their fragmentation products. Hot ions have complex spectra, which are difficult to distinguish from those of neutrals occurring in the same wavelength regions. In the present work, these latter issues are addressed with improved experiments. A pulsed discharge in a supersonic expansion provides high densities of cold ions, which are isolated and studied in pure form with mass-selected infrared laser photodissociation spectroscopy. These methods provide new IR signatures for small carbocations and reveal the role of isomeric structures. Electronic spectroscopy has been successful for several hydrocarbon ions.19−21 For example, CH+ has a well-known electronic spectrum, and was one of the first molecules detected in space.19 However, vibrational spectra usually provide more direct structural information for polyatomic species. Previous vibrational spectroscopy on carbocations has employed infrared
Small organic cations known as carbocations are well-known as fragmentation products in the mass spectrometry of organic molecules.1−4 Indeed, these ions were detected in the earliest days of mass spectrometry.5 Carbocations are also known in organic chemistry as reaction intermediates, for example, in electrophilic aromatic substitution reactions.6−8 Ions such as CH5+ are used as reagents in chemical ionization mass spectrometry.9,10 Carbocations are components of interstellar gas clouds11−14 and planetary atmospheres,15,16 appearing as key players in reactions producing complex polyatomic molecules in space. Carbocations are absolutely fascinating from the standpoint of fundamental electronic structure and bonding. Many of these systems accommodate charge with bridging hydrogen or methyl groups, with two electron−three center (2e−3c) bonding, giving rise to so-called “nonclassical” structures. Others have more than the usual number of four connections to carbon, invoking the term “hypervalent”. “Classical” configurations are also found, having normal single, double, and triple bonds with ordinary connectivity, but the behavior of orbitals adjacent to a charge can introduce subtle effects such as hyperconjugation. All of these structural issues are extremely interesting, providing challenging problems for computational chemistry. It has been recognized for many years that even small carbocations may have more than one isomeric structure, including in some cases competing examples of classical and nonclassical structures.3,4 The structures actually present in any situation can depend on the ionization method and precursors used to produce the ion, as well as the intrinsic stability of the possible bonding configurations and the local environment. Carbocation structures therefore provide compelling targets for modern spectroscopy. © 2012 American Chemical Society
Received: September 11, 2012 Revised: November 6, 2012 Published: November 6, 2012 11477
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fingerprint region using Free Electron Lasers (FELs).56−59 The new experiments described here take advantage of gasphase ion production and mass-selected photodissociation measurements like those already described by the Lee group34,35 and those of Dopfer and Maier50−55 but use improved supersonic molecular beam cooling to produce higher densities of colder ions. Furthermore, new optical parametric oscillator (OPO) laser sources60,61 are employed, which cover a much wider range of infrared wavelengths. These developments allow significant improvements in carbocation spectroscopy. Here, these methods are applied to new infrared spectra for the ions C2H3+,62 C2H5+,63 C3H3+,64 C3H5+,65 the tert-butyl cation,66 protonated benzene,67 and protonated naphthalene.68
measurements of superacid films with counterions present,6−8 photoelectron spectroscopy of radicals,22−24 direct IR laser absorption in plasmas,25−30 rare gas matrix isolation/IR absorption,31,32 IR laser absorption in molecular beams with pulsed discharges,33 or electron impact or discharges followed by mass-selected infrared laser photodissociation.34,35 Each of these methods has disadvantages including low sample densities, high ion temperature, condensed-phase medium effects, or the limited laser tuning ranges available. Nevertheless, significant insights were obtained in these previous studies. Specific structures were identified for certain ions, and evidence for isomers and their specific resonances was obtained. In discharge tubes or plasmas, ions such as CH5+ and C2H3+ have been studied with high-resolution absorption spectroscopy.27,30 However, complications arise in these studies from overlapping transitions from neutral precursor species (methane, acetylene), which are present in much higher concentrations than the ions. Velocity modulation methods were developed to address this issue,36 and these methods have been successful for some ions.25−30,36 A rotationally resolved spectrum of CH5+ was reported by Oka and co-workers, but this spectrum has never been assigned.30 The Nesbitt group has obtained more recent measurements on this ion,33 prompting several theoretical studies of its complex vibrational dynamics.33,37−43 The broader range of the CH5+ IR spectrum was measured by Schlemmer and co-workers using the FELIX free electron laser and action spectroscopy based on vibrationally enhanced proton transfer to CO2.42 However, the bands in the spectrum were quite broad. High-resolution measurements have also been reported in the CH stretching region for C2H3+, but again the spectra were hot, causing significant difficulties in their assignment.27 The group of Maier has produced small carbocations by using a discharge/beam source, mass selecting the ions and depositing them in rare gas matrices, primarily for studies of electronic spectroscopy.21 However, infrared spectra were reported for species such as C3H3+ (propargyl and cyclopropenyl isomers).31 More recent matrix isolation studies by Y.-P. Lee and co-workers have employed solid para-hydrogen matrices for ions such as protonated benzene.32 Some of the most definitive spectroscopy measurements on carbocations have been laser photodissociation studies of massselected ions. Some earlier work employed electronic spectroscopy on these systems,20 but these methods were first applied in the infrared by Y. T. Lee and co-workers to ions such as C2H7+ and CH5+.34,35 The Lee group also first described the method of spectator atom “tagging” to enhance dissociation yields in the infrared,35,44 where covalent bond energies far exceed photon energies. By attachment of a weakly bound molecule such as H2 or a rare gas atom such as neon or argon to the ion of interest, the photodissociation efficiency could be enhanced with minimal perturbation of the spectrum. This tagging method is now used in many laboratories for studies of ion photodissociation spectroscopy.45−49 Because ions are produced in the gas phase and mass selected, these photodissociation measurements eliminate medium effects and overlap with neutral precursor spectra. These methods were employed by the Dopfer and Maier groups to study several carbocations in the CH stretching region.50−55 However, these studies were limited by the infrared lasers available, which until recently covered only the higher frequency region of the CH and O−H stretches. Additional experiments by these workers and others extended the measurements to the
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EXPERIMENTAL SECTION Carbocations are produced by a pulsed discharge synchronized with a pulsed-nozzle gas expansion.62−68 A General Valve (Series 9) provides pulses about 250 μs in duration containing an expansion gas such as helium or argon with a small partial pressure of a hydrocarbon precursor such as acetylene or ethylene. Sometimes a few percent of hydrogen is added to enhance protonation reactions via the formation of H3+. The total backing pressure of the expansion gas mixture is 5−10 atm. The discharge pulse of about 800−1000 V (negative) lasts 1−10 microseconds and is timed to occur in the middle of the gas pulse. The electrodes are either two needles positioned about 2 mm downstream from the opening of the gas valve, with about a 1 mm gap, or two coaxial rings (3−4 mm inside diameter) along the gas flow direction. Variation of the precursor, backing pressure, discharge timing, etc. produces a variety of cold carbocations. Ions traverse the molecular beam apparatus as a neutral plasma, with no acceleration or focusing fields employed until they reach the mass spectrometer. In a differentially pumped chamber at a point 40 cm downstream from the source, pulsed acceleration plates extract the cations into a reflectron time-of-flight mass spectrometer.69 In this device, ions are mass analyzed and size-selected for spectroscopy. Because of the transport through the machine in a plasma, the ion density produced by this source is higher than in many other experiments. The specific configuration of the pulsed discharge and supersonic nozzle causes the cooling to be quite efficient, producing a higher population density in the lowest few quantum states. However, the density after mass selection is still not great enough to allow direct absorption measurements. Photodissociation measurements are therefore required for these studies, but these are also problematic because hydrocarbon bonds energies are higher than the energies of the vibrational fundamentals to be probed. In principle, multiphoton excitation could be employed to accomplish dissociation, but this is not efficient with the 1−10 mJ/pulse infrared laser intensities available in our lab. To enhance photodissociation yields, we therefore employ the method of tagging,35,44−49 usually using argon atoms. To investigate possible perturbations induced by argon, computational studies using density functional theory or MP2 methods examine both the carbocation and its argon complex. Structural isomers are investigated not only in the hydrocarbon framework but also for different attachment sites for the argon tag atoms. For comparison of computed vibrational spectra to those measured, vibrational scaling factors are employed. In some cases, these are developed from comparison to the vibrations of 11478
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corresponding neutral molecules.70 In other cases standard scaling factors from the literature are employed.71 The tunable IR laser is a YAG-pumped optical parametric oscillator/amplifier (OPO/OPA; LaserVision) system. In its normal operating configuration, a KTP oscillator pumped at 532 nm and a KTA difference frequency generation (idler +1064 nm) amplifier provides tunable output in the 2000− 4500 cm−1 region with a line width of about 1 cm−1.60 In a second configuration, another stage of difference frequency generation in AgGaSe2 provides output in the 600−2300 cm−1 range.61
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RESULTS AND DISCUSSION Protonated Acetylene and Ethylene. C2H3+ is one of the smallest and most well-studied carbocations.27,72−89 Its two low-energy structures include protonated acetylene, which has the so-called “nonclassical” structure with a proton in the symmetric π position on the triple bond, and the vinyl cation, which has the classical HCCH2+ structure. Although the energy difference between these structures was hotly contested in earlier computational work,72−78 the protonated acetylene structure is found to be lowest in energy by all modern computational studies, lying 3.7−4.0 kcal/mol below the vinyl cation. Computations at the MP2 level indicate that the classical vinyl cation structure is a transition state, but higher level calculations show that the classical structure is a true minimum, albeit a shallow one, located behind a low barrier.81 Infrared spectroscopy in a plasma by Oka and co-workers studied the CH stretching region,27 finding evidence for the protonated acetylene structure, although the spectra were somewhat warm and difficult to assign. Line intensity alternation from nuclear spin statistics indicated the presence of only two equivalent hydrogens, suggesting dynamic scrambling probably caused by the elevated temperature. Later Coulomb explosion measurements disagreed with the Oka assignment, suggesting instead the vinyl cation structure.88,89 Millimeter wave experiments have also examined the spectrum, analyzing the tunneling splittings and relating these to the barriers for proton transfer.84−87 High-resolution spectroscopy at low temperature is highly desirable for this ion because of its anticipated astrophysical importance. IR spectra in regions other than the CH stretching would of course be valuable to provide further structural insight. An important feature of the protonated acetylene structure is the proton stretch vibration, which has been predicted by theory to occur near 2300−2400 cm−1.73,76,80 Contrasting with this, the vinyl cation should have a CC stretch at lower frequency and a quite different pattern of CH stretches. The expanded coverage of our IR-OPO laser system made it possible to measure the spectrum in this region. Figure 1 shows this spectrum,62 together with the spectra predicted by theory at the MP2(fc)/6-311++G(2p,2d) level for the protonated acetylene and vinyl cation isomers, each considered with and without the attached argon so that its influence can be evaluated. The most prominent feature in the experimental spectrum is the CH stretch at 3146 cm−1, in about the same position noted before by Oka.27 A rotational contour of this band indicates that the temperature is about 100 K. A second strong resonance not seen previously is detected at 2217 cm−1. This frequency is somewhat lower than that predicted previously by high levels of theory for the proton stretch of the nonclassical structure.73,76,80 However, as shown in the figure, tagging is predicted to induce a strong red shift on this
Figure 1. Measured infrared photodissociation spectrum of C2H3+Ar compared to the predictions of theory for the protonated acetylene versus vinyl cation structures, with and without argon.
particular vibration because the argon binds directly on the bridging proton. Although the agreement between theory and experiment is not perfect, it is clear that an argon-tagged species with the bridging proton can explain the spectrum. Additionally, the vinyl cation has no vibrations anywhere near this frequency. This spectrum therefore confirms that C2H3+ has the protonated acetylene structure. None of the vibrational bands predicted for the vinyl cation are detected. Presumably, because it lies behind a low barrier, even if this species were formed initially it could rearrange efficiently to the more stable nonclassical structure in the early stages of ion formation, when the conditions are warmer. At low temperature, therefore, the nonclassical structure of C2H3+ is expected. C2H5+, known as protonated ethylene or the ethyl cation, has structural issues similar to those of protonated acetylene, with classical and nonclassical (bridged proton) isomers.53,90−95 Again, the nonclassical isomer is predicted to be more stable. Dopfer and co-workers measured the infrared spectrum in the CH stretching region via mass-selected ion photodissociation.53 Two bands were detected and assigned to the symmetric and asymmetric stretches of the nonclassical structure. Our spectrum for this ion is shown in Figure 2, with comparison to theory (MP2(full)/aug-cc-pVDZ) for the classical and nonclassical isomers (both tagged with argon), where we again benefit from the extended tuning range of the new OPO laser system. This allows the spectrum to cover the region of the CH stretches seen before and that for the predicted bridging proton stretch. Bands marked with an asterisk are from an impurity of HN2+Ar, whose mass is the same as that of C2H5+Ar, and whose spectrum has been measured previously.96 The two CH stretching modes at 3032 and 3114 cm−1 are quite close to the positions reported by Dopfer and co-workers at 3037 and 3117 cm−1.53 The most interesting new data is the bridging proton stretch vibration, which occurs at 2158 cm−1. This confirms unambiguously that protonated ethylene, like protonated acetylene, has the nonclassical structure. The frequency of the proton stretch for protonated ethylene (2158 cm−1) is lower than that for protonated acetylene (2217 11479
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ported by Dopfer and co-workers in the CH stretching region.52 The bands detected in this latter study were broad with poor signal levels, but evidence was found for the presence of both isomeric structures. The IR spectrum of C3H3+ ions measured with argon tagging is shown in Figure 3.64 Bands are detected throughout the
Figure 2. Measured infrared photodissociation spectrum of C2H5+Ar compared to the predictions of theory for the nonclassical versus classical isomers of protonated ethylene.
cm−1). This is somewhat surprising because the proton affinity for ethylene is greater than that for acetylene (162.6 versus 153.3 kcal/mol, respectively).97 However, as discussed in our original paper,63 the proton stretch vibration in such bridged proton systems is not strongly correlated to the proton affinity values. Instead, it depends more on the hybridization of the host carbon atoms. Thus, proton stretching frequencies should increase for the series sp3, sp2, sp, carbons following the same trend seen for ordinary CH stretches. Other carbocations are also expected to have structures with bridging protons, and these kinds of systems will be interesting for future studies. Notable examples include protonated ethane (C2H7+)34,98,99 and the 2-butyl cation (C4H9+),100−102 which is a less-stable isomer on the global potential surface leading to the tert-butyl cation. Cyclopropenyl and Propargyl Cations. C3H3+ has a long history in mass spectrometry.3,4 This ion occurs frequently in electron impact ionization mass spectra as a fragment from many aromatic molecules.3,4 Two low-energy isomers are recognized corresponding to the cyclopropenyl cation, which is an aromatic ring, and the propargyl cation, which has the linear backbone HCCC+H2 structure. These structures have been investigated extensively by theory.103−109 The more stable cyclic species lies about 25 kcal/mol lower in energy than propargyl; the two structures are separated by an activation barrier for interconversion of about 50 kcal/mol. Ion reaction studies have explored the relative reactivity of the two isomers.110,111 The cyclopropenyl cation is believed to be present in interstellar gas clouds and is proposed to produce the cyclic C3H2 neutral seen widely with radio astronomy via the dissociative recombination reaction with electrons.11−14 The cyclopropenyl species was first isolated in a superacid film by Breslow and co-workers and studied with infrared measurements.112 Other previous vibrational spectroscopy measurements include an IR/Raman study of c-C3H3+ by Craig and co-workers,113 a photoelectron spectroscopy study of the C3H3 propargyl radical,24 and a matrix isolation IR study by Maier and co-workers on both isomers.31 In the first gas-phase data, mass-selected photodissociation spectroscopy was re-
Figure 3. Infrared photodissociation spectrum of C3H3+Ar compared to the spectra predicted by theory for the cyclopropenyl and propargyl cation isomers.
infrared, with a prominent feature at 1445 cm−1, another at 2077 cm−1, and a cluster of five bands in the CH stretching region. No signal is detected below 1000 cm−1, probably because the argon binding energy limits photodissociation here. The lower two traces in the figure show the spectra predicted by theory (DFT/B3LYP/cc-pVTZ) for the argon-tagged isomers of the cyclopropenyl cation (red) and the propargyl cation (blue). Because of its high symmetry, the cyclic species without argon is expected to have only one (degenerate) IRactive CH stretching band, and the propargyl ion should have three. However, when argon attaches to either of these ions, additional bands may result depending on the argon attachment site. In the fingerprint region, propargyl should have a strong acetylenic CC stretch near 2100 cm−1, and weaker bands in the 1000−1500 cm−1 region. Cyclopropenyl should have an asymmetric carbon ring deformation near 1300 cm−1. The most prominent experimental band at low frequency is that at 2077 cm−1, which corresponds well with the prediction of theory for the ν3 (A1) CC stretch of the propargyl cation. This and other distinctive bands (1445 cm−1: ν4 (A1) CH2 scissors) confirm that the spectrum is dominated by the propargyl cation. This makes sense, as the precursor used for this experiment was propargyl bromide. In other previous experiments, the ν3 vibration of propargyl cation was detected in a neon matrix at 2079.9 cm−1 by Maier and co-workers.31 Assignment of this to the propargyl cation was confirmed by the observation of a corresponding electronic transition near the position predicted by theory for this ion (the cyclic isomer has no low-lying excited states). Gilbert et al. reported this vibration at 2122 cm−1 resulting from a photoelectron spectrum of the propargyl radical, but this paper was later retracted because the data could not be reproduced.24a,b A more recent 11480
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then we tried to assign vibrations based on the frequency trends for these two isomers from theory. An additional complication is that the perpendicular-type vibration for the asymmetric stretch of propargyl should have a doublet structure (from Ktype sub-bands), and contour analysis suggested that the pair of bands (3075/3107 cm−1) centered at 3093 cm−1 might fit for this. One of the bands in the CH stretching region (3182 cm−1) grew in intensity together with the 1293 cm−1 band discussed above under “hot” source conditions and was therefore assigned to the asymmetric CH stretch of the cyclic species. However, none of these assignments were definitive. Our initial report on the isomers of C3H3+ stimulated new computational studies by the two groups of Botschwina and coworkers116 and Lee and co-workers.117 Botschwina performed a more careful study of argon attachment positions on the frameworks of both isomers at the explicitly correlated coupled cluster level, finding that our structures were not the lowest in energy.116 Argon attachment sites on CH, which we found, were local minima for both systems, but lower minima were found for argon binding alongside the linear propargyl structure and above the plane of the ring for the cyclopropenyl structure. Lee and co-workers performed a full anharmonic analysis of the vibrations for both isomers.117 Both groups agreed with our main vibrational assignments at lower frequencies (e.g., the 2077 cm−1 band for propargyl and the 1293 cm−1 band for cyclopropenyl) but questioned our assignments in the CH stretching region. However, neither was able to do any highlevel computational study to predict vibrational frequencies including the attached argon, which is how the experiment is done. In light of this new theory, it is apparent that the unexpected number of vibrational bands could arise from a combination of both C3H3+ structures, with more than one argon attachment isomer for either or both. Because of the scrutiny focused on our spectrum in the C H stretching region, we decided to remeasure this. Figure 4
photoelectron spectrum by Hemberger et al. reported this vibration at 1950 cm−1,114 although the spectrum was extremely noisy. Another recent photoelectron spectrum by Ng and coworkers produced an improved ionization potential for the propargyl radical, but no vibrational data for the cation,115a but a very new investigation with velocity map imaging detection of the electrons found this band at 2086 ± 15 cm−1.115b To investigate the possible presence of c-C3H3+, we looked for evidence of the carbon ring distortion predicted near 1300 cm−1. Initially, we were disappointed to find no band with reasonable intensity within 100 cm−1 of the prediction. However, we eventually found a band at 1293 cm−1 whose intensity varied substantially with the discharge conditions. The inset in Figure 3 shows the spectrum here when the discharge is hotter than before, with a higher discharge voltage. The band at 1293 cm−1 is clearly evident, and this corresponds with the prediction of theory. We therefore assign this vibration to the carbon ring deformation (ν5) of the cyclic isomer. Other than the bands in the CH stretching region measured by Dopfer,52 there are no other gas-phase spectra for the cyclopropenyl cation. However, IR bands at 3105, 1276, 908, and 736 cm−1 were reported by Breslow in the initial isolation of this ion in superacid films.112 Later, Craig and co-workers reported the same bands at 3138 (ν4 (E′) asym CH st), 1276 (ν5 (E′) asym CCC st), 908 (ν6 (E′) asym CH bend), and 738 cm−1 (ν7 (A2″) sym CH bend).113 There was some slight variation in these spectra between the two experiments, but our band at 1293 cm−1 is quite close to the corresponding film value for the ν5 vibration. Unfortunately, the neon matrix IR data of Maier and co-workers did not report data for either ion below 2000 cm−1.31 From these data in the fingerprint region, it is clear that we have both isomers present in our spectrum. The propargyl isomer is more prominent because of the linear precursor employed. Because there is a large barrier to rearrangement, the propargyl cation produced initially is likely trapped in this local minimum by the subsequent cooling in the supersonic expansion and cannot interconvert to the more stable cyclic species because of the large activation barrier. A small amount of cyclopropenyl is also produced, presumably by a different formation route in the plasma chemistry rather than by conversion of propargyl ions. This amount could perhaps be increased in future experiments with appropriate cyclic precursors or perhaps by using very different plasma conditions. The CH stretching region of this spectrum is highly problematic to assign. It is clear that there are too many bands to be explained by either propargyl or cyclopropenyl alone, consistent with our conclusions so far that both isomers are present. However, none of the band positions in the CH stretching region agree well with the predictions of theory for either isomer. Complications arise here because of the attachment of argon to this system. If argon attaches on a CH, the vibration(s) at that position are likely shifted; past experience with other systems indicates that the shift will move the vibration to a lower frequency. Another possibility wellknown in infrared spectroscopy is the occurrence of Fermi resonances here from overtones or combination of lower frequency vibrations that may cause unexpected multiplets. In our initial study of this system, we noted this problem but tried to make the best assignment possible with the information available.64 Our computational studies identified argon attachment on the CH positions of both propargyl and cyclopropenyl, as indicated in the structures shown in Figure 3, and
Figure 4. Improved photodissociation spectrum of C3H3+Ar in the CH stretching region.
shows the new spectrum, in which the ions are somewhat colder than before with improved signal levels. Two features are different from before. In the first, we note a small change in the position of the band measured before at 3238 cm−1, which is now seen as a sharper feature at 3235 cm−1. The more interesting change is the emergence of a new band at 3132 cm−1, which was a barely noticeable shoulder before. It is still impossible to make definitive assignments for these bands because of the isomer issues noted above, but the present 11481
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indicates that 78% of the charge is localized on the two CH2 groups.123 As noted by Buzek et al.,126 this structure gives rise to a characteristic CCC stretching vibration near 1580 cm−1, which is intermediate between frequencies expected for single- and double-bonded CC. The central carbon in the 2propenyl cation is sp hybridized, giving rise to a near-linear C CC structure, with individual single and double CC bonds.122 This structure is stabilized by hyperconjugation between the σ electrons of the methyl group CH bond and the empty p orbital on the central carbon. Its charge is also localized primarily on the hydrogen atoms. The vibrational signature of this ion is a strong CC stretch expected near 1880 cm−1. Our spectrum shown in Figure 5 has multiple bands in the CH stretching region, and several sharp bands in the fingerprint region.65 The top trace shows the spectrum produced by an ethylene discharge, with argon tagging, and the second trace of the figure shows a spectrum measured with an allyl bromide precursor and nitrogen (N2) tagging. The lower two traces show the spectra predicted by theory at the MP2(fc)/6-311++G(2p,2d) level for the allyl (red trace) and the 2-propenyl (blue trace) structures. In the top spectrum, a strong isolated band at 1581 cm−1 corresponds to the prediction for the asymmetric CCC stretch of allyl, while another intense feature at 1877 cm−1 corresponds to the CC stretch of 2-propenyl. The 1418 cm−1 band is an in-plane hydrogen scissors bend of the allyl species. These key bands confirm that both isomers are present. The bands in the CH stretching region are also consistent with overlapping vibrations from both isomers. Using computed IR intensities, and comparing measured band intensities, we conclude that the two isomers are present with roughly equal abundance. Bowers and co-workers reached a similar conclusion in their reaction studies of the C3H5+ ions produced from ethylene.119 The second trace shows that variation of the precursor allows us to enhance the abundance of the allyl cation significantly with respect to 2-propenyl. The 1418 and 1581 cm−1 features remain strong, but the 1877 cm−1 band drops significantly in intensity, and the relative intensities of the various CH stretches shift accordingly. This confirms the assignments of all the bands to their respective isomers. This same spectrum was measured with other precursors such as cyclopropyl bromide. As shown, tagging with nitrogen instead of argon produces slight variations in band positions, but the spectrum is qualitatively unchanged. The bands detected in this gas-phase infrared study can be compared to those measured previously with IR measurements on the allyl cation in a superacid film by Buzek et al.126 Of the main features here, our 1418/1421 cm−1 band (Ar/N2 tagging) was detected by Buzek at 1418 cm−1; our 1581/1585 cm−1 band (Ar/N2) was measured in the film at 1578 cm−1; and our main CH stretch at 3110/3113 cm−1 was measured in the film at 3117 cm−1. The differences between the gas-phase results and the film are actually quite small. In both cases the vibrations seen are expected to be quite close to “isolated” gasphase values. Our computational studies of the tagged versus untagged ions indicate that the argon induces red shifts of only 1−3 cm−1 for most of the main bands. The shifts are small because the argon binding is rather weak (
