Vibrational Spectroscopy and Theory of the Protonated Benzene

Article pubs.acs.org/JPCA

Vibrational Spectroscopy and Theory of the Protonated Benzene Dimer and Trimer B. Bandyopadhyay,† T. C. Cheng,† S. E. Wheeler,‡ and M. A. Duncan*,† †

Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, United States Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States

‡

S Supporting Information *

ABSTRACT: Protonated benzene cluster ions, H(C6H6)2+ and H(C6H6)3+, are produced in a pulsed electrical discharge source coupled to a supersonic expansion. Mass-selected complexes are investigated with infrared photodissociation spectroscopy in the 1000−3200 cm−1 region using the method of argon tagging. The IR spectra of H(C6H6)2+−Ar and H(C6H6)3+−Ar contain broad bands in the high frequency region resulting from CH−π hydrogen bonds. Sharp peaks are observed in the fingerprint region arising from the ring modes of both the C6H7+ and C6H6 moieties. M06-2X calculations have been performed to investigate the structures and vibrational spectra of energetically low-lying configurations of these complexes. H(C6H6)2+ is predicted to have three nearly isoenergetic conformers: the parallel displaced (PD), T-shaped (TS), and canted (C) structures [Jaeger, H. M.; Schaefer, H. F.; Hohenstein, E. G.; Sherrill, C. D. Comput. Theor. Chem. 2011, 973, 47−52]. A comparison of the experimental dimer spectrum with those predicted for the three isomers suggests an average structure between the TS and PD conformers, which is consistent with the low energy barrier predicted to separate these two structures. No evidence is found for the C dimer even though it lies only 1.2 kcal/mol above the PD dimer. Although the trimer is also computed to have many low lying isomers, the IR spectrum limits the possible species present.

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spectroscopy.32−35 In the gas phase, a low resolution UV−vis spectrum of mass selected C6H7+ was reported by Freiser and Beauchamp.36,37 Infrared experiments by Solcá and Dopfer and by Maitre and co-workers examined the C−H stretching and the fingerprint region of the benzenium ion.38,39 In more recent experiments, our group has studied this ion in the 700−3400 cm−1 region using the method of argon tagging, revealing a more complete picture of the infrared spectral signature of C6H7+.40 These different spectroscopy studies all indicate that the structure is a σ-complex having a CH2 group on the ring edge with the charge delocalized in the π system. The benzene dimer, a prototype of π−π interactions, has been extensively studied by both theory21−30 and experiments.41−49 Moreover, it exhibits the kinds of structures and interactions expected for protonated benzene clusters. The most recent neutral dimer calculations suggest that there are two nearly isoenergetic minima on the potential energy surface having parallel displaced (PD) and canted (C) structures (see Scheme 1). A T-shaped (TS) isomer is found as a first order saddle point on the neutral surface. The energy differences among these isomers are found to be within 0.1 kcal/mol.28 Although many experimental studies have been performed, there is no consensus in the literature regarding the structure of the gas-phase benzene dimer. A molecular beam deflection

INTRODUCTION Molecular recognition processes involving receptor−ligand interactions play a key role throughout biology and chemistry.1,2 Modern synthetic chemistry and drug design are largely dependent on host−guest interactions in artificial supramolecular complexes.3,4 Much attention has therefore been focused on the measurement and computational studies of noncovalent interactions.5−7 Many noncovalent interactions important in biology involve aromatic rings.1−7 The π electrons of these systems display favorable electrostatic interactions with positively charged groups (cation−π interactions)8−20 and/or dispersion interactions with other aromatic or nonaromatic planar systems (π−π interactions).21−30 The characterization of these interactions through structural determination at the molecular level is critical for a better understanding of macromolecular functionality. The protonated benzene dimer is an interesting system in which the benzenium ion (C6H7+) interacts with benzene through CH−π, cation−π, and π−π interactions in multiple low energy conformers.31 Therefore, this system is expected to exhibit a subtle interplay between electrostatic and dispersion interactions and provide fertile ground for probing these effects. In the present work, we investigate these interactions via infrared spectroscopy and DFT studies of the protonated benzene dimer and trimer. The benzenium ion (also known as protonated benzene, C6H7+) is a well-known intermediate in electrophilic aromatic substitution reactions.32 This prototype ion was first isolated in superacid solutions and investigated with NMR and IR © 2012 American Chemical Society

Received: April 27, 2012 Revised: June 4, 2012 Published: June 7, 2012 7065

dx.doi.org/10.1021/jp304091h | J. Phys. Chem. A 2012, 116, 7065−7073

The Journal of Physical Chemistry A

Article

photodissociation spectroscopy. The experimental spectra and corresponding computational work examine the intermolecular interactions and resulting structures.

Scheme 1

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EXPERIMENTAL DETAILS Protonated benzene cluster ions H(C6H6)2+ and H(C6H6)3+ and their weakly bound argon complexes are produced in a pulsed electric discharge source coupled to a supersonic expansion.40,55 The expansion gas mixture consists of 20% H2 and 80% Ar flowed through a reservoir containing benzene at 0 °C to entrain its vapor. The molecular beam is collimated with a skimmer, and ions are transported to a second differentially pumped chamber containing a specially designed reflectron time-of-flight spectrometer.56 Different configurations of this instrument allow mass analysis of the ions and clusters produced and size selection of these based on their flight time. Mass selection is accomplished with pulsed deflection plates located in the first flight tube section. The selected ions are excited in the turning region of the reflectron field with a tunable infrared optical parametric oscillator/amplifier (IROPO/OPA; LaserVision) to perform photodissociation. The OPO/OPA system pumped by an injection seeded Nd:YAG laser (Spectra Physics model Pro 230) produces tunable light in the 2000−4500 cm−1 region. An additional stage of difference frequency generation in a AgGaSe2 crystal produces far-infrared light in the 1000−2000 cm−1 region. The output pulse energy is typically 100−800 μJ/pulse in the 1000−1800 cm−1 region and 1−2 mJ/pulse in the 1800−3500 cm−1 range. The intensities of one or more fragment ions resulting from IR excitation are recorded versus the IR photon energy to obtain a spectrum. The signal is collected with a digital oscilloscope (LeCroy) interfaced to a computer.

study by Klemperer and co-workers established the benzene dimer as a polar molecule, indicating a TS structure.41 Several electronic spectroscopy studies recorded spectra near the π−π* transition of the monomer, suggesting the possibility of either TS or PD structures.42−45 Felker and co-workers performed Raman vibronic double resonance spectroscopy and reported the structure as TS.46 Arunan and Gutowsky reported the microwave spectra of the dimer, also suggesting the presence of a TS structure.47 However, microwave spectroscopy and molecular beam deflection experiments are only sensitive to polar molecules, and so, nonpolar isomers would not be detected even if they were present in these experiments. Infrared ion dip spectroscopy experiments on the benzene dimer also indicated a TS structure.48 More recently, a direct infrared absorption spectrum confirmed the presence of the TS isomer.49 Thus, although the above spectroscopic studies clearly suggest the presence of a T-shaped isomer, these experiments have not been able to rule out the possible presence of a PD isomer. The structure of the cationic counterpart (C6H6)2+ has also been controversial, and the most recent experiments suggested the presence of both the PD and T-shaped isomers.50−53 Photodissociation spectroscopy in the visible and the near IR region by Nishi and co-workers found evidence for resonant charge transfer from C6H6+ + C6H6 ↔ C6H6 + C6H6+, indicating the presence of a PD isomer.50,51 However, pump−probe hole burning experiments have suggested that, if the TS isomer does exist, its relative abundance is smaller than that of the PD isomer.52,53 Similar to the benzene dimer and its ion, the potential energy surface of the protonated dimer (i.e., C6H7+−C6H6 or benzenium−benzene) has three low energy isomers: PD, C, and TS (Scheme 1).31 However, the relative energies of these structures change compared to those of the neutral dimer as a result of protonation. At the CCSD(T)/aug-cc-pVTZ level of theory, Jaeger et al. predicted the PD conformer to be 0.7 and 1.4 kcal/mol lower in energy than the TS and C dimers, respectively.31 The electronic spectrum of the protonated benzene dimer reported by Jouvet et al. shows a significant red shift for the S0−S1 transition compared to the spectrum of the neutral benzene dimer.54 This experimental study, along with the accompanying ab initio calculations, suggested that the first excited state has charge transfer character. Unfortunately, no structural information about low energy isomers in the ground state could be determined from the electronic spectrum. In the present work, we probe the ground state structures of the protonated benzene dimer and trimer via mass-selected infrared

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COMPUTATIONAL DETAILS Computational studies on these systems were carried out using the M06-2X functional57 and the 6-31+G(d) basis set. This methodology, when paired with this modest basis set, has been demonstrated to provide a reasonable compromise in performance while allowing larger systems to be studied.6,57 An integration grid comprising 99 radial and 590 angular points was used to avoid numerical issues associated with this functional when employing less sparse grids.58,59 The scaling factors recommended by Truhlar for the M06-2X method with other basis sets (0.982)57 were found not to work particularly well here, and so we devised our own scaling factor on the basis of the computed and measured vibrations of the benzenium monomer. In the higher frequency region, we determined a factor of 0.9358 using the CH2 stretches, while, in the low frequency region, a factor of 0.9566 was derived using the 1607 cm−1 carbon ring vibration. For comparison to the M06-2X results, we also did computations using the B97-D dispersioncorrected functional with the same basis set. The scaling factors for this method (derived the same way) were 0.9724 (high frequency) and 1.0082 (low frequency).

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RESULTS AND DISCUSSION Figure 1 shows a comparison of the infrared spectra of the monomer benzenium (bottom) and the protonated benzene dimer (top) in the 1000−3200 cm−1 region, both measured for the argon tagged complexes in the mass channel corresponding to the elimination of argon. The spectrum of C6H7+ was reported in 2008 by our group.40 As documented previ7066



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benzene.60 The pattern here is more complex for benzenium (four bands), and the triplet is therefore associated with the neutral benzene moiety in the complex. In the low frequency region, the dimer has an intense band at 1608 cm−1, which is within 1 cm−1 of the CCC asymmetric stretch band of C6H7+− Ar. The other intense transition at 1449 cm−1, which is close to the 1456 cm−1 band for C6H7+−Ar, can be attributed to the same kind of vibration (asymmetric 3,4,5 CCC stretch) seen here for benzenium. If benzene interacts with benzenium in the region of the CH2 moiety, as suggested above, it is not too surprising that this interaction has little effect on these ring modes of benzenium. In the lower frequency region where the scissor motion of CH2 is expected (1198/1239 cm−1 bands for C6H7+−Ar), the dimer has three bands at 1121, 1183, and 1204 cm−1. The shifts and splittings here are greater, consistent with interactions between benzene and benzenium that perturb the CH2 group of the latter and again suggesting a binding motif involving the CH2 of benzenium. To investigate this complex formation in more detail, we have studied the benzenium ion and its clusters with benzene using the M06-2X DFT functional.57 This method has been demonstrated in the past as an effective approach in the study of noncovalent interactions.6,57 To validate the method and to derive vibrational scaling factors, we first examined the benzenium−Ar complex. This system was studied at the MP2 and DFT/B3LYP levels of theory in our original spectroscopic study.40 More recently, Botschwina and Oswald investigated the potential for argon binding to this system at the explicitly correlated CCSD(T) level,61 predicting that the most stable complex features argon centered above the ring and bound by 550 cm−1. We have performed M06-2X computations with and without argon (see Supporting Information). M06-2X finds the same most stable argon binding position, with an Ar binding energy of 560 cm−1. Consistent with its low binding energy and its binding site above the face of the aromatic ring, argon does not significantly impact the vibrational frequencies of benzenium. The most notable differences involve the CH2 stretches. The ring modes of the benzenium have virtually identical computed frequencies with and without argon. As shown in Figure 2, the predicted vibrational spectrum agrees well with the experimental spectrum. The CH2 stretch vibrations and the 1607 cm−1 bands are used to develop the vibrational scaling factors, and therefore, their computed positions are adjusted to agree with the experiment. With these positions set, the other bands in the predicted spectrum also fall into good agreement with the experiment. The 1456 cm−1 band is predicted to be 13 cm−1 lower than observed, and the band at 1239 cm−1 is predicted 8 cm−1 higher than observed. The relative intensities in the CH2 stretching region are predicted to be lower than those detected in the experiment. This is because the experiment measures the photodissociation yield, which does not exactly track computed absorption intensities. Similar intensity trends are often found for these kinds of systems, regardless of the computational methods employed. The M06-2X method with the scaling factor chosen here therefore performs reliably for the benzenium monomer and should be acceptable for the larger clusters. To provide further insight into the three low-lying dimer structures, and the possible isomerization pathways connecting them, a relaxed two-dimensional potential energy surface (PES) is shown in Figure 3b as a function of the two angles (α and β) depicted in Figure 3a. A one-dimensional minimum energy

Figure 1. Infrared photodissociation spectra measured for the argontagged complexes of protonated benzene monomer (bzH+−Ar) and dimer (bz2H+−Ar) in the 1000−3200 cm−1 region. Both spectra are obtained in the mass channel corresponding to the elimination of argon.

ously,32−35,38,39 protonation occurs on one of the carbons in the benzene ring, resulting in a σ-complex. The hybridization of that carbon changes from sp2 to sp3, and the positive charge is delocalized throughout the π system. The key signature of this kind of protonation is the symmetric and asymmetric sp3 CH2 stretches, which appear as a set of intense overlapping peaks near 2820 cm−1. Several weaker bands are observed in the 3000−3100 cm−1 region, which are assigned to the sp2 CH stretches of the benzene ring away from the CH2. This region also corresponds to the (ν13 + ν16)/(ν2 + ν16 + ν18)/(ν12) Fermi triad of neutral benzene,60 and it is possible to have other resonances here involving the four CH stretches and combinations of CC stretches and CH bends of C6H7+. In the fingerprint region, the spectrum has four strong bands at 1198, 1239, 1456, and 1607 cm−1. The 1607 cm−1 band corresponds to the in-phase asymmetric (2,3,4) and (3,4,5) CCC ring vibration. This motion is analogous to the CCC asymmetric stretch band observed at 1585 cm−1 for the C3H5+ allyl cation.55 For C6H7+, this transition can be viewed as an inphase asymmetric oscillation of two C3 groups in the benzene ring. The (3,4,5) CCC ring stretch appears as another strong peak at 1456 cm−1. The other bands at 1198/1239 cm−1 are assigned to the scissor motion of the CH2 group. All of these modes gain oscillator strength compared to neutral benzene because of the charge in the π system. A qualitative comparison of these spectra already reveals some insight. The dimer spectrum has activity in the fingerprint region and also in the C−H stretching region somewhat like the benzenium ion. The dimer spectrum is most noticeably different from that of benzenium in the region of the sp3 CH2 stretches. Instead of the strong overlapping bands near 2820 cm−1, the dimer has a very broad feature at 2785 cm−1 (line width ≈ 100 cm−1 FWHM) and a somewhat sharper one at 2909 cm−1 (line width ≈ 30 cm−1 FWHM). These bands indicate that the dimer has CH2 stretches similar to those in the C6H7+ moiety, but they are perturbed by the interaction with benzene, suggesting that benzene interacts with benzenium in the neighborhood of the CH2 group. In the higher frequency region, a triplet of bands with outer members at 3052 and 3107 cm−1 is recognized as the Fermi triad pattern for neutral 7067



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Figure 2. Photodissociation spectrum of the benzenium−Ar complex compared to the predictions of theory at the M06-2X/6-31+G(d) level.

pathway connecting these isomers is shown in Figure 3c. Nearly identical one- and two-dimensional PESs are obtained for the argon-tagged dimer (see Supporting Information), demonstrating that Ar does not perturb these dimer structures or their relative energies in any significant way. Different binding configurations of C6H6 with respect to C6H7+ produce three low energy isomers, consistent with previous results for this system.31 The local minimum in Figure 3b near α = 110° and β = 80° is not a minimum on the full PES but is an artifact of the two-dimensional scan. (Starting from this structure, a geometry optimization leads to one of the other minima. On the full PES, that spot is not an actual minimum, it just looks like one when considering the 1-dimensional scan.) In the lowest energy PD configuration, one of the CH bonds of the CH2 moiety of C6H7+ interacts with the neutral C6H6 via a CH−π hydrogen bond, and the charged π-system of the benzenium lies above and is displaced from the neutral benzene. In the TS configuration, both of the CHs of the CH2 group interact with the neutral C6H6 in a bifurcated CH−π interaction. In the C configuration, the CH2 moiety of C6H7+ points away from the neutral benzene, and the main interaction is a CH−π bond involving the sp2 hybridized CH group opposite the site of protonation. The predicted relative energies of these three isomers span 1.9 kcal/mol for the neat species; this range is reduced to only 1.2 kcal/mol for the Ar tagged species. The PD structure is the lowest in energy, and a barrier of less than 2 kcal/mol separates this configuration from the TS structure. A 4 kcal/mol barrier lies between the PD and the C structure. Tables 1 and 2 list the experimental IRPD bands and corresponding predicted vibrations for C6H7+−Ar, and the PD, TS, and C isomers of the dimer (each also tagged with Ar, as in the experiment). The computed structures of these isomers show that argon always binds above the ring of benzenium. Its presence does not change the dimer isomeric structures significantly, and the frequencies predicted with and without the argon are within about 10 cm−1 of each other. We present the spectra predicted for each of the three isomers in Figure 4, where they are compared to the

Figure 3. (a) Definition of intermolecular coordinates used to study the interconversion of the three low-lying benzene−benzenium isomers. (b) Partially relaxed two-dimensional surface for the interaction energy of C6H6 and C6H7+. For each value of (α, β), Rcm was minimized with all internal coordinates constant. (c) Onedimensional potential energy curve (relative to the PD isomer) connecting the three isomers. For each value of α, β and Rcm were optimized with all internal coordinates constant. In panels b and c, the local minimum denoted by the asterisk (*) is not a true minimum on the fully relaxed potential energy surface.

experimental spectrum. It is immediately obvious that no predicted spectrum of any single isomer agrees perfectly with the experiment. We therefore use trends in the monomer versus dimer experimental and computed spectra to tease out which isomers are likely present. We first consider the CH2 stretching region, where the most significant changes occur in the spectrum on going from the monomer to the dimer. Because the C dimer has the CH2 group pointing away from the benzene ring, its stretches are not strongly affected by the interaction with benzene. The symmetric and asymmetric sp3 CH2 stretches for this isomer are predicted at 2804/2833 cm−1, which are quite close to the corresponding predicted frequencies for the monomer benzenium (2793/2820 cm−1). Although the predicted monomer bands do not agree perfectly with the experiment, the trend predicted here is that the C isomer should have CH2 stretches essentially in the same place 7068



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Table 1. Experimental IRPD Bands for Protonated Benzene (C6H7+−Ar) and Three Different Isomers of Protonated Benzene Dimer H+(C6H6)2−Ar Compared to the Prediction of Theory (M06-2X/6-31+G(d)) in the Low Frequency Regiona monomer exptl

theory

1198

1157(22) 1166(21)

dimer exptl

theory PD

TS

C

assignment asym. (3,4,5) ip CH bend iph (5,6) and (2,3) scissor, ooph with sp3 CH2 scissor

1121 1158(19)

1151(290)

1164 (19)

iph (5,6) and (2,3) scissor, ooph with sp3 CH2 scissor

1241(91)

1179(315)

1268 (154)

iph (5,6) and (2,3) scissor, iph with sp3 CH2 scissor

1183 1204 1239 1456

sp3 CH2 scissor, iph with (5,6) and (2,3) CH scissor asym (3,4,5) CCC str.

1247(131) 1443(239) 1402 1426(27)

1397(35)

1419(48)

sym (3,4,5) CCC str of C6H7+ moiety

1423(162)

1428(156)

1460(165)

asym (3,4,5) CCC str of C6H7+ moiety

1460 (13) 1462 (28)

1460(14) 1463(11)

1461 (11) 1463 (47)

1603(92)

1604(85)

1606(174)

sym (3,4,5) CCC str of C6H6 moiety asym (3,4,5) CCC str of C6H6 moiety iph asym (2,3,4) (4,5,6) CCC str iph asym (2,3,4) (4,5,6) CCC str of prot bz

1449 1487

1607

1607(97) 1608

PD = parallel displaced; TS = T-shaped; C = canted. Frequencies and intensities are in cm−1 (km/mol) with a 0.9566 scaling factor (scaled to match the monomer 1607 cm−1 band). a

Table 2. IRPD Bands for Protonated Benzene (C6H7+−Ar) and Three Different Isomers of Protonated Benzene Dimer (C6H7+)2−Ar Compared to the Prediction of Theory (M06-2X/6-31+G(d)) in the High Frequency Regiona monomer exptl

theory

2793

2793(65)

2820

2820(41)

3006 3035 3078 3107

3034(3) 3034(5) 3055(7) 3056(3)

dimer exptl

theory PD

TS

C

2785

2732(342)

2619(703)

2804(64)

2909

2805(39)

2793(236)

2833(28)

3030(5) 3030(5) 3052 (4) 3051(2)

3030(3) 3030(5) 3052(3) 3054(1)

3029(2) 3030(5) 3038(19) 3051 (3)

assignment sym sp3 CH2 str sym sp3 CH2 str of C6H7+ moiety asym sp3 CH2 str asym sp3 CH2 str of C6H7+ moiety asym (2,6) sp2 CH str ooph with asym (3,5) sp2 CH str sym (2,6) sp2 CH str ooph with asym (3,4,5) sp2 CH str asym (3,5) sp2 CH str iph with asym (2,6) sp2 CH str sym (2,3,4,5,6) sp2 CH str; (3,5) largest amplitude

3052 3081 3107 sp2 sp2 sp2 sp2

CH CH CH CH

str str str str

PD = parallel displaced; TS = T-shaped; C = canted. Frequencies and intensities are in cm−1 (km/mol) with a 0.9358 scaling factor (scaled to match the CH2 bands of C6H7+−Ar). a

frequency of 2619 cm−1, while the asymmetric stretch is mostly unperturbed at 2793 cm−1. The predicted split between the CH stretching frequencies is much greater in the TS structure (174 cm−1) than it is for the PD species (73 cm−1). The intensities of these bands are also affected by the interaction of the CH2 group with benzene. Because the IR intensity depends on the dipole moment derivative, the perpendicular configuration of the TS dimer, where both the CH bonds interact with the benzene π cloud, has C−H stretch intensities much greater than those of the PD configuration. In particular, the computed intensity of the symmetric CH stretch of the TS isomer is 703 km/mol, compared to that of the PD dimer (342 km/mol).

as the monomer. Because the experimental bands in this region change dramatically in both position and width (Figure 1), it seems that the C isomer cannot be making any significant contribution to the spectrum. Both the PD and TS isomers bind in the vicinity of the CH2 group and therefore exhibit more significant changes in the CH stretching bands. Because in the PD configuration only one of the CH bonds interacts strongly with benzene, the C−H stretches split into a redshifted vibration for the hydrogen bonding CH (2732 cm−1) and a slightly blue-shifted one for the free CH (2805 cm−1). For the TS dimer, because the CH2 hydrogens are bound to benzene in a more symmetric fashion, the two CH stretches become essentially symmetric and asymmetric CH2 stretches. The symmetric stretch is predicted to shift to a much lower 7069



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corrected functional for comparison to that predicted with the M06-2X functional (Supporting Information, Figure S1). The structures and relative energies obtained with these two methods are very nearly the same. Moreover, the spectra with these two methods are nearly identical in the fingerprint region. However, in the region of the CH2 hydrogen bonding stretch, B97-D predicts a much more intense symmetric stretch shifted to much lower energy (2553 cm−1), and a much weaker asymmetric stretch at much higher energy (2880 cm−1), again looking nothing like the experimental peaks. Clearly, this hydrogen bonding region of the spectrum is highly problematic for theory. Similar problems between theory and experiment affect the spectrum in the fingerprint region. All three structures have a CCC asymmetric ring stretch predicted at about the same position as that observed in the experiment at 1608 cm−1. The CCC stretch involving the C6H6 moiety is predicted at ∼1460 cm−1 for all three complexes. The experimental band at 1487 cm−1 apparently corresponds to this. However, this band is only predicted to have strong IR intensity for the C structure, which is ruled out on the basis of other bands (see above and below). This band is observed within 1 cm−1 of the corresponding ν19 in plane carbon ring distortion of the isolated neutral benzene (1486 cm−1).60 Apparently, the interaction with C6H7+ does not affect the frequency of this vibration significantly. The position and intensity of this vibration is known to vary dramatically in metal ion−benzene complexes with the amount of charge transfer interaction.62,63 It is therefore conceivable here that the enhancement in IR intensity over that calculated could come from a similar charge transfer interaction involving the CH−π bonding that is not handled well by theory. A CCC stretch on the benzenium ring is predicted at 1424, 1429, and 1418 cm−1 for the PD, TS, and C isomers, respectively. This band, which is predicted to be stronger in intensity for the PD and TS isomers, apparently corresponds to the 1449 cm−1 experimental band. It is shifted only slightly from the position of the 1456 cm−1 band of the monomer benzenium. None of these higher energy features in the fingerprint region makes it possible to distinguish clearly between the possible isomers. However, the predicted spectra for the three isomers differ significantly for the scissor vibrations of the CH2 group. Each of these has two scissor vibrations depending on whether this motion is in-phase or out-of-phase with the adjacent in-plane C−H bends. The out-of-phase CH2 scissor vibration is predicted at about the same positions for PD, TS, and C dimers (1158, 1151, and 1164 cm−1, respectively). However, the in-phase scissors vibration varies significantly for the different isomers. The PD species, which has one CH of the CH2 group bound to the neutral benzene, has this vibration predicted at 1241 cm−1, very close to the position predicted for the isolated benzenium monomer (1247 cm−1). For the C isomer, which has a free CH2 group away from the benzene binding, this vibration is predicted at a much higher frequency of 1268 cm−1. In these two cases, only the higher frequency band has strong IR intensity. However, the TS isomer is predicted to have an in-phase band that is red-shifted with respect to the monomer at 1179 cm−1, and both the out-ofphase and in-phase bands are predicted to have strong intensity. Unfortunately again, the experimental spectrum does not agree exactly with any of these predictions. However, there is clearly no signal anywhere near the 1268 cm−1 band predicted for the C isomer, again suggesting that this species cannot be present. Of the other two isomers, the experimental spectrum agrees

Figure 4. Predicted spectra and structures of the parallel displaced (PD), T-shaped (TS), and canted (C) conformers computed using the M06-2X/6-31+G(d) theory (bottom three traces). The top trace shows the measured spectrum of the protonated benzene dimer− argon complex (bz2H+−Ar).

Our measured spectrum in the CH2 region has two bands (2785 and 2909 cm−1) in the region where two bands are predicted for both the PD and TS isomers, but the positions of these do not agree particularly well with those for either isomer. Likewise, the width of the 2785 cm−1 band is much greater than that of the 2909 cm−1 band. Because the 2785 cm−1 band is so broad, its integrated area is about twice that of the 2909 cm−1 band. The two bands detected are significantly shifted from the positions of the bands for the monomer benzenium. However, their positions are not as strongly red-shifted as those predicted by theory for either isomer. In particular, the 2909 cm−1 band is much further to the blue than any band predicted by theory. The measured band positions are closer to those for the PD isomer. Likewise, the splitting measured for the two bands (124 cm−1) is intermediate between the values predicted for the two isomers. The integrated areas under the two bands produces roughly a 2:1 intensity ratio favoring the lower frequency band, which is more like the intensity ratio predicted for the TS species. However, the widths of the two experimental bands are quite different, suggesting an asymmetry to the hydrogen bonding connections, more like that in the PD species. It is also conceivable that the width of these bands could arise from the overlap of bands from both species. Another possibility is that the main isomer has two overlapping bands forming the 2785 cm−1 feature and that the 2909 cm−1 band comes from another isomer, perhaps one with argon bound at another site. However, we have not identified any such isomer, and if it had stronger argon interactions with the CH2 groups this would shift this vibration further to the red. It is therefore clear that several possible alternate assignments are possible for this spectrum, but none are particularly compelling. In light of this, the simplest assignment at present is perhaps the best, and this is that the 2785 and 2909 cm−1 bands are the symmetric and asymmetric stretches of the CH2 group involved in hydrogen bonding. Even though we can rule out contributions from the C species, we cannot distinguish between the PD and TS structures based on these spectra. To investigate this problematic CH2 stretching region further, we have examined the spectrum predicted for the dimer complex in the PD structure with the B97-D dispersion7070



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best with the TS isomer in that there are two relatively strong bands that are more red-shifted. However, the splitting between these two bands in the experiment is more like that for the PD species. Again, it is not possible to make a clear distinction between these species based on the information available. Overall, there seems to be good evidence that the C structure is not present. Although we do not have perfect agreement between predictions and measured spectra, the data in both the high frequency and low frequency regions indicate that benzene binding occurs on the CH2 group of the benzenium. This affects the CH2 stretching modes and the CH2 scissor bending modes, shifting these from the position of the monomer and, in the case of the stretching modes, broadening them consistent with hydrogen bonding. Together these data suggest the presence of either the PD or TS structures or perhaps a dynamic mixture of the two. Considering that the potential is rather flat connecting these two species and that zero point energy is not included in the potential scans, it seems perfectly reasonable that the spectra measured correspond to an admixture of these two structures. We should also note that the temperature of these ions is not well-known. When enough information is available from spectral details such as rotational structure, this cluster source has been shown to produce ions in the 20−100 K temperature range.64 The attachment of argon here and its predicted weak binding energies are also consistent with relatively cold ions. However, it is simply not possible to determine the temperature exactly, and internal energy in these ions could lead to internal motion that tends to average structures between the PD and TS configurations. Figure 5 shows the spectrum measured for the protonated benzene trimer, where it is compared to the spectrum for the

at 1121 cm−1 associated with the scissor motion has disappeared. Computational studies of the trimer are of course far more demanding, as an additional benzene can be added to more than one binding position on each of the isomers identified for the dimer, leading to a number of possible lowlying configurations. Moreover, predicted energies for these complexes are quite close (see Supporting Information). For the sake of this analysis, we select two representative isomers and present their spectra below the experimental data in Figure 5. In isomer 3a (blue trace), one of the benzene−benzenium interactions resembles the TS dimer structure, while the other is analogous to the PD dimer. In 3b (red), the benzene− benzenium interactions resemble the PD and C dimers. It is apparent from these two structures and their spectra that additional binding interactions in the neighborhood of the CH2 group leads to a greater red shift of the CH stretch vibrations here. The broadening in these spectra is generally associated with hydrogen bonding interactions. Therefore, although we cannot make any definitive assignment, it seems that the trimer has greater connectivity in the vicinity of the CH2 groups than the dimer. It also makes sense that greater binding here might limit the CH2 scissor motion, thus explaining the loss of the 1121 cm−1 band observed for the dimer. Structures resembling that of the 3a isomer therefore seem likely for this system.

Figure 5. Infrared spectrum of the protonated benzene trimer−argon complex (bz3H+−Ar) in the 1000−3300 cm−1 region (second trace from the top). The spectrum of the dimer (bz2H+−Ar) is shown in the top trace for comparison. The predicted spectra and structures of two low lying isomers of the trimer computed using the M06-2X/631+G(d) theory are shown in the bottom traces.

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CONCLUSIONS Mass-selected infrared photodissociation spectroscopy is carried out for the argon-tagged complexes of protonated benzene dimer and trimer produced in an electrical discharge/ supersonic expansion source. The IR spectra of these clusters are compared to that of the monomer benzenium ion measured recently in the C−H stretching and fingerprint regions. Computational studies at the M06-2X/6-31+G(d) level are employed to investigate the low-lying isomeric structures of these complexes and their vibrational spectra. These calculations suggest the presence of several isomers for the dimer and trimer resembling structures studied previously for the neutral benzene dimer. Dimer structures include T-shaped, parallel-displaced, and canted isomers all lying within 1.2 kcal/ mol of each other. There is a low energy barrier along the transition from the T-shaped to parallel displaced dimer, and the canted isomer lies at slightly higher energy. Both the dimer and trimer spectra show broad bands indicating CH−π hydrogen bonds, as well as sharp peaks from the ring modes of the C6H7+ and C6H6 moieties. The spectrum of the protonated benzene dimer is best described by a structural average between the TS and PD conformers, while there is little or no evidence for the canted structure. Although the trimer is predicted to have many isomers, its spectrum suggests the presence of one or more configurations having each CH of the CH2 group of C6H7+ interacting with benzene. ASSOCIATED CONTENT

S Supporting Information *

The geometric and energetic parameters for the protonated benzene dimer and trimer computed at both the M06-2X and B97-D levels of theory. This material is available free of charge via the Internet at http://pubs.acs.org.

dimer. The two spectra are quite similar in many respects, with changes occurring only in the region of the CH2 stretches and scissor bend, as well as a new broad structure near the 1607 cm−1 band. The CH2 stretching region now contains a single band centered at 2676 cm−1, which is broader and shifted further to the red from the bands of the dimer. The lower band

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AUTHOR INFORMATION

Corresponding Author

*Fax: 706-542-1234. E-mail: [email protected]. 7071



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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Science Foundation for this work, through grant no. CHE-0956025 (to M.A.D.), and the American Chemical Society Petroleum Research Fund, through grant 50645-DNI6 (to S.E.W.). S.E.W. also thanks the Texas A&M Supercomputing facility for computational resources.

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Vibrational Spectroscopy and Theory of the Protonated Benzene

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