Article pubs.acs.org/JPCA
Excitation Spectra of the Jet-Cooled 4‑Phenylbenzyl and 4‑(4′Methylphenyl)benzyl Radicals Nahid Chalyavi,† Tyler P. Troy,† George B. Bacskay,† Klaas Nauta,† Scott H. Kable,† Scott A. Reid,‡ and Timothy W. Schmidt*,† †
School of Chemistry, The University of Sydney, NSW 2006, Australia Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233, United States
‡
S Supporting Information *
ABSTRACT: The excitation spectra of jet-cooled 4-phenylbenzyl and 4-(4′-methylphenyl)benzyl radicals have been identified by a combination of resonant two-color two-photon ionization mass spectrometry and quantum chemical methods. Both radicals exhibit progressions in the biphenyl torsional mode, peaking near ν = 17. The lowest observed peak for 4phenylbenzyl was observed at 18598 cm−1 and is estimated to be the ν = 3 of the progression, while the lowest observed peak for the 4-(4′-methylphenyl)benzyl radical was observed at 18183 cm−1 and is possibly the origin. The spectra are discussed and compared to other biphenyl and benzyl chromophores.
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substituted phenylbenzyl radical. Nevertheless, the α-phenylbenzyl radical or diphenylmethyl radical has been studied by laser-induced fluoresence spectroscopy,15,16 and the 4-phenylbenzyl radical has been observed in coarse transient spectra from pulsed radiolysis of phenylbenzylchloride.17 Here, we report excitation spectra of two such radicals, 4-phenylbenzyl (PhBz) and 4-(4′-methylphenyl)benzyl (tolylbenzyl, TolBz) radicals. Both of these radicals exhibit striking Franck−Condon envelopes, which are interpreted in terms of a large change in the dihedral angle of the two aromatic rings, as supported by density functional, CASSCF, and CASPT2 computations.
INTRODUCTION The benzyl radical is the prototypical aromatic radical, featuring a delocalized unpaired electron that is shared between the methylene and phenyl moieties. Its electronic structure may be described by several resonance structures, and as such, it is termed a resonance-stabilized radical. Such species are important in energized chemical environments such as combustion1−5 and may play a role in interstellar chemistry.6,7 The radical may be further stabilized by substitution with conjugating groups. Substitution of a methylene hydrogen with an ethynyl or vinyl group yields, respectively, the phenylpropargyl8,9 (α-ethynylbenzyl) and phenylallyl10,11 (α-vinylbenzyl or cinnamyl) radicals. The former of these was observed to be the most fluorescent species in an electrical discharge containing benzene diluted in argon,8 after the ubiquitous C2, C3, and CH fragments, and both species are stabilized with respect to the methyl radical by more than 100 kJ/mol.10 The optical spectroscopy of the benzyl radical and its substituted analogues is of interest as a diagnostic tool of hydrocarbon chemistry. It is also of fundamental interest due to the high symmetry of the benzyl radical. The D1 ← D0 transition of the benzyl radical (Ã 2A2 ← X̃ 2B1) is accidentally forbidden, arising from the superposition of the 3b1 ← 1a2 and 2a2 ← 3b1 one-electron transitions such that the transition moments cancel.12 As such, the origin band is very weak, and the spectrum is dominated by vibronic features. The D2 state (B̃ 2B1) is low-lying and vibronically interacts with the D1 state. This is also the case for the p-methylbenzyl radical (p-xylyl), which is also approximately of C2v symmetry.12,13 The o- and mxylyl radicals exhibit spectra complicated by methyl rotors.14 Despite persistent interest in substituted benzyl radicals, there has been no report of the excitation spectrum of a ring© 2012 American Chemical Society
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THEORETICAL PROCEDURE Electronic Structure. Given the relatively large sizes of the systems of interest and the absence of symmetry in the methylsubstituted molecule, calculations were performed only for the PhBz radical itself, especially because methylation is expected to have minimal effect on the π-electron structure of the chromophore. This is borne out by the strong similarity of the observed spectra of the two species. The ground- and excited-state geometries and harmonic frequencies were obtained by complete active space self-consistent field (CASSCF)18 calculations using the Dalton programs.19 The CASSCF computations, carried out in C2 symmetry using the 6-31G(d) basis, utilize seven active electrons in seven active orbitals (4a and 3b). Single-point complete active space secondorder perturbation (CASPT2)20−22 with a number of different active spaces and time-dependent density functional theory Received: September 10, 2012 Revised: October 15, 2012 Published: October 18, 2012 10780
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(TD-DFT23−25) calculations were carried out at the CASSCF geometries, using the MOLPRO,26 GAMESS(US),27,28 and Gaussian0929 codes. TD-DFT was also employed to explore the effects of the torsional motion of the radical chromophore. In these calculations, the ground-state equilibrium geometries were calculated at the B3-LYP/6-311++G(d,p) level of theory, followed by a relaxed scan of the surface with respect to the dihedral angle between the aromatic rings, θ. At each relaxed geometry, the excited-state energy and corresponding oscillator strength, f(θ), were evaluated using time-dependent DFT (TDDFT) and the 6-311G(d) basis. Spectral Simulations. Due to the highly curvilinear nature of the torsional mode of the PhBz radical, Franck−Condon (FC) factors for all other modes were calculated, in the absence of Duschinsky mixing, by rotating the geometries and modes, as calculated by CASSCF, about the symmetry axis to bring the rings into a coplanar arrangement, minimizing the sum of squared displacements out of the plane. The spectrum arising from these 3N − 7 FC factors was then convolved with a purely torsional spectrum, as calculated below, to yield the full simulation. The Hamiltonian operator for the torsional potential, θ, is given by Ĥ =
−ℏ2(IBz + IPh) ∂ 2 + V (θ ) 2IBzIPh ∂θ 2
extracted vertically and perpendicular to the laser and molecular beam into the time-of-flight tube. Ions were detected with a tandem microchannel plate. The signal was viewed on a digital oscilloscope as a function of laser wavelength and processed using in-house software. Relative instrument timings were controlled by a digital delay generator operated at a repetition rate of 10 Hz.
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RESULTS Theoretical Analysis. The CASSCF optimized groundand excited-state structures of the PhBz radical are shown in Figure 1. Both states exhibit equilibrium geometries of C2
(1)
where IBz and IPh are the moments of inertia of the benzyl and phenyl moieties about the symmetry axis. Wave functions, |ν⟩, and eigenvalues, Eν, were calculated for the ground- and excited-state potentials by diagonalization of the Hamiltonian matrix in a plane wave basis. Vibronic transition moments were calculated as μ(ν′, ν″) = ⟨ν′|μ(θ )|ν″⟩
(2)
Integrals were performed numerically, applying a cubic spline interpolation to the calculated potential energy surface and transition dipole moment functions. The dipole moment function was evaluated in atomic units from the calculated TDDFT oscillator strengths μ(θ ) =
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3f (θ ) 2(Eν ′ − Eν ″)
Figure 1. Ground- (D0) and excited-state (D1) geometries of the PhBz radical, as calculated at the CASSCF/6-31G(d) level of theory. Despite the optimized excited state being nonplanar, the planar structure exhibits a lower zero-point corrected energy. The inter-ring bond is 1.438 Å at the planar geometry. The plot at the bottom shows the change in C−C bond lengths upon electronic excitation.
(3)
symmetry. The most obvious difference at the CASSCF level is that the ground state exhibits a dihedral angle of 46.6°, while that of the excited state is only 18.3°. Indeed, the barrier to planarity is only 56 cm−1 (0.16 kcal/mol), and when zero-point corrected, the energy of the planar system is actually lower by 36 cm−1. The computed dihedral angle in the ground state agrees well with the observed value for the biphenyl molecule of 44°.31 The move toward planarity on excitation is consistent with the higher degree of π delocalization in the excited state; this is readily demonstrated by the application of simple Hückel MO theory. The reduction in the inter-ring bond length from 1.491 to 1.448 Å, that is, increased bond order, is an obvious manifestation of the energetic stabilization brought about by the increased delocalization. The D0 → D1 transition is welldescribed as a combination of HOMO → SOMO and SOMO → LUMO transitions, where the HOMO is the highest doubly occupied orbital and the SOMO is the singly occupied molecular orbital. Inspection of the HOMO reveals a node between the ring systems, while the SOMO is mostly localized
EXPERIMENTAL METHODS Resonant two-color two-photon ionization (R2C2PI) spectra were measured in a two-stage differentially pumped vacuum chamber as used previously.8,30 A pulsed discharge nozzle (PDN) was used to produce the species of interest. A precursor, 4-methyl-1,1′-biphenyl (Aldrich, 98%) or 4,4′dimethylbiphenyl (Aldrich, 97%), was stored in a stainless steel container inside of the source chamber, and in order to form a rich molecular beam, both the sample container and the pulsed PDN were heated to about 120° using wire heaters. A free jet was produced by the PDN, and the emerging radical beam was passed into the ionization region through a 2 mm skimmer. Radicals were ionized by two laser pulses. The first laser pulse, provided by an excimer-pumped dye laser, was tuned over the vibrational levels of the excited electronic state, while the second, fixed wavelength pulse was used to ionize the excited molecules. The 193 nm photons for the ionization step came from the output of an ArF excimer laser. The cations were 10781
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on the benzylic carbon. The LUMO is strongly bonding between the rings, and thus, either of the two transitions that dominate the description of the excitation will serve to strengthen the inter-ring bond. In the C2v point group of the benzyl moiety, both the HOMO and LUMO are of b1 symmetry, meaning that the transition is long-axis-polarized and analogous to the D2 ← D0 transition of the benzyl radical. The excited-state energy is calculated by CASSCF to be 24655 cm−1, accounting for the zero-point energy (planar excited-state geometry). This is significantly higher than the experimental excitation energy of the benzyl radical itself, at 22000 cm−1, but does not account for the major component of dynamical correlation, that is, the contributions of single and double excitations out of the active space. The CASPT2 excitation energies, which do include dynamical correlation contributions computed perturbatively, are ∼6000 cm−1 lower. The value of 18953 cm−1, obtained with the larger 6-311G(d) basis, is in acceptable agreement with the experimental estimate of 18400 cm−1. Table 1 summarizes the calculated T0 values. Table 1. Calculated and Experimental Zero-Point Corrected Excitation Energies of the PhBz Radicala method
T0/cm−1
CASSCF/6-31G(d) CASPT2/6-31G(d)//CASSCF/6-31G(d) CASPT2/6-311G(d)//CASSCF/6-31G(d) experiment
24655 18646 18953 ∼18400
Figure 2. (a) The predicted excitation spectrum for the PhBz radical, excluding the torsional mode. (b) The predicted spectrum with only the torsional mode. (c) The complete FC simulation, computed by convolving (a) and (b).
Figure 3, with a double-well ground electronic state. The ground vibrational level is split into two near-degenerate states
All CASSCF calculations were performed with a [7,7] π-electron active space. a
We found the computed excitation energies to be fairly insensitive to the choice of π-electron active space, once it was larger than [5,5] (up to [11,11]). We settled on a [7,7] active space as a good compromise, yielding reliable accuracy at reasonable computational cost. Due to the large-amplitude displacement calculated for the torsional motion about the inter-ring bond, the FC simulation was performed by separating this mode from the other, rectilinear modes. In Figure 2a, a FC simulation for the D0 → D1 transition is plotted, ignoring the torsional mode. The resulting spectrum is origin-dominated, justifying the treatment of the remaining modes. Activity is seen in several modes, with the most significant FC factors being exhibited by modes in the 1300−1400 cm−1 region. The peak at 1344 cm−1 is due to a combination of inter-ring bond stretch and in-plane wagging of the aromatic hydrogens on the benzylic ring. The peak at 1425 cm−1 is due to a similarly coupled mode involving these motions with significant contribution from the stretching of the benzylic C−C bond. Other modes with notable FC factors include 311 cm−1, which may be described as a “concertina” mode, and 584 cm−1, which is an antisymmetric ring distortion. Calculated frequencies are provided in the Supporting Information. The torsional mode was treated by mapping out the coordinate using TD-DFT as described above. The moments of inertia for the benzyl and phenyl fragments were, respectively, IBz = 90.96 amu Å2 and IPh = 87.16 amu Å2. The resulting excitation spectrum for this mode is shown in Figure 2b. This mode shows the classic FC envelope for a lowfrequency mode with a large distortion and is typical of biphenyl chromophores.31 The TD-DFT calculations predict a planar equilibrium structure for the excited state, as shown in
Figure 3. The torsional potentials calculated by TDDFT. The vibrational wave functions on each state were calculated numerically, with the resulting excitation spectrum plotted at the right.
of even and odd parity, with a tunneling splitting calculated to be just 30 kHz. As such, they are expected to be equally populated in the molecular beam, giving rise to transitions to excited-state vibronic levels of alternating parity. This is indistinguishable from the spectrum arising from an optical isomer, localized in one well, as illustrated in Figure 3. The final form of the predicted spectrum, generated from the convolution of the previous two, is shown in Figure 2c. The 10782
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Figure 4. R2C2PI spectrum of the (a) PhBz radical and (b) TolBz radical. Identified progressions are indicated with various symbols, with wavenumbers of bands provided in the Supporting Information.
⎛ ⎛ 1⎞ 1 ⎞2 G(ν) = ⎜ν + ⎟ωe − ⎜ν + ⎟ ωexe ⎝ ⎝ 2⎠ 2⎠
spectral appearance is dominated by the FC envelopes arising from torsional motion coupled to the FC active rectilinear modes. The spectrum is clean at low wavenumber and becomes increasingly complicated as more and more progressions are commenced, culminating in a complex and congested thicket of vibronic bands. Experimental Spectra. The excitation spectrum measured by monitoring m/z 167 in the products of an electrical discharge containing 4-methylbiphenyl is shown in Figure 4a. While the ionization energies of the PhBz and TolBz radicals were not determined here, they must exceed the 6.4 eV delivered by the second laser pulse and almost certainly are less than the 7.2487 ± 0.0006 eV determined for the benzyl radical.32 The excitation energy, ∼18000 cm−1, is consistent with that of the calculations described above. The spectrum is dominated by a progression-forming mode of frequency of ∼67 cm−1, which exhibits a negative anharmonicity (increasing spacing), typical of torsional modes. This main progression is indicated with solid squares. A second progression commences some 600 cm−1 higher than the first identifiable band, as indicated by open triangles, with more and more progressions becoming apparent (open squares, for instance) as the relative energy increases. The spectrum becomes extremely congested from about 1000 cm−1. From theoretical considerations and comparison to biphenyl chromophores, the progressionforming mode is unambiguously attributed to the torsional mode of the two rings. The first 10 levels calculated from the TD-DFT potentials are well fit by
(4)
where ωe = 64.4 cm−1 and ωexe = −0.21 cm−1. This is in excellent agreement with the observed spacing of ∼67 cm−1. However, the agreement is better still when one considers that the peak of the FC envelope is calculated to be ν ≈ 17 and so the first measurable band is likely to be ν ≈ 3. Assuming this assignment, a fit of the first 10 observed members of the progression yields ωe = 64.8 cm−1 and ωexe = −0.37 cm−1. Guided by the spacings between the members of the other progressions, we can estimate their origin with respect to the unobserved 000 band. The progression marked with open triangles is estimated to commence 326 ± 70 cm−1 above the origin, which agrees with the predicted band at 311 cm−1, assigned to the “concertina” mode. Similar considerations for the progression indicated with open squares suggests a frequency of 300 ± 70 cm−1. However, no additional mode of this frequency is predicted to carry FC activity. The only nearby mode of a symmetry is the methylene group torsion at 332 cm−1. Given the difficulties with unambiguously assigning the clearest progressions, further assignments were not attempted. Furthermore, anharmonic coupling, giving rise to intramolecular vibrational redistribution at higher energies, makes such an exercise ultimately intractable. Nevertheless, the general appearance of the spectrum and the observed progression frequency are in accord with theory, and as such, the identification of the observed spectrum with the PhBz radical is clear. 10783
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Substituting 4-methylbiphenyl with 4,4′-dimethylbiphenyl yielded the spectrum plotted in Figure 4b, as monitored on m/z 181. The similar appearance and wavelength region make its identification with the TolBz radical beyond doubt. The extra methyl group of TolBz has little discernible effect on the excitation spectrum. The improved signal-to-noise allows clearer identification of the progressions, with the first observed peak likely to be ν = 1 or the origin. Identification of the first band with the origin yields ωe = 63.9 cm−1 and ωexe = −0.51 cm−1, which are very close to those observed for the related PhBz chromophore. The other progressions are more clearly identified as originating at 276 ± 70 and 396 ± 70 cm−1 above the origin. The lower-energy mode is likely the concertina mode predicted to lie at 311 cm−1 for PhBz, while the second is likely one of the ring-twisting modes predicted at 440 and 448 cm−1. However, strong FC activity was not predicted for these modes by CASSCF. As for the PhBz radical above, the spectrum of the TolBz radical breaks up into a congested plethora of bands beyond 1000 cm−1, and as such, no assignment is attempted.
Figure 5. The shift of the origin wavenumber for the species R−Ph− CH2, as compared to benzyl, plotted against the shift of R−Ph from benzene.
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DISCUSSION Both of the title radicals are found to exhibit excited-state torsional frequencies, ω′T, near 65 cm−1. In Table 2, the ω′T
depend approximately on the square root of the bond order. This is within the range of values exhibited for the similar chromophores, and thus, the similarity between the radical and closed-shell chromophore is unsurprising. Several diffuse interstellar bands have been noted to be regularly spaced by various frequencies (20−40 cm−1), which has been interpreted as evidence of “pendant rings”.33 The similarity of the torsional frequency of PhBz as compared to that of biphenyl indicates that such moieties will exhibit similar torsional force constants in a range of chemical environments. If one-half of the chromophore is heavy, then the frequency will be reduced by √2, ∼47 cm−1, which agrees well with the observed 49 cm−1 torsional mode of 9-phenylanthracene.34 To reduce this frequency still further, a heavier chromphore is required. Binaphthyls are observed to exhibit frequencies in the 30 cm−1 range.35,36 A radical chromophore built on a binaphthyl-like chromophore should exhibit a torsional frequency in the range consistent with the interstellar band observations. The presently identified spectra are the most red-shifted for any hitherto identified substituted benzyl radical (except analogues built on larger PAHs, such as naphthylmethyl37 and anthracenylmethyl38). Table 3 reveals that most substituted benzyl radicals exhibit excitation spectra in the 21000−22000 cm−1 range. A great number of examples may be found in the literature, and the contribution of the Lee group is notable and too numerous to review here.39,40 The effect of substituents on the benzyl radical origin is somewhat comparable to that on the parent chromophore. For example, fluorobenzene exhibits its origin band at 37813 cm−1, which is shifted ∼200 cm−1 to the red of benzene,41 while p-fluorobenzyl is 479 cm−1 red-shifted from benzyl itself.12 Anisole (trans-p-methoxybenzene) is found to absorb at 36394(2) cm−1,42 which is 1600 cm−1 red of benzene. The corresponding trans-p-methoxybenzyl radical is shifted from benzyl by 2354 cm−1. Cyanobenzene exhibits an origin band at 36513 cm−1, 1500 cm−1 red of benzene, but the cyanobenzyl radical absorbs just 1200 cm−1 lower than benzyl. The title radicals absorb near 18200 cm−1, shifted some 3800 cm−1 from benzyl. The corresponding shift for biphenyl is 2800 cm−1. Thus, it can be seen that the shifts of benzyl radicals induced by substituents are mostly larger than the correspond-
Table 2. Torsional Frequencies of Biphenyl-Containing Chromophores (ref 31) molecule
ωT′ / cm−1
biphenyl 4-methylbiphenyl 4,4′-dimethylbiphenyl 4-fluorobiphenyl 4,4′-difluorobiphenyl
65 66 70 64 70
Table 3. Experimental Origin Positions of Various Substituted Benzyl Radicals radical
origin/cm−1
ref
benzyl pentafluorobenzyl α-methylbenzyl p-fluorobenzyl p-methylbenzyl p-cyanobenzyl trans-p-methoxybenzyl p-phenylbenzyl
22002 21857 21778 21523 21350 20744 19648 ∼18400
12 39 40 12 12 43 44 this work
values for a range of closed-shell biphenyl species are shown. There is no clear effect of substituents on the torsional frequency, except, perhaps, that the disubstituted species tend to exhibit a higher frequency. Neither electron-withdrawing nor -donating groups have a dramatic effect; all torsional frequencies are in the 60−70 cm−1 range. It might thus seem entirely unsurprising that the present radicals exhibit torsional frequencies commensurate with the closed-shell chromophores. Turning to Hückel theory, we find that for the closed-shell biphenyl chromophore, the inter-ring π-bond order is 0.37 in the ground state but increases to 0.49 in the excited state (HOMO → LUMO). In the radical chromophore, the groundstate π-bond order is 0.40, which increases to 0.45 in the excited state. We could thus expect the frequencies in the two chromophores to be within 5% because the frequency should 10784
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(13) Lin, T.; Tan, X.; Cerny, T.; Williamson, J.; Cullin, D.; Miller, T. Chem. Phys. 1992, 167, 203−214. (14) Lin, T. Y. D.; Miller, T. A. J. Phys. Chem. 1990, 94, 3554−3559. (15) Tsuge, M.; Hamatani, S.; Kawai, A.; Tsuji, K.; Shibuya, K. Phys. Chem. Chem. Phys. 2006, 8, 256−263. (16) Okamura, T.; Obi, K.; Tanaka, I. Chem. Phys. Lett. 1973, 20, 90−91. (17) Kimura, N.; Takamuku, S. Bull. Chem. Soc. Jpn. 1993, 66, 3613− 3617. (18) Roos, B. O. Ab Initio Methods in Quantum Chemistry; Wiley: Chichester, U.K., 1987; Vol. II; p 399, and references therein. (19) Dalton, a molecular electronic structure program, release 2.0. http://daltonprogram.org/ (2005). (20) Andersson, K.; Malmqvist, P.; Roos, B. O. J. Chem. Phys. 1992, 96, 1218 , and references therein.. (21) Werner, H. Mol. Phys. 1996, 89, 645. (22) Celani, P.; Werner, H. J. Chem. Phys. 2000, 112, 5546. (23) Bauernschmitt, R.; Häser, M.; Trutler, O.; Ahlrichs, R. Chem. Phys. Lett. 1997, 264, 573. (24) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218. (25) Tozer, D. J.; Handy, N. C. Phys. Chem. Chem. Phys. 2000, 2, 2117. (26) MOLPRO, a package of ab initio programs, version 2009.1; designed by Werner, H.-J.; Knowles, P. J.; Manby, F. R.; Schütz, M.; Celani, P.; et al. 2009. (27) Schmidt, M.; Baldridge, K.; Boatz, J.; Elbert, S.; Gordon, M.; Jensen, J.; Koseki, S.; Matsunaga, N.; Nguyen, K.; Su, S.; et al. J. Comput. Chem. 1993, 14, 1347−1363. (28) Gordon, M.; Schmidt, M. Advances in Electronic Structure Theory: GAMESS A Decade Later. In Theory and Applications of Computational Chemistry: The First Forty Years; Dykstra, C. E., Frenking, G., Kim, K., Scuseria, G., Eds.; Elsevier: Amsterdam, The Netherlands, 2005; pp 1167−1189. (29) Frisch, M. J.; et al. Gaussian 09, revision A.1.; Gaussian Inc.: Wallingford, CT, 2009. (30) 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. (31) Im, H.; Bernstein, E. R. J. Chem. Phys. 1988, 88, 7337. (32) Eiden, G. C.; Weisshaar, J. C. J. Phys. Chem. 1991, 95, 6194− 6197. (33) Duley, W. W.; Kuzmin, S. Astrophys. J. Lett. 2010, 712, L165− L168. (34) Werst, D. W.; Genty, W. R.; Barbara, P. F. J. Phys. Chem. 1985, 89, 729−732. (35) Jonkman, H. T.; Wiersma, D. A. J. Chem. Phys. 1984, 81, 1573. (36) Del Riccio, J. L.; Zhang, F. T.; Lacey, A. R.; Kable, S. H. J. Phys. Chem. A 2000, 104, 7442−7451. (37) Chalyavi, N.; Troy, T. P.; Nakajima, M.; Gibson, B. A.; Nauta, K.; Sharp, R. G.; Kable, S. H.; Schmidt, T. W. J. Phys. Chem. A 2011, 115, 7959−7965. (38) O’Connor, G. D.; Bacskay, G. B.; Troy, T. P.; Nauta, K.; Schmidt, T. W. 2012, in preparation. (39) Lee, S. K.; Baek, D. Y. Chem. Phys. Lett. 1999, 311, 36−40. (40) Lee, G. W.; Ahn, H. G.; Kim, T. K.; Lee, S. K. Chem. Phys. Lett. 2008, 465, 193−196. (41) Butler, P.; Moss, D. B.; Yin, H.; Schmidt, T. W.; Kable, S. H. J. Chem. Phys. 2007, 127. (42) Hoffmann, L. J. H.; Marquardt, S.; Gemechu, A. S.; Baumgartel, H. Phys. Chem. Chem. Phys. 2006, 8, 2360−2377. (43) Fukushima, M.; Saito, K.; Obi, K. J. Mol. Spectrosc. 1996, 180, 389−397. (44) Sakeda, K.; Suzuki, T.; Matsushita, Y.; Ichimura, T. Phys. Chem. Chem. Phys. 2002, 4, 1746−1751.
ing shifts in benzene. The relationship between spectral shifts in substituted benzenes and benzyl radicals is illustrated in Figure 5.
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CONCLUSIONS The 4-phenylbenzyl and 4-(4′-methylphenyl)benzyl radicals have been identified by R2C2PI spectroscopy. They exhibit spectra in the green part of the spectrum, significantly lower in energy than other benzyl radicals hitherto studied. Furthermore, the spectra are characterized by long progressions in the inter-ring torsion, with a frequency at about 70 cm−1, peaking near ν = 17. CASSCF and TD-DFT theories were shown to be of great utility in predicting the general nature of the spectra and in aiding their interpretation.
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ASSOCIATED CONTENT
* Supporting Information S
CASSCF/6-31G(d) frequencies and band positions for PhBz and TolBz band positions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Discovery funding scheme (Project Number DP120102559). N.C. acknowledges the Endeavor International Postgraduate Research Scholarship and the University of Sydney International Scholarship. T.P.T. acknowledges the University of Sydney for a University Postgraduate Award. G.B. wishes to express thanks to the National Computational Infrastructure (NCI) National Facility of Australia for the generous allocation of computer time. S.A.R. acknowledges receipt of a Way-Klingler sabbatical fellowship from Marquette University.
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