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Observation of Vibronic Emission Spectrum of Jet-Cooled 3,5-Difluorobenzyl Radical Seung Woon Lee, Young Wook Yoon, and Sang Kuk Lee* Department of Chemistry and The Chemistry Institute for Functional Materials, Pusan National UniVersity, Pusan 609-735, Republic of Korea ReceiVed: May 7, 2010; ReVised Manuscript ReceiVed: July 11, 2010
We applied the technique of corona-excited supersonic expansion using a pinhole-type glass nozzle to observe the vibronic emission spectrum of jet-cooled benzyl-type radicals from the corona discharge of precursor 3,5-difluorotoluene seeded in a large amount of inert helium carrier gas. The vibronically well-resolved emission spectrum was recorded with a long-path monochromator in the visible region. After subtracting the vibronic bands originating from isomeric difluorobenzyl radicals from the observed spectrum, we identified for the first time the bands belonging to the 3,5-difluorobenzyl radical, from which the electronic energy and vibrational mode frequencies of the 3,5-difluorobenzyl radical were accurately determined in the ground electronic state by comparison with those of the precursor and with those from an ab initio calculation. Introduction Transient species, such as molecular ions and radicals, have long been believed to play an important role as reaction intermediates in chemical reactions.1 For those interested in the fundamental issue of reaction dynamics, the characteristics and structure of transient species should serve at least to define the nature of chemical reactivity.2 The benzyl radical, the prototype aromatic radical, is one of the most important intermediates in aromatic chemical reactions and is the subject of numerous spectroscopic studies.3-6 On the basis of the electronic structure of the benzyl radical, many benzyl-type radicals have been extensively studied to understand the substitution effect by employing the technique of corona-excited supersonic expansion (CESE), which is recognized as a powerful tool for the production of benzyl-type radicals in the gas phase.7 Recently, Lee and co-workers8 significantly improved the efficiency of radical generation by upgrading a pinhole-type glass nozzle for vibronic emission spectroscopy of transient species. The first fluorine-substituted benzyl radicals were produced by Bindley et al. by electric discharge of the corresponding fluorotoluenes,9 in which all three isomers of fluorobenzyl radicals were identified from analysis of the vibronic emission spectra. The laser-induced fluorescence spectra of fluorobenzyl radicals were recorded by monitoring the fluorescence from the decomposition products of precursors with a high power laser system.10 For high resolution work, the spectrum of the room temperature gas phase p-fluorobenzyl radical was simulated to identify the change of rotational constants upon excitation.11 More accurate rotational constants were obtained by CossartMagos and Cossart from analysis of the very cold excitation spectra of p-fluorobenzyl radicals by rotational contour analysis.12 In our laboratory, we have long observed vibronic emission spectra of many fluorine-substituted benzyl-type radicals, from which we determined the electronic energy and vibrational mode frequencies of mono-,13-15 bi-,16-20 and penta-substituted21 radicals. Although fluorine-substituted benzyl radicals are supposed to be suitable candidates for spectroscopic observation * To whom correspondence should be addressed. Fax: +82-51-516-7421. E-mail:
[email protected].
due to their strong fluorescence, a limited number of fluorobenzyl radicals have been identified by various spectroscopic techniques. Of the 6 isomers (2,3-,16 2,4-,17 2,5-,18 2,6-,19 3,4-,20 and 3,5difluorobenzyl radicals), the 2,6-isomer was first successfully identified through vibronic emission spectrum. Subsequently, the other four isomers, except for the 3,5-difluorobenzyl radical, were examined in sequence in our laboratory. In this paper, we present for the first time the observation of visible vibronic emission spectra of the jet-cooled 3,5-difluorobenzyl radical using a modified pinhole-type glass nozzle in the CESE system. From the analysis of the observed vibronic emission spectra, we accurately determined the electronic energy and vibrational mode frequencies of the 3,5-difluorobenzyl radical in the ground electronic state by comparison with those of the precursor and with those from an ab initio calculation. Experimental Section The experimental setup was similar to that described elsewhere.22 A corona discharge was combined with supersonic jet expansion for the generation and excitation of jet-cooled molecular radicals and a long-path monochromator for observation of the vibronic emission spectrum. The experimental conditions are briefly described as follows. The precursor 3,5-difluorotoluene (Aldrich, reagent grade) without further purification was corona discharged with a large amount of helium carrier gas using a pinhole-type nozzle in the CESE. The high purity of the precursor was confirmed by several analytical methods. The vapor of the compound at room temperature was mixed with 2.0 atm of He gas inside the vaporizing vessel. The concentration of the precursor in the mixture was adjusted for the maximum emission intensity as monitored by the strongest band. To ensure longer stability of the corona discharge, we employed a modified pinhole-type glass nozzle (D ) 0.3 mm) made in our laboratory, which significantly reduced the clogging problem by partially allowing the excitation to occur after the expansion.8 An approximate 1.5 kV discharging voltage of 3 mA current was applied to maximize the emission intensity from the strongest origin band
10.1021/jp104161g 2010 American Chemical Society Published on Web 08/02/2010
3,5-Difluorobenzyl Radical and to minimize the production of small fragments, such as C2 and CH radicals that emit very strong fluorescence in the visible region. The Pyrex glass chamber was evacuated by a mechanical vacuum pump to obtain a chamber pressure of about 1.0 Torr during the supersonic expansion with 2.0 atm of backing pressure. With the corona discharge of the precursor, a bluegreen colored jet was obtained, revealing the production of benzyl-type radicals that show strong emission in the visible region. The fluorescence from a jet area about 4 mm below the distance downstream from the nozzle throat was collected through a quartz lens (F ) 50 mm, D ) 38 mm) placed inside the chamber arm and focused onto the slit of a monochromator (Jobin Yvon U1000) equipped with a cooled photomultiplier tube (PMT; Hamamatsu R649). The emission spectrum was recorded by scanning from 18 000 to 22 500 cm-1 over 1.5 h at steps of 2.0 cm-1 with a slit width of 200 µm, with which the effective resolution is less than 1.0 cm-1 in the visible region. The wavenumber of the spectrometer was calibrated using He atomic lines23 recorded at the same time with the spectra and believed to be accurate to within (1.0 cm-1. Since the 3,5-difluorobenzyl radical has many vibrational modes that have not been completely assigned, an ab initio calculation was also carried out to assist the assignments of the observed vibronic bands. The calculation was done with a personal computer equipped with an Intel Pentium 1.2 GHz processor and 2.0 GB of RAM, and with the standard methods included in the Gaussian program for windows package. The geometry optimization and vibrational frequency calculations were performed at the density functional theory (DFT) level and 6-311 g** basis set was employed in all calculations. Results and Discussion It has previously been confirmed12 that a well-controlled corona discharge of substituted toluene with a large amount of inert carrier gas produces corresponding benzyl-type radicals in an excited vibronic state. Although the mechanism for the generation and excitation of benzyl-type radicals in the corona discharge has not been exactly established, it has been suggested8 that, in the use of He as a carrier gas, the metastable He atom in the 1s2s 3S1 state initially excited by the corona discharge transfers its excess energy to the precursor through a collisional process. The highly excited precursor then decomposes to produce corresponding benzyl-type radicals by losing a hydrogen atom from the vulnerable methyl group rather than from the benzene ring. The molecular radicals generated undergo collisional relaxation with He atoms and in the process lose rotational and vibrational energy in the excited electronic states, thereby producing electronically hot, but rovibrationally cold, species. A simulation of rotational contour assuming a Boltzmann distribution showed a rotational temperature of about 40 K, which is relatively higher than that obtained from routine supersonic jet expansion because of the low backing pressure and DC discharge.24 Although the backing pressure was limited by the tolerance of the glass material used for the nozzle, the direction of the transition moment were clearly identified from the contour analysis. The fluorescence emitted from the excited electronic state of benzyl-type radicals in the CESE represent the pure electronic transition energy as well as vibrational mode frequencies in the ground electronic state, in which the origin band shows the strongest intensity at the highest wavenumber. In addition, the spacing between the origin band and the observed vibronic
J. Phys. Chem. A, Vol. 114, No. 34, 2010 9111 TABLE 1: Origin Bands of Fluorine-Substituted Benzyl Radicals of the D1 f D0 Transitiona molecules
origin band
shiftd
benzylb o-fluorobenzylc m-fluorobenzylc p-fluorobenzylc 2,3-difluorobenzyle 2,4-difluorobenzyle 2,5-difluorobenzyle 2,6-difluorobenzyle 3,4-difluorobenzyle 3,5-difluorobenzylf p-fluoro-R-methylbenzylg pentafluorobenzylh
22 002 21 924 21 691 21 527 21 338 21 846 21 048 21 774 21 962 21 182 20 854 21 857
0 78 311 475 664 156 954 228 40 820 1148 145
a Measured in vacuum (cm-1). b Reference 5. c Reference 27. With respect to the origin band of the benzyl radical (22 002 cm-1). e References 16-20. f This work. g Reference 21. h Reference 16. d
bands indicates the vibrational mode frequencies in the ground electronic state.15 Thus, the emission spectrum observed in CESE is similar to dispersed fluorescence spectra while pumping the origin band of the electronic transition. In substituted benzyl-type radicals of a planar structure with seven delocalized π electrons, the interaction between the substituents and the benzene ring must be of second-order compared with that between the methylene group and the benzene ring.25 Thus, the electronic structure of substituted benzyl-type radicals should be similar to that of the benzyl radical, and indeed one might be able to closely relate the two lowest-lying electronic states of benzyl-type radicals to the parental benzyl radical in the 22B2 (D2) and 12A2 (D1) states. The visible emission spectra of benzyl-type radicals are believed to arise from transitions to the 12B2 (D0) ground state from the close-lying 22B2 (D2) and 12A2 (D1) excited electronic states,26 which could be mixed through vibronic coupling. Ring substitution is also expected to affect the energies of the two excited states differently. For the 3,5-difluorobenzyl radical with C2v symmetry, the lowest excited electronic state is the 12A2 state, as is the case of the benzyl radical, showing the B-type band shape for the electronic transition between the 12A2 and 12B2 states.15 However, it is not easy to observe the transition from the second excited electronic state to the ground state because of the very efficient collisional relaxation from D2 to D1 due to the vibronic coupling. The vibronic coupling between these two electronic states has been well described in benzyltype radicals.26 In benzyl-type radicals, the position of the origin band shifts to lower wavenumber upon substitutions into the benzene ring as compared to the parental benzyl radical at 22 002 cm-1. The o-, m-, and p-fluorobenzyl radicals show their origin bands at 21 924, 21 691, and 21 527 cm-1, which are shifted by 78, 311, and 475 cm-1 from the benzyl radical, respectively.27 It has been recently found that the p-fluoro-R-methylbenzyl radical28 also shows red-shift from the benzyl radical by 1148 cm-1. The larger shift suggests that there is a synergic effect by additional substituents for extension of π electron delocalization. The positions of the origin bands of fluorine-substituted benzyl radicals for the D1 f D0 electronic transition are listed in Table 1, together with other isomeric difluorobenzyl radicals. Figure 1 shows a portion of the visible vibronic emission spectrum observed from the corona discharge of precursor 3,5difluorotoluene seeded in a large amount of He inert carrier gas using a pinhole-type glass nozzle in the CESE. Most of the
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Figure 1. A portion of the vibronic emission spectrum observed from the corona discharge of 3,5-difluorotoluene seeded in a large amount of an inert helium carrier gas using a pinhole-type glass nozzle in a corona excited supersonic expansion (CESE). The strong bands belonging to the 3,5-difluorobenzyl radical in the D1(2A2) f D0(2B2) transition are labeled by vibrational modes.
TABLE 2: List of the Observed Vibronic Bands and Their Assignmentsa position
intensity
21 964 21 846 21 696 21 532 21 404 21 182 21 132 21 082 21 048 20 958 20 908 20 734 20 688 20 610 20 590 20 288 20 238 20 204 20 188 20 096 20 030 19 940 19 740 19 708 19 368
m s m vw w vs m w s w w w vs m w w w m m w w m w w m
spacing
0 50 100 224 274 448 494 572 592 944 978 994 1086 1152 1242 1442 1474 1814
b
assignments
c
origin (3,4-) origin (2,4-) origin (m-) 16b01 (3,4-) 9b01 (2,4-) origin (3,5-) 101 (2,4-) origin (2,5-) 10b01 (3,5-) 9b01 (3,5-) 16a01 (3,5-) 6a01 (3,5-) 101 (3,5-) 16b01 (3,5-) 19b01 (2,4-) 7b01 (3,5-) 7a01 (3,5-) 1201 (3,5-) 6a0116b01 (3,5-) 18a01 (3,5-) He atomic 6a017b01 (3,5-) 6a017a01 (3,5-)
a Measured in vacuum (cm-1). b Spacing from the origin band at 21 182 cm-1. c The notation in parentheses indicates the name of isomers of difluorobenzyl radicals.
bands listed in Table 2 were observed with very good S/N ratio in the spectral region of 19 000-22 000 cm-1. The spectrum showed several strong bands to the blue of the strongest band at 21 182 cm-1; that is slightly different from the typical spectra of benzyl-type radicals reported previously. In order to identify the origin of the strong vibronic bands observed to the blue of the strongest one, we compare in Figure 2 the spectrum observed with those from other isomers already reported in previous works. We assume the internal migration of fluorine atoms or methylene groups to adjacent positions on the benzene ring produces these isomers. The migration of a fluorine atom from the m-position to the o- and p-positions produces 2,5- and 3,4-difluorobenzyl radicals, respectively, while migration of the methylene group generates 2,4-difluorobenzyl radicals. From the comparison of the spectra, the strong bands, including the origin band of isomeric difluorobenzyl radicals,
Figure 2. Comparison of the spectra observed from difluorobenzyl radicals with those taken from the corona discharge of the 3,5difluorobenzyl radical. Several strong bands of difluorobenzyl radicals are coincident with those from taken from the corona discharge of 3,5difluorotoluene.
were observed at the same position as the spectrum obtained from the corona discharge of the precursor. Thus, we strongly believe that there is an internal rearrangement of substituents in the transition state during the corona discharge, producing other isomers. Of the isomers obtained, the 2,4-difluorobenzyl radical showed the strongest intensity because of the stability of the bridged cyclic structure using the methylene group, which is a well-known structure used by organic chemists as reaction intermediates. After subtracting the bands belonging to other isomers, we easily identified the vibronic bands originating from the 3,5difluorobenzyl radical, in which the origin band was assigned to the strongest band at 21 182 cm-1. The large shift of 820 cm-1 for the 3,5-difluorobenzyl radical from the benzyl radical at 22 002 cm-1 was attributed to the substitution effect which estimates the shift of multisubstituted benzyl-type radicals from those of monosubstituted benzyl-type radicals.29 The two fluorine substitutions at the m-position can show almost twice the shift of that of the monosubstituted benzyl radical. Although the vibronic assignment of large aromatic molecules cannot be straightforward, the strong observation of modes 1 and 6, well-known vibrational modes in benzene, in benzyltype radical provides more reliable evidence for the identification of the species. Moreover, the rotational contour would serve as additional support for the symmetry of vibrational modes. In the previous work on 2,6-difluorobenzyl radical19 with C2V symmetry, we presented a clear bandshapes of double and triplet for the a- and b-type modes, respectively. However, the band shape observed in this work could not resolve the characteristics of mode symmetry due to the torsional motion of two methyl groups at meta-positions. Thus, the vibronic bands observed in this work were assigned with the help of an ab initio calculation, as well as the known vibrational mode frequencies of the molecules of similar structure, 1,3,5-trifluorobenzene (TFB) and mesitylene.30 The comparison of the vibrational mode frequencies of radicals with those of the known frequencies of similar molecules has been employed in the assignment of the vibrational mode frequencies of many benzyl-type radicals. The very strong band at 494 cm-1 from the origin band in Figure 1 was assigned as mode 6a band, which is the most distinct observation in benzyl-type radicals with mode 6b because of the strongest emission intensity sensitive to the symmetry of molecules. The in-plane, ring deformation modes 6a and 6b are degenerate in benzene and highly symmetric in substituted benzene. With substitution on the benzene ring, these modes are split, giving higher and lower frequencies for modes
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TABLE 3: Vibrational Frequencies (cm-1) of the 3,5-Difluorobenzyl Radical modea
this workb (D0)
ab initioc B3LYP/6-311 g (D0)
mesitylened (S0)
1,3,5-trifluoro-benzene (TFB)d (S0)
symmetry (C2V)
origin 10b 9b 16a 6b 6a 16b 1 7b 7a 12 18a 13
21182 224 274 448 494 494 572 592 944 978 994 1152 1316
228 294 482 494 494 576 587 946 972 1009 1154 1327
225 275 453 516 516 453 579 935 935 995 1104 1298
253 326 595 500 500 595 578 993 993 1010 1122 1350
b1 b2 a2 b2 a1 b1 a1 b2 a1 a1 a1 a1
a
Reference 31. b Measured in vacuum (cm-1). c Multiplied by a scaling factor of 0.982. d Reference 30.
6a and 6b for C2V symmetry species, respectively, while the trend is reversed for Cs symmetry. Further, molecular symmetry strongly affects the intensity of each vibrational mode. In the vibronic emission spectrum of benzyl-type radicals with C2V symmetry, mode 6a is observable with strong intensity whereas mode 6b is detected with much weaker intensity. Becaise the calculation revealed very small splitting for these modes of the 3,5-difluorobenzyl radical (Table 3), the band observed in Figure 1 was considered to represent the overlapping of 6a and 6b, with the intensity mostly from 6a. Another important vibrational mode 1 (ring breathing) was assigned to the fairly strong band at 572 cm-1 since the frequency of this mode should be consistent with those of TFB and mesitylene. The calculation showed very good agreement with the observation for this mode. Mode 10b (C-H out-of-plane bending vibration) was assigned to the weak band at 224 cm-1 because of the excellent agreement with the calculated symmetry and frequency. For bending vibrational modes, mesitylene shows very similar vibrational frequencies to those of TFB. The weak, but wellresolved band at 274 cm-1 was assigned to mode 9b (C-H in-plane bending vibration), because of agreement with the calculation. The weak bands at 448 and 592 cm-1 from the origin band were assigned as modes 16a and 16b (out-of-plane ring deformation vibrations), respectively; these modes are degenerate in mesitylene and TFB. The splitting between modes 16a and 16b increases with increasing size of substituents. For C2V symmetry, mode 16b has a higher frequency than mode 16a, but the trend is reversed for Cs symmetry. The calculation showed very good agreement with observation. Mode 7 (C-H in-plane stretching vibration) is split into modes 7a and 7b with substitution, in which the former always shows strong intensity in benzyl-type radicals and higher frequency. In this observation, modes 7a and 7b were detected, respectively, as medium and weak intensity bands at 978 and 944 cm-1, as predicted. Mode 12 (in-plane ring deformation) was assigned to the strong band at 994 cm-1 because the calculation showed excellent agreement with observation. This mode was strongly observed in the m-fluorobenzyl radical. Other vibrational modes 18a (C-H inplane bending) and 13 (C-H in-plane stretching) were assigned to the weak bands at 1152 and 1316 cm-1, respectively, because of the coincidence of the symmetry and frequencies with those of the calculation as well as of TFB and mesitylene. In this observation, the modes of a1 symmetry showed strong intensity similar to that of other benzyl-type molecules. Conclusion We obtained the vibronic emission spectrum of benzyl-type radicals from the corona discharge of precursor 3,5-difluoro-
toluene seeded in a large amount of inert He carrier gas in the CESE using a pinhole-type glass nozzle. The spectral analysis revealed the presence of many vibronic bands, originating from other isomers; this suggests that the internal rearrangement produces the other isomers. After subtracting the bands belonging to other isomers from the observed spectrum, we identified the bands resulting from the 3,5-difluorobenzyl radical, from which we determined the electronic energy of the 12A2 f 12B2 transition and the vibrational mode frequencies in the ground electronic state for the first time. This work will complete identification of the vibronic spectra of difluorobenzyl radicals in the gas phase. Acknowledgment. This work was supported by the grant No. 2009-0071068 of the General Researcher Program and by grant No. R01-2008-000-20717-0 of the Midcareer Researcher Program through NRF grant funded by the MEST. The Korea Research Foundation Grant (KRF-2008-314-C00161) is also appreciated. References and Notes (1) Carrington, A. MicrowaVe Spectroscopy of Free Radicals; Academic: London, 1974. (2) Tan, X. Q.; Wright, T. G.; Miller, T. A. Electronic Spectroscopy of Free Radicals in Supersonic Jets: Jet Spectroscopy and Molecular Dynamics; Hollas, J. M., Phillip, D, EdsBlackie Academic & Professional, London, 1994. (3) Fukushima, M.; Obi, K. J. Chem. Phys. 1990, 93, 8488. (4) Fukushima, M.; Obi, K. J. Chem. Phys. 1992, 96, 4224. (5) Selco, J. I.; Carrick, P. G. J. Mol. Spectrosc. 1989, 137, 13. (6) Cossart, D.; Cossart-Magos, C. Chem. Phys. Lett. 1996, 250, 128. (7) Suh, M. H.; Lee, S. K.; Miller, T. A. J. Mol. Spectrosc. 1999, 194, 211. (8) Lee, S. K. Chem. Phys. Lett. 2002, 358, 110. (9) Bindley, T. F.; Watts, A. T.; Walker, S. Trans. Faraday Soc. 1964, 60, 1. (10) Charton, T. R.; Thrush, B. A. Chem. Phys. Lett. 1986, 125, 547. (11) Cossart-Magos, C.; Leach, S. J. Chem. Phys. 1972, 56, 1534. (12) Cossart-Magos, C.; Cossart, D. Mol. Phys. 1988, 65, 627. (13) Lee, S. K.; Lee, S. K. J. Phys. Chem. A 2001, 105, 3034. (14) Lee, S. K.; Ahn, B. U. Chem. Phys. Lett. 2000, 321, 25. (15) Lee, S. K.; Baek, D. Y. Chem. Phys. Lett. 1999, 301, 407. (16) Lee, G. W.; Ahn, H. G.; Kim, T. K.; Lee, S. K. Chem. Phys. Lett. 2007, 447, 197. (17) Lee, G. W.; Lee, S. K. Chem. Phys. Lett. 2006, 430, 8. (18) Lee, G. W.; Ahn, H. G.; Kim, T. K.; Lee, S. K. Chem. Phys. Lett. 2008, 454, 207. (19) Lee, S. K.; Baek, D. Y. J. Phys. Chem. A 2000, 104, 5219. (20) Lee, G. W.; Lee, S. K. Chem. Phys. Lett. 2007, 440, 36. (21) Lee, S. K.; Baek, D. Y. Chem. Phys. Lett. 1999, 311, 36. (22) Han, M. S.; Choi, I. S.; Lee, S. K. Bull. Korean Chem. Soc. 1996, 17, 991. (23) Weise, M. L.; Smith, M. W.; Glennon, B. M. Atomic Transition Probabilities; NSRD-NBS4, 1966.
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(24) Suh, M. H.; Lee, S. K.; Rehfuss, B. D.; Miller, T. A.; Bondybey, V. E. J. Phys. Chem. 1991, 95, 2727. (25) Hiratsuka, H.; Mori, K.; Shizuke, H.; Fukushima, M.; Obi, K. Chem. Phys. Lett. 1989, 157, 35. (26) Lin, T.-Y. D.; Tan, X.-Q.; Cerny, T. M.; Williamson, J. M.; Cullin, D. W.; Miller, T. A. Chem. Phys. 1992, 167, 203. (27) Selco, J. I.; Carrick, P. G. J. Mol. Spectrosc. 1995, 173, 277. (28) Lee, G. W.; Lee, S. K. Chem. Phys. Lett. 2009, 470, 54.
Lee et al. (29) Ahn, H. G.; Lee, G. W.; Kim, T. K.; Lee, S. K. Bull. Korean Chem. Soc. 2007, 28, 1993. (30) Varsanyi, G. Assignments for Vibrational Spectra of SeVen Hundred Benzene DeriVatiVes; John-Wiley & Sons: New York, 1974. (31) Wilson, E. B. Phys. ReV. 1934, 45, 706.
JP104161G
