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Vibronic emission spectra of jet-cooled 2,3-difluorobenzyl radical in a corona excited supersonic expansion
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Chemical Physics Letters 447 (2007) 197–201 www.elsevier.com/locate/cplett
Vibronic emission spectra of jet-cooled 2,3-difluorobenzyl radical in a corona excited supersonic expansion Gi Woo Lee, Hyeon Geun Ahn, Tae Kyu Kim, Sang Kuk Lee
*
Department of Chemistry, Pusan National University, Jangjeon-dong, Gumjeong-gu, Pusan 609-735, Republic of Korea Received 4 August 2007; in final form 10 September 2007 Available online 15 September 2007
Abstract We report the spectroscopic observation of difluorobenzyl radical in a corona excited supersonic expansion using a pinhole-type glass nozzle. A jet-cooled 2,3-difluorobenzyl radical was generated and vibronically excited from 2,3-difluorotoluene seeded in a large amount of inert carrier gas He. The vibronic emission spectra of the jet-cooled 2,3-difluorobenzyl radical were recorded for the first time in the visible region and analyzed to obtain an accurate measurement of the vibronic transition energy between the D1 and D0 electronic states. The vibrational mode frequencies in the ground electronic state were determined by comparison with those from an ab initio calculation as well as those of 2,6-difluorobenzyl radical and 1,2,3-trimethylbenzene. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Whereas the benzyl radical, a prototype of the aromatic free radical, has been the subject of numerous spectroscopic interests, halogen-substituted benzyl radicals have been less studied [1–3]. Bindley et al. [4] reported the first vibronic emission spectra of fluorobenzyl radicals generated from fluorotoluenes. Since then, many interesting studies related to fluorobenzyl radicals, employing a variety of spectroscopic techniques, have been reported [5–11]. The rotational contours were analyzed by CossartMagos and Cossart from the high temperature emission spectra of the p-fluorobenzyl radical in the gas phase, from which contours the rotational constants were estimated [12]. Fukushima and Obi [5] have attempted the first full assignment of vibrational modes in the ground electronic state from an analysis of the LIF excitation and dispersed fluorescence spectra. Miller et al. [6] have obtained accurate rotational constants from the high resolution LIF spectra recorded in a supersonic jet expansion. Recently, Lee and *
Corresponding author. Fax: +82 51 516 7421. E-mail address: [email protected] (S.K. Lee).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.024
Baek have extended the vibrational mode assignments of the jet-cooled p-fluorobenzyl [7] and pentafluorobenzyl [8] radicals using a technique of corona excited supersonic expansion. Using the same techniques, the o- and m-fluorobenzyl radicals have been identified from vibronically resolved emission spectra [9,10]. As for bi-substituted fluorobenzyl radicals, Lee and Baek [11] reported the first vibronic emission spectrum of the 2,6-difluorobenzyl radical in the D1 ! D0 transition, in which the well-resolved bandshapes cleared the symmetry of vibrational mode assignments. One of the interesting phenomena observed was the red shift of the origin band of bi-substituted benzyl radicals, in which the 2,6-fluorobenzyl radical exhibits almost twice that the shift of the o-fluorobenzyl radical. The shift of multi-substituted benzyl radicals has been described with reference to the substitution effect. In this work, we present the first observation of the visible vibronic emission spectrum of the jet-cooled 2,3difluorobenzyl radical in a corona excited supersonic expansion(CESE) using a pinhole-type glass nozzle. From an analysis of the observed spectra, spectroscopic data, including the electronic energy of the D1 ! D0 transition, were obtained by comparison with those of the well-known
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G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201
vibrational frequencies of the precursor and those from an ab initio calculation. 2. Experimental The experimental setup used in this work is similar to those described elsewhere [13,14]. Briefly, it consists of a pinhole-type glass nozzle coupled with corona discharge, a high vacuum expansion chamber, and a spectrometer for observing emission spectra in the visible region. The jet-cooled 2,3-difluorobenzyl radical was generated from 2,3-difluorotoluene seeded in a large amount of inert carrier gas He in a corona excited supersonic expansion using a pinhole-type glass nozzle. The reagent grade precursor 2,3-difluorotoluene was purchased from Aldrich and used without further purification. The liquid compound was vaporized at room temperature inside a thick Pyrex glass tube vaporizing vessel under 2.0 bar of carrier gas. The concentration of the precursor in the gas mixture was adjusted for the maximum emission intensity monitored from the strongest origin band, and was believed to be less than 1% in the gas mixture in terms of the vapor pressure. Since the corona discharge of large hydrocarbons produces soot deposits that clog the nozzle throat, we employed in this work a modified pinhole-type glass nozzle of 0.3 mm diameter, developed in this laboratory, which substantially improved the clogging problem by partially allowing the excitation to occur after the expansion [13]. The minimum discharging voltage was applied between the cathode and the anode located inside the nozzle to avoid the production of small fragments such as C2, which could easily be generated by over-voltage discharging. An approximately 5 mA current was applied to the system at a voltage of 2.0 kV to render observable the emission intensity of fluorescence from the origin band as well as to reduce the production of small fragments. The homemade expansion chamber, made of thick Pyrex glass tubing, was evacuated by a mechanical rotary vacuum pump of 800 L/min capacity, obtaining the chamber pressure of about 2 mbar during continuous expansion with 2.0 bar of backing pressure. The 2,3-difluorobenzyl radical was produced in the downstream jet, as was evidenced in the blue–green colored jet from the corona discharge of the precursor. The emission just below the nozzle throat was collected through a quartz lens of 38 mm diameter and 50 mm focal length placed inside the expansion chamber and focused onto the slit of a monochromator (Jobin Yvon U1000) equipped with a cooled PMT (Hamamatsu R649). The vibronic emission spectra were recorded by scanning from 18 000 to 22 500 cm1 at increments of 2.0 cm1 with 200 lm of slit width over 1 h. The spectrometer wavenumber was calibrated according to the He atomic transitions [15] recorded in the same spectral region as the spectra, and is believed to be accurate within ±1.0 cm1. Because the 2,3-difluorobenzyl radical has many vibrational modes whose assignments have not been completely
analyzed, ab initio calculations in the ground electronic state were carried out to assist the assignments of the vibronic emission spectra. The calculations were executed with a personal computer equipped with an Intel Pentium IV of 1.2 GHz CPU processor and 2048 MB RAM, according to the standard methods included in the GAUSSIAN 98 program for Windows package. Geometry optimization and vibrational frequency calculations were performed at the DFT level, and a 6-311 g* basis set was employed in all of the calculations. Each vibrational mode was identified using the HYPERCHEM program with the output of the Gaussian calculation. 3. Results and discussion It has been known that a well-controlled corona discharge of ring-substituted toluenes seeded in a large amount of inert carrier gas He produces corresponding benzyl-type radicals in the vibronically excited states. Although the exact mechanism for generation and excitation of benzyl-type radicals has not been established, it has been suggested that metastable He atoms generated in corona discharge transfer the excess energy to the precursor through a collisional process, resulting in the breaking off of one of the C–H bonds of the methyl group, the weakest bond in the precursor molecule in the gas phase. The weak visible emission spectrum of benzyl-type radicals is believed to arise from transitions to the 12B2(D0) ground state from the close-lying 22B2(D2) and 12A2(D1) excited states, which can be mixed through vibronic coupling [16,17]. Ring substitution is expected to affect the energies of the 22B2 and 12A2 excited states differently [6]. For the fluorobenzyl radicals, the lowest excited electronic state is the 12A2 state, such as that of the benzyl radical, exhibiting a B-type band shape for the electronic transition between the 12A2 and 12B2 states. However, it is not always possible to observe, for many benzyl-type radicals, the transition from the second excited electronic state to the ground state, due to the efficient collisional vibronic relaxation process from the D2 to D1 states during supersonic jet expansion [18]. Since the 2,3-difluorobenzyl radical belongs to the CS point group, the vibronic bands should exhibit a hybrid of A- and B-type bandshapes, according to the vibrational modes. Fig. 1 shows a portion of the visible vibronic emission spectrum of the jet-cooled 2,3-difluorobenzyl radical generated in this work, in which most of the bands were observed with noticeable intensity in the region of 19 000–22 000 cm1. Since the transition dipole moment is of the hybrid-type for CS point group, it is not easy to identify the symmetry of vibrational modes from the bandshapes. The frequency of the transition was measured at the maximum intensity of the band. Similar experiments [19,20] have shown that the jet expansion with corona discharge gives rise to the rotational temperature of 50 K, which is a reasonable value, given by a low backing pressure in a supersonic jet expansion. However, an analysis
G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201 6000 CH2 •
0 00
Intensity
F
4000
6b 10 F
2000
0 19000
12 10 14 10
20000
5 10
6a 10
9a 10
21000
22000
Wavenumber (cm-1)
Fig. 1. A portion of vibronic emission spectrum in the D1(2A2) ! D0(2B2) transition of the jet-cooled 2,3-difluorobenzyl radical produced from 2,3difluorotoluene seeded in a large amount of inert carrier gas He in a corona excited supersonic expansion using a pinhole-type glass nozzle. The origin band shows the strongest intensity due to the collisional vibrational relaxation process in the excited electronic state.
revealed that the rotational contours of the symmetric 2,6difluorobenzyl radical were well established in the assignment of the vibrational modes in the ground electronic state. With a pinhole-type glass nozzle in a corona excited supersonic expansion, vibrational relaxation in the D1 state is so efficient that the vibronic emission spectra should be similar to the dispersed fluorescence spectra observed by pumping the origin band of the electronic transition [5,7,21]. Thus the spacing between the observed vibronic band and the origin band of the electronic transition should represent the vibrational mode frequencies in the ground electronic state. Moreover, collisional relaxation certainly increases the population in the vibrationless state of the lowest excited electronic state, resulting in observation of the strongest intensity from the origin band. The band with the strongest intensity at 21 338 cm1 was deemed to be the origin band of the 2,3-difluorobenzyl radical in the D1 ! D0 transition, followed to lower energies by a series of vibronic bands. The absence of any band of noticeable intensity to the blue of the origin band strongly supports this assignment, since the highly efficient vibronic relaxation process has been observed in the D2 state of the benzyl-type radicals with the CESE system. The shift of the origin band from the original benzyl radical indicates the contribution of substituents to the electronic transition. The 2,6-difluorobenzyl radical, with two substituents in o-positions, shows the origin band at 21 774 cm1 [11], shifted by 228 cm1 from the benzyl radical, which is almost twice the shift of o-fluorobenzyl radicals. A similar tendency has been observed in the 2,6dichloro [22] and 2,6-dimethylbenzyl [23] radicals. The vibronic bands observed in this work were assigned with the help of an ab initio calculation as well as the known vibrational frequencies of the 2,6-difluorobenzyl radical and 1,2,3-trimethylbenzene, since these molecules are subjected to the isodynamic approximation rule that establishes well the correspondence of vibrational mode
199
frequencies and intensity. This rule has already been applied to the vibrational mode assignment of many benzyl-type radicals [7–11]. The moderately strong band at 290 cm1 from the origin band was assigned to mode 9a of C–H in-plane bending vibration. The large shift to a lower wavenumber compared to the benzyl radical reflects the substitution of a heavy atom in place of hydrogen. Strong and well-resolved bands at 496 and 550 cm1 were assigned, respectively, to modes 6a and 6b of in-plane ring deformation, one of the most prominent modes in benzene derivatives. Mode 6a is degenerate with mode 6b in benzene with a mode frequency of 606 cm1. In the vibronic emission spectrum of the 2,6difluorobenzyl radical of C2v symmetry, mode 6a was observable with a fairly strong intensity, whereas mode 6b was barely detected due to a very weak intensity, which is also true for CS symmetry. The calculation predicts the observation very well. The strong band at 686 cm1 was assigned to mode 1 of ring breathing vibration because the frequency of this mode should be consistent with that of the 2,6-difluorobenzyl (696 cm1) as well as less sensitive to the substitution. The p-fluorobenzyl radical followed a similar tendency for this mode. The calculation (696 cm1) agreed well with the observation. The weak but well-resolved band at 786 cm1 was provisionally assigned to mode 11 of C–H out-of-plane vibration band because the calculation provides a very accurate prediction of the out-of-plane symmetry in this region. Molecules of similar structure show a very consistent frequency for this mode. Mode 12 of inplane ring deformation vibration was assigned to the medium band at 828 cm1 from the origin. The calculation agrees well with the observation. Also, this mode shows the only a 0 symmetry in this region. The band of medium intensity at 996 cm1 was assigned to mode 5 of C–H out-of-plane vibration, which is also in accord with the calculation (968 cm1). Mode 18a of C–H in-plane bending vibration splits into modes 18a and 18b with substitution, in which mode 18a is always strong in substituted benzyltype radicals of higher frequency. The calculation and the 1,2,3-trimethylbenzene agree well with the observation. Mode 13 of in-plane C–H stretching vibration was assigned to the medium band at 1186 cm1 since the calculation agreed with the observation as well as 1,2,3-trimethylbenzene. The medium intensity band at 1220 cm1 was assigned to mode 20a of in-plane C–F ring stretching vibration because this mode should be less sensitive to the substitution. The precursor gives a similar frequency to this mode. Finally, mode 14 of C–H in-plane bending vibration was assigned to the strong intensity band 1284 cm1 from the origin. The isomer 2,6-difluorobenzyl radical shows 1268 cm1 for this mode with comparable intensity. This mode is also less sensitive to the substitution. The vibrational mode frequencies obtained in this work are listed, with the assignments, in Table 1. Also, several combination bands, most of them of weak intensity, were observed at the calculated frequencies. These bands are
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Table 1 List of the vibronic bands observed and their assignmentsa
0 00
Position
Intensity
Spacingb
Assignmentsc
21 338 21 288 21 268 21 048 20 842 20 788 20 652 20 552 20 510 20 342 20 292 20 152 20 118 20 054
vs m m m s s s w m m m m m s
0 50 70 290 496 550 686 786 828 996 1046 1186 1220 1284
Origin a b 9a01 6a01 6b01 101 1101 1201 501 18a01 1301 20a01 1401
Measured in vacuum (cm1). Spacing from the origin band at 21 338 cm1. c Greek letters indicate the low frequency sequence bands associated with the strongest origin band.
CH2• F
F
14 10
a
Table 2 Vibrational frequencies (cm1) of the 2,3-difluorobenzyl radical Modea
This workb (D0)
Origin 9a 6a 6b 1 11 12 5 18a 13 20a 14
21 338 290 496 550 686 786 828 996 1046 1186 1220 1284
a b c d e
Ab initioc B3LYP/ 6-311 g (D0)
287 501 551 696 785 826 968 1055 1168 1234 1302
2,6Difluorobenzyl radicald (D0) 21 774 332 470 696
1,2,3Trimethylbenzenee (S0)
Symmetry (CS)
318 485 539 659 768 815 960 1095 1193 1248
a0 a0 a0 a0 a00 a0 a00 a0 a0 a0 a0
1268
Ref. [24]. Measured in vacuum (cm1). Multiplied by a scaling factor of 0.995. Ref. [11]. Ref. [25].
•H2C
6a 10
9a 10
6b 10
F
b
combinations of the bands of strong intensity and the same symmetry, and belong to the in-plane vibration of a 0 symmetry. The observed vibrational mode frequencies are compared with those of an ab initio calculation and those of the precursor, together with the mode symmetries, in Table 2. A total of six isomers of difluorobenzyl radicals can be divided into three different classes: 1,2,3-substitutions consist of 2,6- and 2,3-difluorobenzyl radicals with C2V and CS symmetries, respectively. 1,2,4-substitution contains 2,4-, 2,5-, and 3,4-difluorobenzyl radicals of CS symmetry. Finally, 3,5-difluorobenzyl radical of C2V symmetries belongs to 1,3,5-substitution. The vibronic emission spectrum of 2,3-difluorobenzyl radical was compared in Fig. 2 with that of 2,6-difluorobenzyl, in which both spectra show similar vibrational structure in the ground
1 10
F
Fig. 2. Similarity of the vibronic emission spectrum of between 2,6- and 2,3-difluorobenzyl radicals in the D1(2A2) ! D0(2B2) electronic transition. Both spectra were taken at the same experimental conditions with different precursors. With changing the symmetry from C2V to CS point groups, the band belonging to 6b vibrational mode shows increasing intensity at higher wavenumber, which has also been confirmed in Ref. [25].
electronic state. While the 6b mode was not observed in 2,6-difluorobenzyl radical, the same mode was observed with strong intensity in 2,3,-difluorobenzyl radical. This tendency has been observed in many benzyl-type radicals of CS and C2V point groups. We also observed low frequency sequence bands in the vicinity of the origin band, in which the distance from the origin band is less than 150 cm1. The problem in explaining the origin of low frequency sequence bands remains. Fukushima and Obi [5] have observed several weak bands from the p-fluorobenzyl radical in the vicinity of the strong bands, and attributed them as belonging to the van der Waals molecules. However, it seems unlikely, in the present study, because backing pressure is much lower in the glass-type nozzle scheme. Also the same bands were detected with the other carrier gas Ar, which yielded a much poorer spectral S/N. Cossart-Magos and Cossart [12] have observed several weak low frequency sequence bands near the origin band of the p-fluorobenzyl radical, and assigned them as combination bands coupled with the origin band. Thus, we strongly believe, owing to the incomplete vibrational cooling process in the excited electronic state during supersonic jet expansion, that these bands are from the transition to the excited vibrational state belonging to the D0 state from the excited vibrational state of the D1 electronic state. The distance from the origin band indicates the vibrational energy difference between two electronic states, D1 and D0 states.
G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201
In summary, we generated, for the first time, the jetcooled but vibronically excited 2,3-difluorobenzyl radical from 2,3-difluorotoluene seeded in a large amount of carrier gas He using a pinhole-type glass nozzle in a corona excited supersonic expansion. The visible vibronic emission spectrum of the jet-cooled 2,3-difluorobenzyl radical was recorded, from which the electronic energy of the D1 ! D0 transition and the frequencies of several vibrational modes in the ground electronic state were determined by comparison with those of 2,6-difluorobenzyl radical and 1,2,3-trimethylbenzene as well as with those from an ab initio calculation. Acknowledgement This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-311-C00336 and R14-2003-03301002-0). References [1] X.Q. Tan, T.G. Wright, T.A. Miller, in: J.M. Hollas, D. Phillip (Eds.), Electronic Spectroscopy of Free Radicals in Supersonic Jets: Jet Spectroscopy and Molecular Dynamics, Blackie Academic & Professional, London, 1994.
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[2] T.R. Charlton, B.A. Thrush, Chem. Phys. 125 (1986) 547. [3] H. Hiratsuka, K. Mori, H. Shizuka, M. Fukushima, K. Obi, Chem. Phys. Lett. 157 (1989) 35. [4] T.F. Bindley, A.T. Watts, S. Walker, Trans. Faraday Soc. 60 (1964) 1. [5] M. Fukushima, K. Obi, J. Chem. Phys. 93 (1990) 8488. [6] T.-Y. Lin, X.-Q. Tan, T.M. Cerny, J.M. Williamson, D.W. Cullin, T.A. Miller, Chem. Phys. 167 (1992) 203. [7] S.K. Lee, D.Y. Baek, Chem. Phys. Lett. 301 (1999) 407. [8] S.K. Lee, D.Y. Baek, Chem. Phys. Lett. 311 (1999) 36. [9] S.K. Lee, S.K. Lee, J. Phys. Chem. A 105 (2001) 3034. [10] S.K. Lee, B.U. Ahn, Chem. Phys. Lett. 321 (2000) 25. [11] S.K. Lee, D.Y. Baek, J. Phys. Chem. A 104 (2000) 5219. [12] C. Cossart-Magos, D. Cossart, Mol. Phys. 65 (1988) 627. [13] M.S. Han, I.S. Choi, S.K. Lee, Bull. Korean Chem. Soc. 17 (1996) 991. [14] S.K. Lee, Chem. Phys. Lett. 358 (2002) 110. [15] M.L. Wiese, M.W. Smith, B.M. Glennon, Atomic Transition Probabilities, NSRD-NBS4, Gaithersburg, MD, 1966. [16] M. Fukushima, K. Obi, J. Chem. Phys. 96 (1992) 4224. [17] G.C. Eiden, J.C. Weisshaar, J. Chem. Phys. 104 (1996) 8896. [18] G.W. Lee, S.K. Lee, Chem. Phys. Lett. 423 (2006) 98. [19] M.H. Suh, S.K. Lee, B.D. Rehfuss, T.A. Miller, V.E. Bondybey, J. Phys. Chem. 95 (1991) 2727. [20] M.H. Suh, S.K. Lee, T.A. Miller, J. Mol. Spectrosc. 194 (1999) 211. [21] S.K. Lee, D.Y. Baek, Chem. Phys. Lett. 304 (1999) 39. [22] S.K. Lee, S.J. Kim, Chem. Phys. Lett. 412 (2005) 88. [23] G.W. Lee, S.K. Lee, J. Phys. Chem. A 110 (2006) 2130. [24] E.B. Wilson, Phys. Rev. 45 (1934) 706. [25] G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, Wiley, New York, NY, 1974.
Chemical Physics Letters 447 (2007) 197–201 www.elsevier.com/locate/cplett
Vibronic emission spectra of jet-cooled 2,3-difluorobenzyl radical in a corona excited supersonic expansion Gi Woo Lee, Hyeon Geun Ahn, Tae Kyu Kim, Sang Kuk Lee
*
Department of Chemistry, Pusan National University, Jangjeon-dong, Gumjeong-gu, Pusan 609-735, Republic of Korea Received 4 August 2007; in final form 10 September 2007 Available online 15 September 2007
Abstract We report the spectroscopic observation of difluorobenzyl radical in a corona excited supersonic expansion using a pinhole-type glass nozzle. A jet-cooled 2,3-difluorobenzyl radical was generated and vibronically excited from 2,3-difluorotoluene seeded in a large amount of inert carrier gas He. The vibronic emission spectra of the jet-cooled 2,3-difluorobenzyl radical were recorded for the first time in the visible region and analyzed to obtain an accurate measurement of the vibronic transition energy between the D1 and D0 electronic states. The vibrational mode frequencies in the ground electronic state were determined by comparison with those from an ab initio calculation as well as those of 2,6-difluorobenzyl radical and 1,2,3-trimethylbenzene. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Whereas the benzyl radical, a prototype of the aromatic free radical, has been the subject of numerous spectroscopic interests, halogen-substituted benzyl radicals have been less studied [1–3]. Bindley et al. [4] reported the first vibronic emission spectra of fluorobenzyl radicals generated from fluorotoluenes. Since then, many interesting studies related to fluorobenzyl radicals, employing a variety of spectroscopic techniques, have been reported [5–11]. The rotational contours were analyzed by CossartMagos and Cossart from the high temperature emission spectra of the p-fluorobenzyl radical in the gas phase, from which contours the rotational constants were estimated [12]. Fukushima and Obi [5] have attempted the first full assignment of vibrational modes in the ground electronic state from an analysis of the LIF excitation and dispersed fluorescence spectra. Miller et al. [6] have obtained accurate rotational constants from the high resolution LIF spectra recorded in a supersonic jet expansion. Recently, Lee and *
Corresponding author. Fax: +82 51 516 7421. E-mail address: [email protected] (S.K. Lee).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.024
Baek have extended the vibrational mode assignments of the jet-cooled p-fluorobenzyl [7] and pentafluorobenzyl [8] radicals using a technique of corona excited supersonic expansion. Using the same techniques, the o- and m-fluorobenzyl radicals have been identified from vibronically resolved emission spectra [9,10]. As for bi-substituted fluorobenzyl radicals, Lee and Baek [11] reported the first vibronic emission spectrum of the 2,6-difluorobenzyl radical in the D1 ! D0 transition, in which the well-resolved bandshapes cleared the symmetry of vibrational mode assignments. One of the interesting phenomena observed was the red shift of the origin band of bi-substituted benzyl radicals, in which the 2,6-fluorobenzyl radical exhibits almost twice that the shift of the o-fluorobenzyl radical. The shift of multi-substituted benzyl radicals has been described with reference to the substitution effect. In this work, we present the first observation of the visible vibronic emission spectrum of the jet-cooled 2,3difluorobenzyl radical in a corona excited supersonic expansion(CESE) using a pinhole-type glass nozzle. From an analysis of the observed spectra, spectroscopic data, including the electronic energy of the D1 ! D0 transition, were obtained by comparison with those of the well-known
198
G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201
vibrational frequencies of the precursor and those from an ab initio calculation. 2. Experimental The experimental setup used in this work is similar to those described elsewhere [13,14]. Briefly, it consists of a pinhole-type glass nozzle coupled with corona discharge, a high vacuum expansion chamber, and a spectrometer for observing emission spectra in the visible region. The jet-cooled 2,3-difluorobenzyl radical was generated from 2,3-difluorotoluene seeded in a large amount of inert carrier gas He in a corona excited supersonic expansion using a pinhole-type glass nozzle. The reagent grade precursor 2,3-difluorotoluene was purchased from Aldrich and used without further purification. The liquid compound was vaporized at room temperature inside a thick Pyrex glass tube vaporizing vessel under 2.0 bar of carrier gas. The concentration of the precursor in the gas mixture was adjusted for the maximum emission intensity monitored from the strongest origin band, and was believed to be less than 1% in the gas mixture in terms of the vapor pressure. Since the corona discharge of large hydrocarbons produces soot deposits that clog the nozzle throat, we employed in this work a modified pinhole-type glass nozzle of 0.3 mm diameter, developed in this laboratory, which substantially improved the clogging problem by partially allowing the excitation to occur after the expansion [13]. The minimum discharging voltage was applied between the cathode and the anode located inside the nozzle to avoid the production of small fragments such as C2, which could easily be generated by over-voltage discharging. An approximately 5 mA current was applied to the system at a voltage of 2.0 kV to render observable the emission intensity of fluorescence from the origin band as well as to reduce the production of small fragments. The homemade expansion chamber, made of thick Pyrex glass tubing, was evacuated by a mechanical rotary vacuum pump of 800 L/min capacity, obtaining the chamber pressure of about 2 mbar during continuous expansion with 2.0 bar of backing pressure. The 2,3-difluorobenzyl radical was produced in the downstream jet, as was evidenced in the blue–green colored jet from the corona discharge of the precursor. The emission just below the nozzle throat was collected through a quartz lens of 38 mm diameter and 50 mm focal length placed inside the expansion chamber and focused onto the slit of a monochromator (Jobin Yvon U1000) equipped with a cooled PMT (Hamamatsu R649). The vibronic emission spectra were recorded by scanning from 18 000 to 22 500 cm1 at increments of 2.0 cm1 with 200 lm of slit width over 1 h. The spectrometer wavenumber was calibrated according to the He atomic transitions [15] recorded in the same spectral region as the spectra, and is believed to be accurate within ±1.0 cm1. Because the 2,3-difluorobenzyl radical has many vibrational modes whose assignments have not been completely
analyzed, ab initio calculations in the ground electronic state were carried out to assist the assignments of the vibronic emission spectra. The calculations were executed with a personal computer equipped with an Intel Pentium IV of 1.2 GHz CPU processor and 2048 MB RAM, according to the standard methods included in the GAUSSIAN 98 program for Windows package. Geometry optimization and vibrational frequency calculations were performed at the DFT level, and a 6-311 g* basis set was employed in all of the calculations. Each vibrational mode was identified using the HYPERCHEM program with the output of the Gaussian calculation. 3. Results and discussion It has been known that a well-controlled corona discharge of ring-substituted toluenes seeded in a large amount of inert carrier gas He produces corresponding benzyl-type radicals in the vibronically excited states. Although the exact mechanism for generation and excitation of benzyl-type radicals has not been established, it has been suggested that metastable He atoms generated in corona discharge transfer the excess energy to the precursor through a collisional process, resulting in the breaking off of one of the C–H bonds of the methyl group, the weakest bond in the precursor molecule in the gas phase. The weak visible emission spectrum of benzyl-type radicals is believed to arise from transitions to the 12B2(D0) ground state from the close-lying 22B2(D2) and 12A2(D1) excited states, which can be mixed through vibronic coupling [16,17]. Ring substitution is expected to affect the energies of the 22B2 and 12A2 excited states differently [6]. For the fluorobenzyl radicals, the lowest excited electronic state is the 12A2 state, such as that of the benzyl radical, exhibiting a B-type band shape for the electronic transition between the 12A2 and 12B2 states. However, it is not always possible to observe, for many benzyl-type radicals, the transition from the second excited electronic state to the ground state, due to the efficient collisional vibronic relaxation process from the D2 to D1 states during supersonic jet expansion [18]. Since the 2,3-difluorobenzyl radical belongs to the CS point group, the vibronic bands should exhibit a hybrid of A- and B-type bandshapes, according to the vibrational modes. Fig. 1 shows a portion of the visible vibronic emission spectrum of the jet-cooled 2,3-difluorobenzyl radical generated in this work, in which most of the bands were observed with noticeable intensity in the region of 19 000–22 000 cm1. Since the transition dipole moment is of the hybrid-type for CS point group, it is not easy to identify the symmetry of vibrational modes from the bandshapes. The frequency of the transition was measured at the maximum intensity of the band. Similar experiments [19,20] have shown that the jet expansion with corona discharge gives rise to the rotational temperature of 50 K, which is a reasonable value, given by a low backing pressure in a supersonic jet expansion. However, an analysis
G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201 6000 CH2 •
0 00
Intensity
F
4000
6b 10 F
2000
0 19000
12 10 14 10
20000
5 10
6a 10
9a 10
21000
22000
Wavenumber (cm-1)
Fig. 1. A portion of vibronic emission spectrum in the D1(2A2) ! D0(2B2) transition of the jet-cooled 2,3-difluorobenzyl radical produced from 2,3difluorotoluene seeded in a large amount of inert carrier gas He in a corona excited supersonic expansion using a pinhole-type glass nozzle. The origin band shows the strongest intensity due to the collisional vibrational relaxation process in the excited electronic state.
revealed that the rotational contours of the symmetric 2,6difluorobenzyl radical were well established in the assignment of the vibrational modes in the ground electronic state. With a pinhole-type glass nozzle in a corona excited supersonic expansion, vibrational relaxation in the D1 state is so efficient that the vibronic emission spectra should be similar to the dispersed fluorescence spectra observed by pumping the origin band of the electronic transition [5,7,21]. Thus the spacing between the observed vibronic band and the origin band of the electronic transition should represent the vibrational mode frequencies in the ground electronic state. Moreover, collisional relaxation certainly increases the population in the vibrationless state of the lowest excited electronic state, resulting in observation of the strongest intensity from the origin band. The band with the strongest intensity at 21 338 cm1 was deemed to be the origin band of the 2,3-difluorobenzyl radical in the D1 ! D0 transition, followed to lower energies by a series of vibronic bands. The absence of any band of noticeable intensity to the blue of the origin band strongly supports this assignment, since the highly efficient vibronic relaxation process has been observed in the D2 state of the benzyl-type radicals with the CESE system. The shift of the origin band from the original benzyl radical indicates the contribution of substituents to the electronic transition. The 2,6-difluorobenzyl radical, with two substituents in o-positions, shows the origin band at 21 774 cm1 [11], shifted by 228 cm1 from the benzyl radical, which is almost twice the shift of o-fluorobenzyl radicals. A similar tendency has been observed in the 2,6dichloro [22] and 2,6-dimethylbenzyl [23] radicals. The vibronic bands observed in this work were assigned with the help of an ab initio calculation as well as the known vibrational frequencies of the 2,6-difluorobenzyl radical and 1,2,3-trimethylbenzene, since these molecules are subjected to the isodynamic approximation rule that establishes well the correspondence of vibrational mode
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frequencies and intensity. This rule has already been applied to the vibrational mode assignment of many benzyl-type radicals [7–11]. The moderately strong band at 290 cm1 from the origin band was assigned to mode 9a of C–H in-plane bending vibration. The large shift to a lower wavenumber compared to the benzyl radical reflects the substitution of a heavy atom in place of hydrogen. Strong and well-resolved bands at 496 and 550 cm1 were assigned, respectively, to modes 6a and 6b of in-plane ring deformation, one of the most prominent modes in benzene derivatives. Mode 6a is degenerate with mode 6b in benzene with a mode frequency of 606 cm1. In the vibronic emission spectrum of the 2,6difluorobenzyl radical of C2v symmetry, mode 6a was observable with a fairly strong intensity, whereas mode 6b was barely detected due to a very weak intensity, which is also true for CS symmetry. The calculation predicts the observation very well. The strong band at 686 cm1 was assigned to mode 1 of ring breathing vibration because the frequency of this mode should be consistent with that of the 2,6-difluorobenzyl (696 cm1) as well as less sensitive to the substitution. The p-fluorobenzyl radical followed a similar tendency for this mode. The calculation (696 cm1) agreed well with the observation. The weak but well-resolved band at 786 cm1 was provisionally assigned to mode 11 of C–H out-of-plane vibration band because the calculation provides a very accurate prediction of the out-of-plane symmetry in this region. Molecules of similar structure show a very consistent frequency for this mode. Mode 12 of inplane ring deformation vibration was assigned to the medium band at 828 cm1 from the origin. The calculation agrees well with the observation. Also, this mode shows the only a 0 symmetry in this region. The band of medium intensity at 996 cm1 was assigned to mode 5 of C–H out-of-plane vibration, which is also in accord with the calculation (968 cm1). Mode 18a of C–H in-plane bending vibration splits into modes 18a and 18b with substitution, in which mode 18a is always strong in substituted benzyltype radicals of higher frequency. The calculation and the 1,2,3-trimethylbenzene agree well with the observation. Mode 13 of in-plane C–H stretching vibration was assigned to the medium band at 1186 cm1 since the calculation agreed with the observation as well as 1,2,3-trimethylbenzene. The medium intensity band at 1220 cm1 was assigned to mode 20a of in-plane C–F ring stretching vibration because this mode should be less sensitive to the substitution. The precursor gives a similar frequency to this mode. Finally, mode 14 of C–H in-plane bending vibration was assigned to the strong intensity band 1284 cm1 from the origin. The isomer 2,6-difluorobenzyl radical shows 1268 cm1 for this mode with comparable intensity. This mode is also less sensitive to the substitution. The vibrational mode frequencies obtained in this work are listed, with the assignments, in Table 1. Also, several combination bands, most of them of weak intensity, were observed at the calculated frequencies. These bands are
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G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201
Table 1 List of the vibronic bands observed and their assignmentsa
0 00
Position
Intensity
Spacingb
Assignmentsc
21 338 21 288 21 268 21 048 20 842 20 788 20 652 20 552 20 510 20 342 20 292 20 152 20 118 20 054
vs m m m s s s w m m m m m s
0 50 70 290 496 550 686 786 828 996 1046 1186 1220 1284
Origin a b 9a01 6a01 6b01 101 1101 1201 501 18a01 1301 20a01 1401
Measured in vacuum (cm1). Spacing from the origin band at 21 338 cm1. c Greek letters indicate the low frequency sequence bands associated with the strongest origin band.
CH2• F
F
14 10
a
Table 2 Vibrational frequencies (cm1) of the 2,3-difluorobenzyl radical Modea
This workb (D0)
Origin 9a 6a 6b 1 11 12 5 18a 13 20a 14
21 338 290 496 550 686 786 828 996 1046 1186 1220 1284
a b c d e
Ab initioc B3LYP/ 6-311 g (D0)
287 501 551 696 785 826 968 1055 1168 1234 1302
2,6Difluorobenzyl radicald (D0) 21 774 332 470 696
1,2,3Trimethylbenzenee (S0)
Symmetry (CS)
318 485 539 659 768 815 960 1095 1193 1248
a0 a0 a0 a0 a00 a0 a00 a0 a0 a0 a0
1268
Ref. [24]. Measured in vacuum (cm1). Multiplied by a scaling factor of 0.995. Ref. [11]. Ref. [25].
•H2C
6a 10
9a 10
6b 10
F
b
combinations of the bands of strong intensity and the same symmetry, and belong to the in-plane vibration of a 0 symmetry. The observed vibrational mode frequencies are compared with those of an ab initio calculation and those of the precursor, together with the mode symmetries, in Table 2. A total of six isomers of difluorobenzyl radicals can be divided into three different classes: 1,2,3-substitutions consist of 2,6- and 2,3-difluorobenzyl radicals with C2V and CS symmetries, respectively. 1,2,4-substitution contains 2,4-, 2,5-, and 3,4-difluorobenzyl radicals of CS symmetry. Finally, 3,5-difluorobenzyl radical of C2V symmetries belongs to 1,3,5-substitution. The vibronic emission spectrum of 2,3-difluorobenzyl radical was compared in Fig. 2 with that of 2,6-difluorobenzyl, in which both spectra show similar vibrational structure in the ground
1 10
F
Fig. 2. Similarity of the vibronic emission spectrum of between 2,6- and 2,3-difluorobenzyl radicals in the D1(2A2) ! D0(2B2) electronic transition. Both spectra were taken at the same experimental conditions with different precursors. With changing the symmetry from C2V to CS point groups, the band belonging to 6b vibrational mode shows increasing intensity at higher wavenumber, which has also been confirmed in Ref. [25].
electronic state. While the 6b mode was not observed in 2,6-difluorobenzyl radical, the same mode was observed with strong intensity in 2,3,-difluorobenzyl radical. This tendency has been observed in many benzyl-type radicals of CS and C2V point groups. We also observed low frequency sequence bands in the vicinity of the origin band, in which the distance from the origin band is less than 150 cm1. The problem in explaining the origin of low frequency sequence bands remains. Fukushima and Obi [5] have observed several weak bands from the p-fluorobenzyl radical in the vicinity of the strong bands, and attributed them as belonging to the van der Waals molecules. However, it seems unlikely, in the present study, because backing pressure is much lower in the glass-type nozzle scheme. Also the same bands were detected with the other carrier gas Ar, which yielded a much poorer spectral S/N. Cossart-Magos and Cossart [12] have observed several weak low frequency sequence bands near the origin band of the p-fluorobenzyl radical, and assigned them as combination bands coupled with the origin band. Thus, we strongly believe, owing to the incomplete vibrational cooling process in the excited electronic state during supersonic jet expansion, that these bands are from the transition to the excited vibrational state belonging to the D0 state from the excited vibrational state of the D1 electronic state. The distance from the origin band indicates the vibrational energy difference between two electronic states, D1 and D0 states.
G.W. Lee et al. / Chemical Physics Letters 447 (2007) 197–201
In summary, we generated, for the first time, the jetcooled but vibronically excited 2,3-difluorobenzyl radical from 2,3-difluorotoluene seeded in a large amount of carrier gas He using a pinhole-type glass nozzle in a corona excited supersonic expansion. The visible vibronic emission spectrum of the jet-cooled 2,3-difluorobenzyl radical was recorded, from which the electronic energy of the D1 ! D0 transition and the frequencies of several vibrational modes in the ground electronic state were determined by comparison with those of 2,6-difluorobenzyl radical and 1,2,3-trimethylbenzene as well as with those from an ab initio calculation. Acknowledgement This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-311-C00336 and R14-2003-03301002-0). References [1] X.Q. Tan, T.G. Wright, T.A. Miller, in: J.M. Hollas, D. Phillip (Eds.), Electronic Spectroscopy of Free Radicals in Supersonic Jets: Jet Spectroscopy and Molecular Dynamics, Blackie Academic & Professional, London, 1994.
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