Vibronic emission spectrum of 2-chloro-6-fluorobenzyl radical produced in corona discharge

Chemical Physics Letters 637 (2015) 148–152

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Vibronic emission spectrum of 2-chloro-6-fluorobenzyl radical produced in corona discharge Young Wook Yoon, Sang Youl Chae, Manho Lim, Sang Kuk Lee ∗ Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, Republic of Korea

a r t i c l e

i n f o

Article history: Received 5 May 2015 In final form 5 August 2015 Available online 12 August 2015

a b s t r a c t We generated vibronically excited but jet-cooled 2-chloro-6-fluorobenzyl radical from precursor 2chloro-6-fluorotoluene seeded in a large amount of helium carrier gas using a pinhole-type glass nozzle coupled with a technique of corona excited supersonic jet expansion. From an analysis of the visible vibronic emission spectrum observed, we found evidence of the formation of the 2-chloro-6-fluorobenzyl and 2-fluorobenzyl radicals, and determined the electronic energy in the D1 → D0 transition and the vibrational mode frequencies of the 2-chloro-6-fluorobenzyl radical in the ground electronic state, for the first time, by comparison with ab initio calculations of the precursor molecule. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Transient spectroscopy can provide information on characteristics of transient molecules which are believed to play an important role as reaction intermediates at the transition state. However, spectroscopic observation of transient molecules is extremely difficult to carry out because of very short lifetime and low concentration in the gas phase medium, which requires a more complicated experimental apparatus than stable molecules [1]. Thus, a number of techniques have been developed to produce transient species from stable precursors, in which the most popular method is to use a high voltage DC electric discharge [2]. Benzyl radical [3], a prototype of aromatic free radical, is an important intermediate in aromatic chain reactions and, thus, has been a subject of many experimental and theoretical studies. On the other hand, ring-substituted benzyl radicals have attracted less attention due to the difficulties associated with the analysis of the spectra and the generation of the species from precursors. Although a number of ring-substituted benzyl radicals were identified via spectroscopic observation, most of them belong to methyl-[4] and fluoro-[5] substitutions because they emit fairly strong visible fluorescence from the D1 → D0 transition. To date, spectroscopic studies have been reported for all six isomeric difluoro-[6] and dimethyl-[7] benzyl radicals [6,7]. On the other hand, spectroscopic observation of chlorosubstituted benzyl radicals has been much delayed by their weak visible fluorescence and the possible breakdown of the benzene

ring initiated by the Cl atom liberated from C Cl bond dissociation. Nevertheless, o-, m-, p-, and ␣-chlorobenzyl radicals have been examined for their vibronic assignments [8]. Although it has been expected that the substitution of two Cl atoms into benzene ring further reduces the emission intensity, dichlorobenzyl radicals exhibit an interesting character of red-shift of the origin bands of the electronic transition energies. The shifts strongly depend on the number, kind, and position of substituents on the benzene ring [9]. Generally, the shifts increase with the number of substituents in substituted benzyl radicals regardless of the kinds of substituents [10]. Tzeng and coworkers [11–13] have recently discussed the effect of red-shift on electronic transition energies through analysis of vibronic spectra of difluoroanilines. The red-shifts of electronic transition energies have been further discussed for methyl-, fluoro-, and chloro-disubstituted benzyl radicals, in which the concept of Hückel’s molecular orbital theory was adopted to describe the negligible contribution from the substituents at the 4-position [14]. Recently, the spectroscopic studies of hetero dihalobenzyl radicals have been explored to examine the synergic effect of halo substituents [15]. In this study, we report the observation of vibronic emission spectrum of the 2-chloro-6-fluorobenzyl radical produced by corona discharge of precursor, 2-chloro-6-fluorotoluene and the assignments of vibronic bands to identify the species formed. In addition, we discuss the substituent effect on electronic transition energy of hetero dihalobenzyl radicals. 2. Experimental

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S.K. Lee). http://dx.doi.org/10.1016/j.cplett.2015.08.006 0009-2614/© 2015 Elsevier B.V. All rights reserved.

The experimental setup used in this work is similar to that described previously [16]. Briefly, it adopted a technique of corona

Y.W. Yoon et al. / Chemical Physics Letters 637 (2015) 148–152

excited supersonic jet expansion (CESE) combined with a pinholetype glass nozzle to produce vibronically excited but jet-cooled benzyl-type radicals and a long-path monochromator to observe vibronic emission spectrum. Precursor 2-chloro-6-fluorotoluene (reagent grade) was purchased from Aldrich and used without further purification. It was vaporized inside a Pyrex vaporizing vessel at 3 bar of helium carrier gas. The precursor concentration in the carrier gas was controlled to obtain the maximum emission intensity as monitored from the strongest band in the spectrum, and was calculated to be less than 1% in the mixture in terms of vapor pressure. We adopted a modified pinhole-type glass nozzle which significantly reduces the amount of sticky deposits clogging the glass nozzle throat during corona discharge, thereby improving the longterm stability of corona discharge by partially allowing expansion after excitation. A minimum discharge voltage was applied to reduce the production of C2 molecules [17] that emit strong fluorescence in the same spectral region as benzyl-type radicals. However, it must be noted that the formation of C2 molecules cannot be completely avoided, especially in the case of chlorine-containing precursors. The Cl atoms can be easily produced by dissociation of weak C Cl bond in corona discharge of precursor molecules and act as a chain carrier of the free radical chain reaction to decompose the aromatic hydrocarbons. Pyrex expansion chamber was evacuated by a mechanical vacuum pump, maintaining a chamber pressure of about 3.0 mbar for continuous jet expansion through a 0.3 mm orifice diameter nozzle, the size of which was further reduced by partially inserting the sharpened end of an anode into the nozzle throat. The maximum backing pressure was limited to 3 bar by the tolerance of the glass materials used for the nozzle. In the corona discharge of precursor, a blue-green colored jet indicates the production of benzyl-type radicals. The jet emission from 5 mm below the nozzle throat was collected using a quartz lens (D = 38 mm, F = 50 mm) placed inside the chamber and focused onto the slit of the long-path monochromator (Jobin Yvon U1000) with a cooled PMT (Hamamatsu R649). The vibronic emission spectrum was recorded over 1 h by scanning from 20 000 to 22 000 cm−1 in increments of 2.0 cm−1 with a slit width of 200 ␮m. The frequency of spectrum was calibrated with the He atomic lines observed with the spectra, and was believed to be accurate within ±1.0 cm−1 [18]. For the assignments of vibronic bands of the 2-chloro-6fluorobenzyl radical generated, an ab initio calculation was carried out on the ground electronic state. The calculations were executed on a personal computer equipped with an Intel Pentium IV 1.2 GHz CPU processor and 2048 MB RAM, in accordance with the standard methods included in the Gaussian09 program [19]. Geometry optimization and calculation of the vibrational frequencies were performed at the DFT level with B3LYP function, and a 6311G* basis set was employed in all of the calculations. The motion of each vibrational mode was visualized from the output of the calculation using the HyperChem program.

3. Results and discussion The simplest method producing jet-cooled free radicals is certainly a technique of corona excited supersonic expansion (CESE), originally invented by Engelking and coworker [2], which has been proved to be useful for the generation of jet-cooled benzyl-type radicals with employment of a pinhole-type glass nozzle. The spectral simplification and stabilization of radicals can be obtained by supersonic jet expansion with helium carrier gas. It has been confirmed [20] that vibronically excited but jetcooled benzyl radicals can be produced by dissociation of methyl

149

CH2• F

Cl

F

Cl

Cl

F

+

+ [1]

CH2•

CH2•

[2]

[3]

Figure 1. Three possible generation of 2-chloro-6-fluorobenzyl [1], 2-fluorobenzyl [2], and 2-chlorobenzyl [3] radicals from precursor 2-chloro-6-fluorotoluene by corona discharge.

C H bond in corona discharge of toluene seeded in a large amount of helium carrier gas. Although the exact mechanism for formation and excitation of benzyl radicals from precursors in a CESE system has not been clearly established, it can be suggested that the metastable He atom (1s2s 3 S1 ), about 19.82 eV above the ground state that is initially excited by the corona discharge, transfers its excess energy to the precursor via a collisional process. The collisional energy transfer from metastable He atom to other species is a well-known process in the excitation of Ne atoms as observed in the HeNe laser [21]. Thus, ring substituted benzyl radicals could be produced in a similar way from corresponding toluenes by corona discharge. In benzyl radical, strong vibronic coupling between two excited electronic D1 (2 A2 ) and D2 (2 B2 ) states transfers the population from the D2 to D1 states quickly. Moreover, collisional relaxation of vibrational energies also increases the population of the vibrationless state (v = 0) in the D1 state during supersonic expansion. In addition, the similarity in molecular structure between the D1 and D0 (2 B2 ) states provides large Franck–Condon factor to the origin band of the vibronic transition. Thus, the typical vibronic emission spectra observed in CESE system with a pinhole-type nozzle show the strongest origin band of the D1 → D0 transition at the highest wavenumber, which are similar to the LIF-DF spectrum observed while pumping the origin band of the D1 ← D0 transition, making it possible to measure directly the electronic transition energy. Besides, the spacings of the vibronic bands from the origin band represent the vibrational mode frequencies in the D0 state. Figure 1 exhibits the possible formation of substituted benzyl radicals from precursor, 2-chloro-6-fluorotoluene by corona discharge, in which the 2-chloro-6-fluorobenzyl [1] radicals is a product of a simple dissociation of methyl C H bond, while the 2fluorobenzyl [2] and 2-chlorobenzyl [3] radicals are the products generated by displacement of Cl and F atoms by H atom, respectively. The production of 2-chlorobenzyl radical is believed to be a much less favorable process because of strong C F bond dissociation energy. Figure 2 shows the vibronic emission spectrum observed from the corona discharge of the precursor. It consists of a strong band at 21 926 cm−1 , which is the origin band in the D1 → D0 transition of the 2-fluorobenzyl radical, along with a series of bands of observable intensity in the red region, confirming the production of 2-fluorobenzyl radical in corona discharge. The formation of the 2fluorobenzyl radical was clearly confirmed from the observation of the origin band and other vibronic bands whose frequencies agree with those reported previously [22]. However, we could not detect any evidence of the presence of the 2-chlorobenzyl radical, the origin band of which has been reported to be located at 21 040 cm−1 [8]. Since the bond dissociation energies of benzylic C H, phenylic C Cl and C F bonds are, respectively, 356, 398, and 519 kJ/mol [23], the replacement of strong C F bond by C H bond is thermodynamically less favorable process, rendering impossible the production of the 2-chlorobenzyl radical in corona discharge. It was reported [24] that the 2-chlorobenzyl radical was produced in a greater abundance than the 2,6-dichlorobenzyl radical

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12000

*

Cl

F

Table 1 Origin bands in the D1 → D0 transition of benzyl-type radicals.a

CH2•

CH2•

F

Intensity

Molecules 0 0 0 [1]

[1]

8000

4000

0 1 1 [1]

*

*

0 6a 1 [1]

*

[2]

* 0 7a 1 [2]

c

0 0 0 [2]

0 11 [2]

Benzyl 2-Chlorobenzyld 2-Fluorobenzyle 2-Chloro-4-fluorobenzylf 2-Fluoro-4-chlorobenzylg 2-Chloro-6-fluorobenzylh a

0 6b 1 [2]

b c

0 19500

d

20000

20500

21000

21500

22000

-1

Wavenumber (cm )

e f g h

Figure 2. A portion of the vibronic emission spectrum observed from the corona discharge of 2-chloro-6-fluorotoluene seeded in a large amount of helium carrier gas in a CESE system. The numbers 1 and 2 in the brackets represent the vibronic bands belonging to the 2-chloro-6-fluorobenzyl [1] and 2-fluorobenzyl [2] radicals, respectively. Atomic lines of H and He are marked by asterisks.

from the corona discharge of 2,6-dichlorotoluene, which suggests that the Cl atom at the 2-position can be easily replaced by an H atom of the methyl group. However, the formation of 2fluorobenzyl radical was not detected from the corona discharge of 2,6-difluorotoluene [25], confirming that the bond dissociation energies play a crucial role in the replacement reaction of halogen substituents. We believe that the replacement of the Cl atom at the 2-position can be accomplished in two ways. The first is a two-step process whereby the Cl atom is replaced by the free H atom released from dissociation of methyl C H bond in forming 2-chloro-6fluorobenzyl radical. Although it seems to be reasonable in terms of a simple thermochemical rule involved in the reaction, it cannot be possible because, according to the collision theory, the steric factor is very small for large molecules in the gas phase reaction, and very low concentration of free H atoms in the gas phase medium. The second is the migration of H atom after the C Cl bond dissociation, as shown in Figure 3. Nagano et al. [26] observed the migration of Cl atom from benzylic carbon to benzene ring in generation of benzyl radical by irradiation. This is, in fact, the more feasible process, owing to the two atoms being separated by a very short distance, as well as the large amplitude of vibration of the methyl group in the excited electronic state. Recently, we confirmed the formation of the 2-fluorobenzyl [27] and 4-fluorobenzyl [15] radicals from the corona discharge of 2-fluoro-4-chlorotoluene and 2-chloro-4-fluorotoluene,

H

CH2

F

F

CH2• Cl

Cl

F

[1]

Cl CH3 F

CH2 Cl

F

H

.

CH2• F

[2] Figure 3. A proposed mechanism for the formation of the 2-chloro-6-fluorobenzyl [1] and 2-fluorobenzyl [2] radicals by corona discharge of precursor. A simple dissociation of a methyl C H bond produces the 2-chloro-6-fluorobenzyl radical while the migration of H atom from methyl group to the 2-position after dissociation of C Cl bond generates the 2-fluorobenzyl radical.

Origin band

Shiftb

22 002 21 040 21 926 21 014 21 708 20 868

0 962 76 988 294 1134

Measured in vacuum (cm−1 ). With respect to the origin band of benzyl radical at 22 002 cm−1 . Ref. [20]. Ref. [8]. Ref. [22]. Ref. [15]. Ref. [27]. This work.

respectively. Similarly, the 2-fluorobenzyl [28] and 3-fluorobenzyl [29] radicals were formed from 2-fluoro-5-chlorotoluene and 2chloro-5-fluorotoluene, respectively, suggesting that the distance between the methyl group and the Cl atom on the benzene ring is not an important factor for the replacement process. Thus, the explanation in terms of bond dissociation energy seems to be reasonable for the observation in this work. However, it should be noted that the discharge chemistry will not often follow simple thermochemical rules if excited electronic states are involved. After subtracting the bands belonging to the 2-fluorobenzyl radical from the observation in Figure 2, the strongest band at 20 868 cm−1 was assigned to the origin band in the D1 → D0 transition of the 2-chloro-6-fluorobenzyl radical, because it exhibited strongest intensity at the highest wavenumber. The origin band shows a red-shift of 1134 cm−1 from the benzyl radical at 22 002 cm−1 , which is comparable to that (1040 cm−1 ) obtained by adding the shift of the 2-chlorobenzyl (962 cm−1 ) [8] and that of the 2-fluorobenzyl (78 cm−1 ) [22]. The additivity rule described satisfactorily the red-shifts of the 2,6-dichlorobenzyl [24], 2,6dimethylbenzyl [30], and 2,6-difluorobenzyl [25] radicals. Table 1 lists the substituent effect on electronic transition energy of hetero dihalobenzyl radicals. The 2-chloro-4fluorobenzyl and 2-fluoro-4-chlorobenzyl radicals show similar shift to the 2-chlorobenzyl and 2-fluorobenzyl radicals, respectively, due to negligible contribution from the substituents at the 4- position, the nodal point at the D1 state. Although there is no physical explanation on the substituent effect, to the best of our knowledge, we strongly believe that the substituent bonded to the node shows no connection to benzene ring through ␲ electrons because the node has zero amplitude of ␲ electrons according to Hückel’s molecular orbital theory. In benzyl radical, the first excited electronic state (D1 ) has A2 symmetry, having nodes at the 1- and 4-positions of benzene ring. Thus, the substituted benzyl radicals cannot have any contribution from the substituent at the 4-position in the D1 electronic state. Similar results were found from the dimethylbenzyl radicals [7], which can be useful for identification of each species among isomers. The assignments of the vibronic bands belonging to the 2chloro-6-fluorobenzyl radical were carried out by comparison the observation with the ab initio calculations and the known vibrational mode frequencies of the 2-methyl-6-fluorobenzyl radical which were determined from the stable 3-fluoro-o-xylene because both molecules should show similar vibrational frequencies [31]. For substituted benzenes [32], the well-known vibrational modes are mode 1 of ring breathing and modes 6a and 6b of inplane ring deformation vibrations. The strong band at 20 238 cm−1 , a red-shift of 630 cm−1 from the origin band at 20 868 cm−1 was assigned to mode 1 because of the similarity in vibrational

Y.W. Yoon et al. / Chemical Physics Letters 637 (2015) 148–152 Table 2 List of the vibronic bands observed in this work and their assignments.a Positionb

Intensity

21 926 21 410 21 216 21 170 20 868 20 820 20 662 20 616 20 570 20 480 20 404 20 348 20 316 20 238 20 018 19 938 19 880 19 810 19 770 19 604

vs m s m vs m w w m w m w

a b c

s vw vs w m w m

Spacingc 0[2] 517[2] 756[2] 0[1] 48 1264[2] 252[1] 388[1] 464[1] 520[1] 630[1] 850[1] 988[1] 1058[1] 1098[1] 1264[1]

Assignments Origin of 2-fluorobenzyl 0 6b1 of 2-fluorobenzyl He atomic 101 of 2-fluorobenzyl Origin of 2-chloro-6-fluorobenzyl 7a01 of 2-fluorobenzyl 9a01 of 2-chloro-6-fluorobenzyl H atomic 7a01 of 2-chloro-6-fluorobenzyl 6a01 of 2-chloro-6-fluorobenzyl 0 6b1 of 2-chloro-6-fluorobenzyl He atomic 101 of 2-chloro-6-fluorobenzyl 6a01 7a01 of 2-chloro-6-fluorobenzyl He atomic line 0 6a01 6b1 of 2-chloro-6-fluorobenzyl He atomic line 101 6a01 of 2-chloro-6-fluorobenzyl 102 of 2-chloro-6-fluorobenzyl

Measured in vacuum (cm−1 ). Observation in this work Spacing from the origin band of each product.

frequency between the ab initio calculation and the 2-chloro-6fluorotoluene. A slight decrease from the 2-methyl-6-fluorobenzyl radical is attributed to the substitution of heavy Cl atom instead of methyl group on the benzene ring. The moderate band at

151

2040 cm−1 , a shift of 464 cm−1 and a weak band at 20 348 cm−1 , a shift of 520 cm−1 were assigned to modes 6a and 6b due to the similarities to those of other molecules of similar structure. These bands are degenerate in benzene at 606 cm−1 and split with unsymmetric substitution. The weak band at 20 480 cm−1 , red shifted by 388 cm−1 from the origin band, was assigned to mode 7a of C Cl inplane stretching vibration, since it coincides with the frequency in 2,6-dichlorotoluene (373 cm−1 ), whereas another stretching mode 7b was not detected because of very weak intensity. Both modes 7a and 7b are also degenerate in benzene. Finally, the weak band at 20 616 cm−1 , red-shifted by 252 cm−1 from the origin band was assigned to mode 9a because the frequency of mode 9a should agree with the precursor (242 cm−1 ). The calculation (255 cm−1 ) predicts an excellent agreement for this mode. The same mode was also observed from the 2,6-dichlorobenzyl radical with a strong intensity. From the determination of fundamental bands, several combination bands were assigned in the spectrum at the predicted frequencies. These are the combination of strong vibrational modes. Table 2 lists the vibronic bands observed in this work and their assignments. The H atomic line at 20 570 cm−1 is from the dissociation of methyl C H bond. Table 3 shows the comparison of the vibrational mode frequencies of 2-chloro-6-fluorotoluene and 2methyl-6-fluorobenzyl radical, in which 2-chloro-6-fluorobenzyl radical shows very similar vibrational frequencies to 2-chloro-6fluorotoluene. However, the 2-methyl-6-fluorobenzyl radical gives slightly higher vibrational frequencies for stretching modes due to the mass difference between methyl group and Cl atom. Table 4 lists the calculated vibrational mode frequencies with IR intensities

Table 3 Vibrational mode frequencies of 2-chloro-6-fluorobenzyl radical.a Modeb

This workc (D0 )

Ab initio B3LYP/6-311G* d

Origin 9a 7a 6a 6b 1 a b c d e f

20 868 252 388 464 520 630

2-Methyl-6-fluorobenzyle (D0 )

Symmetry (Cs )

326

a a a a a

d

2-Chloro-6-fluorobenzyl (D0 )

2-Chloro-6-fluorotoluene (S0 )

24 092f 255 392 475 535 642

242 396 470 542 630

468 532 686

Measured in vacuum (cm−1 ). Ref. [33]. Spacing from the origin band. Not scaled. Ref. [29]. TD-DFT calculation with oscillator strength f = 0.0059.

Table 4 List of calculated vibrational mode frequencies at the ground electronic (D0 ) state.a No.

Frequencyb

IR intensity

Symmetryc

No.

Frequencyb

IR intensity

Symmetryc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

3299 3221 3215 3198 3185 1610 1571 1481 1472 1415 1347 1289 1267 1202 1152 1070 1006 960

0.24 2.28 2.56 2.08 0.03 35.60 32.67 54.02 65.61 3.03 3.13 2.78 49.80 14.14 2.21 3.35 58.09 2.53

a a a a a a a a a a a a a a a a a a

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

909 861 836 789 766 716 642 593 535 520 475 466 392 331 288 255 220 113

0.86 25.17 20.13 4.25 64.97 3.42 3.45 0.23 3.16 0.90 3.50 0.01 2.20 2.71 2.15 0.38 0.85 0.82

a a a a a a a a a a a a a a a a a a

a b c

Ab initio calculation with B3LYP/6-311G*. In units of wavenumber (cm−1 ). The a and a symmetries represent in-plane and out-of-plane vibrations, respectively.

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Table 5 Optimized structural parameters for the 2-chloro-6-fluorobenzyl radicals at B3LYP/6-311G* levels. Internuclear distance ˚ (A) C1 –C2 C1 –C6 C1 –Cl14 C2 –C3 C2 –C10 C3 –C4 C3 –F13 C4 –C5 C4 –H7 C5 –C6 C5 –H8 C6 –H9 C10 –H11 C10 –H12

1.434 1.389 1.757 1.430 1.402 1.380 1.350 1.403 1.091 1.401 1.091 1.090 1.088 1.088

Bond angle (◦ ) C2 –C1 –C6 C2 –C1 –Cl14 C6 –C1 –Cl14 C1 –C2 –C3 C1 –C2 –C10 C3 –C2 –C10 C2 –C3 –C4 C2 –C3 –F13 C4 –C3 –F13 C3 –C4 –C5 C3 –C4 –H7 C5 –C4 –H7 C4 –C5 –C6 C4 –C5 –H8 C6 –C5 –H8 C1 –C6 –C5 C1 –C6 –H9 C5 –C6 –H9 C2 –C10 –H11 C2 –C10 –H12

122.6 119.0 118.4 114.4 124.9 120.8 124.1 117.0 118.8 118.8 119.2 122.0 120.2 120.0 119.8 119.9 119.4 120.7 120.8 119.9

Figure 4. Numbering of atoms in the optimized structure of the 2-chloro-6fluorobenzyl [1] radical.

and symmetries. The IR intensity should be compared relatively for each mode. Finally, Table 5 shows the structural parameters for the optimized structure of the 2-chloro-6-fluorobenzyl radicals calculated at B3LYP/6-311G* levels, for which the numbering of atoms are shown in Figure 4. 4. Conclusions We obtained the vibronic emission spectrum from the corona discharge of precursor 2-chloro-4-fluorotoluene using a

pinhole-type glass nozzle in a CESE system. In an analysis of the spectrum, we identified the formation of the 2-fluorobenzyl and 2-chloro-6-fluorobenzyl radicals, for which we propose a possible mechanism. The electronic energy in the D1 → D0 transition and vibrational mode frequencies in the D0 state of the 2-chloro6-fluorobenzyl radical were determined by comparison with ab initio calculations for the first time. It was found that the additivity rule is held for the red shift of the origin band of the 2-chloro-6fluorobenzyl radical. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) funded by the Korean Government (Grant Nos. 2014R1A4A1001690 and 2013R1A1A2061073). 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. [2] P.C. Engelking, Rev. Sci. Instrum. 57 (1986) 2274. [3] M. Fukushima, K. Obi, J. Chem. Phys. 93 (1990) 8488. [4] T.-Y.D. Lin, X.-Q. Tan, T.M. Cerny, J.M. Williamson, D.W. Cullin, T.A. Miller, Chem. Phys. 167 (1992) 203. [5] T.R. Charlton, B.A. Thrush, Chem. Phys. Lett. 125 (1986) 547. [6] S.W. Lee, Y.W. Yoon, S.K. Lee, J. Phys. Chem. A 114 (2010) 9110. [7] Y.W. Yoon, S.K. Lee, J. Chem. Phys. 135 (2011) 214305. [8] S.K. Lee, S.Y. Chae, J. Phys. Chem. A 106 (2002) 8054. [9] E.H. Yi, Y.W. Yoon, S.K. Lee, Chem. Phys. Lett. 608 (2014) 319. [10] C. Branciard-Larcher, E. Migirdicyan, J. Baudet, Chem. Phys. 2 (1973) 95. [11] C.Y. Tsai, W.B. Tzeng, J. Photochem. Photobiol. A 270 (2013) 53. [12] W.C. Huang, P.S. Huang, C.H. Hu, W.B. Tzeng, Chem. Phys. Lett. 580 (2013) 28. [13] Y.H. Huang, W.C. Huang, W.B. Tzeng, Chem. Phys. Lett. 595–596 (2014) 73. [14] Y.W. Yoon, S.K. Lee, Chem. Phys. Lett. 570 (2013) 29. [15] C.S. Huh, Y.W. Yoon, S.K. Lee, J. Chem. Phys. 136 (2012) 174306. [16] M.S. Han, I.S. Choi, S.K. Lee, Bull. Korean Chem. Soc. 17 (1996) 991. [17] H.L. Wallaart, B. Pier, M.-Y. Perrin, J.-P. Martin, Chem. Phys. Lett. 246 (1995) 587. [18] A.R. Striganov, N.S. Sventitskii, Tables of Spectral Lines of Neutral and Ionized Atoms, Plenum Data Corporation, New York, NY, 1968. [19] M.J. Frisch, et al., GAUSSIAN 09, Revision A. 02, Gaussian Inc., Wallingford, CT, 2009. [20] J.I. Selco, P.G. Carrick, J. Mol. Spectrosc. 137 (1989) 13. [21] C.N. Banwell, E.M. McCash, Fundamentals of Molecular Spectroscopy, 4th edn., McGraw-Hill, New York, 1994. [22] S.K. Lee, S.K. Lee, J. Phys. Chem. A 105 (2001) 3034. [23] R.T. Sanderson, Polar Covalence, Academic, New York, 1983. [24] S.K. Lee, S.J. Kim, Chem. Phys. Lett. 412 (2005) 88. [25] S.K. Lee, D.Y. Baek, J. Phys. Chem. A 104 (2000) 5219. [26] M. Nagano, T. Suzuki, T. Ichimura, T. Okutsu, H. Hiratsuka, S. Kawauchi, J. Phys. Chem. A 109 (2005) 5825. [27] S.Y. Chae, Y.W. Yoon, S.K. Lee, Bull. Korean Chem. Soc. 34 (2013) 3565. [28] Y.W. Yoon, S.Y. Chae, S.K. Lee, Chem. Phys. Lett. 608 (2014) 6. [29] S.Y. Chae, Y.W. Yoon, S.K. Lee, Chem. Phys. Lett. 612 (2014) 134. [30] G.W. Lee, S.K. Lee, J. Phys. Chem. A 110 (2006) 2130. [31] Y.W. Yoon, S.Y. Chae, S.K. Lee, Chem. Phys. Lett. 584 (2013) 37. [32] G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, John Wiley & Sons, New York, NY, 1974. [33] E.B. Wilson, Phys. Rev. 45 (1934) 706.