Photodissociation dynamics of CH2BrCl at 234 nm

Chemical Physics 260 (2000) 143±150

www.elsevier.nl/locate/chemphys

Photodissociation dynamics of CH2BrCl at 234 nm Sung-Hae Lee, Young-Jae Jung, Kyung-Hoon Jung * Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology, Taeduck Science Town, Taejon 305-701, South Korea Received 2 May 2000

Abstract The photodissociation dynamics of CH2 BrCl has been studied at 234 nm utilizing the molecular beam-imaging system. In the photolysis, the C±Br bond ®ssion is observed exclusively. The internal energy of the Br(2 P3=2 ) channel consists of two parts which is caused by the interaction between r (C±Br) and r (C±Cl) surfaces. This separation is newly resolved and studied to unravel the e€ect of another chromophore. The energy separation of two peaks is 25 kJ molÿ1 and the ratio of fast to slow one is 3.2. The fast part is interpreted in terms of a rigid impulsive model with no vibrational excitation and the slow part by a soft one with a little vibrational excitation, respectively. The energy gap is suggested to be due to an umbrella motion. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction Alkyl halides with multichromophores yield various fragments from several dissociation channels via n ! r transitions. Extensive studies have been made on both experiments and calculations aiming to observe selective bond dissociations of dihalogenated molecules. Bond-selective photodissociation is to induce the preferential dissociation of a speci®c bond via a speci®ed excitation and is one of the recent major interests in laser chemistry. These selectivities have been observed in the unimolecular dissociations of alkyl halides with two chromophores by several research groups [1±7] and were found to be sensitive to the excitation energy. Several cleavage experiments on molecules with two chromophores on the same carbon have been performed on CH2 BrI, CF2 BrCl,

*

Corresponding author. E-mail address: [email protected] (K.-H. Jung).

and CH2 BrCl systems. In the work on CH2 BrI [1], the speci®c bond cleavage was controlled preferentially by selecting the n(I) ! r (C±I) or n(Br) ! r (C±Br) transition at various excitation wavelengths. This result was combined with those of 1,2-C2 F4 BrI [2] and 1,3-C3 H6 BrI [3] to unravel the e€ect of separation distance between C±I and C±Br on coupling and energy transfer. In the CF2 BrCl photodissociation at 193 nm [6], the reaction proceeded mostly on the C±Br bond cleavage and the branching ratio of C±Br to C±Cl was 6. In the CH2 BrCl study at 193 and 248 nm [7], the branching ratio at 193 nm was 4.5 in favor of C±Br bond ®ssion and sole C±Br bond cleavage at 248 nm. Since these experiments, aimed at observing the bond selectivity, have been monitored by photofragment translational spectroscopy, some possible uncertainties for the resolutions of the ground and excited state halogen atoms were not eliminated completely in the case of a small velocity di€erence between fragments. In order to understand the reaction dynamics and the

0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 2 1 6 - 0

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character of excited states well, the spin±orbit states of products have to be resolved. Recently, photodissociation of CH2 BrCl with ground and excited Br discerned was ®rst reported in the range of 248±268 nm [8]. The nonadiabatic coupling e€ect on bond selectivity in CH2 BrCl photodissociation has been studied via the time-dependent wave packet calculation on diabatic potentials and ab initio molecular orbital calculation [9]. Although the spin±orbit e€ect was neglected in this calculation and only C±Br and C±Cl bond lengths were varied assuming a pseudotriatomic molecule, the result is somewhat helpful in understanding reaction dynamics better. An absorption pro®le of a molecule can be assigned to various electronic transitions by comparing it with a similar molecule whose pro®le has already been analyzed. The absorption peak of CH2 BrCl at 203 nm [10] has been assigned at zeroth order wave function to the n(Br) ! r (C±Br) transition, comparing it with that of CH3 Br. Since no absorption spectrum below 185 nm of this molecule has been studied, an absorption peak via n(Cl) ! r (C±Cl) electronic transition may be expected near 164 nm by comparing it with that of CF2 BrCl, which has shown an absorption peak at this wavelength [11] and primarily been assigned to the n(Cl) ! r (C±Cl) transition. In order to infer the C±Br bond ®ssion dynamics and the e€ect of the other chromophore in the CH2 Cl±Br system, CH3 Br is used as a reference material, based on similar absorption behavior. CH3 X has been investigated experimentally at various wavelengths [12±14] and theoretically [17,18] to verify a nonadiabatic transition probability. During the photodissociation process, the point group of CH3 X has been changed from C3v to Cs symmetry due to vibrational motion and has often been deviated from the one-dimensional Landau±Zener model. Since the bent structure of CH3 Br with Cs symmetry is similar to that of CH2 BrCl, the dynamic properties of CH3 Br can be used to deduce that of CH2 BrCl by comparing these two molecules. In this article, we report on the unimolecular dissociation dynamics of CH2 BrCl at 234 nm aiming to verify the character of the excited states by distinguishing excited and

ground state halogen atoms and the in¯uence of another chromophore Cl by comparing it with CH3 Br. 2. Experimental The experimental setup has been described elsewhere in detail [19]. In brief, the reaction vacuum chamber was equipped with a molecular beam setup. Photolysis and state-selective multiphoton ionization were achieved within a single laser pulse. A linearly polarized laser beam is focused on the interaction region with a molecular beam by a lens with a 16 cm focal length. An image was obtained from the gated signals during over 6000 laser shots and then the background was removed by subtracting the image obtained at an o€-resonance wavelength under the same condition. The halogen atoms were detected via the (2 ‡ 1) resonance-enhanced multiphoton ionization (REMPI) technique. In the (2 ‡ 1) REMPI scheme, Br(2 P3=2 ) and Br (2 P1=2 ) use 6p 4 P3=2 and 6p 2 S1=2 intermediate states, and Cl(2 P3=2 ) and Cl (2 P1=2 ) do 4p 2 D3=2 and 4p 2 P1=2 , respectively. In order to cover the velocity Doppler pro®les of halogen atoms, their wavelengths were scanned across these pro®les. Photoionized fragments were accelerated along the time-of-¯ight (TOF) axis by an electrostatic zoom lens consisting of a repeller, an extractor, and a ground electrode [20]. The molecular beam was produced by expanding the gas mixture (1% sample seeded in 1.5 atm He) into the source chamber using a pulsed nozzle valve (general valve). The valve driver was operated at 10 Hz and synchronized with the laser pulse. The linewidth of the dye laser was 0.1 cmÿ1 and the pulse duration was 10 ns. The polarization of the UV laser light was obtained using a half-wave retardation plate.

3. Data analysis and results A primary dissociation channel of CH2 BrCl at 234 nm is the ®ssion of the weakest bond CH2 BrCl ‡ hm ! CH2 Cl ‡ Br=Br :

…1†

S.-H. Lee et al. / Chemical Physics 260 (2000) 143±150

145

Table 1 Some dynamic properties of CH2 BrCl State 

Br Br

Eava (kJ molÿ1 )

b (0:1)

v (m sÿ1 )

Et (kJ molÿ1 )

Et =Eava

182.53 227.37

1.5 1.0

972.0 940.1 1064.4

98.0 91.5 116.5

53.7% 40.2% 51.2%

Another process competes with this dissociation channel CH2 BrCl ‡ hm ! CH2 Br ‡ Cl=Cl

…2†

Each velocity value of resolved Br, Br , Cl, and Cl atoms is obtained from the corresponding image. Some dynamic properties obtained from photoimages are given in Table 1. The dissociation energies of each bond are 285 kJ molÿ1 for CH2 Cl±Br and 331 kJ molÿ1 for CH2 Br±Cl, respectively [7]. The available energies are determined from the energy conservation relation, p ˆ Et ‡ Eint ; Eavl ˆ hm ÿ D0 …C±X† ‡ Eint    ; Eavl ˆ Eavl ÿ ESO …X† ˆ Et ‡ Eint

where hm is the photon energy, D0 (C±X) the disp the internal sociation energy of the C±X bond, Eint

energy of the parent molecule, and ESO the spin± p is orbit energy of the halogen atom X. The Eint assumed to be zero because of the supersonic molecular beam condition. Translational energies of photofragments are calculated from the energy conservation and momentum conservation relations using the velocities obtained from images. The Br and Br raw images of CH2 BrCl are given in Fig. 1(a) and (b), respectively. The Br translational energy distributions of CH2 BrCl are displayed in Fig. 2 and the angular distributions are given in Fig. 3. The convoluted velocity of ground state Br is deconvoluted using two Gaussian curves. The double peak of ground Br has not been reported till now and it is newly resolved in this experiment [7,8]. The velocities are 940 and 1064 m sÿ1 , respectively, and the ratio of large velocity to small is 3.2.

Fig. 1. Raw images of (a) Br and (b) Br for CH2 BrCl system.

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S.-H. Lee et al. / Chemical Physics 260 (2000) 143±150

Fig. 2. Translational energy distributions of Br fragments in the center-of-mass coordinates of CH2 BrCl system.

Fig. 3. Angular distributions of Br fragments in the CH2 BrCl system.

Two primary dissociative processes, (1) and (2), make CH2 Cl and CH2 Br radicals. Bond energies

of radicals are calculated using thermodynamic data and are listed in Table 2.

S.-H. Lee et al. / Chemical Physics 260 (2000) 143±150

147

Table 2 Standard enthalpies of formation and dissociation energies

a b

Species

DHf0 (kJ molÿ1 )

Cl Br CH2 CH2 Cl CH2 Br

121:301  0:008a 111:87  0:12a 390.4a 120b 169b

…i†

Reaction

D0 (kJ molÿ1 )

CH2 ±Br CH2 ±Cl

327:1  10 385:4  10

Ref. [21]. Ref. [22].

CH2 Cl ! CH2 ‡ Cl CH2 Br ! CH2 ‡ Br

…3† …4†

Since the dissociation energies of CH2 ±Cl and CH2 ±Br are larger than the available energies, the reactions (3) and (4) do not proceed further. By the absorption of additional photons, CH2 Cl and CH2 Br can decay further. CH2 Cl ‡ hm ! CH2 ‡ Cl=Cl

…5†



…6†

CH2 Br ‡ hm ! CH2 ‡ Br=Br

The CH2 Cl radical has a maximum absorption cross section at 205 nm, r ˆ …11:7  1:6†  10ÿ18 cm2 per molecule [23]. The broad distribution of Cl atom suggests that there is no direct dissociation channel of C±Cl bond. The spatial distribution of the Cl-channel shows the propensity to be aligned in parallel directions of electric ®elds and supports the source, CH2 Cl, since both Br and Br are strongly anisotropic. The absence of broadly distributed Br atoms in the images supports the fact that reactions (2) and (6) have nearly no roles. Even though the reaction (2) proceeds, Cl atom via this channel is buried in the photoproducts of CH2 Cl due to its small concentration. So, the reaction (6) makes nearly no contribution, although the CH2 Br radical has a maximum absorption cross section near 230 nm, r ˆ …8:76  1:31†  10ÿ18 cm2 per molecule, and still has a large absorption cross section at 234 nm [24]. We have observed no evidence of other channels. If three body dissociation occurs, CH2 ‡ Br ‡ Cl, both Br and Cl should show near zero velocity. However, the Br image does not show a zero velocity component. This channel is also not

valid energetically, calculating the thermodynamic data of Table 2. Although BrCl elimination, CH2 ‡ BrCl, is also possible, BrCl undergoes a perpendicular transition at 234 nm [25] and the perpendicularly distributed products have not been observed. The branching ratios of halogen atoms are proportional to ion signals N …X † S…X † ˆk ; N …X† S…X†

…7†

where S…X† is the signal intensity of a halogen atom X, N …X†, the number of atoms produced in the photodissociation, and k, the proportionality constant. The k value of Br and Br is obtained from Br2 molecule under the same condition and k ˆ f …Br†=f …Br † ˆ 0:42  0:02 [26]. Since the k value is sensitive to the experiment condition, care is taken to maintain the same condition. The branching ratio of Br and Br is obtained to be N …Br †=N …Br† ˆ 0:54  0:08. 4. Discussion The fragment Br atom has exhibited two peaks in its ground state product distribution of the translational energy. These two peaks have shown an extraordinarily broad bandwidth, in this newly resolved high resolution imaging detection technique, compared with Br . The energy di€erence between two peaks is 25 kJ molÿ1 and the ratio of fast to slow one is 3.2. This phenomenon has been interpreted in terms of the energy partitioning. The translational energy partitioning can be explained by the impulsive models, i.e., rigid and soft models. The rigid impulsive model, neglecting the

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fragment vibrational excitation, gives an upper limit of the translational energy and predicts the fast part of Br peaks in agreement with slightly lower value from the observed value [8]. It predicts 0.4 as the translational energy contribution, Et =Eava , which is calculated using the optimized geometry in Fig. 5 and is also slightly lower than the experiment value. This discrepancy may be due to simpli®ed treatment of CH2 Cl radical as a pseudo-diatom in the model. Owing to movements of other atoms, the radical is apt to be less rotationally excited than a diatomic system. The slow part by the soft impulsive model suggests each fraction of the translational, vibrational, and rotational energy to be 0.34, 0.08, and 0.58, respectively. According to this model, the vibrational energy for Br is 20 kJ molÿ1 , which is in good agreement with the energy gap between two peaks of Br which is 25 kJ molÿ1 . The fraction of translational energy, 0.34, is also in reasonable agreement with the observed value, 0.40. For further analysis of the internal modes of the system, we look into the corresponding vibrational motion. In the photodissociation of an analog, CH3 Br, symmetric stretching (m1 ), which was not dominant, and umbrella (m2 ) motions for CH3 radical were reported [15,16]. The energies of m2 and m1 modes for CH2 Cl radical are 17 and 36 kJ molÿ1 , respectively, and the vibrational frequencies of the radical are listed in Table 3. Taking into account the excess translational energy of the fast part of Br from the upper limit by the rigid impulsive model, we propose that the translational energy di€erence of two peaks has increased from 17 to 25 kJ molÿ1 . As a consequence, the correTable 3 Vibrational frequencies of CH2 Cl Mode

Frequency (cmÿ1 )

m6 m4 m3 m2 m1 m5

397a 826a 993b 1391a 3054b 3185b

a

Ref. [29]. Calculated using factor 0.9061 [30].

b

GAMESS

[27,28] and multiplied by a scaling

sponding vibration is suggested to be an umbrella motion. Since two peaks of Br were not observed in CH3 Br [14], this must result from another chromophore Cl. The wave packet calculation on C±Br(n ÿ r ) surfaces [9] also provides a reasonable explanation. In the calculation, wave packets related to C±Br bond dissociation consisted of two parts. One part was assigned to a direct dissociation and the other came over a crossing seam from a r (C±Cl), which was a little internally excited in the course of crossing the seam. In light of this, two parts of Br internal distribution, shown in Fig. 2, can be interpreted as being one from a wave packet of r (C±Br) and the other over the crossing seam from r (C±Cl). Since the faster part dissociates directly, it has less internal and larger translational energy. It is consistent with the interpretation by the rigid and soft impulsive models. The ratio of large to small one is observed to be 3.2. It means that the ratio of two wave packets is 3.2 and C±Br bond is broken directly at least 3 times favorably. Since the spin±orbit e€ect did not come into the calculation, the agreement is restricted to the ground Br. By the ®ssion, the slow Br part of CH2 BrCl has a larger internal energy partitioning, contrary to the behaviors with similar Et =Eava ratios for Br and Br . It also supports the idea that the slow Br of CH2 BrCl has di€ernt origin, which is internally excited in the course of crossing the seam. Some information related to reaction dynamics of CH2 BrCl can be obtained by comparing the photolysis of the molecule with that of CH3 Br. Since the observed bond-selectivity guarantees the degree of the localized electronic transition, the shape of excited r (C±Br) surfaces of CH2 BrCl will be similar to those of CH3 Br. The excited states can be inferred by those of bent CH3 Br. Although all the well ordered energy states are not obtained, the order of some critical states is. The electronic con®gurations of ground and 2 2 the ®rst two excited states are


…a00 † …a0 † , 00 2 0 0 00 0 2 0


…a † …a †…a †, and


…a †…a † …a †, respectively. The energy di€erence between the ®rst two excited states is small. The symmetry of each con®guration is A0 , A0 , and A00 , respectively. For the case of small spin±orbit coupling, the ®rst excited band consists of 3 A0 , 1 A0 , 3 A00 , and 1 A00 . If the spin±orbit

S.-H. Lee et al. / Chemical Physics 260 (2000) 143±150

e€ect is large, 3 A0 is split into A0 , A0 , and A00 and 1 0 A is changed to A0 . The highest state is 4A00 , which originates from 1 A00 without spin±orbit coupling. If dissociating limit states are correlated to each electronic state, only one 5A0 state is correlated to Br . In a recent study, a correlation diagram of CH2 BrCl originated from bent CH3 Br with Cs symmetry is proposed [8]. Because CH2 BrCl belongs intrinsically to Cs symmetry and the states forbidden to CH3 Br with C3v symmetry are accessible, it is unreasonable to apply the oversimpli®ed diagram for CH3 Br with Cs symmetry to the CH2 BrCl system. The correlation diagram of CH2 BrCl in Fig. 4 is newly drawn assuming electronic energy levels, containing the excluded states, are similar to that of bent CH3 Br with a Cs symmetry. In Cs symmetry, the degeneracy of the excited surfaces of CH3 Br is broken since E symmetry is changed to A0 and A00 and A1 to A0 [17]. Though the orders of 1 A0 and the split states of 3 A00 may be exchanged, there is no essential change since all other states except one 5A0 are correlated to Br. One 5A0 state correlated with Br intersects with the 4A00 state and a curvecrossing will occur between these states. In the photolysis of CH2 BrCl, the anisotropy of Br indicates the surface linked to Br is repulsive

Fig. 4. Correlation diagram of CH2 BrCl. The dashed lines represent avoided curve-crossing due to the reduction of symmetry.

149

and the 5A0 state has a parallel transition character. Since the absorption peak of CH2 BrCl is a little blue-shifted compared with that of CH3 Br and 1 Q1 state of CH3 Br is not accessible in the experiment wavelength [14], we expect that the 4A00 state does not contribute to the Br channel. Because of the symmetries of transition moments, the A0 transition should lie in the Br±C±Cl plane A0 and the A00 A0 transition perpendicular to this A0 transition moments lie in plane. Although A0 the Br±C±Cl plane, the angles between these moments and the C±Br bond are arbitrary and the A0 transition can be 2 limiting anisotropy for A0 to ÿ1. The limiting anisotropy parameter for A0 transition is ÿ1. We believe that A0 A0 A00 transition moments are approximately parallel to the C±Br bond, considering the anisotropy of Br and the contribution of accessible states. The angular distribution of Br with a strong anisotropic character is interpreted that more A0 than A00 states are correlated with Br and the A00 state shows a nominal perpendicular transition. However, the portion of two transitions in the Brchannel cannot be obtained because the numbers of A0 and A00 states correlated to Br are unknown. The angle a between the parallel transition dipole and a C±Br bond is obtained to be about 21 through the relation b ˆ 2P2 (cos a) [31], where bk ˆ 1:6 is used to consider the lifetime and rotation of the parent molecule. The geometry of CH2 BrCl is obtained using the G A M E S S program [27,28] at the MP2 level with 6-31G basis sets, as given in Fig. 5. Since a direct Cl-channel was not

Fig. 5. Equilibrium geometry of CH2 BrCl. The transition di1A0 and lies in the Br±C±Cl plane. pole l is 5A0

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observed in the experiment, it is deemed necessary to investigate the Cl-channel via n ! r (C±Cl) transition and the study is in progress aiming to verify the excited electronic states and dissociation dynamics in our lab. Acknowledgements The authors gratefully acknowledge the Korea Research Foundation for its support of this research by the Korea±Germany Joint Project, 1998±2001. References [1] L.J. Butler, E.J. Hintsa, S.F. Shane, Y.T. Lee, J. Chem. Phys. 86 (1987) 2051. [2] D. Krajnovich, L.J. Butler, Y.T. Lee, J. Chem. Phys. 81 (1984) 3031. [3] J.E. Stevens, D.C. Kitchen, G.C.G. Waschewsky, L.J. Butler, J. Chem. Phys. 102 (1995) 3179. [4] A. Yokoyama, T. Takayanagi, G. Fujisawa, J. Chem. Phys. 103 (1995) 1710. [5] A. Yokoyama, K. Yokoyama, T. Tagayanagi, J. Phys. Chem. A 101 (1997) 6647. [6] G. Baum, J.R. Huber, Chem. Phys. Lett. 213 (1993) 427. [7] W.B. Tzeng, Y.R. Lee, S.M. Lin, Chem. Phys. Lett. 227 (1994) 467. [8] W.S. McGiven, R. Li, P. Zou, S.W. North, J. Chem. Phys. 111 (1999) 5771. [9] T. Tagayanagi, A. Yokoyama, Bull. Chem. Soc. Jpn 68 (1995) 2225. [10] P. Cadman, J.P. Simsons, Trans. Faraday Soc. 62 (1966) 631. [11] J. Doucet, R. Gilbert, P. Sauvageau, C. Sandorfy, J. Chem. Phys. 62 (1975) 366.

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