Fluorine Substitution Effects on the Photodissociation Dynamics of

J. Phys. Chem. 1996, 100, 7989-7996

7989

Fluorine Substitution Effects on the Photodissociation Dynamics of Iodobenzene at 304 nm Jennifer A. Griffiths, Kwang-Woo Jung,† and M. A. El-Sayed* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: September 12, 1995; In Final Form: NoVember 13, 1995X

The photodissociation dynamics of pentafluoroiodobenzene are investigated by state-selective one-dimensional translation spectroscopy at 304 nm. We have determined the one-dimensional recoil distribution and the spatial distribution in the form of the anisotropy parameter, β, as well as the photodissociation relative yields of both ground-state I(3P3/2) and excited-state I*(2P1/2) iodine photofragments. The results are compared to those observed for iodobenzene at 304 nm. As in iodobenzene, two velocity distributions were observed for the dissociation channel which gives ground-state iodine: a sharp, high recoil velocity peak assigned previously to n,σ* excitation and a slow recoil velocity distribution peak assigned previously to π,π* excitation. Unlike in C6H5I, the I* distribution is relatively strong and its spatial anisotropy can be measured. The fluorine perturbation has led to a number of different observations that can be summarized as follows: (1) The high velocity distribution has a lower average value and much broader width, suggesting more rapid energy redistribution to the fluorinated phenyl ring prior to and during the dissociation process, resulting from stronger coupling between the n,σ* and π,π* states and/or a longer excited-state lifetime; (2) the slow distribution is weaker and has an almost isotropic spatial distribution (the anisotropy parameter β ≈ 1.0), while in the iodobenzene spectrum β is correlated with the recoil velocity; (3) the I* quantum yield for C6F5I is 14 times larger than that for iodobenzene; and (4) β is correlated with the velocity in the I* spectrum found for C6F5I which is not observed for iodobenzene. These observed fluorine perturbations are attributed to an increased mixing between the charge-transfer state (resulting from electron transfer from the iodine nonbonding electrons to the π* orbitals of the fluorinated benzene ring) and both the n,σ* and the ring π,π* states. This leads to two effects: (1) a decrease in the nonbonding electron density on the iodine, which decreases the spin-orbit interaction between the n,σ* states themselves, resulting in a decrease in the curve-crossing probability (thus increasing the I* yield) and (2) an increase in the coupling between the repulsive n,σ* states and the fluorinated phenyl π,π* states, leading to an increase in the rate of energy redistribution.

I. Introduction The photodissociation dynamics of alkyl and aryl halides have been intensively investigated recently because of the wide variety of systems available for study, their applicability as models for larger molecular systems, and the useful properties of their upper-state potential surfaces. Alkyl iodides have an absorption continuum in the region 200-300 nm, the A band, which arises from an σ* r n transition localized on the C-I bond. There are five spin-orbit states resulting from the σ* r n transition, resulting in three possible transitions to the 3Q0, 3Q , and 1Q states. As Mulliken had predicted, the transition 1 1 to the 3Q0 is polarized parallel to the C-I bond axis, while the 3Q and 1Q transitions are polarized perpendicular to the C-I 1 1 bond axis.1 At infinite separation (dissociation), the 3Q0 state correlates with the excited state 2P1/2 of the iodine atoms (I*), while the 3Q1 and 1Q1 states correlate with the 2P3/2 ground state of the iodine atom (I) production. Photodissociation studies show that parallel polarized light yields both ground- and excited-state iodine atoms.2-12 The production of ground-state iodine atoms with parallel polarized light was attributed to curve crossing from the 3Q0 state to the 1Q state. The dissociation is rapid and occurs on a subpicosecond time scale. Aryl halide dissociation dynamics are of interest due to the possible coupling between phenyl ring excitation and the C-I localized σ* r n transition typical of the nonaromatic part of the molecule. The fluorinated derivatives of aryl halides provide * Author to whom correspondence should be addressed. † Permanent address: Department of Chemistry, Wonkwang University, Iri 570-749, Korea. X Abstract published in AdVance ACS Abstracts, April 1, 1996.

S0022-3654(95)02662-1 CCC: $12.00

added insight into the role of the aromatic ring in the photodissociation process, as the replacement of the hydrogen atoms by fluorine atoms is known to perturb both the σ and π molecular orbitals of the benzene system. In addition, fluorination of alkyl iodides is known to result in an increase in I* production over the unfluorinated derivatives. In this work, we report results obtained using state-selective time-of-flight (TOF) photofragment spectroscopy to probe the photodissociation dynamics of pentafluoroiodobenzene at 304 nm. II. Electronic States of C6H6, C6H5I, and C6F5I The spectroscopy of most benzene derivatives is closely related to unsubstituted benzene. There is a considerable amount of experimental and theoretical work on the electronic states of benzene.13,14 Benzene has three singlet-singlet transitions occurring at about 4.90, 6.20, and 6.95 eV which have been assigned to the 1A1g f 1B2u (Lb), 1A1g f 1B1u (La), and 1A1g f 1E 1 1u ( B) transitions, respectively. These give rise to the absorption bands that are designated as the 1Lb, 1La, and 1B bands using the Platt notation.15 Three other transitions occurring at 3.89, 4.85, and 5.69 eV have been assigned to singlet-triplet transitions.16a,b The first and third are identified as 1A1g f 3B1u and 1A1g f 3B2u transitions, while the second is tentatively assigned to the 1A1g f 3E1u transition.16b-d The electronic spectroscopy of fluorobenzene derivatives has been well investigated. The fluorobenzenes are found to have three singlet-singlet transitions analogous to the three singletsinglet transitions of benzene.15,17-19 For ease of discussion, the symmetry notation of benzene will be retained. Electron impact studies by Frueholz et al. indicate that increasing © 1996 American Chemical Society

7990 J. Phys. Chem., Vol. 100, No. 19, 1996 fluorination of the benzene ring results in little change in the 1B transition energy as evidenced by the experimental transi2u tion energies of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene reported to be 4.80, 4.87, and 4.79 eV, respectively. Hexafluorobenzene exhibits a weak broad band in the 290470-nm region assigned by Fukuzimi et al. to the S1 r S0 transition. The higher singlet states do show some shift to higher energies with increasing fluorination.19-21 In general, the benzene electronic states oscillate about 1 eV around their original value with increasing fluorination of the benzene ring. The spin-forbidden transitions have not been as extensively studied.16a,b,17,22-25 Phillips and co-workers used O2 perturbation methods to study the S1-T1 energy gap in several benzene derivatives and observed that T1 occurred at 3.66 eV in benzene and 3.62 eV in hexafluorobenzene.17 Electron impact studies show similar results for the T1 state. Doering et al. observed that the S1 r S0 transitions and the T2 r S0 transition overlap in benzene and place the T2 energy level at 4.85 eV.16 Electron impact studies place the T2 level at 5.59 eV. Interestingly, the T2 state is not observed in the higher substituted fluorobenzenes. Frueholz et al.14 suggests that either it is an extremely weak transition or it has shifted to higher energies. In the latter case, if the singlet-triplet transition is at higher energies than 5.9 eV, it will be obscured by the intense S2 r S0 benzene transition. One interesting feature evident in the higher substituted fluorobenzenes but not observed for benzene itself is the so-called C band at 5.32 eV for hexafluorobenzene. Frueholz et al. propose14 that this band is due either to a π-σ* transition or to a charge transfer of a fluorine-excited π electron to a carbon π* orbital. There are no ab initio studies on the electronic structure of the fluorobenzenes, although there have been several semiempirical studies performed.16,26-32 In general, these studies overestimate the known energies by about 1 eV and are not effective in reproducing the behavior of the benzene energy levels with increasing fluorination. Parr-type semiempirical studies predict a steady stabilization of the benzene 1B2u state with increasing fluorination. III. Experimental Section A. Effusive Beam. The experimental method and apparatus are described in detail elsewhere.33 Briefly, gaseous C6F5I molecules from an effusive beam are photodissociated by linearly polarized nanosecond laser pulses to produce aryl and iodine radicals. Photodissociation experiments are performed at polarizations parallel and perpendicular to the detection axis, i.e., polarization angles of R ) 0° and R ) 90°, respectively. For the 304-nm dissociation experiments, the iodine atoms are produced in either the ground or spin-orbit excited states and are state-selectively ionized within the same pulse (304.67 or 304.02 nm for I and I*, respectively). The iodine ions are then allowed to move in a field-free region for a delay time τ of 1.5 µs. During the delay time, the photoions spread out from their initial positions with the same velocities as the recoil velocity of their respective atoms since the electron carries the excess photon energy above the ionization threshold. The photoions are accelerated by applying a pulsed acceleration voltage of approximately -1500 V for 1 µs to an extraction electrode in a home-built time-of-flight mass spectrometer (TOF-MS). In addition to propelling the ions to the detector, the acceleration increases the kinetic energy of the photoions which in turn is used to amplify the signal at the detector and enhances detection sensitivity. The ions travel across a second field-free region to a discrimination pinhole 6.00 mm in diameter placed on the detection axis. When the acceleration field is applied, the ions

Griffiths et al. acquire new velocities dependent on their initial position in the extraction region. Consequently, the arrival time of a photoion at the detector is directly related to its recoil velocity component along the detection axis (a line connecting the laser focus and the center of the discrimination pinhole). The pinhole eliminates ions that have a large component of velocity perpendicular to the detection axis (greater than 120 m/s) and thus permits detection of only those ions that are aligned along the detection axis. Typical TOF spectra consist of two peaks, with the positive velocity peak corresponding to the photoions with their initial recoil direction toward the detector and the negative velocity peak corresponding to the photoions with their initial recoil direction away from the detector. B. Molecular Beam. The instrument can also be run in conjunction with a supersonic molecular beam in order to vibrationally and rotationally cool the parent molecule for comparison with the effusive beam experiment. The molecular beam source chamber is a differentially pumped chamber with a 2-mm skimmer located 1 in. from a General Valve Corp. pulsed nozzle. The backing pressure for the pulsed valve is kept low (300-500 Torr) to avoid cluster formation in the nozzle expansion. The pressure in the source chamber is 6 × 10-7, and the pressure in the ionization chamber (after the skimmer) is 4 × 10-7. The laser power was 70 µJ/pulse in these experiments. It is not possible to use deflection plates to guide the ion packet due to the distortion which would be created in the dissociated ion cloud by ion shielding and inhomogeneous electric fields. Any distortion of the spatial relationship of the ions in the ion packet would create errors in the data analysis. C6F5I was obtained from Sigma and used without further purification. Prior to use, the sample underwent several freezepump-thaw cycles to remove lower vapor pressure contaminant molecules. Experiments were performed at pressures on the order of 1.0 × 10-6 Torr and laser power of 15-25 µJ/pulse. To ensure that there were no significant multiphoton processes occurring, experiments were performed with similar laser power at wavelengths slightly off resonance from the I and I* resonance adsorption wavelengths of 304.7 and 304.0 nm, respectively. No multiphoton dissociation was observed. IV. Results and Analysis A. Center-of-Mass Translational Energy Release. The 304-nm experiments were performed at two wavelengths, 304.02 and 304.67 nm, allowing for both dissociation and selective ionization of either I* or I within the same pulse.

C6F5I f C6F5 + I* (304.02 nm)

(1)

C6F5I f C6F5 + I (304.67 nm)

(2)

The angular distribution and translational energy of both the ground- and excited-state iodine fragments were determined by examination of the photofragment TOF distributions. The TOF distributions of the iodine photofragments were measured at two polarization angles with respect to the detection axis, R ) 0° and R ) 90°, and at two wavelengths, 304.02 and 304.67 nm. The TOF results were subsequently transformed to laboratory (lab) velocity distributions. The procedure for this transformation is described elsewhere.33 The velocity distributions for the I and I* atoms produced in the effusive beam dissociation of C6F5I at 304 nm are shown in Figure 1b and Figure 2b. The velocity distributions for the corresponding dissociation channels in C6H5I are shown for comparison in Figures 1a and 2a. The velocity distribution peaks for the parallel polarization (R ) 0°) of the I channel are

F Substitution Effects on Dynamics of Iodobenzene

Figure 1. Lab recoil velocity distribution measured for resonantly ionized I atoms produced from the photodissociaton of (a) iodobenzene (C6H5I) and (b) pentafluoroiodobenzene (C6F5I) at 304.67 nm at two laser polarization angles R ) 0° and 90° with respect to the detection axis. The iodobenzene distribution clearly shows two overlapping recoil distributions: a sharp high-velocity distribution and a broader, slower velocity distribution. The C6F5I distribution has a broader, high-velocity band and a weaker, more diffuse low-velocity band. (c) The lab recoil velocity distribution measured for jet-cooled C6F5I molecules for the I dissociation channel. Comparison with the results for the effusive beam dissociation experiment reveals a decrease in the peak width of the fast dissociation peak (from 9.7 to 4.3 kcal/mol in the pulsed beam).

of two types: a narrow, high-velocity peak and a low-intensity broad isotropic peak centered at zero velocity. The I* channel shows two overlapping peaks with much slower recoil velocity. The positive and negative velocity peaks correspond to iodine atoms with their initial recoil direction away from and toward the detector, respectively. For the perpendicular polarization (R ) 90°), the I channel shows two weak intensity peaks at approximately the same recoil velocity as the parallel dissociation, while the I* channel exhibits a broad isotropic peak indicative of slow dissociation. The effusive beam dissociation results for C6F5I are compared with the molecular beam results in Figure 1c. Cooling of the parent molecule prior to dissociation by expansion leads to greater than a factor of 2 decrease in peak width for the fast recoil velocity peak (from 9.7 kcal/mol FWHM to 4.3 kcal/ mol). The velocity distributions are deconvoluted to obtain the center-of-mass velocity dependence of the anisotropy parameter,

J. Phys. Chem., Vol. 100, No. 19, 1996 7991

Figure 2. Lab recoil velocity distribution measured for resonantly ionized I* atoms produced in the photodissociation of (a) iodobenzene (C6H5I) and (b) pentafluoroiodobenzene (C6F5I) at 304.02 nm at two laser polarization angles R ) 0°and 90° with respect to the detection axis. The I* channel is only weakly evident in C6H5I (φ* ) 0.005). For C6F5I, the I* channel has a higher quantum yield (φ* ) 0.07) as compared to C6H5I and the I* atoms have a lower average recoil velocity.

β(ν), and the speed distribution, g(ν). The methodology has been described in detail elsewhere.33 Figure 3 shows the photofragment recoil velocity dependence of β(ν) and g(ν) for I and I* production at 304 nm. The I channel speed and anisotropy parameter distributions are deconvoluted to allow independent examination of the high-velocity and low-velocity peaks. The speed distribution is deconvoluted by assuming that the sharp peak is approximately symmetric around the peak maximum to give gh(ν) and gl(ν) for the higher and lower velocity peaks, respectively. β(ν) is deconvoluted to βh(ν) and βl(ν) taking into account the anisotropic broadening of the distribution to the low-velocity side resulting from thermal velocity of the parent molecule and using the following relation:

βl(ν) )

g(ν)β(ν) - gh(ν)βh(ν) g(ν) - gh(ν)

(3)

From Figure 3, it can be observed that β for the high-velocity peak is nearly independent of velocity, while β for the lowvelocity peak changes with changing velocity. The different behavior of the anisotropy parameter can be attributed to the dynamics of the dissociation process. β is related to the dissociation time of the molecule and the rotation period of the parent molecule. A rapid dissociation process will show high anisotropy due to the initial alignment of the dissociating molecule by the polarized photolysis light, and β will be near the limiting values of -1 and 2 for purely perpendicular and

7992 J. Phys. Chem., Vol. 100, No. 19, 1996

Griffiths et al. The use of the correlation between rotational time and anisotropy factor to estimate the dissociation time has been treated by many authors. Most recently, Hwang et al.35 used a variation of the methods of Yang and Bersohn36,37 to find the dissociation time of C6H5I. Briefly, in this model, it is assumed that depolarization is lost purely due to rotation of the molecule during dissociation and that bending vibration excitation, whether resulting from thermal excitation or Franck-Condon factors, has no significant effect on β. While these effects might not contribute significantly to changes in β in rigid molecules such as iodo aromatics, it will not be the case for floppy molecules that change geometry upon excitation like I-CN.38 If we assume that the change in β is a result of only rotational depolarization, the value of the anisotropy factor at reduced time t*, β(t*), can be related to the initial value at time zero, β0, as follows36,37

β(t*) ) β0〈D200(t*)〉

(6)

where 〈D200(t*)〉 is a rotational correlation function averaged over the rotational ensemble. This average depends both on the temperature T and the symmetry parameter b ) (I - Iz)/Iz, where for a symmetric top molecule I is the perpendicular moment of inertia and Iz is the moment of inertia about the symmetric top axis. b was calculated to be 2.33. β0 can be expressed as

β0 ) 2κP2(cos χ)

Figure 3. Velocity dependence of the anisotropy parameter β(ν) and g(ν) for (a) the I(2P3/2) dissociation channel and (b) the I*(2P1/2) dissociation channel of C6F5I.

parallel transitions, respectively. A slow dissociation allows rotation of the parent molecule prior to dissociation and loss of anisotropy in the velocity distribution, with β approaching zero. Thus, the results in Figure 3 indicate that there are two dissociation processes occurring: one dissociation mechanism that is faster than molecular rotation and one that is slower. B. Energy Partitioning, Dissociation Lifetime, and Quantum Yield. The photofragment distributions can be examined as a function of the center-of-mass translational energy by transformation of g(ν) and β(ν) into the energy domain using energy conservation relations. From this transformation, one can obtain the average center-of-mass translational energy, 〈Et〉. The available energy for the ground-state iodine formation channel, Eavl, is related to 〈Et〉 as follows:

Eav1 ) hν - D0(C6F5-I) + Eint(p) ) [〈Et〉 + Eint]fragments (4) while for I*, the Eavl* is less than that for I by the amount of electronic energy left in the I* (the spin-orbit energy)

Eav1* ) Eav1 - Eso

(7)

where κ e 1 is a parameter that takes into account any decrease in the observed β due to both vibrational motion and parent molecule velocity. Taking the highest value of β obtained using our experimental method, the lower limit of κ ) 0.88 is found to correspond to a dissociation with no rotational depolarization. As discussed earlier, β is nearly independent of velocity for the I channel. Comparing the measured β with the time dependence of the rotational correlation function and using the relation

t* ) (kT/I)1/2t the dissociation time is estimated to be < 0.56 ps for the fast recoil velocity I channel, 0.62-1.75 ps for the slow recoil velocity I channel, and 0.80-1.5 ps for the I* channel. The upper limit on the dissociation time for the fast dissociation channel of I is a result of the fact that its β value is limited by the apparatus. Finally, the quantum yield φ* of I* can be determined from the relative ion signal, s*/s, for the I and I* atoms produced at 304.67 and 304.02 nm, respectively, using the relation

φ* )

n*/n n*/n + 1

(8)

where n*/n is the number ratio of the excited-to-ground state iodine atoms produced. This ratio can be determined from the relative ion signal and the relative ionization efficiency for the I* and I, f*/f,

(5)

D0(C6F5-I) is the energy of the dissociation of ground-state pentafluoroiodobenzene into C6H5 and I radicals at 0 K (66.2 ( 1.0 kcal/mol),34 Eso is the spin-orbit splitting between ground- and excited-state iodine atoms (21.2 kcal/mol), and Eint(p) is the internal thermal energy of the parent molecule. Table 1 summarizes the Et, Eint, Eavl, and ratio of Eint/Eavl.

s* n*f* ) s nf

(9)

The relative ionization efficiency is determined from the dissociation of I2 at the same wavelength and laser pulse energy. At this wavelength, I2 dissociates to I + I*; thus, n*/n ) 1 and f*/f ) s*/s.

F Substitution Effects on Dynamics of Iodobenzene

J. Phys. Chem., Vol. 100, No. 19, 1996 7993

TABLE 1: Summary of Important Values for the I and I* Channels from the Dissociation of Pentafluoroiodobenzene I fragment

φ*

λ, nm

〈Eavl〉a

〈Et〉a

〈Eint〉a

〈Eint〉/〈Eavl〉

FWHMb

t, ps

β

-dEt/dtc

I* I (fast) I (slow)

0.07

304.02 304.67 304.67

6.7 27.9 27.9

1.4 15.7 ∼1-6

5.3 12.2 ∼22-26

0.80 0.44 0.80-0.95

9.7 3.3

0.8-1.6 0.56 0.62-1.75

0.4-1.2 1.5 0.12-1.4

∼25

a

b

∼20

c

All energies are in kcal/mol. This is the full width at half-peak maximum for the G(Et) distribution. Results are in kcal/mol. Units kcal/(mol

ps).

f*/f was determined to be 1.3 ( 0.2. Calculation of the quantum yield at 304 nm gives φ* ) 0.07. This is approximately 14 times the I* quantum yield observed for iodobenzene photodissociation at the same wavelength. V. Discussion Several groups have investigated the effect of the phenyl ring on the photodissociation pathway of aryl iodides. Bersohn and co-workers39,40 photodissociated molecular beams of various aryl halides resulting from excitation to the S1, S2, and S3 states of the aryl chromophore. At low photon energies between 265 and 320 nm, it was observed that the product translational energy distribution was independent of wavelength and produced mainly ground-state iodine. It was concluded that the aryl halides dissociate from an indirect process involving the π system of the aryl ring.39 Fluorination of iodobenzene decreased the kinetic energy release of the photofragments at 193 nm.39 This decrease was attributed to the formation of a distorted ring structure prior to dissociation. It has been proposed by Iredale and co-workers41 that the iodobenzene 1Lb absorption intensity can be explained as a combination of the benzene S1 r S0 transition with max ) 200 LM-1 cm-1 at λ ) 254 nm and the σ* r n C-I localized transition with max ) 400 LM-1 cm-1 at λ ) 260 nm. The dissociation dynamics of iodobenzene has been investigated by Hwang and El-Sayed at 304 nm.35 Only the groundstate iodine channel was probed due to the extremely low (0.005%) quantum yield for the I* channel. The experimental results suggested that there are two dissociation channels to give ground-state I. The first channel gives fast iodine atoms formed from absorption to the 3Q0 state followed by curve crossing to the repulsive 1Q1 state. This is the rapid dissociation mechanism. The second channel produces ground-state iodine with a slower velocity distribution. This channel is postulated to involve an initial absorption to a predissociative π,π* state followed by crossing to an C-I localized n,σ* repulsive state(s). This is the predissociatiVe mechanism which results in a range of dissociation times (0.5-1.3 ps) all of which are longer than those for the n,σ* repulsive state, leading to more energy redistribution prior to dissociation and a broad velocity distribution. Furthermore, if the range of dissociation lifetimes is in the range of the molecular rotation times, β becomes correlated with the velocity, as observed.35 A comparison of the recoil velocity distribution of the I produced in the photodissociation of C6F5I with that from C6H5I (Figure 1a,b) reveals differences. The sharp and broad bands observed for C6H5I (Figure 1a) are now a much broader highvelocity band and a weak broad and diffuse band for C6F5I (Figure 1b). Two different assignments of the two peaks observed for C6F5I can be made. The first is that the two sharp high-velocity distributions in C6F5I and C6H5I correspond to a similar rapid dissociation mechanism and the broad diffuse lowvelocity distributions correspond to the predissociation mechanism (absorption to the π,π* state(s) and crossing to the n,σ* repulsive state). The second assignment could be that the broad central band in C6F5I arises from multiphoton statistical dissociation. In this case, the broad high-velocity band in C6F5I

could be a superposition of the high-velocity sharp band and the broad lower velocity band observed for C6H5I. This would suggest that the slow dissociation in C6H5I became more rapid in C6F5I. The relatively high symmetry of the distribution places some doubt about the possible overlapping of two different bands. Of course, it is possible that the mixing between the n,σ* and the π,π* states becomes sufficiently strong and that the excitation involves only one component of the two mixed components, the one with the highest absorption intensity or else the one with the much broader distribution. In order to distinguish between the two possibilities outlined above, we used our recently built supersonic nozzle system to photodissociate jet-cooled C6F5I to obtain better resolution in the fast dissociation peak and attempt to resolve any overlap of the fast direct dissociation channel with the predissociative channel. These results are shown in Figure 1c. The cooling of the parent molecule leads to a decrease in the FWHM of the fast dissociation peak from 9.7 kcal/mol in the effusive beam to 4.3 kcal/mol in the pulsed beam. In this spectrum, there is no indication of two overlapping peaks. In light of these results, we shall use the first assignment in the discussion below, i.e., that the high velocity distribution and the broad low-velocity distribution correlate with one another in the two molecules. The dissociation mechanism of pentafluoroiodobenzene is discussed below at 304 nm in terms of the direct dissociation mechanism and the predissociative mechanism. The effect of the perturbation of the fluorination is discussed in terms of the changes in the spin-orbit coupling, the relative excited-state surfaces, and the mixing between the charge-transfer states, the π,π* excited states, and the n,σ* C-I localized states. A. Effect of Fluorine Substitution on the I*/I Branching Ratio. In contrast to the dissociation of C6H5I at 304 nm, the dissociation of C6F5I at the same wavelength has an increased dissociation probability for the I* channel. φ* increases from 0.005 for C6H5I to 0.07 for C6F5I. This corresponds to a 14fold increase in φ* as a result of the fluorination of iodobenzene. The enhanced I* quantum yield for fluorinated vs unfluorinated compounds is not unusual. It has been found that in general the perfluoro analogues of alkyl iodides have significantly higher dissociation quantum yield of I*.10,43 There are two possible explanations for this interesting behavior as observed in C6F5I: (1) fluorination of the benzene ring changes the spin-orbit coupling of the 3Q1 and 1Q dissociative states or (2) low-lying charge-transfer states mix with the dissociative states and influence the curve-crossing probability. Each of these cases is discussed below. The spin-orbit interaction energy ∆E in atoms is known27 to depend on the nuclear charge, Z, as given by

∆E ∝ Z4/h3 The absorption of alkyl iodides in the A band is due to a σ* r n transition. Thus, the spin-orbit interaction is governed by the extent to which the excited electron is localized on the iodine atom, which has the largest nuclear charge. For alkyl iodides, any electron-withdrawing substituent on the alkyl group will draw electron density away from the iodine atom, thus reducing

7994 J. Phys. Chem., Vol. 100, No. 19, 1996 SCHEME 1 A1 + hν

3Q

0(n,σ*)

P*diss

I*

Pcurve crossing 1Q

1(n,σ*)

I

the effectiveness of the spin-orbit coupling on the iodine atom (the atom with the highest Z). Ivanov et al.26 have shown this to hold true for some derivatives of iodoalkanes (CF3I, C2F5I, C3F7I, and CF3CFICF3) by using the MO LCAO SCF (molecular orbitals, linear combination of atomic orbitals, self-consistent field) method to calculate the spin-orbit splitting, ∆E, between the 3Q1 and 3Q0 states. Their results show that an increase in the extent to which the excited σ* orbital is localized on the I atom results in an increase in the spin-orbit interaction between these two states. Fluorine as a substituent is electron withdrawing and thus will decrease the electron density on the I atom. The result is a decrease in the coupling strength between the 3Q0 and 1Q states. Thus, the probability of crossing from the 3Q0 surface (which correlates with the I* product) to the 1Q1 surface (which correlates with the I product) is decreased, and more of the excited molecules dissociate to the excited-state iodine photoproduct. It has been noted by Donohue and Wiesenfeld44 that the φ* quantum yield correlates approximately with the ionization potential of the radical photofragment. It is suggested that the mixing of low-lying charge-transfer states with the Q branch may influence the curve-crossing probability. The internuclear distance, r, of the zeroth-order crossing between a chargetransfer state and the dissociative state can be related to the ionization potential of the radical, I(R), and the electron affinity of a halogen atom, E(X), by

e2/r ≈ I(R) - E(X) Thus, as the ionization potential of the radical increases, the internuclear distance at which the charge-transfer state interacts with the dissociative state becomes smaller. The I* product is assigned to direct dissociation along the 3Q surface, which correlates with excited-state I* production. 0 This dissociation process is in competition with curve crossing to the 1Q1 state, which produces the observed high-velocity distribution of the ground-state iodine, I as shown in Scheme 1 The relative quantum yield of φ*/φ depends on P*diss/ Pcurve crossing in Scheme 1, where P signifies the probability of the process and the “diss” is for the dissociation process. The fact that fluorine atoms decrease the electron density in the σ* orbital on the iodine atom should decrease Pcurve crossing and increase φ*. The fact that the effect in pentafluoroiodobenzene is more pronounced than in aliphatic iodides could result from a better charge-transfer mixing in the former. The chargetransfer state in pentafluoroiodobenzene should be lower in energy than in either aliphatic iodides or in iodobenzene. This would bring it energetically in close proximity with the n,σ* state, which would result in better mixing with it. This could account for the much greater enhancement in the observed quantum yield for I* in the pentafluoroiodobenzene relative to that in iodobenzene as compared to the aliphatic compounds. B. Effect of Fluorine Substitution on the Direct Dissociation Channel Producing I(2P3/2). The center-of-mass velocity distribution of the ground-state iodine channel (Figure 3) exhibits two distributions: a sharp distribution at 722 m/s and a broad, slow-velocity distribution. The slower velocity distribution is discussed later in terms of a predissociative mechanism. The

Griffiths et al. behavior of the sharp, high-velocity distribution, however, is similar to that observed for iodoalkanes. The high anisotropy parameter (∼1.5 at the peak maximum) suggests an initial electronic transition that has predominately parallel character (with a transition moment parallel to the C-I bond), while the narrow velocity spread suggests that the excited state is repulsive with a short lifetime that minimizes energy redistribution prior to dissociation. The anisotropy parameter is constant with velocity (Figure 3), indicating that the dissociation via this channel occurs faster than the rotation period of the parent molecule. Thus, this distribution is assigned to absorption due to an initial parallel transition to the 3Q0 state, followed by curve crossing to the 1Q1 surface to give ground-state iodine as shown in Scheme 1. Fluorination of the benzene ring has an interesting effect on this direct dissociation channel. The FWHM has increased from 8.3 kcal/mol in iodobenzene to 9.5 kcal/mol in pentafluoroiodobenzene. In addition, the value of 〈Eint〉/Eavl has increased from approximately 0.24 in iodobenzene to 0.40 in pentafluoroiodobenzene. The increase of both the FWHM and 〈Eint〉/Eavl indicates that fluorination of the benzene ring has resulted in an increase in the coupling between the C-I localized state and the benzene π,π* states. This results in a longer dissociation lifetime calculated for this channel (∼0.56 ps) as compared to the analogous dissociation channel in iodobenzene (∼0.34 ps). This increased dissociation lifetime, as well as the increased density of the pentafluoro phenyl states, results in more energy redistribution and thus a larger FWHM. The value of the anisotropy parameter is lower than the limiting value of 2 for a rapid, direct dissociation on a repulsive surface with an initial parallel transition. It is possible that this decrease is due to the presence of more than one absorption with opposite polarizations. The relative contributions of the parallel and perpendicular transitions to the mixed transition can be calculated using the following relation

β ) X||β|| + X⊥β⊥

(10)

where X|| and X⊥ are the fractional contributions of the parallel and perpendicular transitions, respectively. It is necessary to know β|| and β⊥. For a purely parallel transition, β|| ) 2. However, β|| is taken as 1.63 due to apparatus depolarization effects. Assuming a similar fractional decrease in β for β⊥ gives β⊥ ≈ 0.8-0.9. Using these values gives a mixed transition with a composition of 85% parallel and 15% perpendicular character. However, it should not be discounted that the lower value of β could be due to overlap of the predissociative distribution (with a low β value) with the direct dissociation distribution, introducing a small error into the calculation of the anisotropy parameter. In this case, the value of the anisotropy parameter is taken as the lower limit. C. Effect of Fluorine Substitution on the Energy Redistribution Prior to Dissociation. The behavior of the rapid dissociation channel in C6F5I giving rise to I* is different from the analogous dissociation channel in iodobenzene in that the energy redistribution prior to dissociation seems to be more effective. This is shown by its broader velocity distribution (see Figure 2) and its lower average velocity value. In addition, Figure 3b shows the dependence of β on velocity for the lower portion of the velocity distribution peak (a contribution of instrumental factors discussed previously could be made at these lower velocities). The values of Eint/〈Eavl〉 for the I and I* dissociation channels of C6H5I are 0.24 and 0.18, respectively, while for C6F5I these values are 0.40 and 0.80 for the I and I* channels, respectively. For C6F5I, there is a strong increase in the partitioning of available energy into internal energy as

F Substitution Effects on Dynamics of Iodobenzene compared to C6H5I. In addition, while for C6H5I the I channel has a higher percentage of available energy in internal modes, the opposite is true for C6F5I in which the I* channel has a higher value of Eint/〈Eavl〉 than the I channel. All of these observations suggest that the I* distribution results from a combination of dissociation channels: a direct dissociation channel akin to the alkyl iodide dissociation channel as discussed earlier and a new contribution from the predissociative π,π* excitation which gives I* with velocity dependence on β. The predissociative mechanism in this molecule is discussed below for both the I and I* product channels. A similar channel to the latter could also be operative in C6H5I, but due to the low quantum yield of I*, it is not detected. Figures 1 and 3 show that fluorination of iodobenzene has three important effects on the ground-state iodine distribution: (1) an increase in the bandwidth of the rapid distribution from 3.0 to 10.0 kcal/mol (for C6H5I) but a decrease in the average velocity; (2) an increase in the relative yield of the rapid-toslow velocity distribution; and (3) a relatively small β value for the slow distribution with weak dependence on velocity. Observations 1 and 2 can be explained by an increased coupling between the n,σ* and the phenyl π,π* states together with an increased energy redistribution efficiency. This can result from an increased mixing between both of these states with the nπ(I) f π* charge-transfer state. The fluorine atoms could lower the energy of this state, allowing it to interact more strongly with the n,σ* and π,π* states, which in turn leads to more effective mutual mixing between them. This leads to elongating the lifetime of the n,σ* repulsive state and allowing for more energy redistribution prior to and during its dissociation. This could explain observation 1. Due to the enhanced mixing between the n,σ* and π,π* states, molecules absorbing to the π,π* state could undergo more rapid crossing to the repulsive n,σ* state and give rise to iodine atoms within the rapid distribution. This could explain observation 2. The third observation could result from an overlap of different iodine distributions resulting from absorption to more than one triplet π,π* states having opposite polarizations. Assuming that the correlation between the recoil velocity and β is a result of the effect of molecular rotation, the approximate rate at which the electronic energy of excitation is redistributed into the normal modes of the phenyl ring can be calculated using the center-of-mass energy dependence of β and the correlation of β and reduced time as was previously done for iodobenzene.35 The relationship is approximately linear, with the slope, (dEt/ dt*), which can be converted to t (ps) using the relation t* ) t(I/kT)-1/2. For C6F5I, (I/kT)-1/2 is calculated to be 2.13 ps. This gives a dEt/dt of ∼25 kcal/mol ps for the ground-state iodine dissociation channel. This value is similar to that observed during the predissociative channel, giving rise to the groundstate iodine in iodobenzene (∼23 kcal/(mol ps)). This suggests there is no change in the rate of energy redistribution by fluorination. This is unexpected since if indeed there is coupling between the π,π* states with the charge transfer and the n,σ* states as concluded from all the previous observations, one would expect a more rapid energy redistribution rate in the predissociative channel in C6F5I as compared to C6H5I. This might suggest that the method of using rotation clocking is not applicable. This could happen if the changes in β are due to other factors, e.g., a change in the phenyl-C-I angle or the presence of more than one absorption with different polarizations and rates of energy redistributions, as was recently shown for C6H5I.45 The mechanism of producing the slow I* channel is assigned to a large contribution from predissociation from the strongly

J. Phys. Chem., Vol. 100, No. 19, 1996 7995 SCHEME 2 mechanism I A1

+ hν

(π,π*)

mechanism II A1

+ hν

3Q (rapid) 0

3Q (rapid) 1

I*

I

mixed π,π* doorway states. As with the I channel, the I* channel is observed to have an anisotropy parameter that depends on velocity (Figure 3). As for the conclusion made for the slow I channel, this could be a result of multiple excitation to states of different polarization and rates of energy redistribution. D. Conclusion. In summary, the dissociation dynamics of C6F5I at 304 nm show different interesting behavior from that of the dissociation of C6H5I at the same wavelength. Most important are the following: (1) an increased I* production and a dependence of its β on velocity, suggesting a new observed dissociation channel; (2) an increase in the bandwidth of the rapid I(2P3/2) distribution but a decrease in the average recoil velocity, suggesting an increase in the energy redistribution efficiency prior to and during dissociation process; (3) an increase in the relative yield of the rapid:slow I(2P3/2). The above results can be accounted for by proposing an increase in the electron-transfer probability between the pπ nonbonding iodine electrons and the π* molecular orbitals of the phenyl ring as a result of stabilizing the resulting state by the electronegative fluorine atoms on the ring. This leads to (1) a decrease in the spin-orbit coupling between the n,σ* states, resulting in a decreased curve-crossing probability and an enhanced I* signal, and (2) an enhanced mixing between the n,σ* states with the phenyl π,π* states. This increases the efficiency of the energy redistribution prior to and during dissociation via either the rapid or the predissociative channels. The overall possible dissociation mechanisms for this molecule are summarized in Scheme 2. In this scheme, the processes indicated by the solid arrows are enhanced, while that represented by a broken line is diminished by fluorine substitution. Acknowledgment. Jennnifer A. Griffiths thanks Dr. Hyun Jin Hwang and Dr. John Freitas for their assistance. The financial support of the National Science Foundation is greatly appreciated. K.-W.J. thanks the Korea Science and Engineering Foundation for partial support. References and Notes (1) Mulliken, R. S. J. Chem. Phys. 1940, 8, 382. (2) Riley, S. J.; Wilson, K. R. Faraday Discuss. Chem. Soc. 1972, 53, 132. (3) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1985, 83, 1996. (4) Hwang, H. J.; El-Sayed, M. A. Chem. Phys. Lett. 1990, 170, 161. (5) Gedanken, A.; Rowe, M. D. Chem. Phys. Lett. 1975, 34, 39. (6) Gedanken, A. Chem. Phys. Lett. 1987, 137, 462. (7) Dzvonik, M.; Yang, S.; Bersohn, R. J. Chem. Phys. 1974, 61, 4408. (8) van Veen, G. N. A.; Baller, T.; de Vries, A. E.; van Veen, N. J. A. Chem Phys. 1984, 87, 405. (9) van Veen, G. N. A.; Baller, T.; de Vries, A. E.; van Veen, N. J. A.; Shapiro, M. S. Chem. Phys. 1985, 93, 277. (10) Barry, M. D.; Gorry, P. A. Mol. Phys. 1984, 52, 461. (11) Paterson, C.; Godwin, F. G.; Gorry, P. A. Mol. Phys. 1987, 60, 729. (12) Sparks, R. K.; Shobatake, K.; Carlson, L. R.; Lee, Y. T. J. Chem. Phys. 1981, 75, 3838.

7996 J. Phys. Chem., Vol. 100, No. 19, 1996 (13) For example, see: Lassettre, E. N.; Skerbele, A.; Dillon, M. A.; Ross, K. J. J. Chem. Phys. 1968, 48, 5066. (14) See, for example: (a) Freuholz, R. P. F.; Flicker, W. M.; Hosher, O. A.; Kupperman, A. J. Chem. Phys. 1979, 70, 3057. (b) Petruska, J. J. Chem. Phys. 1961, 34, 1111. (15) Platt, J. R. J. Chem. Phys. 1949, 17, 484. (16) (a) Doering, J. P. J. Chem. Phys. 1979, 67, 4065. (b) Doering, J. P. J. Chem. Phys. 1969, 51, 2866. (c) Hay, P. J.; Shavitt, I. J. Chem. Phys. 1974, 60, 2865. d) Karawowski, J. J. Mol. Struct. 1973, 19, 143. (17) Fukazumi, T.; Nakashimi, K.; Ogawa, T. Bull. Chem. Soc. Jpn. 1991, 64, 2323. (18) Sponer, H. J. Chem. Phys. 1954, 22, 234. (19) Cooper, C. J. Chem. Phys 1954, 22, 503. (20) Trundell, B.; Price, S. J. Can J. Chem. 1979, 57, 2256. (21) Brundle, C. R.; Robin, M. B.; Kuebler, N. A. J. Am. Chem. Soc. 1972, 94, 1466. (22) Phillips, D. J. Chem. Phys. 1967, 46, 4679. (23) Evans, D. F. J. Chem. Soc. 1959, 2753. (24) Metcalfe, J.; Rockles, M.; Phillips., D. Faraday Trans. I 1974, 70, 1660. (25) Kimura, K.; Nagakura, S. Mol. Phys. 1964, 9, 117. (26) Ivanov, V. S.; Kozlov, A.; Pravilov, A.; Smirnov, E. SoV. J. Quant. Electron. 1980, 10, 556. (27) Yadav, J. S.; Mishva, P. C.; Rai, D. K. Mol. Phys. 1973, 26, 193. (28) (a) Kiss, A. I.; Martin, A. Chem. Phys. Lett. 1973, 19, 104. (b) Kiss, A. I.; Martin, A. Chem. Phys. Lett. 1973, 22, 390. (29) Khetrapal, C. L.; Rai, D. K. Theoret. Chim. Acta 1969, 13, 308. (30) Duke, C. P.; Yip, K. L.; Ceasar, G. P.; Potts, A. W.; Streets, D. G. J. Chem. Phys. 1977, 66, 256.

Griffiths et al. (31) Ford, B. Theoret. Chim. Acta 1968, 10, 342. (32) Chalvet, O.; Leibovici, C. Theoret. Chim. Acta 1968, 13, 295. (33) Hwang, H. J.; Griffiths, J. A.; El-Sayed, M. A. J. Mass Spectrom. Ion Process. 1994, 131, 265. (34) Taken from: Drech, M. J.; Price, S. J. W.; Sapiano, H. J. Can. J. Chem. 1977, 55, 422. (35) Hwang, H. J.; El-Sayed, M. A. J. Chem. Phys. 1992, 96, 856. (36) Yang, S.; Bersohn, R. J. Chem. Phys. 1974, 61, 4400. (37) Zare, R. N. Mol. Photochem. 1972, 4, 1. (38) Griffiths, J. A.; El-Sayed, M. A. J. Chem. Phys. 1994, 100, 4910. (39) Dzvonik, M. J.; Yang, S.; Bersohn, R. J. Chem. Phys. 1976, 66, 2647. (40) Freedman, A.; Yang, S. C.; Kawasaki, M.; Bersohn, R. J. Chem. Phys. 1980, 72, 1028. (41) (a) Durie, R. A.; Iredale, T. J. Chem. Soc. 1950, 1181. (b) Dunn, T. M.; Iredale, T. J. Chem. Soc. 1952, 1598. (42) (a) Brewer, P.; Das, P.; Ondrey, G.; Bersohn, R. J. Chem. Phys 1983, 79, 720. (b) Hess, W. P.; Kohler, S. J.; Haugen, H. K.; Leone, S. R. J. Chem. Phys. 1988, 84, 2143. (c) van Veen, G. N. A.; Baller, T.; de Vries, A. E.; Shapiro, T. J. Chem. Phys. 1984, 81, 1759. (43) (a) Gerck, E. J. Chem. Phys. 1983, 79, 311. (b) Krajnovitch, K.; Butler, L. J.; Lee, Y. T. J. Chem Phys. 1984, 81, 3031. (44) Donohoe, T.; Wiesenfeld, J. R. J. Chem. Phys. 1975, 63, 3130. (45) Jung, K. W.; Griffiths, J. A.; El-Sayed, M. A. J. Chem. Phys., in preparation.

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