9085
J . Phys. Chem. 1991, 95, 9085-9091 (i) Since the T L signal is detected as the decrease of the probe beam intensity, the decrease of the intensity by the T-T absorption (process e)) cannot be separated from the T L signal. The contribution of this effect is represented by
Gr!(r)= A l c x l p d i s c ~ x P ( - ( ~+T kq[Ozl)t) = ql)exp(-(kT + k,[O,]))
where ko is the rate constant of the decay of singlet oxygen in the media. (iv) The time dependence of the T L signal due to heat from the decay processes of the higher excited triplet state (process (e)) is given by fi42(t) = BIexI~tT~+i,h~p(l - exp(-(kT
+ k,[O,])t)}E
(‘41)
G4)11- ~ x P ( - ( ~+T kq[021)t)J (A4)
where A is a constant that depends on an experimental configuration, I , is the intensity of the probe beam, is an extinction coefficient of the T-T absorption, kT is the intrinsic triplet lifetime, k, is the quenching rate constant of the triplet state by oxygen, and [O,] is the concentration of oxygen in the solution. (ii) The time dependence of the T L signal due to heat from the triplet decay (processes (d) and (g)) is given by
where hu, is the photon energy of the probe beam. (v) The TL signal intensity of the fast-rising component (processes (b) and (c)) is expressed by
11 - exp(-(kT
PTL
(‘45)
Since the observed TL signal is given by the sum of these contributions, the signal intensity should be expressed by eq 4. 2. Intensity Ratio of the Slow Decay Component. The total intensity of the T L signal after completion of all the processes is given by
+ kqP21)t)l= 65*V - exp(-(kT + k,[021)tN
(A2) where B is a constant that reflects the sensitivity of the TL signal detection. (iii) The time dependence of the T L signal due to heat from the decay of singlet oxygen (process (h)) is given by
= ~ ~ e x z p ( h v e x- @iscET)
UT,,
E ZTL(~)
= & + 06’’= BlcxZph~ex+ a4’ (A6)
The intensity ratio of the decay component to the total intensity ( R ) is defined as
RI
I T L (-~ ITL(-) ) ITL(W)
65” - BIexIp@i&T - a4’ BZexZphucx + G4)
(A71
c4)
Since and the other terms are proportional to Z,Z and Zp, respectively, R should increase with decreasing the power of the H e N e laser. Experimentally, however, R is nearly constant even when the He-Ne laser power is decreased by about I/5o. Therefore, we conclude that the term is negligible under our experimental conditions. Then, one can approximate (A7) as eq
c4)
5. Registry No. Car 99685-96-8; 02,1182-44-7.
Substituent Effects on Gas-Phase Photodissociation Dynamics: Resonance Raman Spectra of Ethyl Iodide, Isopropyl Iodide, and terf-Butyl Iodide David L. Phillips,+ Barbara A. Lawrence,$and James J. Valentini**l Department of Chemistry, University of California, Zrvine, California 9271 7 (Received: April 10, 1991; In Final Form: July I , 1991)
We report resonance Raman spectra of photodissociating ethyl, isopropyl, and tert-butyl iodide in the gas phase in order to study the effects that changes in the mass and branching of the alkyl group have on the dissociation. These resonance Raman spectra at 266-nm excitation are compared with preresonant spectra taken with 355-nm excitation and previously published 266-nm spectra of methyl iodide. The dissociation coordinate appears to be primarily in the direction of the C-I stretch motion, but there are significant contributions from bending motions about the a-carbon. The bending motions appear to become more important as the alkyl group mass and the branching about the a-carbonincrease. These gas-phase resonance Raman spectra are compared with similar measurements made in solution. This comparison reveals that overall the short-time dissociation dynamics of the isolated and solvated molecules are similar, but there are some significant differences.
Introduction Theoretical developments’-3 over the past decade and the application of these ideas to experimental absorption and resonance Raman (emission) spectrac1’ have produced information about short-time (=IO fs) photodissociation dynamics. By viewing the ‘Present address: Department of Chemistry, University of Rochester, Rochester, N Y 14627. *Presentaddress: Department of Chemistry, Wellesley College, Wellesley, MA 02181. I Present address: Department of Chemistry, Columbia University, New York, N Y 10027.
0022-3654191/2095-9085$02.50/0
light absorption and emission in photodissociating molecules with a time-dependent formalism, intuitive connections can and have (1) Heller, E. J. J . Chem. Phys. 1975, 62, 1544. (2) Lee, S.-Y.;Heller, E. J. J . Chem. Phys. 1979, 71, 4777.
(3) Heller, E. J.; Sundberg, R. L.; Tannor, D. J . Phys. Chem. 1982.86, 1822. (4) Imre, D. G.; Kinsey, J. L.; Field, R. W.; Katayama, D. H. J . Phys. Chem. 1982,86, 2564. ( 5 ) Imre, D.; Kinsey, J. L.; Sinha, A.; Krenos, J. J. Phys. Chem. 1984.88, 3956. .. - -. ( 6 ) Hale, M. 0.;Galica, G. E.; Glogover, S. G.; Kinsey, J. L. J . Phys. Chem. 1986, 90, 4997.
0 1991 American Chemical Society
9086 The Journal of Physical Chemistry, Vol. 95, No. 23, 199I
been made between the spectroscopy and the underlying early-time dynamics of the photodissociation. This resonance Raman spectroscopy approach to the characterization of photodissociation dynamics has been applied to the study of a variety of small molecules over the past few years, including CH31,'-' CD31,598N02,9-10H 2 0 ( D 2 0 and HDO),"J2 C H 2 I 2 H 2 S9 I4-I6 CH3N02," and n-propyl iodide.'* In this publication we describe resonance Raman spectra of photodissociating ethyl iodide, isopropyl iodide, and tert-butyl iodide. We selected this series of primary, secondary, and tertiary alkyl iodides to examine substituent effects on the photodissociation process. Since we made these measurements, Myers and cow o r k e r ~have ' ~ reported resonance Raman spectroscopy of these same alkyl iodides in solution. The short-time dissociation dynamics of these relatively complex molecules may show significant solvent effects (in contrast to the dissociation of methyl iodide for which the solvent effects are sma11k8),so it is useful to compare the dissociation dynamics of the isolated molecules in the gas phase with the dissociation dynamics of the solvated molecules. Resonance Raman spectra provide information on the short-time dissociation dynamics and thus an opportunity to make this comparison. The present study can focus on substituent effects in the dissociation because the identity of the alkyl group R in R-I appears to have little influence on the absorption into the dissociative excited state. Excitation in the lowest electronic absorption band, the A band peaked near 260 nm, leads to a prompt breaking of the R-1 bond in ethyl, propyl, and tert-butyl iodides. The A-band absorption spectra of all three molecules are very similar,20 implying that the electronic transitions involved are nearly the same. The similarity is not surprising, since the 260-nm absorption band is associated with excitation of an electron from a lone pair nonbonding orbital on the iodine atom to the u* antibonding orbital localized on the C-I bond. The strong spin-orbit interaction caused by the iodine atom gives rise to five states from the u* configuration of the molecule. Only three of these states have appreciable dipole-allowed transitions from the ground electronic state and they are called the 'QO,3Ql,and 'QI states in Mulliken's notation.*I The transition to 'Qo is polarized parallel to the C-I bond while those to 'QI and to 'QI are both polarized perpendicular to the C-I bond. Ged a n k e r ~ ~has ~ . ~carried ' out magnetic circular dichroism experiments on methyl, ethyl, and terr-butyl iodide to determine the relative contributions of these transitions to the A-band absorption. The 'QOabsorption accounts for 70-80% of the oscillator strength of the A band and dominates the absorption near the band maximum, although the contribution from the 'Q1and IQ1states is appreciable at the band edges. Photofragment translational spectroscopy experiments2e31have 3
(7) Lao. K. Q.; Person, M. D.; Xayayiboun, P.; Butler, L. J. J . Chem. Phys. 1990, 92, 823. (8) Markel, F.; Myers, A. 9. Chem. Phys. Leu. 1990, 167, 175. (9) Rohlfing, E. A.; Valentini, J. J . Chem. Phys. Leu. 1985, 114, 282. (IO) Rohlfing, E. A.; Valentini, J. J. J . Chem. Phys. 1985, 83, 521. ( I 1) Scnsion, R. J.; Brudzynski, R. J.; Hudson, 8. S.Phys. Rev. Lett. 1988, 61, 694. (12) Sension, R. J.; Brudzynski, R. J., Hudson, B. S.; Zhang, J . Z.; Imre, D. G.Chem. Phys. 1990, 141,393. (13) Zhang. J.; Imre, D. G. J . Chem. Phys. 1988,89, 309. (14) Kleinermanns, K.; Linnebach, E.; Suntz, R. J . Phys. Chem. 1987, 91, 5543. (15) Person, M. D.; Lao, K. Q.; Eckholm, B. J.; Butler, L. J. J . Chem. Phys. 1989, 91, 812. (16) Brudzynski, R. J.; Sension, R. J.; Hudson, 9. S. Chem. Phys. Leu. 1990, 165, 487. (17) Lao, K. Q.;Jensen, E.; Kash, P. W.; Butler, L. J. J . Chem. Phys. 1990, 93, 3958. (18) Phillips, D. L.; Lawrence, B. A.; Valentini, J. J. J . Phys. Chem. 1991, 95, 7570. (19) Phillips, D. L.; Myers, A. B. J . Chem. Phys. 1991, 95, 226. (20) Kinmura, K.; Nagakura, S.Spectrochim. Acra 1961, 17, 166. (21) Mulliken, R. S.J . Chem. Phys. 1935, 3, 506. (22) Gedanken, A.; Rowe. M. D. Chem. Phys. Lerr. 1975, 34, 39. (23) Gedanken, A. Chem. Phys. Lerr. 1987, 137,462. (24) Riley, S.J.; Wilson, K. R. Faraday Discuss. Chem. SOC.1972, 53, 132.
Phillips et al. revealed the photofragment anisotropia and determined that the photodissociation of the alkyl iodides is fast relative to molecular rotation. The completely structureless A-band absorption suggests that the photodissociation is also faster than vibrational motion along any Franck-Condon active bound coordinate. Real-time ob~ervation'~ of the alkyl iodide photodissociation indicates that dissociation is complete in less than 100 fs. One of the most interesting aspects of the photodissociation of the alkyl iodides in the A band is the branching between the two spin-orbit states of the iodine atom 2P3/2(I) and 2Pl,, (I*), and its dependence on the identity of the alkyl group in the RI molecule. Although the n u* A-band absorption spectra in the 230-280-nm region are very similar to one another for all of the alkyl iodides, the quantum yield for production of I* varies greatly31,33-39 as a function of the nature of the alkyl radical. In general, there is a decrease in the I* quantum yield as the alkyl group becomes more massive and branched. For example, upon 248-nm photolysis, methyl iodide produces 78% of the iodine atoms in the excited state, while tert-butyl iodide yields only 4% of the iodine atoms in the excited state.38 The 'QOstate correlates to I* product, while the 'QI and lQI states correlate to I product. A substantial amount of I product has been observed at wavelengths where the 'QOstate carries almost all the oscillator strength of the absorption. This has been ascribed to a curve crossing of the )Qo and 'QI potential energy surfaces. The internal energy of the alkyl photofragment is also dependent on the nature of the alkyl group. Measurement of photofragment translational energies shows that an increasing fraction of the available energy goes into the internal (vibrational and rotational) degrees of freedom of the alkyl radical fragment as the alkyl group gets heavier and more branched about the c r - ~ a r b o n . ~ ~ - ~ ' Mode-specific information on the energy disposal of the photodissociating alkyl iodides into the alkyl fragments is available for methyl iodide from several experimental techniques. Highresolution time-of-flight molecular beam experiments have shown most of the methyl radical internal energy appears in excitation of the umbrella vibrational mode.2s-26More recent experiments have used MPI,363m4 diode l a ~ e r ,and ~ ~coherent , ~ ~ anti-Stokes Raman scattering (CARS)47to measure the vibrational and rotational energy distributions of the methyl radical from the
-
(25) Sparks, R. K.; Shobatake, K.; Carlson, L. R.; Lee, Y. T. J . Chem. Phys. 1981, 75, 3838. (26) Van Veen, G.N . A.; Baller, T.; Devries, A. E.; Van Veen, N . J. A. Chem. Phys. 1984,87,405. (27) Barry, M. D.; Gorry, P. A. Mol. Phys. 1984, 52, 461. (28) Paterson, C.; Godwin, F. G.; Gorry. P. A. Mol. Phys. 1987,60.729. (29) Godwin, F. G.;Paterson, C.; Gorry. P. A. Mol. Phys. 1987,61,827. (30) Black, J. F.; Powis, I. Chem. Phys. 1988, 125, 375. (31) Zhu, Q.;Cao, J. R.; Wen, Y.; Zhang, J.; Huang, Y.; Fang, W.; Wu, X . Chem. Phys. Lert. 1988, 144,486. (32) Knee, J. L.; Khundar, L. R.; Zewail, A. H. J . Chem. Phys. 1985,83, 1996. (33) Hess, W. P.; Kohler, S.J.; Haugen, H. K.; Leone, S. R. J . Chem. Phys. 1986, 84, 2143. (34) Donohue, T.;Wiesenfeld, J. Chem. Phys. Lerr. 1975, 33, 176. (35) Donohue, T.; Wiesenfeld, J. J . Chem. Phys. 1975, 63, 3130. (36) Ogorzalek-Loo, R.; Hall, G.E.; Haerri, H.-P.; Houston, P. L. J . Phys. Chem. 1987, 92, 5. (37) Pence, W. H.; Baughcum, S.L.; Leone, S.R. J . Phys. Chem. 1981. 85, 3844. (38) Brewer, P.; Das, P.; Ondrey, G.;Bersohn, R. J . Chem. Phys. 1983, 79, 720. (39) Godwin, F. G.; Gorry, P. A.; Hughes, P. M.; Raybone, D.; Watkinson, T.M.; Whitehead, J. C. Chem. Phys. Leu. 1987, 135, 163. (40) Black, J. F.; Powis, I. Laser Chem. 1988, 9, 339. (41) Powis, 1.; Black, J. F. J . Phys. Chem. 1989, 93, 2461. (42) Chandler, D. W.; Thoman, J. W.; Janssen, M. H. M.; Parker, D. H. Chem. Phys. Leu. 1989, 156, 151. (43) Chandler, D. W.; Janssen, M. H. M.; Stolte, S.;Strickland, R. N.; Thoman, J. W.; Parker, D. H. J . Phys. Chem. 1990, 94,4839. (44) Ogorzalek-Loo, R.; Haerri, H.-P.; Hall, G. E.; Houston, P. L. J . Chem. Phys. 1989, 90,4222. (45) Hall, G.E.; Sears, T. J.; Frye, J . M. J . Chem. Phys. 1988,89, 580. (46) Sears, T.J.; Frye, J. M.; Spiro, V.; Kraemer, W. P. J . Chem. Phys. 1989, 90, 2125. (47) Tnggs, N . E.; Zahedi, M.; Nibltr, J. W.; Debarber, P. A.; Valentini, J . J . J . Chem. Phys., submitted for publication.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9087
Gas-Phase Photodissociation Dynamics
ISOPROPYL IODIDE
ETHYL IODIDE VI0
I I
355 NM
355 NM
266 NM
500
1500
2500
500
RAMAN SHIFT (CM-1)
Figure 1. Raman spectra of ethyl iodide taken with excitation wavelengths of 355 and 266 nm. The 355-nm spectrum is preresonant to the A-band absorption, while the 266-nm spectrum is resonant with the A-band absorption. The nominal C-I stretch progression is given by the shaded peaks in the figure. photodissociation of methyl iodide. However, to our knowledge no mode-specific information has been reported for the fragment energy distributions of the alkyl radicals from A-band photodissociation of higher chain length alkyl iodides.
Experimental Section The experimental a paratus and methods have been discussed in detail e l s e ~ h e r e ~ ~ -we ~ ~will a n dgive only a brief description here. The experimental apparatus consists of a pulsed ND:YAG laser, a gas flow cell connected to a vacuum system, a single monochromator, a photomultiplier detector, and data acquisition electronics. The laser (Quanta Ray DCR-I A) produces pulses of 7-ns duration and is operated at a repetition rate of 10 Hz with 1-10 mJ per pulse at its fourth harmonic (266 nm). The light is weakly focused into the gas cell such that the beam diameter in the interaction region is approximately 2 mm. The emitted light is collected at 90° to the incident laser light by an fl collection lens made of UV-grade fused silica, and then imaged into the 1-m single monochromator by two UV-grade fused silica lenses. The monochromator is operated with a spectral resolution of 30-50 cm-]. The signal is detected by a photomultipler tube and sent to an oscilloscope, which averages the signal that is subsequently digitized and stored by a microcomputer. The oscilloscope is triggered by a fast photodiode that receives a small part of the laser beam incident on the gas cell. The ethyl iodide, isopropyl iodide, and rerr-butyl iodide obtained from Aldrich Chemical Co. had stated purities of 99%, 97%, and 95%, respectively. The samples of the alkyl iodides were flowed through the gas cell a t static pressures of 0.5-2.0 Torr at room temperature and at a rate sufficient to replenish the interaction region between laser shots. Experiments were carried out at pulse energies of 1, 5, and 10 mJ and the Raman signal intensities were found to be a linear function of the laser pulse energy. The laser power and sample pressure were stable to within 10% over the course of a single scan. The spectra were scanned five to six times and then averaged. Raman intensities were extracted from the averaged spectra by graphical integration of the areas under individual peaks, with corrections for peak overlap and baseline variations. Peak positions were calibrated with known mercury emission lines from a lowpressure mercury lamp. The instrument response as a function (48) Lawrence, 1990.
B. A. Ph.D. Dissertation, University of California, Imine,
1500
2500
RAMAN SHIFT (CM-I)
Figure 2. Raman spectra of isopropyl iodide taken with excitation wavelengths of 355 and 266 nm. The 355-nm spectrum is prercsonant to the A-band absorption, while the 266-nm spectrum is resonant with the A-band absorption. The nominal C-I stretch progression is given by the shaded peaks in the figure. TERT-BUTYL IODIDE
I
"8
/1i/
500
355 NM
1500
2500
RAMAN SHIFT (CM-1)
Figure 3. Raman spectra of rerr-butyl iodide taken with excitation wavelengths of 355 and 266 nm. The 355-nm spectrum is preresonant to the A-band absorption, while the 266-nm spectrum is resonant with the A-band absorption. The nominal C-I stretch progression is given by the shaded peaks in the figure. of wavelength was determined by use of an intensity-calibrated standard deuterium lamp (Optronics Model UV-40). The instrument response function was used to correct the Raman spectra for the wavelength-dependent sensitivity of the apparatus. Results The Raman spectra of vapor-phase ethyl iodide, isopropyl iodide, and rert-butyl iodide obtained with excitation wavelengths of 266 and 355 nm are shown in Figures 1-3. The 355-nm spectra are preresonant to the lowest electronic absorption of the 230300-nm A band and therefore display significant intensity only in fundamental peaks. The 266-nm spectra are resonant with the A-band absorption and show rich progressions of overtones and combination bands. These overtones and combination bands reveal the short-time photodissociation dynamics of these alkyl iodides.
Phillips et al.
9088 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 TABLE I: Alkyl Iodide Ground-State Normal Modes exptl calc freq, freq,
mode
cm-l
cm-'
calc PED'
262 498 1203
260 505 1203
250
257
499 879 1 I53 1210
496 877 1 I49 121 1
49% BCI, 26% CI, 17% CBH, 13% BCB 71% CI, 36% BCI, 27% CBH 48% BC, 40% CBH 47% CBH, 13% HBH, 1 1 % BC 35% CBH, 27% HCI. 17% BCH
259 487 806 1143
246 487 80 1 1 I57
52% CI, 17% BCB, 1 1 % BCI 51% C1, 39% BCB, 24% BCI 68% BC, 17% CBH 63% CBH, 1 1 % HBH
TABLE 11: Frequencies and Relative Intensities in the 266-nm Ethyl Iodide Resonance Raman Spectra
vibration
ethyl "I I
VI0 Ul
63% BCI, 19% CI 83% C1, 20% BCI 56% BCH, 16% HCH
isopropyl UIO
Y8 Ul
US y4
tert-butyl us Ul
u6
US
'Diagonal force constants contributing 10% or more to the total potential energy of the normal mode are given. "C" represents the acarbon atom and "B" represents a methyl carbon atom. Values are
26 1 761 1271 1770 2267
relative intensity gas phasecd cyclohexane solne 86 100 55
41 21 45 33 38 24 26
107 100 61 28 17
f
35 24 14 6.2
O I n the gas phase, from this work. buncertainty in peak positions is i t 0 cm-'. eThis work. duncertainty in relative intensities is -10%. From ref 19. 'Not reported.
TABLE 111: Frequencies and Relative Intensities in the 266-nm Isopropyl Iodide Resonance Raman Spectrum
vibration
from refs 19 and 49.
Raman
shift,".b cm-I 512 1018 1517 2014 2509
Raman shift,',b cm-I
relative intensity gas phaseCd cyclohexane solne
UP 502 92 63 We have assigned most of the Raman peaks in the spectra of 999 100 100 Figures 1-3 on the basis of normal coordinate c a l ~ u l a t i o n s ~ ~ ~ ~ ' ~ ~ 1494 61 59 and previous experimental nonresonant Raman s p e ~ t r a . ~ *Table -~~ 1988 29 38 I gives the results of the normal coordinate calculations, showing 2479 39 f the internal coordinates that give the major contribution to the 262 50 50 potential energy distribution of the normal modes we see in our 65 52 763 Raman spectra. We will use these results to discern what internal 61 408 1260 coordinate motions are important for a particular normal mode. 31 20 1755 Since the C-I bond is broken upon excitation in the A-band 17 2249 33 absorption, we would expect the nominal C-I stretch progression 20 7.0 893 to be prominent in the 266-nm resonance Raman spectra. The 21 24 I392 shaded peaks in the spectra of Figures 1-3 are the nominal C-I 15 35 1890 stretch fundamental ( u I o for ethyl iodide, u8 for isopropyl iodide, 1151 35 30 and u1 for tert-butyl iodide) and overtones. This is the strongest 652 20 24 progression in all of the 266-nm spectra, but the intensity of this 6.0 2151 18 progression relative to the total Raman intensity decreases as the 1206 308 24 alkyl group becomes heavier and more branched. The next largest progression in the 266-nm spectra is the nominal "bending" 'In the gas phase, from this work. *Uncertaintyin peak positions is 1 1 0 cm-l.
