High overtone resonance Raman spectra of photodissociating

J. Phys. Chem. 1991,95,7164-7171

7164

shape (peaked 1000 s-l above the background). The integrated decay rate of the SIorigin ks,(Sl0 4 ) is evaluated by integrating over the Lorentzian part of the total decay line profile: * (3.8/2)2 dE ks,(Sl04)= 1 0 0 0 ~ ~ AE2] -_ [(3.8/2)2

+

N

lo6 s-I

(15) This value can be regarded as the hypothetical total decay rate of the unperturbed SI0 4 state to the So ground state. The dilution of the SI state by the triplet manifold reduces the actual decay rate to our measured values. The radiative rate of the SIstate can be estimated from the integrated oscillator strengths3 to be 2.5 X lo5 s-l. Thus, the So Sl(O-O)integrated internal conversion (IC) rate klc(SI0-0) can be extracted by subtracting the SIradiative rate from the total decay rate: klc(SI0-0) N 7.5 X IO5

-

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 24, 2015 | http://pubs.acs.org Publication Date: September 1, 1991 | doi: 10.1021/j100172a015

-

In contrast, the So T ISC rate is associated with the constant part of the total decay rate line profile. The 650-s-l decay rate of the background includes the radiative as well as the ISC rates. The radiative rate of the triplet manifold in the vicinity of the SIorigin was measured to be 0.63 of the k, at the TI origin (section D3). This enables a crude estimate for the ISC rate of the (TI T2}levels to S,,. Using 2.2 ms as the TI M) radiative rate results in 360 s-l for the ISC rate in the vicinity of the SIorigin. Together, the above analysis produces a consistent and comprehensive picture of a single allowed excited state coupled to an almost "dark" sparse background. Our findings are consistent with the predictions of Bixon and J ~ r t n e r . ~ ' Actually, the applicability of the theory of radiationless transitions is not obvious for benzaldehyde. In real systems, like benzaldehyde, where the states width 6 H 10" cm-I is much smaller than the levels average energy distance of vibronic states, e = p-l > 3 X IO-' cm-', and the coupling strength varies randomly, the simple model of equally spaced equally coupled states33may be inadequate. This fact was demonstrated in the high-resolution domain, in the case of pyrazine SI excitation^.^^*^ It is gratifying

+

(60) Kommandeur, J.; Majewski, W.A.; Meats, Annu. Rev. Phys. Chem. 1987, 38,433.

W.L.; Pratt. D. W.

that a 0.3-cm-I excitation resolution averages completely the individuality of the states dynamics (only 45-90 states averaging). This means that the applicability of the simple model in predicting the general coupling and dynamics lies beyond the limits of its basic assumptions. Finally, we would like to emphasize that although the oscillator strength of a doorway state is several orders of magnitude larger than that of the "dark" manifold, the contribution of the dark states to the pure radiative rate is not necessarily negligible. The fact that the k, wavelength-dependent profile of an excitation can be a superposition of a Lorentzian and a constant calls for precaution in the analysis of the time-dependent decay of coherently excited Lorentzian bundle of states (excitation width, Aw larger than A). The coherent excitation of a Lorentzian bundle of states may result in a biexponential decay. The short component of that decay cannot be explained solely by the observed coupling width measured from either absorption or excitation experiments.

5. Conclusions The radiative dynamics in the 0-3750-cm-l energy interval above the benzaldehyde T I origin was explored. The details of the dynamics were rationalized as an interplay in the coupling of three electronic states. The TI(3nr*)-T2(3rr*)vibronic coupling increases irregularly in the region between 400 and -2OOO cm-I. Above the energy of the SI origin (1730 cm-I), the SI excitations are coupled to the sparse Tl-T2 background. The line profile and the dynamics of the SImanifold in the 1730-25OO-cm-' region can be described in terms of the interaction of an optically active state with a background of almost dark states, in accordance with the theory of radiationless transitions. The SI content in the coupled states is diluted along the 1730-2500-cm-' region with the congestion of the density of the triplet states. Finally, with larger excess energy, the T2 character is eroded probably due to the larger density of T I states. Acknowledgment. We thank Joshua Jortner and Aviv Amirav for stimulating discussions throughout this project. This research was supported by the Fund for Basic Research, administered by the Israel Academy of Sciences and Humanities and the James Franck German-Israeli Binational Program in Laser-Matter Interaction. Registry No. Benzaldehyde, 100-52-7.

High Overtone Resonance Raman Spectra of Photodissociating Nitromethane in Solution David L. Phillips and Anne B. Myers* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: February 5, 1991; In Final Form: May 1, 1991)

Resonance Raman spectra of nitromethane have been obtained in cyclohexane, acetonitrile, and water solvents with excitation at 218 and 200 nm and in the vapor at 218 nm. Fully deuterated nitromethane has also been examined in both vapor and solution phases. Resolvable Raman lines are observed at energies up to 15ooO cm-l, which is approximately the.lowest dissociation limit (to ground-state C H 3 0 + NO). The spectra in solution and in the vapor are qualitatively similar in that overtone progressions in the NO2 symmetric stretch dominate, but the higher signal-to-noise ratio of the solution-phase data allows many weaker transitions to be observed as well. The vibrational frequencies and bandwidths are interpreted qualitatively to explore solvation effects on the ground-state potential surface. The resonance Raman intensities are modeled with a simple theoretical treatment employing wave packet propagation on a single electronic surface. This approach does a reasonable job of reproducing the relative and absolute solution-phase intensities, but some deviations between experimental and calculated combination band intensities are observed.

Introduction Nitromethane belongs to a class of small molecules having relatively intense and structure]- absorption s w r a in the quartz 'Author to whom correspondence should bc addressed.

0022-3654/91/2095-7164$02.50/0

ultraviolet that photodissociate with near-unity quantum yields. The photodissociation dynamics of such m o k u l e s are ideally suited for study by a combination of ground-state resonance Raman spectroscopy, which is most sensitive to the dynamics on that part of the excited-state potential surface closest to the Q 1991 American Chemical Society

Photodissociating Nitromethane in Solution ground-state geometry,’J and other methods that probe the spatial anisotropy, translational energy, and/or quantum state distribution of the photoproducts f ~ r m e d . ~ The - ~ UV absorption spectrum of nitromethane is dominated by a broad band having a maximum near 198 nm with a molar extinction coefficient of about 4000 L mol-’ cm-l in the vapor, which has been assigned as a T*+T transition localized on the NO, group.61o However, the dominant photodissociation channel forms CH, N02,11.12indicating the energy must flow from the initially excited NO2 moiety into the C-N bond, and the featurelessnes of the optical absorption implies that the energy flow and subsequent dissociation are rapid. The 193-nm photodissociation of nitromethane vapor was studied in detail through product emission spectroscopy and photofragment translational and more recently Butler and coworkers examined the resonance Raman spectrum of the vapor using 200- and 218-nm e~citati0n.l~They interpreted these data in terms of a two-channel model in which the NO, product can be formed in either of two electronic states, although the emission polarization results could not be fully explained. The present study reports the resonance Raman spectra of nitromethane in the vapor and in three solvents of widely varying properties: cyclohexane, acetonitrile, and water. These data are of interest for several reasons. First, the vapor phase spectra of ref 13 were obtained at low spectral resolution and exhibit a fairly low signal-to-noise ratio, meaning that weak but informative transitions might simply be missed. With our multichannel detection system and at the higher number densities achievable in solution, weaker Raman lines can be observed, perhaps shedding additional light on the dissociation dynamics. Second, comparison of the vapor and solution spectra might reveal solvation effects on the photodissociation dynamics due either to collisional effects or to solvent-dependent modification of the relevant electronic surfaces.14J5 Finally, the ability to observe transitions to highly excited overtone levels also makes it possible to evaluate in detail the effects of solvation on the ground-state potential energy surface and vibrational dephasing dynamics at energies closely approaching the ground-state dissociation limit.16


+

Experimental Section CH3N02(Aldrich, 99+%) and CD3N02 (Aldrich, 99% isotopically pure) were used as received. The cyclohexane and acetonitrile solvents were spectroscopic grade; water was distilled and deionized. Solution-phase resonance Raman spectra were collected in a backscattering geometry from a flowing stream of 5-18 mM nitromethane in solution. Vapor-phase spectra were collected in a 90’ geometry from a fused silica cuvette attached to a 500-mL bulb. The laser was polarized perpendicular to the scattering plane, and both polarizations of the scattered light were collected. The excitation and detection system has been described in detail e1~ewhere.l~ Typical pulse energies at the sample were ( I ) Heller, E. J.; Sundtcrg, R. L.; Tannor, D.J . Phys. Chem. 1982, 86, 1822. (2) Myers, A. B.; Mathies, R. A. In Biological Applications of Raman Spectroscopy; Spiro, T. G.,Ed.; Wiley: New York, 1987; Vol. 2, p I . (3) Bcrsohn, R. J . Phys. Chem. 1984,88, 5145. (4) Hall, G.E.; Houston, P. L. Annu. Rev. Phys. Chem. 1989, 40, 375. (5) Leone, S.R . Annu. Reu. Phys. Chem. 1984, 35. 109. (6) Nagakura, S.Mol. Phys. 1960, 3. 152. (7) Harris, L. E. J . Chem. Phys. 1973, 58, 5615. (8) Flicker, W. M.; Mosher, 0. A.; Kuppermann, A. Chem. Phys. Leu. 1979, 60, 5 18. (9) Kleier, D. A.; Lipton, M. A. J . Mol. Srrucr. (Theochrm) 1984, 109, 39. (10) Mijoule, C.; Odiot, S.; Fliszar, S.; Schnur, J. M. J . Mol. Srrucr. (Theochem) 1987, 149, 3 11, ( I I ) Butler, L. J.; Krajnovich, D.; Lee, Y . T.; Ondrey, G.;Bersohn, R. J . Chem. Phys. 1983, 79, 1708. (12) Blais, N . C. J . Chem. Phys. 1983, 79, 1723. (13) Lao, K. Q.;Jensen, E.; Kash, P. W.; Butler, L. J. J. Chem. Phys. 1990, 93, 3958. (14) Markel, F.; Myers, A. 8. Chem. Phys. h a . 1990. 167, 175. (15) Ci, X.; Pereira, M. A,; Myers, A. B. J. Chem. Phys. 1990, 92.4708. (16) Myers, A. B.; Markel, F. Chem. Phys. 1990, 149, 21.

The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7165 n

N

O S

.-0E Y

0.20

-

----

0 W

d..”’..

r h cn cn

0.1s

-

u

z

0.10

-

E 0

0.05

-

. I

Y

t s: P

42000

4

44000

48000

48000

50000

52000

Frequency (cm-1)

Figure I . Absorption spectra of CH,NOz vapor (solid curve) and of CH3NOZin cyclohexane (dot-dashed), acetonitrile (dotted), and water (dashed). The two excitation wavelengths employed in this study are indicated.

5-10 pJ/pulse with a spot size of 0.5 mm. Slit widths were 75-125 pm, giving an instrumental resolution of 12-16 cm-I. The displayed spectra and tabulated data have been corrected for the spectral response of the collection and detection system and for reabsorption of the scattered light. Vibrational frequencies were calibrated relative to solvent Raman lines and emission lines from a low-pressure Hg lamp and a Pt/Ne hollow cathode lamp. Absolute Raman cross sections for the deuterated species in cyclohexane were determined by reference to the C H stretching bands of the solvent” as described elsewhere? using depolarization ratios of p = 0.26 for cyclohexane and 0.33 for CD3N02.

Theoretical Section The absorption spectrum and absolute resonance Raman intensities were modeled by using Heller’s time-dependent wave packet approachi+’*as detailed in our recent work on the alkyl iodides19 and elsewhere., The absorption spectrum is computed from the expression AWL) =

4*e2MZEL d t (OJO(r)) exp[i(EL + co)t/h] 3nh2c Re l m 0

where M is the electronic transition length, which is assumed to include any local field factors, ELis the excitation energy, n is the solvent refractive index, to is the ground-state zero-point vibrational energy, and l O ( t ) ) = exp(iHt/h)lO) is the initial multidimensional vibrational wave function propagated by the excited-state vibrational Hamiltonian. The resonance Raman crass sections are given by “O-.dEL) =

where Es is the scattered photon energy and If) is the final state in the Raman process. The ground- and excited-state potential surfaces were modeled as harmonic oscillators with their potential minima separated by an amount A (in ground-state dimensionless normal coordinates) and having different ground- and excited-state frequencies where necessary (see Results) but no Duschinsky rotation of the normal coordinates. The multidimensional overlaps (O(O(r)) and ( f ( O ( t ) ) then factor into products of one-dimensional harmonic oscillator overlaps. Thermal population of initial states other than 10)was ignored; although nitromethane has several vibrations low enough in frequency to have significant thermal population, these modes (17) Li, B.; Myers, A. B. J . Phys. Chem. 1990, 94, 4051. (18) Lee. SPY.;Heller, E. J. J . Chem. Phys. 1979, 71,4777.

Phillips and Myers

7166 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991

t

t

I

I

6000

3000 1300

4500

2900

6100

7700

Raman Shill (cm-1)


Figure 2. Resonance Raman spectrum of CH3NOzvapor at 218 nm, pressure IO Torr. Daggers indicate features due to stray laser light.

1

mark solvent subtraction artifacts. t

I

I 0

12,000

Raman Shift (em-1) Figure 4. Resonance Raman spectrum of CH3N02at 218 nm in cyclohexane. Daggers indicate features due to stray laser light, and asterisks t

T

9000

I

3000

6000

9000

12,000

15,000

Raman Shift (cm-1) Figure 3. Resonance Raman spectra of CH3NOzat 200 nm in cyclohexane (top), acetonitrile (middle), and water (bottom). Daggers indicate features due to stray laser light, and asterisks mark solvent subtraction artifacts.

0

9000

6000

3000

12,000

Raman Shift (cm-1) Figure 5. Resonance Raman spectrum of CD3NOl vapor at 218 nm, pressure IO Torr. Daggers mark features due to stray laser light. t

t

are not strongly Franck-Condon active. Instead of including an explicit damping function to represent the effects of excited-state population decay and/or electronic pure dephasing, we simply truncated the time integrals in eqs 1 and 2 at a time after the wave packet had initially moved far enough from the origin to no longer have significant overlap with any of the final states of interest, as discussed in ref 19. The absence of structure in the experimental gas-phase absorption spectrum suggests that the total electronic dephasing is dominated by dissociation prior to the first vibrational recurrence.

Results Figure 1 shows the U V absorption spectra of nitromethane in the vapor and in cyclohexane, acetonitrile, and water solvents. The optical spectrum red-shifts, sharpens, and increases its intensity somewhat upon solvation. Our spectra in the vapor and in water are in good agreement with those previously reported? Figure 2 displays the resonance Raman spectrum of nitromethane vapor at 218 nm. This spectrum is essentially in agreement with that of ref 13, but our higher resolution enables us to separate the scattered laser light at 41 55 cm-I from the nearby nitromethane Raman lines and to report more accurate vibrational frequencies. Unfortunately we were not able to obtain high-resolution spectra in our stationary cell with 200-nm excitation due to strong scattering from photoproducts. Figure 3 presents the resonance (19) Phillips. D. L.; Myers, A. B. J . Chcm. Phys. 1991, 95, 226.

1

0

I

3000

6000

9000

12,000

Raman Shift (cm-1) Figure 6. Resonance Raman spectra of CD3NOZat 21 8 nm in cyclo-

hexane (top), acetonitrile (middle), and water (bottom). Daggers mark stray laser light.

Raman spectra in cyclohexane, acetonitrile, and water at 200 nm, and Figure 4 shows the spectrum in cyclohexane at 218 nm. The spectra in different solvents and at the two excitation frequencies are quite similar, although the bands tend to be broader in the more polar solvents, and the combination bands and higher overtones are weaker at 218 nm than at 200 nm. Distinguishable

The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7167

Photodissociating Nitromethane in Solution

TABLE I: Resonance R 8 m n Tnnsitlons of CH\NO, ~~~

assign:

vapor re1 int freqb 218 nm

freqb

cyclohexane re1 int 218 nm 2Wnm

frwb

~

~

acetonitrile re1 int 218 nm 2Wnm

frwb

~

water re1 int 218 nm 2Wnm

~~

I376 2762 4121 5479 6815 8151

100 69 48 51 26 9


918'

657'

1584'

1381 2753 41 12 5469 6803 8125 9442 10742 12016 13294 14550 912 2293 365 1 5008 6347 7673 8998 10305 1 I606 12903 14162 647 2037 3406 4768 6109 7442 8749

100 70 40 27 18 14 9.4 6.2 4.6

1562 3104 4479 5817 7158 8468

4 1 1 2 1

100 80 56 37 31 22 18 15 13 12 11 0.9 2.5 4.8 4.3 5.2 7.0 6.1 5.1 4.4 3.9 3.6

2.7 2.2 1.8 2.1 2.2 2.3 2.1 2.3 2.2 0.6 1.2 1 .o 1.1

2.1 2.5 2.4 1.1

0.9 0.5

7 3 7 6 3

1393 2773 4141 5504 6856 8188 9515 10831 12126 13399 14659 914 2306 3671 5034 6389 7725 9054 10374 11678 12985 14245 654 2042 3423 4796 6152 7497 8831 10156 1561 31 12 4478 5840 7169

100 71 53 39 29 22 18 13 11

2.1 2.2 2.6 3.7 4.2 4.0 4.4 2.9

0.7 1.8 1.4 2.3 2.6 2.2 1.6 1.o 3 2 2 2 2

100 75 61 45 39 29 26 21 10 7.6 5.7 1.6 3.5 4.6 4.9 6.2 7.7 6.6 4.4 4.1 3.8 4.0 0.7 1.1 1.6 1.3 2.8 1.8 1.2

2786 4161 5527 6879 8212 9545 10864 12170 13459 14740 894 2312 3683 5055 6410 7755 909 1 10413 11714 13002 14290 2054 3449 4800 6156 7519 8847 10181 1569 3133 4483 5854 7191

4 1 2 3 3

68 48 34 27 20 15 13

IO

67 50 37 30 23 20 15 12 11

2.5 2.3 3.3 3.9 4.5 2.9 2.6 2. I

1.2 3.6 4.4 4.2 5.3 6.0 5.2 3.9 4.1 2.9 2.2

1.7

1.5

3.6 0.8 1 .o 0.6

I

1.5 2.0 0.5 0.8 3 2

3 2 3

2 2

4

I

.u2 refers to the strongly overlapped pair of u2 and ~2 (see text). bFrequencies are apparent band maxima in wavenumbers, which in general do not correspond to single Raman transitions (see text). 'Data from ref 22. t

1 1250

1350

I

0

I 3000

6000

9ooo

12,000

Raman Shift (cm.1)

Figure 7. Vertically expanded plot of spectrum of CD3N02 in water at 218 nm.

Raman bands are observed a t energies up to nearly 15 OOO cm-I in the 200-nm spectrum. Finally, Figure 5 shows the spectrum of deuterated nitromethane vapor at 218 nm, while Figure 6 displays the corresponding solution-phase spectra and Figure 7 gives a vertically expanded plot of the spectrum of CD3N02in water. Table I summarizes the peak positions and relative Raman intensities for CHtNOl at 218 and 200 nm, together with the probable vibrational assignments. Table I1 gives the corresponding

1550 Shift (cm-1)

1450

Raman

1650

Figure 8. High-resolution resonance Raman spectra at 240 nm of CHJN02vapor (top), CHpN02in water (middle), and CDlN02 in water (bottom) showing the near coincidence of v2 and up in the parent mole-

cule.

data, together with the Raman bandwidths, for CD3N02at 218 nm. In all cases the dominant progression involves a vibration near 1380 cm-' as reported previously." This band actually consists of two overlapped, strongly mixed modes, the "symmetric NOz stretch" ( u Z ) assigned at 1402 cm-' in the liquid, and the "methyl deformation" (up) at 1379 cm-1.20 Figure 8, obtained with 240-nm excitation at a spectral resolution of about 6 cm-I, (20)

462.

Miller, P. J.; Block, S.;Piermarini, G. J. J . Phys. Chcm. 1989, 93,

7168 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 TABLE II:

Reclonmnce Rima

Tnnsitiow of CDaO,‘ ~


assign.

frw

Phillips and Myers

vapor int

I386 2761 4125 5480 6823 8156 9475 10785 12085 894’ 2265 3641 4995

100 63 57 45 30 22 12 7 3

767 1 8997 10315

3 2 2

625‘ 2010 3388 4736 6089 7433 8766

1 1 3 3 2 2

1572’ 3128 4489 5833 71 59

3 3 4 6

9802

3

2 3 3

~

~~~~~

~~

~~

cyclohexane width

frw

int

width

frea

21 26 27 33 42 40 41 44 28

1382 2752 41 13 5466 6807 8135 9449 10754 12055 889 2264 3630 4986 6329 7657 898 I 10288 11602 617 2000 3375 4729 6073 7410 8736 10038 11354 1555 3093 4445 5792 7125

100 61 56 43 34 20 11 7.6 4.0 1.6 1.3 1.7 2.7 3.6 4.7 3.0 2.3 1.7 0.8 1.o I .5 2.3 1.9 2.3 1.3 1.1 0.8 0.9 2.2 1.5 2.5 1.7

18 20 25 28 35 44 50 58 67 14 17 18 24 36 40 38 50 61 14 15 26 28 30 40 43 66 93 9 19 27 39 39

1396 2779 41 52 5510 6860 8196 9528 10842 12139 914 2284 3658 5023 6377 7719 905 1 10368 11678 660 2017 3396 4763 61 I 9 7468 8802 10121 1I435 1546 3080 4446 5801 7147

1.2 1.2

44 46

9782 11082

9744 11050

~~~~~

acetonitrile int 100 64 50 34 24 23 12 7.8 4.5 1.7

width

frw

1.8 2.2 2.0 2.5 2.7 2.6 2.5 2.0 1.o 1.9 1.9 2.1 2.5 1.9 1.3 1.6 1.2 1.8 0.9 1.2 1.5 2.0

15 22 24 35 44 65 69 81 97 15 18 23 27 41 45 56 68 68 11 21 25 29 40 52 67 92 96 12 18 25 37 41

1400 2787 4163 5526 6880 8219 9550 10868 12177 917 2293 3667 5039 6396 7743 9079 10408 11722 664 2024 3391 4775 6135 7484 881 1 10138 11447 1543 3078 4447 5804 7152

1.5 1.4

71 101

1 1097

9793

~

water int 100

~

width

67 55 52 38 31 16 12 8.3 1.7 2.2 3.0 3.4 4.3 4.0 3.4 4.4 4.2 1.2 1.7 3.0 3.2 3.4 3.4 1.8 1.7 1.6 2.5 1.2 I .7 2.3 2.6

17 24 34 49 65 97 112 143 187 15 26 32 37 49 66 88 124 181 20 24 39 54 65 81 93 106 136 20 26 36 55 55

2.3 2.1

108 186

“11 data are at 218 nm. Frequencies and widths are in wavenumbers; intensities are relative to the u2 fundamental. Widths are full widths at half-maximum of spectra obtained without polarization selection (IH + IL), corrected for instrumental response. ’Data from ref 22.

demonstrates clearly that two transitions of comparable intensity are present in both vapor and solution phases. The relative intensities change somewhat upon solvation, most likely due to solvent induced changes in the normal-mode character of these strongly mixed modes. In our slightly lower resolution 21 8- and 200-nm data on CH3NOZtwo lines are usually discernible in the fundamental region, although they are not clearly separated. The apparent positions and widths for overtones of this band cannot be interpreted simply, as they undoubtedly involve contributions from multiple overlapped vibrational transitions. This is not a problem in CD3NOZ,where the methyl deformation has been shifted out of the region to 1075 cm-’,m leaving a single clean line in the symmetric NO2 stretching region as shown in Figure 8. Thus we limit our analysis of the vibrational frequencies, bandwidths, and quantitative intensities to the deuterated species. We do not observe the deuterated methyl deformation in the Raman spectrum, indicating that in the parent molecule it gained most of its intensity through mixing with the NOz stretch. In solution we observe extended progressions in not only u2/u9 but also its combination bands with the CN stretch (v4), the NOz symmetric bend ( u s ) , and two quanta of the NOz antisymmetric stretch (u,; see below). The combination bands involving u4 are somewhat more intense at 200 nm, but they are still clearly present at 218 nm. In the 218-nm vapor-phase spectra these combination bands are much less evident, but this is due largely to the reduced signal-to-noise ratio in the vapor compared with solution. In the deuterated molecule, where we obtained slightly higher quality vapor-phase spectra, many of the combination bands are weakly detectable, and Table I1 shows that their intensities are fairly comparable to those observed in solution. Our results are not inconsistent with the report of Lao et al. that combination bands involving u4 appear at 200 nm but not at 218 nm,I3 as transitions having only I-3% the intensity of the U J Y ~fundamental probably

could not have been detected in those spectra. The weak band near 1560 cm-’ is assigned as the antisymmetric NOz stretching fundamental (u7), observed in the Raman spectra of the pure liquid at 1561 and 1545 cm-’ in C H 3 N 0 2 and CD3NOz, respectively,2*2z and about 15 cm-’ higher in, the corresponding vapor-phase infrared spectra.z1-2zThe persistence of intensity in this band as the excitation is tuned away from resonance to 240 nm strongly suggests assignment as a fundamental, and most other possible assignments are precluded by the relatively small deuterium shift of this band. This is a non-totally symmetric vibration that in principle should not appear in resonance Raman, but it is allowed and fairly intense in the normal Raman spectrum.20 Its appearance in the resonance Raman spectrum presumably arises from a “B-term” enhancement mechanism involving vibronic coupling between the resonant electronic state and other higher states.23 A breakdown in the symmetry of the molecular environment could also induce intensity in this mode in solution, but it is hard to explain the vapor-phase intensity in terms of such a mechanism unless significant dimerization, which may be important in condensed also occurs in the vapor. We also observe in both the normal and deuterated molecule a progression of combination bands of nu2/u3 with an interval near 3 110 cm-I, which we tentatively assign as 2 quanta of the NOz antisymmetric stretch. Overtones of u7 would be expected to have at least some intensity on resonance due to the anticipated change in the vibrational frequency of this mode (21) Smith, D. C.; Pan, CY.;Nielsen, J. R. J. Chem. Phys. 1950,/8,706. (22) McKcan, D. C.; Watt, R. A. J. Mol Spectrosc. 1976, 61, 184. (23) Albrecht, A. C. J. Chem. Phys. 1961.34, 1476. (24) Verderame, F. D.; Lannon, J. A.; Harris, L. E.;Thomas, W. G.: Lucia, E. A. J. Chem. Phys. 1972,56, 2638. (25) KnBzinger, E.;Kollhoff, H.;Wittenbeck, R. Ber. Bunsen-Ges. Phys. Chem. 1982,86,929.

Photodissociating Nitromethane in Solution

The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7169 TABLE III: Potential Panmeters for CDJ+J02in CyclobexrneO ground-state excited-state mode frea. cm-I frw, cm-I A y2 1382 1382 3.30 us

889 617

889 617

Y1

1555

850

u4

A.

0.60 0.60 0.70

'Zero-zero energy Eo = 42 250 cm-I. Transition length M

0.665

~~

218 nm transition y2

2v2 3~2


44

5~2 6u2 7~2 8y2

9u2 y4

+ ~2 +2~2 u, + 3u2 ~4 + 4Y2 ~4 + 5 ~ 2 ~4 + 6 ~ 2 u4 + 7Uz +8~2 us us + u2 us + 2u2 + 3Y2 US + 4 ~ 2 + Sui US + 6 ~ 2 Us + 7U2 US + 8 ~ 2 ~4

~4

-

0

0

1

4

6

8

1

0

n Figure 9. Vibrational frequency shift, u,,, - urF, as a function of vibratonal quantum number for the nu2 progression of CD3N02in cyclohexane (top), acetonitrile (middle), and water (bottom), together with the best linear fit.

Y(

YJ

YJ

CXDt lo0 (2.5 X lod) 61 56 43 34 20 11 7.6 4.0 1.6 1.3 1.7 2.1 3.6 4.7 3.0 2.3 1.7 0.8 1.0 1.5 2.3 1.9 2.3 1.3 1.1

0.8

Yl

0.9

2u7 2u7 2u7 2u7 2u7 2u7 2u7

2.2

+ uz +2q + 3u2 + 4u2 + 5u2 + 6u2

1.5

2.5 1.7 1.2 1.2

200 nm Calc

CXDt

100 100 (2.7 X le) (7.1 68 59 51 51 40 41 32 31 27 25 22 21 18 16 14 11 1.6 1.o 2.2 1.3 2.5 2.8 3.5 2.6 2.7 3.9 2.7 5.2 5.8 2.8 2.6 5.6 2.3 4.6 0.8 1.1 1.3 0.8 1.4 I .7 1.4 I .8 1.4 2.6 2.1 1 .5 I .4 2.4 1.3 2.7 0.6 1.1 1.4 1.6 1.2 I .6 0.9 1.5 1.4 0.8 1.4

Calc 100

X

le)

(6.0 X IOd) 71 54 43 35 29 24 21 17 1.4 2.0 2.3 2.5 2.6 2.6 2.6 2.5 2.3 0.7 I .o 1.1 I .2 1.3 1.3 1.3 1.3 1.2 0.5

1.2 1.6 I .8 1 .8 1.8

1.7 1.6

'Calculated from cq 2 and the parameters of Table 111. Intensities are relative to u,; absolute Raman cross sections for u2 in A~/molecu~eare given in parentheses.

0

20

40

60

80

100

n2

Figure 10. Vibrational bandwidth (fwhm of Lorentzian after correction

for instrumental response) as a function of vibrational quantum number squared for the nu2 progression of CD,N02 in cyclohexane (top), acetonitrile (middle), and water (bottom), together with the best linear fit. in the excited state. Significant anharmonic coupling is required to make this assignment tenable, as the frequencies of the bands we assign to nu2/u3 + 2u7 do deviate considerably from the sum of the frequencies of nu2Iu3 and 2u7.

Figure 9 plots thc vapor to solution vibrational frequency shifts versus quantum number for the first nine members of the symmetric NOz stretching progression in CD3N02. In each solvent the frequency shift is approximately linear with vibrational quantum number as predicted by simple theories based on a cubically anharmonic oscillator coupled through quadratic terms to the solvent.16*z6The frequency shifts are small and negative in cyclohexane, larger and positive in acetonitrile, and even larger and positive in water. Figure 10 displays the quantum number dependence of the bandwidth of the uZ progression in the three solvents examined. The scaling of bandwidth with quantum number is much more nearly linear with nz than with n, and the widths are slightly greater in acetonitrile and much greater in water than in cyclohexane. The bandwidths in the vapor vary only slowly with n and probably have a large component from the underlying rotational structure, which we have not attempted to estimate. Table 111 gives the potential surface parameters used in eqs 1 and 2 to model the experimental absorption spectrum of CD3N02 in cyclohexane as well as the absolute resonance Raman intensities at 218 and 2 0 0 nm. Figure 1 1 compares the experimental and calculated absorption spectra, and Table IV compares the Raman (26) Ben-Amotz, D.; Zakin, M.R.; King, H. J. Phys. Chem. 1988, 92, 1392.

E.,Jr.; Herschbach, D.R.

Phillips and Myers

7170 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 h

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. 3

8

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8

3 0 E

*

. I

0.15

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0.001 40000

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42500

45000

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47500

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50000

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Frequency (cm-1)


Figure 11. Absorption spectrum of CD3N02in cyclohexane (solid) and spectrum calculated from eq 1 and the parameters of Table 111 (dotted).

intensities. For these calculations we have assumed that all of the Raman intensity arises from an “A-termn enhancement mechanism and that the intensity of the antisymmetric stretching fundamental (u7) must therefore arise from a nonzero displacement A between ground- and excited-state potential minima. A relatively large change in the frequency of this mode upon excitation is required to reproduce the observed intensity ratio of transitions involving 2u7 to the u7 fundamental. The effects of frequency changes in the other Franck-Condon active modes were also explored but did not substantially improve the quality of the fits. The featureless absorption spectrum is reproduced quite well by the calculation, as are the absolute cross sections of the u2 fundamental and the relative intensities of the first 5-6 members of the nu2 progression. The combination band series u4 + nu2 and us nu2 are not fit quite as well, the experimental increase in intensity from n = 0 to n = 5-6 being somewhat greater than predicted by the model. The calculations also fail to reproduce the increased intensity in the members of the u4 + nu2 progression between 218- and 200-nm excitation.

+

Discussion Our ability to obtain ground-state vibrational frequencies and line widths at energies up to 15 000 cm-’ points out the utility of the resonance Raman technique for examining vibrational dynamics in highly excited states of molecules in dilute solution. We observe resolvable Raman transitions at energies all the way up to the lowest dissociation limit in the gas phase (1 5 050 cm-’ to form ground state CH30 + NO),” and it is tempting to ascribe our difficulty in observing sharp vibrational transitions above this energy to the ground-state dissociation, although we do not know the energy barrier to this dissociation channel. Most theoretical attention has focused on the channel involving breaking of the C-N bond to form CH3 + NO2, which occurs at 21 030 cm-’ in the vapor phase” and is probably barrierless in the ground ~tate.’~.~~ Within the context of simple theories of nonspecific solventsolute interactions, increases or dccteases in vibrational frequencies upon solvation are interpreted as demonstrating the dominance of the repulsive or attractive parts, respectively, of the intermolecular intera~tions.~~.~~*~**~~ In the present case, the large frequency increases upon solvation in water and acetonitrile probably result from more specific dipolar and/or hydrogen-bonding int e r a c t i o n ~ , ~with ~ * ~the ) nitromethane intramolecular vibrations being pushed up in frequency by coupling with lower frequency intermolecular vibration^.^^ Simple theories of vibrational line broadening in solution imply that the scaling of overtone bandwidths with quantum number is diagnostic of the time scale for (27) Kaufman, J. J.; Hariharan, P. C.; Chabalowski, C.; Hotokka, M. h r , J . Quantum Chcm.: Quantum Chcm. Symp. 1986, 19, 221. (28) Schweizer, K. S.; Chandler, D. J. Chrm. Phys. 1982. 76, 2296. (29) Zakin, M.R.;Herschbach, D. R. J . Chem. Phys. 1986. 85, 2376.

fluctuations in the solvent-solute interactions that perturb the vibrational f r e q ~ e n c y . l ~The * ~ ~approximately quadratic dependence of the nu2 bandwidth on n would then indicate that the relevant interactions are in the fast modulation limit; that is, the time scale on which the broadening interactions fluctuate is fast compared with the inverse bandwidth, which in water implies fluctuation times on the order of 50 fs or faster. However, it is not clear that these simple theories are applicable to a polyatomic molecule, in which intramolecular anharmonic couplings also come into play, or to one undergoing specific hydrogen-bonding interactions with a solvent. It should also be noted that these are not isotropic bandwidths (we measure I, + I,) and thus still contain a component due to orientational motion. This probably accounts for the fact that our line widths for u2 and 2u2 in CD3N02 in cyclohexane are 2-3 times larger than the corresponding widths reported for C H 3 N 0 2 in C C 4 , where only I, was detected.” As mentioned above, the fact that we seem to observe more combination band transitions in the solution phase than in the corresponding vapor spectra is largely a consequence of the better signal-to-noise ratio in solution. It does not appear that the combination band intensities are actually much different in vapor and solution phases, although high-quality data in both phases a t several excitation wavelengths would be required to confirm this. The present data provide no clear evidence for effects of solvation on the excited state photodissociation dynamics. The observation of a strong progression in u2 as well as intensity in combination bands involving u4, us, and u7 indicates that the potential surface for the resonant excited state differs from that of the ground state principally in the N-O bond length but also (to a lesser degree) in the C-N bond length, the C-N-O bond angle, and the N-0 force constant, as shown by the fitting parameters in Table 111. This is qualitatively consistent with the photoexcitation being relatively localized on the NO2 moiety. The combination band progressions nu2 + u4 and nu2 + u5 are seen to reach an intensity maximum near n = 5 . Lao et aLI3also noted this trend and suggested that it could result from curvature in the excited-state potential surface such that motion along the C-N stretching and NO2 bending coordinates is maximized only after a significant degree of motion along the NO2 symmetric stretch has developed. However, it is interesting to note that our numerical simulations, which include no such curvature, also predict intensity maxima near n = 5 in these combination band progressions, although agreement with experiment is not quite quantitative. Anharmonicity of the ground-state potential surface may also be important in determining the intensities of the higher overtones and combination as may breakdown of the Condon approximation at large vibrational amplitudes. Thus we draw no definite conclusions about coupling between motions along the NOz symmetric stretch and other coordinates in the excited state. While the “interesting” (Le, reactive) aspects of the excited-state dynamics in this system may depend on interactions among multiple electronic surfaces, this does not necessarily invalidate the use of a single electronic surface model to simulate the absorption and Raman spectra. If the surface crossing occurs far from the Franck-Condon region and with a sufficiently high crossing probability, both the absorption and the Raman spectra will be determined mainly by the motion of the wave packet lO(t)) during its first excursion away from the origin on the initially excited surface. Both the absence of structure in the experimental absorption spectrum, even in the vapor phase, and the reasonable agreement between experimental and calculated absolute Raman cross sections suggest that this model may be adequate; any recurrences of the wave packet to the Franck-Condon region would result in a structured absorption spectrum and larger absolute Raman intensities than observed. However, the observed enhancement of the nu2 + u4 combination band intensities at 200 nm relative to 218-nm excitation, not predicted by our simple (30)Battaglia, M.R.;Madden, P. A. Mol. Phys. 1978, 36. 1601. (31) Arndt, R.;Yarwood, J. Chrm. Phys. Lerr. 1977, 45, 155. (32) Sension, R. J.; Brudzynski, R. J.; Hudson, B. S.;Zhang, J.; Imre, D.

G . Chem. Phys. 1990, 141, 393.

J . Phys. Chem. 1991, 95, 7171-7180 model, does suggest that more than one electronic surface might have to be taken into account. Simulation of the spectra with explicit inclusion of more than one potential surface will require more sophisticated computational methods that are presently being developed and applied by several group^.^^-^^ (33) Coalson, R. D. Chem. fhys. Lcrr. 1988, 147, 208. (34) Heather, R.; Metiu, H. J . Chem. fhys. 1989, 90,6903. (35) La, S.; Freed, K. F. J . Chem. fhys. 1989, 90, 7030. (36) Stock, 0.;Domcke, W. J. Chem. fhys. 1990,93, 5496. (37) Manthe, U.; Kappcl, H. Chem. fhys. Lerr. 1991, 178, 36. (38) Waldeck, J. R.; Campos-Martinez,J.; Coalson, R. D. J. Chem. fhys. 1991, 94, 2773.

7171

Acknowledgment. This work was supported in part by a grant from the N S F (CHE-8709485). A.B.M. is the recipient of a Dreyfus Distinguished New Faculty Award, an NSF Presidential Young Investigator Award, a Packard Fellowship in Science and Engineering, and a Sloan Research Fellowship. We thank Professor Laurie Butler for transmitting a copy of ref 13 prior to publication. Registry NO. CHSN02, 75-52-5; DZ,7782-39-0. (39) Manthe, U.; KBppel, H. J. Chem. fhys. 1990, 93, 345.


Triplet-Sensitized and Thermal Isomerlzatlon of All-Trans, 7-Cls, SCIs, 13-Cls, and 15-Cis Isomers of &Carotene: Configurational Dependence of the Quantum Yield of Isomerlzatlon via the T, State Michitaka Kuki: Yasusbi Koyama,*It and Hiroyosbi Nagae* Faculty of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan, and Kobe City University of Foreign Studies, Gakuen Higashi-Machi, Nishi-ku, Kobe 651 -21, Japan (Received: February 13, 1991)

The productsof triplet-sensitized photoisomerization (excitation at 337 nm of the sensitizer, anthracene) and thermal isomerization of &carotene in n-hexane, starting from the all-trans, 7-cis, 9-cis, 13-cis, and 1 5 4 s isomers, were analyzed by HPLC. Direct photoisomerization (excitation at 488 and 337 nm) was also examined for comparison. Three different isomerization patterns were found in both triplet-sensitized and thermal isomerization: pattern A, cis to trans isomerization around each cis bond; pattern B, trans to cis isomerization in the central part of the conjugated chain; and pattern C, cis to another cis isomerization. In the TI state, the pattern A isomerization was predominant even for the peripheral-cis (7-cis and 9-cis) isomers and its efficiency was extremely high for the central-cis ( 1 3 4 s and 1 5 4 s ) isomers. In the So state, the pattern B isomerization, instead, was predominant for the peripheral-cis isomers, and the pattern A isomerization was predominant only for the central-cis isomers. The quantum yields of triplet-sensitizedisomerization (decrease of the starting isomer per triplet species generated) were determined to be as follows: all-trans, 0.04; ‘Ids, 0.12; 9-cis,O.15; 13-cis, 0.87; and IS-&, 0.98. In direct photoisomerization, the quantum yield of isomerization at 488-nm (337 nm) excitation was 4 (3) orders of magnitude lower than the above values, the relative values among the isomers being similar to the above. Further, the overall isomerization patterns of direct photoexcitation were similar to those of triplet-sensitized isomerization, supporting the idea that isomerization takes place via the TI state even in the case of direct photoexcitation. Carbon-carbon 7r bond orders of model polyenes in the TI and S, states were calculated by using the Pariser-Parr-Pople CI theory; bond lengths were optimized by using a bond order-bond length relationship. Isomerization characteristics in the TI and So states observed were discussed based on the results of the calculations.

Introduction Carotenoids (Car) in photosynthetic systems have dual functions of photoprotection and light-harve~ting,’-~ and natural selection of carotenoid configurations, in relation to these functions, has been found in the igment-protein complexes of purple photosynthetic bacteria!” In thc reaction center (RC), where the photoprotective function involving both triplet energy transfer from bacteriochlorophyll (BChl) and dissipation of the transferred energy by the carotenoid is most important, the 15-cis configuration is selected. In the light-harvesting complex, where the light-harvesting function involving singlet energy transfer from carotenoids in BChl is most important, the all-trans configuration is selected (see a review by Koyamalz). It is an intriguing question as to why the 15-cis configuration has been selected by the RC. The configuration must be advantageous Over all the possible cis-trans configurations in order to carry out the photoprotective function, and the selection should be ascribed to its TI-state property. The TI-state properties have been compared among cis-trans configurations by use of a set of isomers of @-carotene: Transient Raman spectroscopy showed that the all-trans, 7-cis, 9-cis, and 13-cis isomers generate their

’*Kobe Kwansei Gakuin University. City University of Foreign Studies. 0022-3654/91/2095-7171$02.50/0

own TI species (we call them the “all-trans”, “7-cisn, “9-cisn, and ‘13-cis” TI), but that the 15-cis isomer generates the “all-trans” TI.” Transient absorption spectroscopy supported the observation; ( I ) Mathis, P.; Schenck, C. C. In Carorenoid Chemistry and Biochemistry; Britton, G., Goodwin, T. W., Eds.; Pergamon Press: Oxford, U.K., 1982; p 339. (2) Siefermann-Harms, D. Physiol. Plant. 1987, 69, 561. (3) Cogdell, R. J.; Frank, H. A. Biochim. Biophys. Acra 1987, 895, 63. (4) Lutz, M.; Kleo, J.; Reiss-Huson, F. Biochem. Biophys. Res. Commn. 1976, 69, 7 1 1 . (5) Lutz, M.; Agalidis, I.; Hervo, 0.; Cogdell, R. J.; Reiss-Husson, F. Biochim. Biophys. Acra 1918, 503, 287. (6) Koyama, Y.; Kito, M.; Takii, T.; Saiki, K.; Tsukida, K.; Yamashita, J. Biochim. Biophys. Acra 1982, 680, 109. (7) Koyama, Y.; Takii, T.; Saiki, K.; Tsukida, K. fhorobiochem. fhorobiophys. 1983, 5, 139. (8) Lutz, M.; Szponarski, W.; Berger, G.;Robert, 6.; Neumann, J.-M. Biochim. Biophys. Acra 1987, 894, 423. ( 9 ) Koyama, Y.; Kanaji, M.; Shimamura, T. fhorochem. fhorobiol. 1988, 48, 107. (IO) Koyama. Y.; Takatsuka, 1.; Kanaji, M.; Tomimoto, K.; Kito, M.; Shimamura, T.; Yamashita, J.; Saiki, K.; Tsukida, K . fhorochem. fhorobiol. 1990, 51, 119. ( I I ) Koyama, Y. In Carotenoids: Chemisrry and Biology; Krinsky, N . I., Mathews-Ross, M. M., Taylor, R. F., Eds.; Plenum Press: New York, 1990; p 207. (12) Koyama, Y. J . fhotochem. Phorobiol. 1991, 9, 265.

0 1991 American Chemical Society

High overtone resonance Raman spectra of photodissociating

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