J . Phys. Chem. 1988, 92, 4015-4019 the available time domain data. The transient hole-burned spectra for glasses 1-111 indicate that IX)* and lZ)* are responsible for roughly 30% and 70%, respectively, of the P960 absorption intensity. None of the existing electronic structure calculations8 predict the existence of two close-lying states characterized by this intensity distribution. However, these calculations are semiempirical in nature and are continually undergoing refinement. Our data suggest that the calculations may significantly underestimate the intermolecular electron-exchange integrals between P* and B on the active side. It is reasonable to attempt to interpret ( X ) *and lZ)* as states arising from strong coupling between P* (viewed as a special pair state with mixed neutral exciton and charge-transfer (CT) character39) and one or more of the C T states IP+B-H)*, IPB+H-)*, and IP+BH-)*. Data (not given), from experiments in which white or laser light was used to reduce PBHQ- to PBH-Q-, indicate that the latter two C T states do not contribute significantly to lZ)* and lX)*.29 The data are consistent with lZ)* and IX)* simply undergoing comparable blue shifts from the reduction to PBH-Q-. These data do not exclude the possibility that lZ)* and IX)* are strong admixtures of P* and IP+B-H)*, with P* being the source of their oscillator strengths. Provided the primary decay channel of (Z)*is to IX)* and provided that this decay is faster than the decay of IX)* to IP+BH-)* (viewed simply as a diabatic state), such an adiabatic two-state model for P960 would be consistent with the low-temperature time domain datal3-" which pertain to the primary donor state. Furthermore, these datal6,'' indicate only that the diabatic state JP+B-H)*does not appear as a distinct intermediate at ultrashort times; cf. Introduction. The analysis of bleaching and electrochromic shifting of the Q, transition of B would take on a different complexion in the strong coupling adiabatic picture, especially when significant C T character for P* itself is admitted. This picture, however, presents several questions which will need to be addressed, e.g., the mechanism for strong coupling between P* and IP+B-H)* and (39) First paper of ref 8.
4015
whether the model is consistent with the 77 K Stark data25-27which indicate that the dipole moment change is constant throughout the P960 profile. Another is whether the X,Y,Z hole structure (intensities and widths) can be accounted for by the model. With regard to the first question, it would seem important to further explore the possibility that40 low frequency and relatively large amplitude intermolecular pigment (protein) vibrations (phonons) enhance electron exchange between P* and B. The questions just posed are obviously difficult since they cannot be addressed without consideration of the static and dynamic roles of the protein in electron transfer. Nevertheless, it is hoped that they will be explored in the near future. The important conclusion from this work is that two close-lying electronic states contribute to the P960 absorption profile. The zero-phonon hole data in combination with the time domain data suggest that the lower energy and less strongly absorbing state is the emitting and precursor state for the formation of IP*BH-)* (at least at low temperatures). The data indicate that the coupling between P* (viewed as a dimer state) and B is very strong and, consequently, that the primary charge separation process must be understood in terms of coupled adiabatic (rather than diabatic) states.
Acknowledgment. Ames Laboratory is operated for the U S . Department of Energy by Iowa State University under Contract W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Science. The research at the Argonne Laboratory was supported by the Division of Chemical Sciences, Office of Basic energy Sciences, U S . Department of Energy, under Contract W-31-109-Eng-38. We are grateful to W. E. Catron for supporting J.K.G. through a research fellowship. G.J.S. thanks G. R. Fleming, W. W. Parson, and A. Warshel for several stimulating discussions and G. R. Fleming for making available to us ref 16 and 17. We would like to thank J. M. Hayes for his advice on experimental design. (40) Warshel, A. Proc. Natl. Acad. Sci. U.S.A. 1980, 7 7 , 3105
Angular Momentum Correlation in the Photodissociation of H202at 193 nm Reinhard Schinke Max- Planck-Institut fur Stromungsforschung, 0-3400 Gottingen, FRG (Received: April 7 , 1988)
We investigate the correlation between the rotational states of OH radicals coincidently prepared in the photodissociation of H202at 193 nm. The calculations use classical mechanics and ab initio potential energy surfaces for the two lowest excited states. For very low Hz02 temperatures the two OH rotamers are found to be highly correlated (jI j,) and the correlation is gradually lifted as the temperature rises. This effect can be explained by the influence of initial H20zrotation about the 0-0 axis. Comparison with preliminary, only partly resolved measurements is satisfactory.
-
Introduction If a four-atomic molecule is dissociated through the absorption of an UV photon according to ABCD + hv ABQ,) + CDQ2) (1)
rotational-state distributions of both fragments are resolved. Although such experiments represent the current state-of-the-art in molecular photodissociation studies, they involve-in principle-substantial averaging. The most detailed information
each diatomic product is prepared in a specific rotational state ( i = 2, (vibration be ignored throughout this letter)' In a experiment One determines the probability P161)for finding rotor 1 in state i , irrespective of the rotational state of rotor 5 and likewise P2&). Such averaged rotational-state distributions are usually measured by laser detection methods (LIF, REMPI), and nice examples are reported for the photodissociation of NCNO,' H 2 C 0 , 2and H N C 0 . 3 In each case the averaged
(1) Qian, C. X. W.; Noble, M.; Nadler, I.; Reisler, H.; Wittig, C. J . Chem. Phys, ,985, 83, 5573, Reisler, H,; Noble, M,; Wittig, C ,In Mo[ecu[ar Photodissocation Dynamics; Ashfold, M. N. R., Bagaott, J. E., Eds.; The Roval Societv of Chemistrv: London, 1987; Cha~teF-5 i2) Bamfird, D. J.; Filskth, S. V.; Foltz, M. F.: Hepburn, J. W.; Moore, c. B. J . Chem. Phys. 1985,82,3032. Debarre, D.; Lefebvre, M.; Pealat, M.; Taran, J.-P. E.; Bamford, D. J.; Moore, C. B. J . Chem. Phys. 1985,83,4476. (3) Spiglanin, T. A.; Perry, R. A.; Chandler, D. W. J . Chem. Phys. 1987, 83, 1568. Spiglanin, T. A,; Chandler, D. W. J . Chem. Phys. 1987,83, 1577.
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ii
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The Journal of Physical Chemistry, Vol. 92, No. 14, 19618
is contained in the microscopical probability matrix P(jlJ2) including the correlation of the two rotamers which are coincidently formed in the same dissociation process. P ( j l j 2 )is the probability for finding rotor 1 in state j , if rotor 2 is in state j 2 . The traditionally measured probabilities P , ( j , ) and P2(j2)are simply averages of P ( j l J 2 )according to
PIG,) = CPO’Ij2)
(2)
J2
and likewise for P(j2). If the two fragments are completely uncorrelated, Le., if the rotational probability for molecule 1 is independent of j,, the probability matrix can be written as POIj2) = pI0.l) P20‘2)
(3)
In this extreme case the averaged probabilities P I and P2 contain the maximum of information and determining the full probability matrix P is redundant. An example of uncorrelated product rotations is the photodissociation of H,CO; Le., the CO rotational distribution is independent of the particular rotational state of H2 and vice versa.4 The photodissociation of H 2 C 0 is, however, a very special case. Two distinctly different dynamical mechanisms determine the fragment rotations: The rotation of H2 is governed by the rotational Franck-Condon principle4 (no final state interaction in the exit channel) whereas the rotation of CO is determined by strong forces during the fragmentation process5 (the rotational reflection principle6,’). If, on the other extreme, the two rotamers are strictly correlated, the probability matrix can be written as PO’,j2) = 6,,J* P*Gd
(4)
Le., the two molecules are produced in identical states. In this case it also suffices to merely measure the averaged probability P I or P2. In general, however, one cannot expect that the microscopical probability matrix Pfollows eq 3 or 4. If in such cases only the averaged probabilities P I and/or P2 are measured, information might be lost which would be necessary to unambiguously characterize the dissociation dynamics. In this letter we investigate the correlation of product rotations in the dissociation of H 2 0 2 at 193 nm. We will show (and explain) that the degree of correlation is high at low H 2 0 2 temperature and decreases with increasing temperature.
Photodissociation of HzOz The photodissociation of H 2 0 2has been amply investigated by several groups in recent years.*-” The measurements include scalar properties (Le., OH-rotational-state distributions) as well as vector properties (Le., correlation between joH and the recoil velocity v) at several photolysis wavelengths (193, 248, and 266 nm) and two H 2 0 2temperatures (beam and bulk). We started recently to study the photodissociation of H 2 0 2theoreticallyI2using ab initio potentia! energy s_urfaces for the two lowest excited electronic states, A’A and B1B.13 The dynamical calculations for 193 nm are solely based on classical trajectories including five degrees of freedom. According to the experimentallo and t h e ~ r e t i c a l ’ ~analysis .’~ the photodissociation of H 2 0 2at 193 nm can be summarized as follows: ( 1 ) The dissociation is fast and direct. (2) The A and (4) Schinke, R. J. Cbem. Phys. 1986, 84, 1487. (5) Schinke, R. Cbem. Phys. Lett. 1985, 120, 129. (6) Schinke, R. J . Chem. Phys. 1986, 85, 5049. Schinke, R.; Engel, V. Faraday Discuss. Chem. Soc. 1986, 82, 11 1. (7) Schinke, R., to be published in Annu. Rev. Cbem. Pbys. ( 8 ) Ondrey, G.; van Veen, N.; Bersohn, R. J . Chem. Pbys. 1983,78, 3732. (9) Jacobs, A.; Wahl. M.; Weller, R.; Wolfrum, J. Appl. Phys. B 1987, 42, 173. (IO) Gericke, K.-H.; Klee, S.; Comes, F. J.; Dixon, R. N. J . Chem. Pbys. 1986, 85, 4463. Grunewald, A . U.; Gericke, K.-H.; Comes, F. J. J. Chem. Phys. 1987, 87, 5709. (11) Docker, M. P.; Hodgson, A,; Simons, J. P. Cbem. Phys. Lett. 1986, 128, 264; Faraday Discuss. Cbem. SOC.1986, 82, 25. (12) Schinke, R.; Staemmler, V. Cbem. Phys. Lett. 1988, 145, 486. (13) Meier. M.; Staemmler, V.; Wasilewski, J., to be published.
Letters the B states contribute with roughly two-thirds and one-third, respectively, to the absorption. (3) O H rotational excitation originates primarily from torsional motion about the 0-0 axis which is induced by the dependence of the excited-state potentials on the dihedral angle. (4) The OH distributions are nice examples of the rotational reflection p r i n c i ~ l e . ~( 5. ~) The rotaLiona1 distributions for dissociation via the A state and via the B state are roughly identical. All experimental observations are nicely reproduced by our classical calculations. This underlies the quality of the calculated potential surfaces and the applicability of ordinary classical mechanics for this system, at least for dissociation at 193 nm. One of the remaining questions is the correlation of the two O H rotamers as discussed in the Introduction.
Classical Calculations The trajectory calculations are fully described in ref 12, and therefore we repeat only those details which are necessary to follow the subsequent discussion. The classical Hamiltonian in the excited states is given byI2sL4(j = 1, 2)
where R is the vector which joins the centers of mass of the two O H radicals and rloHare the two intramolecular OH vectors. (6’,,p,)are the polar angles of rloHwith respect to R . The z axis of the body-fixed frame is directed along R. The potentials depend on R,4, 02, and the dihedral angle cp = cpI - p2. m is the reduced mass of the OH-OH system, and B is the rotational constant of OH. The momenta corresponding to the five coordinates are PR, Po,,and P,, respectively. The absolute value of the final rotational angular momentum vector j, is given by
and its projection on the body-fixed z axis is simplyj,,, = Pq,. The equations of motion following from the Hamiltonian in eq 5 are
0, = 2BP0, (b, =
av
p.
R. = -p R m
R -
av
P , = -80,
2B P sin2 Oi
,
dR 2B cos 0,
+7 p,2 s1n3 8, av
P = --
acpi
(7)
where E = E, + hu - Dois the available energy in the excited state, X is the wavelength, and T is the temperature of the H202parent molecule. The integration extends over the 10-dimensional initial ~ ~ ~ , Pand ~ ~J 2) (.7 ’ ) phase space and 7’ = ( R o , c p , 0 , 8 , 0 , ~ ~ o , Jl(7O) are the two classical excitation functions defined as J,(ro) = j,( t---17’) They are obtained by solving Hamilton’s equations and depend on all independent initial variables. The excitation functions are exclusively determined by the motion in the exit channel and “reflect” the dynamical forces d V / d p , and aV/a8,. In the actual calculations the integration in eq 8 is performed by Monte Carlo methods. Nine of the initial variables are chosen randomly and R‘ is fixed by energy conservation. This eliminates the first &function in eq 8. The other two &functions are approximated by boxes of unit length centered around j , = 0, 1, 2, .... The calculations in this letter are identical with those of ref
.’
(14) Bersohn, R.; Shapiro, M. J . Chem. Phys,. 1986, 85, 1396
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4017
Letters 1 Or
T:
I
I
I
OK
T -300K
I
\
rotational state Figure 1. Averaged rotational-statedistributions PG) ( 0 )as defined in eq 2 for three H202 temperatures. The experimental data (0)for 0 K are taken from ref 15 (beam), and those for 300 K are taken from ref 10 (bulk). In order to compare theory and experiment we used the relation j = N - 1.
12 except for the total number of trajectories. In ref 12 we analyzed only the averaged probabilities Pi(ji) and ca. 1000 trajectories were sufficient for convergency. In this Letter we analyze the complete probability matrix P(j,J,)and ca. 10000 trajectories are calculated for each electronic state. The function Wgr weights each trajectory according to the (quantum mechanical) motion of the parent molecule. It is fully described in ref 12. Since overall rotation of H,02 about the 0-0 axis leads directly to final OH rotation, it is essential to incorporate it into the dynamical treatment. The total angular momentum for rotation about the 0-0 axis is M = PqI+ P,, and its initial distribution is approximately given by the Boltzmann distribution
(
wM(M)= exp --Mz 2:T
)
(9)
M is a constant of motion. The two O H entities within the H,Oz parent molecule are assumed to rotate about the 0-0 axis in the same direction with the same angular velocity (Pn0 = P O) such that the total angular momentum is equal to M = 2P,,OZ 2P-O. The initial distribution function for the momenta P,: is therefore given by’,
I
J2
j2
Figure 2. Rotational-statedistributions for molecule 2 and fixed rotational state ji. All distributions are normalized to the same value at the maximum. The H202 temperatures are 0 and 300 K. The lines are d r a w by hand through the data points.
corresponding result for dissociation in the bulk.I0 The experimental results of ref 9 and 10 are essentially the same. As found before,I2 the agreement between theory and experiment is excellent. The theoretical distributions are marginally narrower than the experimental distributions which is possibly due to the neglect of overall rotations of H 2 0 2except for rotation about the 0-0 axis. Figure 2 depicts the complete microscopical probability matrix P ( j l J , )for T = 0 K and T = 300 K. Plotted are rotational-state distributions for rotor 2 for a given rotational statej, of rotor 1. Because of an insufficient number of trajectories, the distributions exhibit some “noise” especially for T = 300 K. The j, distributions for T = 0 K are relatively narrow and peak at or at least very close toj, = j 1 . An exception is the distribution forj, = 16, which peaks at j , 13. However, in this case the averaged probability Pl(jI=16) is very small (Figure l ) , and therefore the uncertainty of the Monte Carlo calculation is quite large. The j , distributions for 300 K are generally much broader than for 0 K. The width decreases slightly with increasing j l . In contrast to T = 0 K, however, the peak center does not significantly shift with j,. Thus, the two rotamers are highly correlated for very low H 2 0 2temperatures and the correlation diminishes with increasing T. This tendency can be explained quite easily on a dynamical basis. In addition to overall rotation of H 2 0 2about the 0-0 axis, O H rotation can be caused dynamically either by (1) bending motion associated with the two H-0-0 angles or by (2) torsional motion associated with the dihedral angle p. In the first case an appreciable torque about the O H center of mass is induced by strong 0-0 repulsion, and in the second case OH rotation is induced by a significant p dependence of the excited-state potentials. If OH rotation would originate exclusively from bending motion j, would be perpendicular to the recoil velocity v ( a v j= 90’) whereas j, would be parallel to v (a,i = Oo) if OH rotation would be induced solely by torsional motion. Here a,j denotes
-
Initially the rotation of OH molecule 1 and the rotation of O H molecule 2 are exactly correlated. As we will show below, this correlation might be partly destroyed during dissociation depending on the initial H202temperature.
Results The calculations are performed for the A state and for the B state separately. All results in this letter are averages over the two electronic states incorporating the theoretical branching ratio 0.7/0.3.In Figure 1 we show, for three H 2 0 2 temperatures, averaged OH-rotational-state distributions Pi(ji),eq 2, irrespective of the rotational state of the other molecule. Since the two fragment molecules are the same, the two averaged probabilities Pi and P, are of course identical. The distributions are smooth and inverted. They can be nicely interpreted by the rotational reflection principle.12 The full width at half-maximum changes 4 for 0 K to about A j 6 at room temperature from A j whereas the peak center is not affected by the increase of the H,02 temperature. The theoretical distribution for 0 K is compared to the measured distribution obtained for dissociation in a molecular beam,15 and the 300 K distribution is compared to the
-
-
( I S ) Grunewald, A. U.; Gericke, K.-H.; Comes, F. J., to be published.
4018
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
Letters
I
,/--
,
151 /
---____
41
151
--'--,\
i=I
/'
T = OK
i
I
A
",. V
I 51
/
/ ,
15- T = 150K
A
.,"
1
10-
V
5-
-151
25
30
35
LO
L. 5
Figure 3. Angular momenta P,, and P , vs 0-0separation R, calucated on the A-state potential energy surface. Initial momenta are P,,O = PO , = 0 and 4, respectively. All other initial momenta (PRO,Pelo.and Po:) are 0. The initial dihedral angle is qo = I IOo.
the angle between ji and v . The average value of avjis -23' as obtained from the trajectory calculations12 and -26O as determined experimentally.'0 This indicates very strongly that, for dissociation at 193 nm, torsional motion is dominantly responsible for the rotational excitation of the two O H radicals. For a qualitative interpretation of the correlation effects seen in Figure 2 it is therefore sufficient to consider only the torsional degree of freedom and neglect bending motion. If bending motion is completely ignored and if we assume that sin 8, = 1, it follows from eq 6 that ji= IP,(t)l for all times. For T = 0 K the initial momenta Pv,"and hence j p are zero. Since dV/dcp, = -dV/'/dcp, (the potential depends only on cp = cp, - p2), Hamilton's equations predict that P,,(t) = -P,(t) and hence j,(t) = j 2 ( t ) for all times; Le., the two rotors are strictly correlated throughout the trajectory. A typical trajectory for T = 0 K (P,,O =P O, = 0) is depicted in Figure 3. The variation of P,, and P,, with Roo, the 0-0 separation, is shown. Since V A ( R ~[and ~) likewise V B ( R ~ is) ]very steep, the dissociation is direct and rotational excitation starts immediately after the electronic transition. The maximum of lP,l is reached within a short distance beyond the classical turning point and a slight deexcitation occurs at larger 0-0 separations. Thus, for T = 0 K and without including bending motion the microscopical probability matrix is diagonal as given in eq 4. The deviations of the calculated distributions in Figure 2 for T = 0 K from a perfectly diagonal matrix therefore reflect the influence of bending motion which is of course included in these calculations. If T f 0, nonzero initial momenta P,,O = P,,O contribute to the cross section in eq 8 and this destroys the "symmetry" of the trajectories which exists for P," = 0. A typical trajectory for PPIo = Pao = 4 ( M = 8) is also shown in Figure 3. It is merely shifted upward by four units. Initially the two molecules rotate with equal angular velocity in the same direction. When the fragmentation starts, the angular motion of rotor 1 is accelerated and P,,(t) increases whereas rotor 2 is first deaccelerated and P,(t) decreases. After a short time molecule 2 comes to a stop (P, = 0), and then it starts to rotate in the opposite direction. Because the total
0
5
10
15
'1
Figure 4. Mean rotational state ( j , ) of rotor 2 (eq 1 1 ) as a function of the rotational state j , of rotor 1 for three H,O, temperatures. The experimental data are taken from ref 17. The straight lines ( j , ) = j , represent complete correlation of the two OH molecules.
angular momentum is conserved, P,, and Pa are related to each other by P,, = M - P,, at all times, where M can be positive or negative. Therefore, the asymptotic values for j , = IPpIIand j 2 = IM - P,,I can differ by as much as IMI = 21P,,01. For the example in Figure 3 M = 8 and j , 15 and j 2 7 . For M = -8 (P,,O = Pe0 = -4) the situation would be reversed. This leads naturally to a broadening of the j 2 distributions for any fixed rotational state j , . A more quantitative estimate is difficult to make. In conclusion, the strict correlation between j , and j 2 is gradually lifted as IM/ increases. Since the distribution of possible total angular momentum states becomes broader with increasing temperature, this effect becomes more pronounced as T rises. The addition of angular momenta induced by thermal motion within the parent molecule and induced by dynamical forces during fragmentation also explains the broadening of the averaged probabilities Pi(j,) in Figure 2 although there the temperature dependence is much less prominent.
-
-
Comparison with Experiment Fully resolved microscopical-state distributions P ( j , j 2 )have not yet been measured, neither for H 2 0 2dissociation nor for any other molecule. Recently GerickeI6 proposed a new technique in which the Doppler profile of the laser induced fluorescence lines is analyzed. Because of energy conservation, the translational recoil energy depends o n j 2 for any fixed rotational state j , probed by LIF. If the energy spacing between adjacent levels is not too small and if the probe laser is sufficiently narrow, coincidence (16) Gericke, K.-H. Phys. Rev. Lett. 1988, 60,561.
J . Phys. Chem. 1988, 92, 4019-4022
Conclusions In this letter we investigated the correlation of product rotation in the photodissociation of H 2 0 2at 193 nm. The classical calculations are based on two ab initio potential energy surfaces for the lowest excited states. Completely resolved, microscopical probability matrices P ( j , j 2 )are calculated for three H 2 0 2temperatures. For T = 0 K the two O H rotamers are highly correlated; i.e., they are dominantly populated in identical states. The correlation is gradually destroyed as the temperature increases. The classical theory provides a simple explanation of this effect. Hamilton’s equations of motion are symmetric with respect to the two O H rotations about the common 0-0 axis. If the initial conditions are also symmetric, as it is the case for rotationless H 2 0 2 , the symmetry is retained throughout the dissociation process. If the initial conditions are not symmetric because of initial H 2 0 2rotation, the perfect correlation is destroyed. This effect becomes gradually more prominent as the H 2 0 2temperature rises. Initial rotation of H 2 0 2also explains the broadening of the averaged distributions PiGi) as the temperature increases. Our arguments are very similar to those given by Gericke et al.” The agreement with preliminary experimental data based on the average rotational state (j,) for fixed rotational state j , is satisfactory. The most rigorous test of our predictions, however, requires the measurement of the fully state-resolved probability matrix P ( j 1 j 2 ) .
measurements should be possible, at least in principle. In a first attempt Gericke et al.I63” applied this method to the photodissociation of H202. Since the probe laser was not narrow enough, only a mean value u 2 ) for fixed rotational statej, rather than P(jlJ2) could be resolved. The experimental results for 193 nm and dissociation in the beam ( T = 0 K) and in the cell ( T = 300 K) are compared in Figure 4 with the theoretical results which are defined as
( j , ) = cj2pOlJ2)/m0’2J,) Jz
Ji
(11)
-
If the two rotors were perfectly correlated, i.e., P ( j l J 2 ) S(jl
- j2), (j,) would be identical with j , . Such a constraint is approximately found for T = 0 K. The theoretical data points slightly deviate from the diagonal at higher j , states, where either the overall probability is too low and the trajectory calculations are not fully converged or where bending motion, which can destroy the correlation, becomes more prominent. Similar arguments might explain the rather large deviation from the diagonal of t h e j , = 5 experimental data point. As the H202temperature rises, the correlation is gradually destroyed and the data points are significantly shifted away from the diagonal. Low j , states correlate with higher ( j , ) values and vice versa. Taking into account the experimental uncertainties for 300 K, we claim that the agreement between theory and experiment is fairly satisfactory. Most-importantly, the general trend of decreasing cokelation with increasing H 2 0 2temperature is observed experimentally as well as theoretically. (!7) Gericke, K,-H,; Grunewald, A, U,;Klee, S , ; Comes, F, published.
J,,
4019
Acknowledgment. Many stimulating discussions with K.-€1. Gericke (Frankfurt) on this topic are gratefully acknowledged. I am also grateful to F. J. Comes. K.-H. Gericke. S. Klee. and A. U. Gruiewald (Frankfurt) and V. Staemmler (Bochum) for permission to use parts Of their work prior to publication.
to be
Registry No. H202,7722-84-1; OH, 3352-57-6.
Real-Time FTIR Spectroscopy as a Quantitative Kinetic Probe of Competing Electrooxidation Pathways for Small Organic Molecules Lam-Wing H. Leung and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: April 11, 1988)
The application of real-time FTIR spectroscopy to probe the quantitative kinetics and mechanisms of competing electrochemical pathways on a voltammetric time scale is illustrated for the electrooxidation of ethanol, ethylene glycol, and glycolaldehyde in 0.1 M HC104 at platinum. A simple procedure is outlined whereby the required ratio of molar absorptivities in the thin-layer cavity and bulk solution, eerT/cb, can be evaluated. This approach enables the proportion of CO, and partial oxidation products formed during voltammetric sweeps to be determined reliably and the role of adsorbed CO and other chemisorbed fragments in the electrocatalytic mechanisms to be evaluated.
The electrocatalytic oxidation of small organic molecules is a topic of widespread significance.’ Although conventional electrochemical methods can provide a sensitive monitor of kinetics and adsorption for such systems, the identification of reaction pathways is necessarily limited by the nonspecific nature of these techniques. Of the various spectroscopic methods developed recently for in situ electrochemical applications, infrared reflection-absorption spectroscopy (IRRAS) offers considerable promise given the rich molecular structural information provided by vibrational techniques. While the potential-difference surface infrared techniques commonly employed to minimize the solvent interference2 can (1) For example: Rudd, E. J.; Conway, B. E. In Comprehensiue Treatise of Electrochemistry; Conway, B. E., Bockris, J. O.’M., Yeager, E., Khan, S . U. M., White, R. E., Eds.; Plenum: New York, 1983; Vol. 7, Chapter 10. (2) For recent reviews, see for example: (a) Bewick, A,; Pons, S . In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: New York, 1985; Vol. 12, Chapter 1. (b) Foley, J. K.;Korzeniewski, C.; Dashbach, J. L.; Pons, S . In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, p 309.
0022-3654/88/2092-4019$01.50/0
provide a sensitive means of examining redox-induced molecular transformations, the common use of repeated potential modulation restricts their application to reversible processes. By use of a Fourier transform spectrometer, however, in some cases it is feasible to obtain surface spectra by using only a single potential excursion during the spectral acq~isition.~.~ This “single potential alteration infrared” (SPAIR) approach can enable potential-induced changes in the thin-layer solution composition as well as at the interface itself to be monitored during the evolution of irreversible electrode proce~ses.~”Most conveniently, a sequence (3) Corrigan, D. S.; Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252. (4) (a) Corrigan, D. S.; Weaver, M. J. J . Electroanal. Chem. 1988, 241, 143. (b) Leung, L.-W. H.; Weaver, M. J. J . Electroanal. Chem. 1988, 240, 341. (5) Corrigan, D. S.; Weaver, M. J. Langmuir 1988, 4 , 599. (6) Other published applications of this general procedure include: (a) Pons, S.; Datta, M.; McAleer, J. F.; Hinman, A. S. J . Electroanal, Chem. 1984, 160, 369. (b) Christensen, P. A,; Hamnett, A,; Travellick, P. R. J . Electroanal. Chem. 1988, 242, 23.
0 1988 American Chemical Society
