The Journal of
Physical Chemistry
0 Copyright, 1987, by ihe American Chemical Society
VOLUME 91, NUMBER 15 JULY 16, 1987
LETTERS The Origin of Fragment Rotation in ICN Photodissociation C. H. Dugan* and D. Anthony Physics Department, York University, Toronto, Canada M3J 1 P3 (Received: February 2, 1987; In Final Form: April 14, 1987)
CN rotational distributions observed in recent long-wavelength photodissociation experiments on ICN are compared with predictions of several theoretical descriptions of the process. An elementary impulsive model is shown to yield results that agree with experimental data and support one view of the dynamics of dissociation.
Dissociation of ICN leading to ground-state fragments I(2P,jz) and CN(X) is one of the simplest photochemical events involving triatomic molecules. In spite of the simplicity, the experimental observations of the rotational distribution of the C N fragment do not have a generally accepted explanation. We discuss several possible origins of the fragment rotation and show that results from a very elementary model, which agree with experiment, lead to an intuitive understanding of the process. The experimental results to be discussed were obtained in experiments on ICN at long wavelengths, where only the ground state of iodine will occur.’-4 Reference 1 contains a current bibliography of experiments on this widely studied problem.
Explanation of Fragment Rotation The absorption spectrum of ICN in the wavelength region of our experiments is featureless so we have very little information about the upper state involved in the dissociation. The dissociation is “direct”, occurring in one or two periods of a typical bending vibration of the m ~ l e c u l e . ~Early * ~ attempts to explain the ex-
perimental results used a linear model of the molecule, considering the inevitable bending motion in the ground state. In that configuration there are several sources of fragment rotation following dissociation: prior parent molecule rotation, bending motion of the parent molecule; and torque exerted by the repulsive force during recoil of the diatomic fragment from the iodine atom, in the event that the parent molecule is not linear. The first two sources provide rotation to C N prior to dissociation, about its own center of mass; we suppose that angular momentum of C N present prior to the dissociation event will survive the disruptive dissociation but will be modified by torques developed during that event. Several writers have suggested that the angular momentum of parent rotation is converted efficiently into angular momentum of the C N fragment7-I0 but it is recognized that, even if that were true, it requires additional sources of rotational energy to explain the observations. Pattengill’s classical trajectory calculations]I based on necessarily arbitrary potentials led him to conclude that ( l ) , in ICN dissociation, the fragment rotational state does not depend on the
(1) Hall, G.; Sivakumar, N.; Houston, P. J. Chem. Phys. 1986,84, 2120. (2) Nadler, I.; Reisler, H.; Wittig, C. Chem. Phys. Lett. 1984, ZO3,451. (3) Fisher, W.; Carrington, T.; Filseth, S.; Sadowski, C.; Dugan, C. Chem. Phys. 1983,82, 443. (4) Fisher, W.; Eng,R.; Carrington, T.; Dugan, C.; Filseth, S.; Sadowski, C. Chem. Phys. 1984, 89, 457.
( 5 ) Ling, J.; Wilson, K. J . Chem. Phys. 1975, 63, 101. (6) Knee, J.; Khundkar, L.; Sewail, A. J. Phys. Chem. 1985, 89, 5141. (7) Morse, M.; Freed, K.; Band, Y . J . Chem. Phys. 1979, 70, 3604. (8) Bersohn, R. IEEE J . Quantum Electron. 1980, QE-16, 1208. (9) Long, S . R.; Reilly, J. J. Phys. Chem. 1982, 86, 56. (10) Halpern, J. B.; Jackson, W. M. J. Phys. Chem. 1982, 86, 973. (11) Pattengill, M. Chem. Phys. 1983; 78, 229. 1984, 87, 419.
0022-3654/87/2091-3929$01.50/0
0 1987 American Chemical Society
3930 The Journal of Physical Chemistry, Vol. 91, No. 15, I987
Letters
parent molecule rotational state and (2) that a model of ICN Equation 2 associates rotational quantum state N with angle 0; the angular range A0 associated with the interval between dissociation based on a bending linear molecule, rotating, does not give sufficient rotational energy to explain the observations successive rotation states is obtained from eq 2: on rotational distributions of CN(X). We reach the same conclusion from the quantum-mechanical calculations of Beswick and A8 = N + t/z tan 8 ( 1 sin2 8 ) Gelbart .I N ( N 1) It is now generally recognized that the experiments on ICN In this classical model of a quantized system, we determine the dissociating to give CN(X) require us to suppose that the excited relative population of each rotational level N as molecule dissociates from a bent state. (The dissociating state is unstable, so the loosely used term “bent state” means for a fixed P(N) = P ( 0 ) A8 r(1-C) the potential energy as a function of bond angle has a minimum at some angle other than lSO’.) But even if we accept The calculation proceeds from B = 0’ (the linear configuration the bent configuration, several factors may contribute to C N which yields no C N rotation) to larger values of 8. The largest rotation. For example, Baronavski, Waite, and their c o - ~ o r k e r s ~ ~ J ~N value possible is associated with 0 = 90’. Finally, we have and Schinkels concluded that the C N rotation arises from the calculated the average rotational energy: forces acting in the upper state to bend the molecule away from the linear arrangement (the upper state potential anisotropy). Since changing the bond angle will also change the effect of recoil for each distribution to compare with the same quantity obtained forces exerted between I and C, at least one additional way of earlier for the experimental result^.^ exciting C N rotation may be involved. In this paper I argue for Because the nature of the paper state potential surface is not the importance of the torque exerted by the repulsive force by known in these molecules, we cannot proceed in a stepwise analysis showing that some elementary calculations based on a classical of the dissociation involving the Franck-Condon factors for the impulsive approximation reproduce the features of the measured transition from the lower to the upper state. The experimental rotational distributions. The model is explained next. results at one wavelength (337 nm in Figure 1) enable us to c h m e values of 80 and the width of the distribution of bending angles An Elementary Model in the dissociating state. Having made that choice for one value We describe a simple classical model of the dissociation that of available energy, we then calculate rotational distributions for is appropriate for the case of the heavy cyanogen halides. It is the other available energies without changing the parameters of an elaboration of the impulse model of Ling and Wilson5 and the upper state. Tuck.16 An impulsive force acts between the iodine atom and Figure 1 shows a comparison of ICN photodissociation data the C atom in the CN. This force does not act through the center with calculations in which the dissociating state is characterized of mass of the C N fragment when the ICN is bent, so its effect by a Gaussian distribution in angles having 80 = 18’. If the is to accelerate the C N and to rotate it. An impulsive model is molecule were linear (on average) rather than bent, then the a “sudden” approximation to a full classical trajectory analysis, distribution would show a maximum value at N = 0 instead of analogous to approximations widely used in quantum mechanical having a maximum value a t some N > 0 as the data show. It studies of dissociation and rea~ti0n.l~ We use this simple, limiting is a distinctive feature of the experimental results that the peak approximation to estimate the rotation imparted to the retreating of the rotational distribution lies a t higher N values for higher C N fragment during the motion of the molecule on the steep EAv, and also that the average energy in the distribution increases repulsive potential surface in the excited state (ref 11, for example) with increasing E A V . This result is obtained from our calculations and approximate the angular portion of the motion by the distoo, as mentioned above. The average rotational energies for the tribution appropriate for motion in a harmonic angular potential calculated distributions are given in Figure 1. Since the calculated anisotropy about a bent conformation. and experimental curves are in moderately good agreement it is The rotational energy of the C N fragment arising from the not surprising that experimental average energies and calculated action of an impulsive repulsive force between I and C is given average energies agree well. to a good approximation by this relationship3: At the wavelengths used in the experiments, the absorption is caused by molecules with one or two quanta of bending vibration; no dissociation arises at these wavelengths from molecules in the ground state.2 This is due to unfavorable vibrational wave function overlap near 0 = Oo, not to energy limitations. The classical where EAVis the available energy (energy of the dissociating turning points for the first two excited vibrational states are 14.4’ photon plus thermal energy minus dissociation energy), 8 is the and 18.7’ so that we can understand how the absorption from supplement of the bond angle, and the masses given are those of these states can populate a state having a minimum in its angular the C and N atoms. In terms of the angular momentum J of the dependence near 18’, its equilibrium angle. C N fragment Other Sources of CN Rotation E,,, = p/21 We have not included the rotation derived from parent rotation because it can be shown readily, using a simple classical a r g ~ m e n t , ~ and the states of the rigid rotor will be characterized by the that there is very little rotation of C N about its center of mass quantum number N according to in the rotation of the parent molecule. Most of the angular momentum of ICN appears as orbital angular momentum of the J = fi [ N ( N 1)3l/* C N fragment about the ICN center of mass, after the dissociation. cgs and express the This is the reason that Pattengill” and Beswick and Gelbarti* For C N we use the value I = 1.42 X found parent rotation to be unimportant; for ICN virtually all energies in cm-I to obtain the expression the rotation comes from other sources. In a different formulation N ( N + 1) = 0.51K(O) E A v (2) this conclusion allows us to understand the results of Guest, O’Halloran, and Zare” in which the angular momentum alignment of C N from ClCN was found to be independent of CN (12) Beswick, J. A.; Gelbart, W. M. J. Phys. Chem. 1980, 84, 3148. angular momentum. The most basic source of variation of (13) Waite, B.;Helvajian, H.; Dunlap, B.; Baronavski, A. Chem. Phys. fragment alignment is rotation of the parent molecule during Lett. 1984, 1 1 1 , 544.
+
+
2
+
(14) Waite, B.; Dunlap, B. J. Chem. Phys. 1986, 84, 1391. (15) Schinke, R.; Engel, V. J. Chem. Phys. 1985, 83, 5068. (16) Tuck, A. F. J. Chem. SOC.,Faraday Trans. 2 1977, 73, 689.
(17) Guest,J.; O’Halloran, M.; Zare, R. Chem. Phys. Lett. 1984, 103, 261.
3931
Letters L
350
A 350
337
308 OUANTUM NUMBER N
0
DO
2;
00
4;
00
G 0I
60 , 00
30 8
s
0
G
G
W
W
G 0
W
00
20 0 0
40 0 0
60 0 0
t
337
E: c a
N
N
0
CI CI
c,
0
r
20 P O
40 0 0
60 00
OUANTUM NUMBER N
00
C.O
2 C 00
LO C.O
bO CJC
OUANTUM NUMBER N
Figure 1. Rotation distributions of CN from ICN photodissociation at 308, 337, and 350 nm (counterclockwise from lower left). Populations are all relative. Average rotational energies are compared with experimental values (in parentheses). For each graph the same molecular parameters were used, with different values of available energies (for which values are 6960,4070, and 2340 cm-', respectively). Experimental data from ref 3 are shown as crosses. and C and they show proportionality between the available energy dissociation; however, any value of fragment angular momentum and the rotational energy in the fragment. correlated with every value of parent angular momentum, and so all C N fragments have essentially the same dissociation history. Conclusions In ref 12 "final state interactions" were neglected for compuIn demonstrating that the observed C N rotation can be untational convenience but zero-point bending vibration and parent derstood as a result of the repulsive force developed between the rotation were included. The classical calculation" considers both dissociating C N and I fragments acting in a bent arrangement, of those sources and an approximation to final state interactions. we have not shown that the alternative explanation advanced in Neither yields results that match the experimental data because the literature-bending force in the upper state-is incapable of they restricted consideration to linear molecules. explaining the results. However, there is no demonstration that Waite and DunlapI4 analyzed the problem by a classical trasuch an explanation is adequate; the classical trajectory calculation jectory method and assigned the CN rotation to bending forces by Waite and Dunlap14does not yield rotational distributions that in the upper state; the Hamiltonian they used helps in reaching agree convincingly with experimental data as the results shown this conclusion because it separates into a bending force component in Figure 1 do. On the other hand, it is obvious that the bending and a recoil component that acts between the iodine atom and force in the upper state is involved in our explanation of the the C N center of mass. Forces arising from such a potential will process: the ground-state molecule is linear, excited probably with not yield a linear relationship between available energy and roa few quanta of bending vibration,2 and it appears that the distational energy; in contrast, Pattengill's calculations1' are based sociating state is bent. But it is our claim that the rotational energy on a Hamiltonian that provides a recoil force acting between I
J. Phys. Chem. 1987, 91, 3932-3934
3932
in the fragment derives from the repulsive forces acting along the I-C axis, and in the present special case of ICN, this is the principal source. Although bending vibration energy is efficiently disposed to CN fragment rotation, as elementary calculations show, our comparison with the experimental data suggests that this source is not of central importance for molecules with a mass
distribution like the one we are studying. A similar analysis of BrCN dissociation shows similar agreement with experimental data.4 It should be realized that this simple treatment is useful only for molecules like ICN; it is not adquate when considering HCN. For that molecule, all three of the sources of fragment rotation must be considered.
Infrared Spectra and Dimer Structure of Reduced Vioiogen Compounds M. Ito,* H. Sasaki, and M. Takahashi Department of Chemistry, Faculty of Science and Technology, Keio University, Kohoku- ku, Hiyoshi, 223, Japan (Received: April 3, 1987) The self-dimer structure of electrochemically reduced methylviologen was confirmed by observing the remarkably strong totally symmetric modes of vibrations (a,) in the infrared spectra. The charge-transfer process in polymer-coated viologen on a Pt electrode was also studied by measuring the intensity growth of the ag bands.
Introduction The viologens were originally investigated as redox indicators in biological studies and, more recently, as electron mediators in biological systems.] On the basis of many available data2,3there are good grounds for expecting dimeric association of the cation radical, although no direct evidence for this structure has been reported. It is important to note that intermolecular interactions of viologen molecules or polymer viologens are closely related to the electron-transfer process. There have been a number of reportsks on the in situ Raman characterization of methylviologen (MV) generated at electrode surfaces. However, no infrared spectroscopic work has been reported on reduced MV. Girlando et aL9q10 have reported electron-molecular vibration interactions in many organic charge-transfer crystals. They showed that the totally symmetric Raman-active (a,) modes can couple with electronic wave functions, the coupling giving rise to vibronic absorptions in the IR. In this paper we report for the first time the infrared spectra of the three reversible oxidation states of some viologens and present results which show the vibronic activation of the totally symmetric (a,) modes of the viologen monocation based on the FergusonPerson vibronic model.iiJ2 The vibronic effects in various polymer viologens are also found and discussed.
Experimental Section A Perkin-Elmer 9 8 3 0 infrared spectrometer with a reflection attachment is used. The electrochemical celli3 was inserted in the sample beam. The broad absorption in Figure 1 around 1640 cm-’ is due to the absorption of a thin aqueous solution layer between the window and the electrode. A gradual cutoff due to the CaF2 appears below 1100 cm-’ in each spectrum. The spectra were recorded under potentiostatic conditions. (1) Michaelis, L.; Hill, E. S.J . Am. Chem. SOC.1933, 55, 1481. (2) Datta, M.; Jansson, R. E.; Freeman, J. J. Appl. Spectrosc. 1986, 40, 251. (3) Forster, M.; Girling, R. B.; Hester, R. E. J . Raman Spectrosc. 1982, 12, 36. (4) Regis, A.; Corset, J. J. Chim. Phys. 1981, 78, 687. (5) Lee,P. C.; Schmidt, K.; Gordon, S.; Meisel, D. Chem. Phys. Lett. 1981, 80, 242. (6) Lu Tianhong; Birke, R. L.; Lombardi, J. R. Langmuir 1986, 2, 305. (7) Ohsawa, M.; Nishijima, K.; Suetaka, W. Surf. Sci. 1981, 104, 281. (8) Benchenane, A,; Bernard, L.;Theophanides, T. J . Raman Spectrosc. 1974, 2, 543. (9) Girlando, A.; Bozio, R.; Pecile, C. Phys. Reu. B 1982, 26, 2306. (IO) Bozio, R.; Pecile, C. Solid State Commun. 1981, 37, 193. ( 1 1) Ferguson, E. E. J. Chim. Phys. 1964, 61, 257. (12) Friedrich, H. B.; Person, W. B. J. Chem. Phys. 1966, 44, 2161.
0022-3654/87/2091-3932$01 .50/0
All the reagents were used as received (Wako Chemical Co.). The redox polymer viologens, (poly(p-xylylviolgen dibromides) @-PXVBr2), and a mixed polymer complex composed from p PXVBr2 and potassium poly(styrenesu1fonate) (PXV-(PSS)2) were used. The syntheses of polymer viologens were prepared as described by the 1 i t e r a t ~ r e . IThe ~ ~ ~PXV-(PSS)2 ~ film on Pt was prepared by a dipcoating method.14 The film of the polymer layer was allowed to completely dry, and the thickness was roughly estimated from the amount of polymer used (0.5-1.0 pm). A saturated calomel electrode (SCE) was used as a reference electrode. Argon gas was bubbled through the solutions in order to prevent reactions of the reduced films with oxygen. The cyclic voltammetry of MVC1, was examined in an aqueous phosphate buffered solution (pH 7.2).
Results and Discussion Methyluiologen. Cyclic voltammetry a t a Pt electrode in deaerated 0.1 M MV in 0.1 M phosphorous acid buffered aqueous solution scanned between 0 and -1.4 V showed the first (-1.0 V) and the second (-1.3 V) electron reduction peaks and the corresponding oxidation peaks (-0.3, -1 .O V). Therefore, we chose two reduction potentials, -1.0 and -1.4 V, at which the spectrum of the two reduced species are measured. Figure 1 shows the reflection absorption spectra (IRRAS) of electrochemically reduced methylviologen in 50 mM MV2+ aqueous solution. The spectrum at 0 V where no reduction occurs is also included in the figure. The bands at 1605, 1511, 1340, 1201, and 1184 cm-I appeared with large intensities. All of those bands begin to glow at -0.4 V and reach maximum intensities at -1.0 V and decrease in intensities at more negative potentials, while the frequencies of those bands remain unchanged throughout the potential regions. The intensity changes against the potentials are rapid and mostly reversible. Monomer cation radical species cannot exist stably in an aqueous solution phase, although an adsorbed monomeric species on the electrodes from dilute solution (less than 2 mM of MV2+) has been reported. Since a highly concentrated solution (0.1 M MV2+)is used in the present work, the precipitated species in the solution may be the “dimer species” as indicated by both the surface-enhanced Raman scattering and the resonance Raman However, there have been so far no detailed structural data for the dimer species. ESRI6 or UV-visible s p e c t r o ~ c o p y ~ ~ ~ ~ ” (13) Kitamura, F.; Takahashi, M.; Ito, M. Chem. Phys. Lett. 1986, 123, 273. (14) Factor, A.; Heinshon, G. E. J . Polym. Sci., Part B 1971, 9, 289. (1 5) Wiley, R. H.; Smith, N. R.; Ketterer, C. C. J. Am. Chem. SOC.1954, 76, 720. (16) Ivanov, V. F.; Grishnia, A. D.; Shapiro, B. I. Izu. Akad. Nauk SSSR, Ser. Khim. 1976. 1383.
0 1987 American Chemical Society
