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J . Phys. Chem. 1991, 95, 2430-2434
sothermal conditions. The essential points in the models are the positive feedback provided by the autocatalytic reaction or substrate inhibition and the displacement of the system far from equilibrium by the photochemical reactions. The use of closed systems with illumination has some advantages over a CSTR; it is easier and more convenient to control the intensity of illumination than to control the flow rates of chemicals. Chemical instabilities in closed system may have some potential applications in practice, for example, the transformation of radiation energy of light into mechanical energy.I2 Berry and his colleagues have explored this p ~ s s i b i l i t y " * ~in~ ~a *simple ~ dimerization reaction' run in a system with variable volume. On the basis of thermokinetic instabilities in a closed system predicted theoretically and confirmed e~perimentally,~-* they found that if the volume of the reactor can vary, the original closed illuminated system with a thermokinetic instability of multiple stationary states may exhibit temperature or pressure oscillations. Isothermal pressure oscillations may produce work; that is, one (22) Watowich, S.J.; Hoffmann, K. H.; Berry, R. S.J . Appl. Phys. 1985, 58, 2893. (23) Watowich, S.J.; Hoffmann, K. H.; Berry, R. S.Nu000 Cimento 1989, 104B, 131.
may build a machine to turn radiation energy of the light into mechanical energy, if the volume of the system may vary. Temperature oscillations may produce work in a system at constant volume by flow of heat to a thermally connected piston and cylinder combination. The experimental verification of chemical instabilities in closed, illuminated systems remains to be shown. Functionally, the photochemical reaction step (for instance the last step in reaction scheme (l), which transforms the species D (the net production of other steps) to A (the reactant)) plays a role similar to the input process of species A in the CSTR mode. In fact, the kinetic equations (6) for reaction scheme (1) have a form very similar to those for the first two reaction steps of this scheme run in a CSTR.% Thus, a system that may show chemical instability under CSTR condition is likely to be able to show similar chemical instability in closed, illuminated system if a photochemical reaction may transform the products into the reactants.
Acknowledgment. This work was supported in part by the Department of Energy/BES Engineering Sciences. We thank Professor Jean-Pierre Laplante for helpful comments. (24) Li,
R.S.;Li, H.J. Chem. Eng. Sci.
1989, 44, 2995.
Proton Transfer in Naphthol/Ammonia Complexes: Deuteration Effects T.C. Swinney and D. F. Kelley* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 (Received: August 27, 1990)
The effect of deuteration on the spectroscopy and proton-transfer dynamics of 10 K matrix-isolated a- and &naphthol-(NH3), ( n = 0, 1 , 2, 3) complexes has been studied. Large changes in the &:trans rotamer ratio of &naphthol are observed upon deuteration. These observations can be explained in terms of the energetics and relaxation dynamics of both forms. The and b-naphth~l(-OD)-(ND~)~ complexes were determined to be deuteron-transfer times in the a-naphth~l(-OD).(ND~)~ longer than several nanoseconds. Comparison with the corresponding proton-transfer rates indicates a kinetic isotope effect of 2200. These results can be understood in terms of a simple two-dimensional tunneling model.
Introduction Proton-transfer reactions are of fundamental importance in many areas of chemistry. Excited-state proton-transfer (ESPT) reactions in solute/solvent microclusters are of particular interest, as the study of these reactions yields detailed information on solvation effects. These reactions are also of interest because observed deuteration effects can be easily compared with those predicted by theory. Intermolecular ESPT reactions have been studied in several gas-phase and matrix-isolated solute/solvent systems. The most thoroughly studied solute/solvent intermolecular ESPT system is naphthol/ammonia. a-Naphthol.(NH,), complexes have been extensively studied in the gas-phase' and in low-temperature argon It has been shown that the gas-phase n = 4 complex undergoes ESPT, but the n < 4 complexes do not. These results were interpreted in terms of the excited-state a-naphthol acidity and the basicity of ammonia clusters. Similar studies on matrix-isolated a-naphthol.(NH3), and @-naphthol.(NH3), complexes have also been reported. In contrast to the gas-phase results, these studies show that, for both a-and j3-naphthol.(NH3),, the n = 3 complex undergoes ESPT. The difference between the ma-
trix-isolated and gas-phase results was explained in terms of matrix stabilization of the n = 3 ion pair, which is absent in the gas ESPT times have been measured for the matrix-isolated a-and @-naphth~l.(NH,)~ complexes. Values about 20-25 ps were obtained in both cases. These ESPT times were temperature independent over the 10-24 K range, which indicates that proton transfer occurs by a tunneling mechanism. A variety of theoretical treatments of proton tunneling have been reported. The simplest treatments are one-dimensional and consider only the proton motion! In these treatments, the barrier to proton transfer is often taken to have some specific functional form (Eckhart, parabolic, etc.). The calculated proton tunneling rate is proportional to the extent to which the wave function penetrates the barrier. Proton motion potential curves can be constructed such that the calculated tunneling rates are in reasonable agreement with experiment over a limited range of temperatures. However, these potential curves are in gross disagreement with known spectroscopic potential curves, indicating a basic failing of the one-dimensional model.s The physical origin
(1) Cheshnovsky, 0.;Leutwyler, S.Chem. Phys. k t r . 1985,121;J. Chem. Phys. 1988, 88, 4127. Knochenmuss, R.; Cheshnovsky, 0.;Leutwyler, S. Chem. Phys. Lett. 1988, 144, 317.
(4) Bell, R. P. The Proton in Chemistry, 2nd ai.;Comell University Res: Ithaca, 1973; Chapter 12; The Tunnel/ Eflecr in Chemistry; Chapman-Hall: New York, 1980; Chapter 2, and references therein.
0022-3654/9 1/2095-2430$02.50/0
(2) Brucker, G. A.; Kelley, D. F. J . Chem. Phys. 1989, 90, 5243. (3) Brucker, G. A,; Kelley, D. F. Chem. Phys. 1989, 136, 213.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2431
Proton Transfer in Naphthol/Ammonia Complexes of this failure is the neglect of heavy-atom motion. As the heavy atoms vibrate, the proton-transfer distance is modulated. Changes in the proton-transfer distance also change the barrier height and thus dramatically alter the calculated proton-transfer rate. Integration over the the thermally averaged heavy-atom positions (Le., integration over the thermally averaged proton-transfer potential surfaces) yields proton-transfer rates in agreement with experiment, using realistic proton motion potential surfaces. Several theoretical treatments based on these ideas5" and on golden rule considerations' have been published. Similar ideas have also been applied to proton-transfer reactions in solution, where the solvent polarization can also alter the proton-transfer rate.*-1° Importantly, all theories of proton tunneling make the same qualitative predictions about kinetic isotope effects. Proton transfer is predicted to te orders of magnitude faster than deuteron transfer at very low temperatures, and this large isotope effect decreases with increasing temperature. In this paper we report proton- versus deuteron-transfer rates in a-and 8-naphthollammonia complexes isolated in low-temperature matrices. A large isotope effect is seen in the rates, with the deuteron transferring a factor of 2200 slower. We show that this result is consistent with a simple two-dimensional model of proton tunneling.
Experimental Section The matrix isolation and time-resolved emission apparatus have been described in detail elsewhere."J2 Briefly, the matrix isolation system was based on an Air Products CSA-202E closed-cycle displex, evacuated by a turbomolecular pump. Static spectra were excited with a 150-W Xe lamp dispersed through a 1/8-mOriel double monochromator (resolution 0.5 nm). Detection of the static spectra was accomplished with a 0.65-m ISA monochromator (resolution 0.2 A) with 1200 groove/" grating coupled to a Hamamatsu R943-02 Ga-As PMT with single photon counting electronics. All spectra were taken with quartz optics and are uncorrected for instrument response. Time-resolved measurements were made with a Hamamatsu C979 streak camera coupled to a PAR 1254E SIT vidicon and interfaced to an IBM PC A T compatible computer. Wavelength selection was made with either spectral interference filters or a '/s-m Spex monochromator with a 150 groove/" grating. The time-resolved excitation source was based on a tunable, amplified dye laser synchronously pumped by an active/passive mode-locked Nd:YAG laser.12 Frequencydoubled ultraviolet pulses of 22 ps and 50 pJ were focused to a 1-mm spot size on the matrix. a-NpOD was prepared by dissolving a small amount of aNpOH (Kodak) in anhydrous ether and washing several times with D 2 0 (Cambridge). Ether from the ether/naphthol mixture was then pumped off, and sample vacuum sublimed several times to ensure the absence of D20. BNpOD was prepared by a similar procedure. In both cases, deutereated samples were kept under vacuum until matrix deposition. Gas-phase a-NpOD spectra were obtained by use of a supersonic expansion and time-of-flight mass spectrometer. The spectra show that deuteration was very nearly complete. Argon/ND3 gas mixtures were prepared by mixing high-purity (>99.998%) argon passed through a dry ice/acetone trap, with freshly distilled ND3 (MSD Isotopes) at a ratio of 2000:l (Ar:ND3). Samples were deposited on a 10 K copper block at a rate of 10 cm3 Torr/s. Deposition times were typically 1.5 h. ~
(5) Ziebrand,
~~
~
W.;Wildman, T. A.; Zierski, M.F. Chem. Phys. Lett. 1983,
98, 108. (6) LeRoy, R. J.; Murai, H., Williams, F. J . Am. Chem. Soc. 1980, 102, 2325. (7) Siebrand, W.; Wildman, T. A.; Zgierski, M. Z. J . Am. Chem. Soc. 1984, 106,4093,4089. (8) Morillo, M.; Cukier, R. 1. J . Chem. Phys. 1990, 92, 833. (9) Borgis, D. C.; Lee, S.;Hynes, J. T. Chem. Phys. Lett. 1989, 162, 19. (10) Brucker, G . A.; Kelley, D. F. J. Phys. Chem., submitted for publi-
cation.
(1 1) Brucker, G. A.; Kelley, D. F. J . Phys. Chem. 1987, 91, 2856, 2862. (12) Brucker, G. A.; Young, M. A.; Kelley, D. F. Rev. Sci. Inrtrum. 1985,
56, 2205.
0.3
~
Wavelength/nm
Figure 1. Absorption spectra of fl-NpOD in a 10 K argon matrix. The origins a t 325.6 and 328.8 nm are assigned to the cis and trans rotamers, respectively.
The naphthol/ammonia complexes were formed by annealing the matrix at temperatures of 24-32 K, and complex formation was followed through steady-state spectroscopy. This matrix annealing technique of forming hydrogen-bonded complexes has been previously discussed in detai1.2*3."
Results and Discussion Bare Molecule Spectroscopy. The absorption spectrum of matrix-isolated deuterated &naphthol (j3-NpOD) is shown in Figure 1. The origin at 325.6 nm shows three resolved peaks separated by about 65 cm-'. Almost identical structure has been observed in matrix-isolated BNpOH and was assigned to different sites in the argon matrix. Each peak has an inhomogeneous width of -50 cm-I. Due to this width, little or no H / D spectral shift could be detected. The deuterated sample also shows similar structure in a -300-cm-' red-shifted feature. The most intense peak in this feature is at 328.8 nm. Within experimental uncertainties (f0.2 nm), no other changes were observed in the absorption spectra upon deuteration. Ito et al.I3 have reported the detection of cis-trans rotational isomers (rotation of the OH group) of 8-NpOH in a supersonic expansion with the electronic origins of the isomers separated by 318 cm-I. Johnson et aLI4 have also observed two electronic origins of 8-NpOH separated by 317 cm-l in a supersonic expansion and assigned the blue and red origins to cis and trans rotamers, respectively. On the basis of these assignments, we assign the intense origin at 325.6 nm to the cis rotamer and the 300-cm-I red-shifted origin to the trans rotamer. The intensity ratio of the 8-NpOD rotamers (cis:trans) in the 10 K argon matrix is approximately 6:l and is temperature independent over the range 10-32 K. The trans rotamer is almost totally absent in the 8-NpOH spectrum; the ratio is >100:1. The absorption spectrum of matrix-isolated a-NpOD displays the same site structure and vibrational progressions upon deuteration as a-NpOH. Within experimental uncertainties (f0.2 nm), the absorption spectrum is not shifted with respect to aNpOH. Only one rotamer is observed in both a-NpOH and a-NpOD. The a-NpOH and a-NpOD origins are both 318.8 nm, and based on the assignments in ref 14, this corresponds to the trans rotamer; no cis rotamer is observed in the 10 K argon matrix. It is somewhat perplexing that only the cis rotamer is seen in the P-NpOH spectrum while both cis and trans rotamers are observed in the P-NpOD spectrum. It is also perplexing that only the trans rotamer is observed in both the a-NpOH and a-NpOD spectra. However, these results can be understood in terms of the ground-state energetics reported in ref 14. In 8-NpOH, the (13) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M.J. Phys. Chem. 1984,88,
5180.
(14) Johnson, J. R.; Jordan, K. D.; Plusquellic, D. F.; Pratt, D. W. J. Chem. Phys., in press.
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The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
TABLE 1: Summary of NpOH-(NH,), 1390-cm-' Vibrational Features species
a-NpOH a-NpOH.( N H3) I
0: excitation. nm
318.8 319.5 321.8 322.7
333.6 334.3 336.8 337.9
323.7
339.1
2
324.7
340.3"
1
324.1 325.6 326.4 333.0 333.6 334.3
340.1 341.1 341.9 349.5 350.1 350.9
site 1 2 1
2 3
P-NPOH*(NH~)I
2 3
1 1
2 3
337.5
354.6
337.8
355.6b
1
P-NPOH~NHJ)~
1
I
1
0-NpOH
P-NPOHWH,),
I
1
2
a-NpOH*(NH3)3
= 0, 1, 2, 3) Origins and
emission Deak. nm
2
a-NpOH*(NH3)2
(D
Swinney and Kelley
2 3
Corresponds to the 1390-cm-I vibrational feature from the neutral emission from the a-NpOD.(ND,), complex. bCorresponds to the 1390-cm-I vibrational feature from the neutral emission from the bNpOD.(ND,), complex. a
trans form is 353 cm-l higher than the cis form, whereas in a-NpOH the cis form is 912 cm-l higher than the trans form. In the case of a-NpOH, the high-energy cis form is not observed simply because it is not populated in the vapor immediately prior to matrix deposition. The Boltzmann population in the cis form is calculated to be only about 1% of that in the trans form in the room-temperature vapor from which deposition occurs. However, this is not the case in 0-NpOH. The trans form of 8-NpOH is only 353 cm-' higher in energy than the cis form, and therefore the cis:trans population ratio in the room-temperature vapor is about 6: 1. The ratio is maintained in the deposition of @-NpOD, but not in 8-NpOH. We speculate that trans-P-NpOH can rapidly interconvert to the lower energy cis-BNpOH as the molecule cools upon deposition into the matrix. This interconversion would occur by a tunneling process that would be far slower in 6-NpOD, resulting in negligible trans-to-cis interconversion. The observed deuterated &:trans ratio simply reflects the gas-phase populations. NaphthollAmmonia Complex Spectroscopy. The spectroscopy of matrix-isolated naphthol/ammonia complexes has been discussed in previous papers.*v3 These complexes are formed by deposition of an argon/ammonia/naphthol mixture onto a 10 K copper block, followed by matrix annealing. Complexes of increasing stoichiometry are formed as annealing proceeds. The sequential nature of the formation process facilitates assignment of the stoichiometry of the complex, giving rise to each peak observed in the spectra. The assignment of stoichiometries is made from matrix annealing studies, combined with the comparison of observed and simulated spectra. This procedure is described in detail in ref 2. The results for both a- and P-naphthol-(NH,),, complexes are summarized in Table I. Upon deuteration, there were no detectable spectral shifts in the fluorescent excitation (FE) spectra of the a-NpOD.(ND,),, (n = 1, 2, 3) complexes. In previous studies we showed that a-NpOH.(NH,),
