J . Phys. Chem. 1987, 91, 1400-1404
1400
Effect of Reactant Rotation on Reactlvlty: Comparlson of Exact Coplanar Results and a Model Calculation for H 4- H2 Howard R. Mayne* and Steven K. Minick Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824 (Received: August 18, 1986)
A simple classical model was proposed to study the effect of reactant rotation on reactivity for atom-diatom collisions. The model was able to reproduce trends recently reported in the reaction cross section for H + H2 and other systems. Stretching of the target diatom bond during the collision was found to enhance the effectiveness of rotation in promoting reaction. This suggested that rotation may promote reaction more readily on surfaces with late barriers. The model results were found to be particularly sensitive to the long-range anisotropy of the potential, implying that rotational excitation of reactants may be a means of probing that part of the potential.
Introduction The effect of reactant translation and vibration in gas-phase atom-diatom reactions has received considerable attention in the past few decades. (For a review, see ref 1.) Much less attention has been paid to the role of reactant rotation. The reason seems to be that changes in reactant rotational quantum number were believed to produce small changes in reactivity compared with the much more spectacular changes induced by translation and vibration. However, some recent experiments2 on direct reactions with a significant barrier to reaction have revealed that the effect of rotation may not be so small after all. (For a review of the field see ref 3.) On the theoretical side, recent trajectory studies4 on Li + HF(u=2j) showed rather startling (and seemingly discontinuous) changes in reaction cross section as j was increased. A more recent careful study of the same system by Loesch5 has revealed these discontinuities to be an artifact of the potential. Loesch’s results as a function of roshow that the reaction cross section, SG), tational quantum number, j , decreased at low translational energy but was roughly constant at higher energy. A recent very detailed trajectory study6 on the accurate7 LSTH potential for H H2 showed a complicated behavior for SrG): near threshold, S‘ decreased somewhat, reaching a minimum at j = 4, after which it increased rapidly. At higher energies, j increased the reactivity monotonically. Similar behavior has also been reported for H + C12.8 Loesch’s explanation5 for the results he sees in the Li + H F system is based on the anisotropy of the entrance channel. The highly anisotropic potential “steers” trajectories into the reaction “cone of acceptance”. However, the H H2 potential is rather isotropic at large atom-diatom distances, and it is difficult to imagine its being able to “focus” trajectories into the cone of acceptance at high j . The aim of this work is to attempt to gain some understanding of the role played in reaction by molecular rotation in general, and the trend noted in the H H2G) cross sections in particular. One of the greatest conceptual aids in the understanding of the effects of reactant vibration and translation on reactivity has been the collinear model. For this case, the dynamics can be intuitively , a skewed understood by following the trajectory, ( r ( l ) , R ( f ) )on
+
+
+
(1) Bernstein, R. B. Chemical Dynamics via Molecular Beam and Laser Techniques; Oxford University Press: New York, 1982. ( 2 ) Hoffmeister, M.; Potthast, L.; Loesch, H. J. Chem. Phys. 1983, 78, 369. (3) Sathyamurthy, N. Chem. Reu. 1983, 83, 601. (4) NoorBatcha, I.; Sathyamurthy, N. J . Am. Chem. SOC.1982,104,432. NoorBatcha, I.; Sathyamurthy, N . Chem. Phys. 1983, 77, 67. (5) Loesch, H. J. Chem. Phys. 1986, 104, 213. (6) Boonenberg, C. A.; Mayne, H. R. Chem. Phys. Lett. 1984, 108, 67. (7) Truhlar, D. G.; Horowitz, C. J. J . Chem. Phys. 1978,68,2466. 1979, 71, 1514E. Siegbahn, P.; Liu, B. J . Chem. Phys. 1978, 68, 2457. (8) Connor, J. N. L.; Jakubetz, W.; Laganl, A,; Manz, J.; Whitehead, J. C. Chem. Phys. 1982, 65, 29.
0022-3654/87/2091-1400$01.50/0
and scaled collinear potential ~ u r f a c e . ~The advantages of such a model are obvious. In lower dimensionality, calculations are cheaper, exact quantum calculations are feasible, and the sensitivity of effects to changes in the potential surface can readily be assessed. So far few such simple models for studying the effect of rotation have been available. L o e s ~ h using , ~ the rotational ) r fixed sliding mass model, maps the trajectory ( R ( t ) , y ( t ) with for a coplanar collision. This method provided great insight into the role of anisotropy in the entrance channel. In a recent study, MayneIo (adapting a model originated by Schatz”) considered a model system designed to simulate a coplanar collision with zero impact parameter in which the symmetric stretch vibration was considered adiabatic. In this model, reaction occurs along a reaction coordinate, s, proceeding from reactants (s = -m) to products (s = m). The only other degree of freedom is the angular motion of the atom with respect to the diatom center of mass. This motion corresponds to rotation of the diatom at large s and bending vibrations of the triatom near the transition state. The model is similar in spirit to that of L o e ~ c h but , ~ differs in detail-most significantly in that the diatom bond length is allowed to adjust adiabatically in our model, while it is fixed in Loesch’s. The model potential used there was unrealistic in that there was no barrier to reaction for the collinear (y = 0) geometry. In addition, the model does not explicitly conserve angular momentum. In this paper we correct both those deficiencies. In the next two sections we describe the model and the potential used. In the fourth section we compare the results of the model calculations with exact trajectory calculations carried out on the LSTH surface under conditions as close as possible to those of the model. Conclusions are in the final section. Model Hamiltonian The Hamiltonian for an A-BC collision can be written qyite generally as12 1 1 (1) H = PR’PR + Z P r ’ P r + V(r,R) ~PA,BC
where R is the displacement from A to the BC center of mass, and r is the BC bond displacement, pA,Bcis the translational reduced mass, and pBcis the diatom reduced mass. By decomposing the vectors into components parallel to and perpendicular to r and R, this can be written as H = (pR2+ I.I/R2) -(p1: j - j / r 2 )+ V ( r , R , y )
+
~PA.BC
+
2PBC
(2) (9) Polanyi, J. C.; Schreiber, J. L. Physical Chemistry-An Advanced Trearise, Vol. 6A, Eyring, H., Jost, W., Henderson, D., Ed.; Academic: New York, 1974; Chapter 6. (10) Mayne, H. R. Chem. Phys. Lett. 1986, 130, 249. (11) Schatz, G. C. Chem. Reu., in press. (12) Karplus, M.; Porter, R. N.; Sharma, R. D. J. Chem. Phys. 1965,42, 3259.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
Effect of Reactant Rotation on Reactivity
the collinear path. Values of m$ relative to its asymptotic value are given as a function of s in Table I. We note that at the saddlepoint, s = 0, m 3 has increased by 58%. We therefore model I ( s ) = mr2(s) with a Gaussian of the form I ( # ) = Z , ( 1 a exp(-(bs)*)), where I , is the moment of inertia for the isolated Hz molecule at its equilibrium geometry.
TABLE I: Ratio of Moment of Inertia of Diatom, mr2, to Asymptotic Value as a Function of Reaction Coordinate, s 0 0.5 1.o 1.5 2.0
1.76 1.57 1.50 1.46 1.42 1.40
m
+
1.58 1.26 1.15 1.09 1.03 1.oo
where I = R X p~ is the orbital angular momentum, j = r X pr is the diatom rotational angular momentum. The total angular momentum is J = I j. Substituting for I in ( 2 ) yields
+
H=
+ (J - j)2/R2) + -(p:1
1
-(pRz
2pBC
~PA,BC
+ j 2 / r Z )+ V(r,R,y) (3)
We now transform the terms in pr and pRinto natural collision
coordinate^^^*'^ 1
1
+ -pr2
-PR2
2pBC
~CLA,BC
1 = -(pS2/v2
~IJA*BC
+ pp2)
1401
(4)
wherep, is the momentum conjugate to s, the reaction coordinate which goes from --m (reactants) to (products), and pp is the momentum conjugate to p, the vibrational coordinate transverse to s. The term 9 = 1 k(s)p where k(s) is related to the curvature of the reaction path at the point s. We now introduce our first approximation. If we assume that there is zero curvature, then 9 = 1. If, further, we assume that there is no motion in p during the entire collision, then we can ignore the term in pp. This is equivalent to demanding that the trajectory remain on the minimum energy path during the course of the collision, that is, it assumes there is no “ b ~ b s l e d d i n g ” . ’ ~ The ~ ’ ~ validity of this assumption will be discussed in the Results section. Our Hamiltonian becomes QJ
+
We now introduce a further approximation to obtain a functional form for R(s,y). For collinear H Hz at the saddle point R = ( 3 / 2 ) r # , where r# is the bond length a t the saddlepoint. Since we constrain the trajectory to remain on the minimum energy path, this is the minimum value that R can attain. At the saddlepoint s = 0. As the system leaves the saddlepoint, whether or not it is reactive, R increases. We also ignore the dependence of R on y. Choosing a particularly convenient and simple form, we set Rz= Rhz + s2. (The actual results given here are very insensitive. to the particular choice of R2.) If we now restrict ourselves to planar collision geometries, then our final Hamiltonian is
+
Z:
where we have replaced /LA,BC with p , and pBc with m . The rotational quantum number j is a momentum variable conjugate to y. The term in ( J - j ) which forces conservation of angular momentum was not present in our earlier model ca1culation.l0 In that work, the moment of inertia of the diatom I = m$ was given a functional form in s. We found in that work that an increase in I from its asymptotic value was necessary to reproduce the trends seen in the H Hztrajectories. Since we intend to model our system using the LSTH potential surface,’ and since we restrict our motion to the minimum energy path we can obtain r as a function of s and y. To simplify the functional form we shall neglect the y dependence of r and give simply the value of r along
+
(13) Marcus, R. A. J . Chem. Phys. 1968, 45,4493. (14) Child, M. S.Molecular Collision Theory; Academic: New York, 1974.
Potential Function and Trajectory Calculations The potential energy function V ( s , y )is determined as follows. The profile along the collinear reaction coordinate (y = 0 or T ) is fitted by an Eckart function, with a maximum, E,, given by the classical barrier height of the LSTH surface,’ 0.42 eV. The potential dependence on y was then obtained by fitting the bending potential of LSTH at several points along the collinear reaction coordinate. We require that the potential be simple and flexible in form so that it can be changed for systematic studies of the effect of parameters. It was difficult to fit the entire angular range with a simple form. We therefore weighted the points near y = 0 more heavily in the fit. The sin2 y form chosen fits quite well up to y = 30 degrees, after which it always underestimates the potential. The model potential used was
V(s,y) =
E, cosh2 (cs)
B sinZy
+-cash (ds)
(7)
The values of the constants used were p = 1224.7 au, I , = 1800 au, a = 0.58, B = 0.67 eV, E, = 0.42 eV, c = 1.0 ao-’, d = 2.5 ao-’. Classical calculations were carried out for the model system by numerically integrating Hamilton’s equations of motion with s originally -3.0 ao, sufficiently large that the potential was zero. The total angular momentum, J , was held constant at the initial value of j . The only variable not fixed by the initial conditions was y,which was systematically varied between 0 and T in 50 equal increments. The reaction probability was calculated by plotting the finalj as a function of initial y and then refining the reactive-nonreactive bo~ndaries.’~That is, between the values of y where the dynamics changes from reactive to nonreactive we run a further four trajectories equally spaced in y. The probability of reaction is thus established to f 0 . 0 0 4 . To be able to compare our model calculations with exact trajectories on a realistic potential surface, it was necessary to decrease the dimensionality of the trajectories, in order that as few variables as possible had to be averaged over. Thus, trajectory calculations were carried out for the coplanar geometry, and with zero impact parameter. Furthermore, the trajectories were also run with no initial vibrational energy, in order that they remain as close as possible to the minimum energy path. To achieve this, we set the initial (classical) vibrational quantum number, u, of the H2molecule to -l/z: that is, there was no zero point vibrational energy in the molecule. Thus, under adiabatic conditions we would expect the system to remain on the local minimum energy path of the potential energy surface for a given bending angle, y. The phenomenon of “ b ~ b s l e d d i n g ” ’ will ~ ~ ’ ~of course induce some vibration in the products, but this is expected to be slight. (The justification of this assumption is discussed in the Results section.) We therefore believe that the trajectories experience a dynamical environment consisting only of reaction along the minimum energy path, together with the bending motion of the complex, as is used in the model. Results and Discussion In Figure 1 we show the LSTH trajectory results for the cocase. Here reactant planar, zero impact parameter, u = rotation increases S at all energies, although the initial increase is rather less for energies near threshold. Except for the disappearance of the dip near threshold, the results of Figure 1 are similar to those of the 3D data. In general, the greatest deviation from vibrational adiabaticity was found to occur for the j = 0 collinear cases, since here the ~~
~
(1 5 ) Wright, J. S.; Tan, K. G.;Laidler, K. J. J. Chem. Phys. 1976, 64, 970.
Mayne and Minick
1402 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 I
1.01
z "i e
O
8
O
Figure 3. Reaction probabilities for model at 1 a The error bars are about the size of the dots. 2
OO
0.45, 0.5, 0.6, and 1.0 eV.
10
4
' j Figure 1. Coplanar (zero impact parameter) reaction probabilities for H + H2(-1/2j)on the LSTH surface. Energies are in eV. The error bars are about the size of the dots.
\ ' \ \\
1 4
7-" 1 r-1
-*LSTH
, 0.6
0.8
t
1.0
Et ran s /e" Figure 2. Comparison of reaction probabilities for coplanar (zero impact parameter, u = -*/2) trajectories on the LSTH surface and model. Rotational quantum number, j , is zero. The error bars are about the size of the dots.
speed along the reaction coordinate is greatest, and bobsledding is most likely to occur. At 1.0 eV, the largest value of u'after a reactive collision was 0.8, with the mean value approximately 0.2. For 1.0 e V , j = 8, the maximum v'was 0.5,with the average value less than 0. At the lower energies, the maximum value of u' never exceeded 0 (e.g. for 0.7 eV, urmax= -0.23, for 0.6 eV, u',,, = -0.41). We therefore feel that, at least for energies less than 1.O,the trajectories remained approximately vibrationally adiabatic. That is, the major dynamics lay in motion along the reaction coordinate, s, and in bending motion, y, only. We now consider the results of the model calculations. First, in order to establish the accuracy of the model potential, we compare results at j = 0 using the model with those obtained by running trajectories (coplanar, zero impact parameter, u = -1/2) on the LSTH surface. The comparison is shown in Figure 2. Although there are differences-particularly at high energies-the results are in sufficiently good agreement to convince us that the potential experienced by the dynamics in both cases was similar. The agreement up to fairly high energies probably indicates that the width of the reaction valley is a good approximation to that of LSTH. We have compared our results using Hamiltonian (6) with the model suggested previouslyloin which the (J -j ) zterm was absent. For the case considered here, where the initial orbital angular momentum is zero, the difference between the results for the two models is negligible. This is not surprising since (Rfin2+ s2) > 6.97, making the denominator for the ( J - j ) * term much greater than that for the other terms. Thus the conclusions reached with the simpler modello are still valid here. In Figure 3 we show the reaction probabilities as a function of j for the same energies as used in Figure 1. The results are qualitatively very similar to those of Figure 1: At energies near
-n/2'
'
-1.5
..
I
-1
,
s/a,
I
- .5
I
,
0
Figure 4. Comparison of trajectories. A and B are trajectories for the model system; A has a = 0, B has a = 1. Trajectory C was run on the LSTH surface. The potential contours shown are for the entrance channel of the model potential.
threshold there is a slight dip and then a rise; at higher energies there is a monotone increase. The largest discrepancy between the model trajectories and the trajectories on LSTH occurs for 1 eV, where the model consistently overestimates the reaction probability. However, this is not surprising when we recall that the potential is least accurate (and too low) at the larger angles which the more energetic trajectories will sample. Nevertheless, considering the simple nature of the model and the potential used, the resemblance between these data and the full 3D calculation is striking. It is encouraging that the trends noted in 3D should be reproducible in a model, where the possibilities of understanding the detailed dynamics are so much greater. The reason that the reactivity at high j can increase above that of j = 0 is because more total energy is available at higher j . Thus more energy is available to be converted into motion along s. The potential alone, however, is more efficient at converting s motion into y motion. The crucial factor in the increase of P with j is the fact that Z(s) increases as the saddlepoint is neared. In Figure 4, we illustrate the effect of changing Z(s) for a single trajectory. Trajectory A is run with E = 0.5 eV, j = 4 on the model surface with a = 0 (constant r). As it approaches the saddlepoint, y increases due to the force acting in the y direction. The trajectory is deflected and cannot react. Trajectory B is initially identical with A, but has a = 0.58. The same force in the y direction acts on B, but because Z is higher near the saddlepoint, the acceleration in y is less, hence the trajectory is deflected less, and goes on to react. For comparison, we also show a trajectory C with identical initial conditions, but which was run on the LSTH surface (the trajectory has been reflected through the y = 0 line for clarity in the figure.) The resemblance between B and C is striking.
Effect of Reactant Rotation on Reactivity
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987
1403
d=3.5
d=2.5
Figure 5. Probability of reaction as a function of j and B. The lines are for translational energy 0.5 eV, the upper lines are for 1.0 eV. The value of B is indicated in eV. Note that for 1.0 eV, all the B = 0.54 results are unity.
The effect of changing the parameter a was investigated in the earlier studylo on this system in which there was no barrier to reaction. It was seen there that for a = 0 the Pro.) decrease for translational energies near threshold, and at higher energies remain almost constant. For negative a the decrease near threshold is even more dramatic. This was also found to be the case for all model trajectories we ran on other model surfaces (unpublished). In no case could we obtain an increase in PO.)unless the moment of inertia of the diatom increased as the barrier was approached. It is interesting to compare these findings with Loesch’s results.5 Using the sliding mass model5 with constant r, he finds trends very similar to our u = 0 data. Since the increase in Z(s) seems to be the one vital element for rotation to be able to enhance reactivity, it may be possible to predict how the position of the barrier to reaction governs its behavior in this respect. For instance, for a system with a “late” barrier? i.e. (roughly speaking) one where the barrier occurs in the exit valley, then r increases considerably from its asymptotic value by the time the barrier has to be overcome. On the other hand, for an “early” barrier? where (again, roughly speaking) the barrier is in the entrance channel, r is much closer at the barrier to its asymptotic value. One might, therefore, predict, on the grounds of the Z(s) behavior alone, that rotation would enhance reactivity more for a late barrier than for an early one. Similar conclusions were reached by Polanyi and co-workers.16 We now investigate the sensitivity of PO.)to other factors. If we increase or decrease B, the strength of the bending potential (eq 6 ) , we should expect the reactivity to decrease or increase accordingly. That this is indeed the case is shown in Figure 5 , where we consider values of B = 0.54 and 0.82 eV, as well as 0.67 eV which was used in the previous calculations. More interesting is how the trend varies with translational energy. At 0.5 eV, where most of the reactive trajectories sample the region near y = 0, the change in reactivity with changing B is slight, reflecting the broadening or narrowing of the y valley near the saddlepoint. It is also clear that the effect is greater for high j . The differences between the Pro)with B at high translational energy are much more pronounced, even at low j . This is really an artifact of the particular potential form chosen, since at high energy we sample rather large y values, where we expect the difficulties noted in the section on fitting the potential to be apparent. Much more revealing is the behavior of PO.)with d, the range of the potential. Contour plots of the potential for d = 1.5 and 3.5 a i l , together with the d = 2.5 ao-I used in the previous calculations, are given in Figure 6 . It can be seen that the smaller d is, the longer ranged the anisotropy of the potential is. Trajectory results for PO.)on these surfaces are given in Figure 7 . The results for d = 3.5 ao-’ are identical with those for d = 2.5 ao-’, indicating that slight changes in the anisotropy of a short-ranged (16) Blackwell, B. A.; Polanyi, J. C.; Sloan, J. J. Chem. Phys. 1978, 30, 299.
d= 5I.
-1 wa, 0 Figure 6. Contour plots of the entrance channel (s < 0) part of the potential energy function with values of d = 1.5, 2.5, and 3.5 sol. The potential contours are (a) 0.82 eV, (b) 0.54 eV, (c) 0.27 eV, (d) 0.14 eV. !
I
1.5,
----
__
i
2.5
-e--
O’
I
1
I
2
I
6
i
a
Figure 7. Reaction probabilities as a function of j and d . Lower lines are for translational energy 0.5 eV, upper lines are for 1.O eV. The values of d are indicated in ao-’. The results for d = 3.5 aC1are identical with those for d = 2.5 a0-l.
potential are not crucial. The long-range anisotropy, on the other hand, while much weaker than in the Li H F case,5 is extremely important. At high energy (1 .O eV) increasing the range of the potential increases reactivity for low j , but has little effect at high j . This is due to a long-range weak focussing effect toward the y = 0 valley. This focussing effect is also present for j = 0 at low energy (0.5 eV) where the reactivity is likewise enhanced. However as j increases further, the reactivity begins to decline. This can be understood by considering trajectories such as those shown in Figure 4. As the trajectories move across the (s,y) plane, they are deflected by the potential in the direction of increasing y. The larger the initial y component (Le. the larger j ) , the more marked the effect. The longer the time the potential has to act on the trajectory, the greater will be the deflection. Thus, there is a considerable drop in reactivity with increasingj for the longer range potential, d = 1.5 ao-’.
+
Conclusions We have proposed a model system to help us understand the trends seen in recent trajectory calculations6Ss on the effect of rotation on reactivity. In order to test the model, we ran coplanar classical trajectories on the LSTH potential surface for H + H2
1404
J . Phys. Chem. 1987, 91, 1404-1407
with zero impact parameter, and with the diatom vibrational quantum number, u, initially set equal to to suppress vibration in the molecule. These results showed the same trends as the 3D calculations. The model, using a potential energy function similar to the LSTH near the saddlepoint, was able to reproduce these trends successfully. Near threshold Pro.) for low j may increase slowly, or even decrease, since motion in the y coordinate tends to deflect the trajectory away from the saddlepoint. At higher values ofj, however, the extra energy available due to the rotation makes more of the valley in the saddlepoint region of the bend potential energetically accessible, and reaction occurs readily. For higher translational energies, the ability of the potential to deflect trajectories is reduced, and increase in rotation always enhances reactivity. These trends have been seen in 3D studies for H H: and H Cl,.*
+
It was found to be necessary that the moment of inertia of the target diatom increase as the saddlepoint is approached. This led us to predict that rotation would enhance reactivity most effectively in systems with a late barrier to reaction. The results were found to be sensitive to changes in the bending potential. A softer bend obviously allows reaction to occur more easily. More interestingly, the results were found to be sensitive to the long-range anisotropy of the bending potential. It therefore seems possible that rotationally excited reactants could be used to probe this part of the potential surface. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support to H.R.M. of this research. Registry No. H, 12385-13-6; H,, 1333-74-0
+
Excited-State Intramolecular Proton Transfer in Jet-Cooled 2,5-Bis( 2- benzothiazolyl) hydroquinone N. P. Emsting,* A. Mordziiiski,+and B. Dick Max-Planck- Institut fur biophysikalische Chemie, Abteilung Laserphysik, 0-3400 Gottingen, Federal Republic of Germany (Received: August 26, 1986)
The fluorescence of 2,5-bis(benzothiazolyl)hydroquinone was studied for isolated molecules cooled in a supersonic free neon jet. It consists of a strongly Stokes-shifted fluorescenceband, and it is assigned to the molecule which is formed by excited-state intramolecular proton transfer. The fluorescence excitation spectrum shows a progression in a 1 14-cm-’ vibrational mode of the excited state. The observed spectrum is more congested than that of the parent oxazole under otherwise identical experimental conditions. It is concluded that the thiazole has no significant intrinsic barrier to excited-state intramolecular proton transfer. Weak vibronic bands were observed by fluorescence excitation in the adjoining long-wavelength spectral region. They could indicate “nonvertical” transitions to the proton-transferred excited molecule.
Introduction Aromatic molecules having a phenolic hydroxyl group with an intramolecular hydrogen bond to a nearby heteroatom of the same chromophore often show fluorescence with an anomalously large Stokes shift. This fluorescence arises from the product of an excited-state reaction, namely excited-state intramolecular proton transfer (ESIPT, for example, see Figure 1). The renewed interest in ESIPTIw3concerns mainly the mechanism and rates of the reaction in solution. It was shown that ESIPT usually has rates greater than 10” s-l, independent of H / D exchange. Some investigators suggest that the reaction occurs partly from the hot, vibrationally unrelaxed m o l e c ~ l e s . The ~ ~ ~established conclusion is that ESIPT has no significant energy barrier. The reaction in solution may be complicated by specific Hbonding solvation or by the existence of distinct conformers having different internal H-bonding sites. Thus it is often difficult to separate the intrinsic ESIPT reaction from solvent effects.6.’ Some of these problems may be overcome by the study of cold, isolated molecules in supersonic jets or in noble gas matrices. Fluorescence excitation studies of methyl salicylate8-10 and 1,5-dihydroxyanthraquinonei1 have shown three main results. First, the intensity of the electronic origin band is relatively low, and long FranckCondon progressions involving a low-frequency mode of the excited state are found. Second, a strong increase of fluorescence intensity and spectral density is observed if the excess vibrational excitation energy is increased above some threshold value. Third, the fluorescence lifetimes are identical across the entire emission spectrum, and the rise time of the tautomer fluorescence could ‘On leave from the Institute of Physical Chemistry of the Polish Academy of Sciences, 01-224 Warsaw, Poland.
0022-3654/87/2091-1404$01.50/0
not be resolved when excitation pulses with duration of 15 ps were used. It was concluded that the absorption of a photon leads directly to an adiabatic excited state which is formed by a strong H-bonding interaction between the original and proton-transferred excited forms.* A different situation exists for 2,5-bis(2-benzoxazolyl)hydroquinone (Figure 1, with both S exchanged for 0). A blue fluorescence band and a red fluorescence band were observed in ~ o l u t i o n . ’ ~ ~Contrary ’~ to previous cases, a marked dual fluorescence was also found for the jet-cooled molecule excited near the electronic origin.14 The ratio of the quantum yield for red fluorescence to the quantum yield for blue fluorescence de~
~
~~
(1) KlBpffer, W. Adu. Photochem. 1977, 10, 317. (2) Huppert, D.; Gutman, M.; Kaufmann, K. J. Adc. Chem. Phys. 1981, 47, part 2, 643. (3) Flom, S . R.; Barbara, P. F. J . Phys. Chem. 1985, 89, 4489. (4) Barbara, P. F.; Rentzepis, P. M.; Brus, L. E. J . Am. Chem. SOC.1980, 102, 2786. ( 5 ) Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. J . Am. Chem. SOC.1980, 102, 5631. ( 6 ) Sandros, K. Acta Chem. Scand., Ser. A 1976, 30A, 761. (7) McMorrow, D.; Kasha, M. J . Phys. Chem. 1984, 88, 2235. (8) Goodman, J.; Brus. L. E . J . Am. Chem. SOC.1978, 100, 7412. (9) Felker, P. M.; Lambert, Wm. R.; Zewail, A. H. J . Chem. Phys. 1982, 77, 1603. (10) Heimbrook, L. A,; Kenny, J. E.; Kohler, 8 . E.; Scott, G.W. J . Phys. Chem. 1983, 87, 280. (1 1) Van Benthem, M. H.; Gillispie, G. D. J . Phys. Chem. 1984,88,295. (12) Mordzinski, A,; Grabowska, A,; Kiihnle, W.; Krowczynski, A. Chem. Phys. Lett. 1983, 101, 291. (13) Mordzinski, A,; Grabowska, A.; Teuchner, K. Chem. Phys. Lett. 1984, I l l , 383. (14) Ernsting, N. P. J . Phys. Chem. 1985, 89, 4932.
0 1987 American Chemical Society
