J. Phys. Chem. 1993,97, 12485-12490
Polanyi Rules for Ultrafast Unimolecular Reactions: Simulations for HCo(C0)4( ‘E) * CO(C0)4+
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C. Daniel’ and M.-C. Heitz Laboratoire de Chimie Quantique, UPR 139 du CNRS, Universitt Louis Pasteur, 4, Rue Blaise Pascal, F-67070 Strasbourg, France
L. Lehr’ and J. Manz Institut fur Physikalische und Theoretische Chemie, Freie Universitiit Berlin, Takustrasse 3, D - 141 95 Berlin, Germany
T. Schrader Max-Planck-Institut fur Stromungsforschung, Bunsenstrasse 3, 0-37073 Gottingen, Germany Received: May 28, 1993’
The traditional Polanyi rules for control of bimolecular reactions by selective investment of energy, e.g. preferentially translational, not vibrational energy for early-downhill reactions on attractive potential energy surfaces, are extended to ultrafast unimolecular reactions. Specifically, we consider photodissociations of the metal-hydrogen bond of HCo(C0)4( lE), occurring on a time scale of approximately 20 fs, much faster than competing intramolecular vibrational energy redistribution (IVR). Here the reaction path toward the products H C O ( C O ) is~ hindered by a barrier located in the H Co(CO)4 exit valley of the potential energy surface. In order to overcome this barrier and, therefore, to increase reactivity, vibrational energy should be invested selectively into the bond to be broken, i.e. [H-Co], not into complementary “spectator” modes, e.g. [CO-Co]. The required energetic preparation of reactants may be achieved by selective IR UV two-photon excitations or by alternative techniques including frequency-selective UV single-photon photodissociation. The Polanyi rules for unimolecular reactions are demonstrated by fast Fourier transform (FFT) propagations of representative wave packets.
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1. Introduction
The purpose of this paper is to extend the traditional Polanyi rules from bimolecular’.*reactions to ultrafast unimolecular ones. These rules provide a key to the control of branching ratios of reactive versus nonreactive processes by selective investment of energy into the reactants. Accordingly, in the traditional case of bimolecular collisions, one distinguishes between early versus late barriers on the potential energy surface, which are located in the exit valleys toward reactants and products. Overcoming of early barriers requires investment of translational energy into the reactants, whereas selective vibrational excitation is needed for the crossing of late ones. Both rules may be reduced to a unique condition: The reactants should be prepared selectively with momentum along, not perpendicular to, the reaction path, as the reactants approach the potential barrier toward the products. For a recent survey, see ref 3. As our working hypothesis, we assume that the same, generalized Polanyi rule should also apply to unimolecular reactions, provided they proceed on an ultrashort time scale, typically less than lo0 fs. The restriction to ultrafast unimolecular reactions is essential: otherwise selectivelyinvested energy would be dissipated via relaxation processes, e.g. vibrational energy redistribution (IVR),before the reactants have a chance to approach the energy barrier on their path toward products. t Dedicated to John C. Polanyi on the occasion of his honorary doctoral degree from Freie UniversitHt Berlin, March 2, 1993. *Abstract published in Advance ACS Absrracrs. November 1, 1993.
In order to verify the generalized Polanyi rule in a prototype unimolecular reaction, we consider laser control of the decay of HCO(CO)~(~E)*, which can be reactive or nonreactive:
hv
HCO(CO)~(’A~)
HC~(CO),(~E)*
(1 a)
+ Co(CO),(‘E)*
(lb)
d y v H
HCO(CO)~(’E)* HCO(CO)~(’E)
(IC)
Initial photoabsorption-either by continuous wave (cw) lasers or by short laser pulses-prepares electronically (‘E) and vibrationally (*) excited reactants (la), which may dissociate directly into H Co(CO), (lb) or relax via IVR (IC), followed by sequel processes such as intersystem crossing (ISC) to HCo(C0)4(3E)and subsequent dissociation toward the products HCo(CO)3 CO. The branching ratio of the two channels 1b and IC should depend on selective preparations of reactants. We consider this type of laser control of the prototype reaction 1, because it is both a rather simple and instructive example for the validity of the Polanyi rules for unimolecular reactions and because it plays an important role as the primary step of the catalytic cycle of oxosynthesis;4 correspondingly, it has been examined in a number of experimental5 and theoretical studies.a Here we center attention on the initial steps la-lc; for a detailed analysis of the sequel processes, see ref 9. More specifically,we investigate the effects of different energetic preparations of the reactant HCo(C0)4(1E)* on barrier crossings toward dissociated products (1b), in order to confirm the generalized Polanyi rule.
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QO22-3654/93/2097-124855Q4.QQ/Q 0 1993 American Chemical Society
12486 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
A brief summary of the methods is presented in section 2. The results and conclusions are in sections 3 and 4, respectively.
2. Model and Methods For our demonstration of the Polanyi rules for the unimolecular reaction 1, we employ a simple, pseudotriatomic model of HCo(CO)4(1E)*in terms of two coordinates 40,qb for the metalligand bonds, q, = [H-Co] and 46 = [CO-Co],,, respectively. Allother modes are assumed to bedecoupled. This approximation is reasonable for the ultrafast dissociations ( i d i s I100 fs; see below), much shorter than competing IVR processes (typically, TIVR L 100 fs'o). Similar simplistic, two-dimensional models have been used successfully for instructive, semiquantitative simulations of photodissociations of other polyatomic molecules; see ref 11. The evaluations of selective preparations of initial states l a and the subsequent specific molecular reaction dynamics l b and I C employ the potential energy surfaces (PES) MAl(q,, q b ) and VlE(q0,qb) for the electronicground ('AI) and excited ('E) states. These PES are adapted from refs 6-9. In order to simulate selective investments of energy into reactants, we assume that the initial HCo(C0)4('E)* is prepared by IR + UV two-photon absorption of ground-state HCo(C0)d('AI) molecules, as in refs 8,9, and 12-19; see also the conclusion section. In our model, the IR photons serve to excite HCo(C0)d('AI) selectively into vibrational states with essentially u, plus U b quanta in modes q, and 46. The corresponding wave functions are denoted by (qa,qbl'Al,UaUb) E * I A , , u y b ( ~ & 6 )
(2a)
and they are evaluated as eigenstates for the PES VIAl(q&,) by expansions in terms of the Morse oscillator wave functions for the coupled bonds q, = [H-Co] and 46 = [CO-Co],,, as in refs 8, 18, and 19. In practice, we consider only local modes where most of the vibrational energy is stored in a single bond, q, or 46; thus u, # 0,06 = 0 or u, = 0,ub # 0. For reference, we also consider the vibrational ground state, u, = 0 , U b = 0, to be excited by UV single-photon absorption. Next, the UV photons induce the transitions of the groundstateor vibrationally excited H C O ( C ~ ) ~ ( ~ A I , U molecules , $ ~ ) from the 'AI electronic ground to the 'E excited state. In accord with the Franck-Condon (FC) approximation, we assume vertical transitions between the relevant PES VIA^ and VIE,and we neglect any variations of the electronic transition dipole function in the FC region. As a consequence, the vibrationally excited wave packets 2a represent our selective initial states for the subsequent unimolecular dynamics of the HCo(C0)4('E) molecules. For convenience, the corresponding wave packets \klE~#~(q,,,qb,f) are also labeled by the initial vibrational quantum numbers u,, 0 6 , indicating selective investments of vibrational energies, thus (qo,qbl'E,uaub,t=O)
E
*i~,~~b(q,,q6,t=o)
(2b) = ( q,*qbllA1.uyb) The time evolutions of the wave packets *lE&#b(q,&brt) on the relevant PES ViE(q,,,qb)are evaluated by fast Fourier transform (FFT) propagations, as in refs 8, 9, 11, 12, and 17-21, subject to the initial conditions (2b). The results are presented as snapshots showing equidensity contours
Daniel et al.
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(IE,u,$b)* to dissociative products H CO(CO)~, or via inelastic IVR processes to vibrationally relaxed reactant molecules HCo(CO)4(IVR). Below, we shall center attention on control of the branching ratio for the dissociative and IVR channels, depending on the reactant preparation (U&,). The branching ratios are evaluated as follows: First, inspection of the snapshots (3) of *lE~&~(q,&,,t) and of the corresponding autocorrelation function (4) at sufficiently long times (typically t 1 20 fs, see below) indicates the separation of the wave packet into two orthogonal parts, representing the dissociative and IVR channels, l'E,U,Ub,f) = I'E,u,ub,diss,r)
+ l'E,u,ub,IVR,t) =0
( 'E,U,U,,diSS,tl'E,U,Ub,IvR,t)
(5) (6a)
Relations 5 and 6a are general, including also special, limiting cases where one of the two channels may be entirely dominant, e% *.1E.u#b(q0,q6,t) * *iE,uyb,diss(4.,4brt), and *lE,upaIVR(q&brt) = 0. Second, it is helpful and instructive to evaluate the overall time evolutions of the dissociative and IVR parts of the overall wave packet, simply by propagating *lE,v#adisr(qa,q6rt)r and separately \kiE,u#aIVR(qo,qbrt)r first backwards ( t 0) and then forwards (O+ t- "m") in time. These two "partial" wave packets are always orthogonal, in particular for t = 0, due to the unitarity of the time evolution operator U ( t ) ,
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( 'E,u,~~,di~~,tl'E,u,u~,IVR,t) =
(U(t)'E,u,u,,diss,t =O~U(t)'E,u,v,,IVR,t=O)
= ( 'E,U,U~,~~SS,~=O~'E,~,U~,IVR,~=O) = 0 (6b) As a consequence, the normalization of the overall wave packet allows the time-independent definition of two probabilities P(-dissllE,u,ub) and P(-IVR('E,upb) for the dissociative and IRV channels, 1 = ('E,U,Ub,fl'E,U,Ub,t)
+
= ( 'E,u,u,,diss,t~'E,u,ub,diss,t)
( 'E,u,u,,IVR,tJ'E,U,U,,IVR,t)
P(+dissI'E,u,ub)
+ P(+IVRI'E,U,u,)
(7)
Thirdly, the branching ratio for processes 1band 1c is evaluated in terms of the corresponding probabilities for the dissociative and the IVR channels, as defined in eq 7. Using self-explanatory notations, we obtain [+di~~I'E,u,u~] :[+IVRI'E,U,U~] = P(-+dissl' E,u,u6):P(+IVRI1 E,u,u6) (8a) In addition, it is also instructive to evaluate the individual contributions of the dissociative and IVR parts of WE,"#, to the autocorrelation functions, i.e. ( 'E,u,u,,t=O('E,u,u,,ch,t) =
, f , f d q a dqb * * ' E , ~ y b ( q a * q b ' ~ = ~ )*lE,uy,.~h(qa,qb,~) (9)
or by (absolute values of) autocorrelation functions
for the two channels, "ch = diss" or "ch = IVR". In particular, the branching ratio 8a may be re-expressed, using relations 6 and 7 in terms of the initial values of these partial autocorrelation functions ( 9 ) ,
( 'E,U,Ub,t = OI'E,U,Ub,t?O)
[+dissI1E,u,u6]: [+IVRI1E,u,ub]
PlE,u#b(qa,qb,f)
,f ,fdq,
= I*'E,yDb(q,&6,t)12
constant
(3)
=
dq6 * * l E , ~ y b ( q d ? b * ~ ' ~ ) * ' E , ~ y b ( q a d ? b , ~ ~ O )(4) *lE##b(q,&rf) represent the entire molecular reaction dynamics of the competing processes 1band IC,from reactants HCO(CO)~-
( 'E,u,u6,t=O11E, u,u6,diss,t): ( 'E,u,u6,t =OI'E,u,u,,IVR,t)
(8b) All calculations are carried out in atomic units with conversion
Polanyi Rules for Ultrafast Unimolecular Reactions
The Journal of Physical Chemistry,
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Figure 1. (a) Time evolution of the wave packet *IE,W(q,&,f). where q. and are the [H-Co] and [CO-Co] bonds, respectively, of HCo-
(CO)d(lE). Snapshots of the wave packets are shown by equidensity .02 i = contours, l*1E.W(q#,qb9t)1* = max ~ \ k l ~ ~ ( ~ . , ~ b , f ' o ) ~ *+( oO.12i), 0-8, superimposed on equipotential contours V'E(qa,qb,t) = (-0.6 + 0.017i)Eb,i = 0-5. The branching of *lE,W(q.,qb,t) intodissociative and IVR parts simulate the competing processes HCO(CO)~(~E,OO) H + (b) Autocorrelation functions Co(C0)r versus HCO(CO)~(~E,IVR). (*'l&OOf=01*lE,W*f)v(*~I?,OO,~=Ol*lE,W,diu,~) and (*.~E.W.~'~~*%,?~.IYR,~) for the total wave function *iE,OI)(q,,,qb,t) and its dissociative (dm) and IVR parts, presented by absolute values versus time. The initial values of the dissociative and IVR parts yield the corresponding branching ratio, e.g. 0.36:0.64 for PIE,^.
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factor h/Eh = 2.419 X 1O-l'~. The spatial and temporal grids for the wave packets are Aq. = A q b = 0 . 1 ~ 0and , At = 0.1 h f Eh. These rather coarse grids suffice for accurate FFT propagations of wave packets and for evaluations of overlap integrals such as (9) by the trapezoidal rule. Needless to add, the grid also determines the rather low resolutions of the graphical representations 3.
3. Results In order to demonstrate the Polanyi rules for unimolecular reactions, we present exemplarily our model simulations for reaction 1, starting from four different preparations of initial states. For reference, let us first consider the case of reactants with zero vibrational quanta, u. = ub = 0. The time evolution of the corresponding wave packet *lE,@)(q&,,t) is illustrated in Figure l a by snapshots (for additional snapshots taken at different times t , see ref 9). Apparently, the initial Franck-Condon transition puts *1E,00(q&b,t'O) on the steep repulsive slope of the PES. From there, it is accelerated toward the potential well of VIE(17.46). As a consequence, it gains momentum along and
60 timeifs
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Figure 2. Time evolutionand autocorrelation functionsof *lE,ol(qu,qb,t). The notations are as in Figure 1.
perpendicular to the reaction path 5. As a consequence, the wave packet *lE,00(qa,qb,t) separates into two parts, *'E,00,diU(q0rqb,f) and \klE,00,IVR(q.,qb,t). The former has sufficient momentum along 5 in order to cross the potential barrier of &(qa,qb) toward dissociative products H C O ( C O ) ~in, contrast to the latter which remains in the strong interaction region of V'E(q&b). There it starts to oscillate along both q. and q b coordinates; i.e. internal energy is redistributed over the vibrational modes of our model (IVR). The vibrations along q. and q b are difficult to perceive from the snapshots of the wave packets shown in Figure l a (and in similar figures below), but they may be recognized more easily by drawing additional lines of reference; e.g. 4 6 = 4a0 or 4. = 3.2~0 close to the local minimum of the potential energy surface. The molecular reaction analysis of *iE,@)(qu,qb,t), Figure la, is confirmed by a plot of the autocorrelation functions 4 and 9 in Figure lb. I( lE,OO,t=OJiE,OO,diss,t)l showsaveryrapid (within less than 20 fs) decay to zero, in accord with ultrafast dissociation into the products H C O ( C O ) ~ . In contrast to that, I( lE,OO, t=O1lE,OO,IVR,r)l shows a rapid but incomplete decay, followed by oscillations, in accord with IVR. The branching ratio is determined from the autocorrelation functions, see eq 9 and Figure 1b. Accordingly,
+
+
[-.diss)'E,OO]:[-.IVRI'E,OO] = 0.36:0.64
(loa)
for reactants H C O ( C ~ ) ~ ( ~ E , U . = O , U ~ = ~ ) . Next, let us consider the effect of additional vibrational energy, invested into the bond = [Co-co], of reactants HCo(C0)d(lE,Ua=O,Ub= 1). Thecorresponding molecular reaction dynamics (1) are simulated by the representative wave packet *lE,oi(q&brt)r which is illustrated in Figure 2a by snapshots and in Figure 2b
Daniel et al.
12488 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
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Figure 3. (a,b) Time evolution and autocorrelation function of the total wave function *1&20(q&q,,t). The notations are as in Figure 1. (c) Vibration of the metal-carbonyl bond Co-CO. The value of the corresponding coordinate q b is determined from the maximum of the absolute value of the nondisswiative part of the wave function. (d) “Norm” of *IE,2O(q&,,?), evaluated as integrated density in the domain of propagation (see a). The decay of the norm indicates the flux of the wave packet out of the domain of reactants toward products. The structure of the decay curve indicates alternation of dissociative versus nondissociative periods, which correlate well with the structures seen in band c, in accord with the sequential dissociation mechanism: see the text.
by autocorrelation functions. Comparison with Figure la,b yields entirely analogous behavior of *1E,W(q&6,f) and \klE,01(q&,,f) qualitatively and even quantitatively; Le. the branching ratio 10a is noted effected, and thus
corresponding autocorrelation function in Figure 3b. Obviously, already two vibrational quanta in bond qr invert the branching ratio in favor of product channel 1b; instead of (loa) and (1Ob) we obtain
[-~liss)~E,01]:[+1VRI~E,Ol] = 0.36:0.64
[+dissl’E,20]:[+IVR11E,20]= 0.98:0.02 (1Oc) The wave function \k’lE,zo(q&b.t) proves to be almost completely dissociative, implying perfect control of reaction 1. A more detailed analysis of the time evolution of ~IE,20(q&6,t) reveals a fascinating sequential mechanism of the dissociation process: First, the original wave function separates into two parts for direct dissociation, plus “temporary IVR”. Then the temporary IVR fraction separates into another dissociative part plus a smaller fraction representing temporary IVR. Afterward the remaining temporary IVR part separates again, etc. The time between subsequent dissociations correlates with the period of the CoCO vibration; see Figure 3c,d: Apparently, this vibration drives the part of the wave packet representing temporary IVR first
(lob)
for HCo(C0)4(’E,ua=O,~~= 1). The inefficiency of vibrational energy investment into bond qb = [Co-CO], is explained readily in terms of the Polanyi rules: Bond 46 acts as a spectator mode perpendicular to the bond to be broken, qo = [Co-HI. Vibrational excitation of bond will, therefore, increase momentum along 46, i.e. perpendicular to, not along, the reaction path. Therefore, it cannot help crossing the barrier toward products H CO(CO)~. Let us now turn to the effect of vibrational energy invested into bond qcr= [Co-HI, the bond to be broken. Exemplary results for reactants HCo(C0)4( 1E,uo=2,ub=O) are shown by snapshots of the wave function \klE,20(q&q,,f) in Figure 3a and by the
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Polanyi Rules for Ultrafast Unimolecular Reactions
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The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12489
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The branching ratio
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[+dis~l’E,05]:[+IVR~~E,05] = 0.39:0.61 (10d) is also similar to the corresponding values given in (loa) and (lob): Apparently, even five vibrational quanta invested into spectator modes are nothing but a waste of energy, without any meaningful effect on reactivity. Finally, we point to an important effect which is documented in all Figures la, 2a, and 3a but most prominently in Figure 4a: The dissociative wave packets *lE,upbdias(q.,qb,t) have ub nodes along coordinate qb, and this nodal pattern appears to be conserved during the dissociation process. For example, \klE,O5,disr(qcr,qbrt) starts out from five nodes along q b , and these are still visible in the final snapshot shown in Figure 4a. As a consequence, both the reactants and the nascent products have the same number of quanta in the spectator modes. Dissociation 1b is therefore much faster (120 fs) than competing IVR IC between the promoting and spectator modes. This observation may be considered an a posteriori confirmation of the important prerequisite of the Polanyi rules: they apply to ultrafast reactions, on a femtosecond time scale. 4. Conclusions
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The present demonstration of the Polanyi rules for the unimolecular reaction 1 is entirely analogous to previous verifications for bimolecular reactions.’-3 In both cases, selective investment of energy into promoting modes yields enhanced reactivity, whereas spectator modes provide useless sinks of reactant energy. More specifically, the present unimolecular reaction 1 may be compared with a “late-downhill” bimolecular reaction of the type A + BC AB + C: In both cases the reactants have to cross a potential barrier which is located on the reaction path in a “late” position, between the strong interaction region and the product configuration. Crossing these barriers calls for momentum in the bonds to be broken: By analogy, these are the promoting modes in both the bimolecular and in the unimolecular systems. As a consequence, the bimolecular reaction is supported by vibrational excitation of the reactant BC, not by an increase of translational energy of the collision partners A + BC.1-3 Likewise, unimolecular dissociation 1b profits from vibrational excitation of the metal-hydrogen bond, not by excitations of any other bonds, e.g. [Co-CO]. Extending the analogy, both bi- and unimolecular reactions should be ultrafast, i.e. either reactive molecular collisions or photodissociations which proceed on a femtosecond time scale (5100 fs), faster than competing IVR processes. With this prerequisite, the Polanyi rules suggest efficient control of bi- and unimolecular reactions by selective excitations of promoting modes. The present simplistic, two-dimensional model simulation has been instructive, yet it has to be confirmed by extensions to more realistic, multidimensional models, ultimately by experimental verification. We anticipate some modifications of the present quantative results, e.g. of the values of branching ratios lOa-d, whereas the qualitative results should not be affected, e.g. similar branching ratios irrespectiveof the excitations of spectator modes (cf. eqs lOa,b,d), in contrast with strong enhancement of the dissociative channel -H C O ( C O ) upon ~ excitations of local H-Co vibrations (cf. eq 1Oc). In general, effects of IVR will be more prominent in multidimensional models than in twodimensional ones. For example, IVR could start to compete with the sequential dissociation mechanism discovered for HCo(C0)4(lE,u,=2,ub=0), cf. Figure 3, at least during later times t 2 100 fs, and this would reduce branching ratio 1Oc for the dissociative versus IVR channels from its present value of approximately 0.98:0.02 to smaller values. An important prerequisite for achieving a high yield of dissociative products are favorable times scales for the two competing processes: In the present case, the frequency of the vibration along the dissociative coordinate qo is much higher than the frequency of the vibration
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Figure4. Time evolution and autocorrelation functionsof qlE,oS(q.,qb,f). The notations are as in Figure 1.
away from and then back to the favorable exit valley toward products H + CO(CO)~, thus switching off and on the dissociation process, respectively. Similar “sequential” dissociation processes are described in ref 22. The large effect of vibrational energy invested selectively into bond q. = [Co-HI may also be explained easily in terms of the Polanyi rules: q. acts as a promoting mode toward the desired dissociation, +H + Co(CO)4. Additional energy in q. increases momentum along the’reaction coordinate, supporting the reactants’ approach to and crossing of the barrier toward products H + Co(CO)4. One might argue that the preceding example wave packets, *‘IE,W(qa,qb,r)r *‘E,OI(qrrrqb,f), and *‘E,20(qmqb,t), did not Provide a sufficient proof of the Polanyi rules, since they represent reactant states with increasing energies EIE,OO IE I E , O I 5 ElE,20. The enhanced reactivity of *IE,20(q.,qb,~), in comparison with \ ~ ~ E , o o (f&,qb,f) could then just be due to the increasing overall energy in the reactants and would thus have to be considered a statistical effect, not mode selectivity. This hypothetical counterargument is rejected, however, by systematic studies of alternative preparations of reactants: selective excitations of local modes, i.e. HCo(C0)4( lE,u.>O,ub=O)r always support the dissociative product channel, whereas selective excitations of the local spectator modes, Le. HCO(CO)~(IE, us=O, ub>O), have negligible effects, irrespective of the total energy of the reactants. As confirmation of the general rule we present, as our final example, the result for reactants HCo(C0)4( lE,05): The time evolution of the wave function, illustrated by snapshots of \ k l ~ , 0 5 (q.,qb,t) in Figure 4a and by autocorrelation functions 4 and 9 in Figure 4b, is entirely similar to analogous behaviors of \~Q,oo( q & b , f ) and *lE,OI(q.,qb,?) shown in Figures 1 and 2, respectively.
+
12490 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
Daniel et al.
References and Notes
along 46 which induces IVR. As the wave packet slowly vibrates along q b , it moves to the right position close to the exit toward dissociation, and there one of many vibrations along q,, will be at the right time to cause ultrafast (-20 fs) dissociation. The present results should be stimulating for experimentalist^;^^ e.g. they should verify the predicted increase of dissociative products 1b upon excitations of the metal-hydrogen bond. In principle, the technique of excitations is irrelevant. In practice, the IR + UV two-photon t e c h n i q ~ e ~ is J ~highly - ~ ~ suggestive, for the following reasons: (i) In favorable cases, the metal-hydrogen vibrations appear as isolated peaks in IR or Raman spectra;23(ii) decoupling of the metal-hydrogen bond from other modes is supported by heavy-atom blocking of energy flux between different light ligands;24(iii) it should be possible therefore to exite local metal-hydrogen modes; (iv) using narrow band lasers, it should even be possible to exite these local modes as vibrational eigenstates; (v) in practice, these "eigenstates" should of course decay, e.g. by IR emission. Nevertheless, the lifetime of local metal-hydrogen modes should be sufficiently long (greater than nanoseconds) so that additional UV photons have a chance to hit the vibrationally excited molecules and to excite them electronically. It should, however, be mentioned that several ones of items i-v do not necessarily apply to other metal-ligand bonds. In fact, the present system HCO(CO)~,with its metal-hydrogen bond, has been chosen rather carefully as candidate for successful demonstrations of the Polanyi rules in unimolecular reactions. Needless to add, similar molecules should also be considered as favorable candidates. The results presented above show that the Polanyi rules should provide a key to the control not only of bimolecular reactions1-' and dissociativeadsorptions on surfaces25 but also of unimolecular reactions. A promising technique for providing sufficient momentum along the reaction path and across the potential barrier toward the desired products, as requested first by Polanyi,lJ is vibrationally mediated photochemistry by IR UV two-photon adsorption.l2-*9 The present results provide an extension of this technique from triatomic molecules (HOD12-15)via small organic molecules (CH30H18)to simple organometallics (HCo(C0)d); see also refs 8, 17, and 19. Alternatively, one may employ a combination of an IR continuous wave excitation of local Co-H vibrations in the electronic ground state 'Al of HCo(C0)4, together with an ultrashort UV laser pulse inducing efficient photoexcitations of the Co-H bond by 'Al 'E transitions. An even simpler method is suggested, for the present favorable case, by Fourier transform of the autocorrelation functions, as carried out in ref 9: The resulting UV single-photon absorption spectra indicate that high and low frequencies induce different vibronic transitions which predominantly yield dissociation of the Co-H bond and IVR, respectively. In conclusion, the Polanyi rules for unimolecular reactions may be verified by various approaches to selective photodissociation.
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Acknowledgment. We express our thanks to Dr. Elke Kolba for stimulating discussions and helpful calculations in the initial stage of this work. We are also very grateful to Britta Meissner, Boris Proppe, and Michael Dohle for generating video movies of dissociating HCo(C0)d. These helped us analyze the dynamics and select exemplary snapshots for Figures 1 4 . Financial support of our French-German research project by PROCOPE is also gratefully acknowledged. The calculations have been carried out on the Cray-2 computer of the CCVR (Palaiseau, France) through a grant of computer time from the Conseil Scientifique de Centre de Calcul Vectoriel pour la Recherche and on our H P 710 work stations at the Freie Universitlt Berlin.
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