J. Phys. Chem. 1994, 98, 9823-9830
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9823
Photodissociation Dynamics of Organometallic Complexes: Model Simulation for H cO(c0)4 HCo(C0)4* HCo(C0)3 CO
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C. Daniel,*$?E. Kolba,"* L. Lehr,l J. Manz,*s$J and T. Schroder$3J The Laboratoire de Chimie Quantique, UPR 139 du CNRS, Institut le Bel, F-67000 Strasbourg, France; The Institut f i r Physikalische Chemie, Universitat Wiirzburg, Marcusstrasse 9-11, 97070 Wiirzburg, Germany; and Freie Universitat Berlin, Institut f i r Physikalische und Theoretische Chemie, Takustrasse 3 14195 Berlin, Germany Received: June 10, 1994; In Final Form: June 15, 1994@
The photochemistry of HCo(C0)4 has been studied through dynamical calculations based on ab initio potential energy surfaces for the metal-hydrogen bond homolysis and for the dissociation of the axial carbonyl ligand. The dynamics of the two competitive primary pathways are simulated by adiabatic motions of representative wave packets on the CASSCF/CCI potential energy surfaces corresponding to the lowest excited states by means of the fast Fourier transform (FFT) technique. The present study suggests the following sequential mechanism: (i) initial excitation of the molecule by UV photons from the 'A1 ground state (preferably around 229 nm) to the 'E 3ds o* excited state; (ii) from this excited state, dissociation to the primary products H Co(C0)4 in the 'E excited state on an ultrashort time scale (ca. 10 fs) competes with intramolecular vibrational energy redistribution (IVR) of the rest of the molecule HCo(C0)d in the 'E state on a longer time scale; (iii) intersystem crossing (ISC) from the vibrationally relaxed HCo(C0)4 ('E) molecule either to the 3A1o o* excited state or to the 3E 3ds o* excited state; (iv) ultrafast dissociation into dominant product channels H Co(C0)4 (10 fs from the 3A1state) or HCo(C0)3 CO ('100 fs from the 3E state); (v) intramolecular vibrational energy redistribution (IVR) of the remaining fraction of nondissociative HCo(C0)4 in the 3E state, with possible transition back to the ground state of the molecule. This sequential reaction mechanism (i-v) of the title reaction does account for some experimental results obtained by Sweany in low-temperature matrices experiments, and it does predict important details of the absorption spectra, product distribution, and femtochemistry which may be tested experimentally.
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1. Introduction
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L,M-X
Organometallics have a rich and interesting photochemistry which has been extensively used in the past 10 years to generate unsaturated and very reactive species. As pointed out by Meyer "this extensive photochemistry has generally been based only on the product and quantum yield studies. There is a little insight in this area into excited-state dynamics, detailed photochemical mechanisms, on the nature of the excited state or states responsible for the photochemistry."' Until recently, the current understanding of the photochemical reactions of organometallics has been based on molecular orbital diagrams coupled with an analysis in terms of th bonding and antibonding character of the orbitals involved. The approach based on state correlation diagrams and potential energy surfaces that connect the ground and excited states of the reactant and those of the primary products has enabled us to overcome an important step in the understanding of the mechanism of the photochemical reactiom2 Two important classes of photochemical reactions in organometallic chemistry are the homolysis of a bond originating from the metal atom (metal-hydrogen, metal-alkyl, or metal-metal bond^):^-^ +
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Laboratoire de Chimie Quantique.
* Institut fur Physikalische Chemie, Wiirzburg.
Present address: Siemens Nixdorf-AG, 8 1739 Munchen, Germany. Present address: Institut fur Physikalische und Theoretische Chemie, Berlin. II Present address: Max Planck Institut fur Stroemungsforschung, Bunsenstrasse 6-10, 37073 Goettingen, Germany. Abstract published in Advance ACS Abstracts, August 15, 1994. 8
@
0022-365419412098-9823$04.50/0
hv
L,M -I- X
(X = H, R, L,M)
(1)
and the heterolytic loss of a carbonyl ligand: L,M-CO
hv
L,M
+ CO
(2)
A number of organometallics, upon irradiation, undergo both photochemical reactions, either at a unique wavelength3 or at different wavelength^.^,^ To explain the mechanism of photodissociation and the existence of two concurrent dissociative channels (carbonyl loss vs metal-hydrogen bond homolysis) in transition metal hydrides, we have investigated the excited states and the correspondingpotential energy surfaces for HCo(co)4 and HMn(CO)5.9-15 From these studies, it was concluded that the homolysis of a metal-hydrogen bond, which is a rather general reaction in monohydrides, results from the dissociative character of the potential energy curve for the triplet state corresponding to a OM-H (T*M-H excitation. The dissociative character of the curve associated to the d U*M-H excitation is responsible for the ligand dissociation. Unfortunately, without a time-dependent approach, the estimate of the relative quantum yields for the different pathways is beyond reach. Although the field of molecular photodissociation dynamics has expanded tremendously during the last decade,16-22the application of quantum time-dependent wave-packet propagation methods to multidimensional systems is still a challenge. In the present study, we extend the model proposed by Imre for competitive bond breakings from simple systems19to HCo(C0)4 which undergoes two primary processes after irradiation at 254 nm:3
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0 1994 American Chemical Society
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Daniel et al.
9824 J. Phys. Chem., Vol. 98, No. 39, 1994 HCo(CO), HCo(CO),
-H + hv
hv
CO
Co(CO),
(3)
+ HCo(CO),
(4)
According to the experiments carried out by S ~ e a n yusing ,~ matrix isolation techniques, if one assumes that neither the photolyzed CO nor the H atom retum to the metal, the branching ratio for the primary products has an upper limit of 8:1.3b Even if the various experimental3 and theoretical9-' 1,14~15 studies reported on reactions 3 and 4 have already discovered many hv important details, the derivation of a consistent overall reaction mechanism remains a challenging puzzle of rather a high degree of complexity. From previous theoretical studies?-' three u* and 3A1 u u* will excited electronic states 's3E 3ds play a key role in the photodissociation process with a variety of open channels to different primary products, either in the hv ground state or in excited states. Moreover, these states and channels may be coupled by a variety of transitions and 'E relaxations processes such as intersystem crossing (ISC), internal conversions (IC) combined with intramolecular vibrational 3E energy redistribution (IVR). The mechanism may account for 3 direct dissociations (DD), typically occurring on an ultrafast A1 time scale (10- 100 fs), or for indirect dissociations (ID), e.g., Figure 1. State diagrams for HCo(C0)d based on the results of ref vibrational predissociation,22 occurring on longer time scales 9-11. ('1 ps) due to slow tunneling or IVR processes preceeding 2. Model and Techniques ultimate dissociations. Several of these steps may occur simultaneously or may compete to each other. The complex For the sake of simplicity, the molecule HCo(CO)4 is modeled network of these elementary processes is described in Figure 1 as a pseudotriatomic molecule, with two collinear dissociative (schematic state diagram, where energy conservation's rules are bonds, qa = [H-Co] and q b = [Co-CO,]. All other "spectator" modes are decoupled in this zero order approximation. This respected). mode should be reasonable, at least for ultrafast time A few of these paths have been considered p r e v i ~ u s l y ~ ~ ~ - * decoupling ~~'~ scales (5.100 fs) when the initial energy remains stored in the without any correlation between the experimental data and the dissociative bond(s). During later times, IVR will tend to theoretical results. Even if a direct comparison with the scramble vibrational energy among additional models, and this experimental data is unreachable, mainly due to the influence effect will be described by a phenomenal modification of the of the matrix on the observed photodissociation (cage effects) present two-dimensional model. The relevant Hamiltonian is on the one hand and to the limited accuracy of the potential thus energy surfaces on the other hand, the present model simulation should give us a semiquantitative view of the elementary He = T + V, (5) processes which contribute to the photochemistry of the molecule. The first step is the reduction of Figure 1 from a depending on the electronic state e ('AI, IE, 3A1, or 3E). rather complex network of all possible interrelated and competExplicitly ing processes to a simple one. The resulting reaction mechanism should incorporate the previous detailed experimental and theoretical results for some of the individual processes which where pa, p b are the momenta conjugate to qal qb, and pa = are indicated in Figure 1. Similar investigations may be applied msmcd(ma + mco), clb = mcdncd(mco + mco) are the to other organometallic molecules which undergo concurrent corresponding reduced masses. Similar simplistic low-dimenprimary reactions upon irradiation at a unique wavelength or at sional Hamiltonians have been employed previously for phodifferent wavelengths. For this class of compounds, this study todissociations of other pseudotriatomic systems,2°,28as starting may serve as prototype discovery, exploiting the available models for more sophisticated studies. theoretical techniques. The potential energy surfaces V(qa,qb). corresponding to the The model and techniques are explained briefly in section 2. initially photoexcited 'E 3d u* state, to the excited 3E 3d Essentially, we employ quantum chemical ab initio CASSCF/ u*, 3A1u u* states and to the 'A1 ground state, have been CCI method^^^^^^ for the evaluation of the potential energy evaluated by the CASSCFKCI m e t h ~ d . *The ~ $ ~ab~initio points surfaces, combined with fast Fourier transform propagations of were obtained from a multireference CCI (contracted configurarepresentative wave packets for simulations of the primary tion interaction) c a l ~ u l a t i o nbased ~ ~ on a CASSCF (complete The reactions dynamic^*^-^' and absorption spectra. active space SCF) reference wave function correspondmg to results are presented in section 3, including potential energy the selected 3A1 u a*'' (a and u* denote the molecular surfaces for the 1,3Eand 3A1electronic excited states together orbitals that are respectively bonding and antibonding with with the wavepackets' dynamics. As a summary, the resulting respect to the metal-hydrogen bond). The CASSCF space was photodissociation mechanism and the conclusion are presented limited to six active orbitals corresponding to the 3ds orbitals of the cobalt, the 4ds orbitals which correlate them, and the c7 in section 4.
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19d,28929
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J. Phys. Chem., Vol. 98, No. 39, 1994 9825
Photodissociation Dynamics of Organometallic Complexes
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TABLE 1: CCI Energy Values (in E h = Hartrees and Relative to -1832) along the 3E (3da @) Potential Energy Curve for the Reaction HCo(C0)4 H f cO(c0)4as a Function of the Coordinate qa = [H-Co] in Angstroms for Different CASSCF Reference Wave Functions (See Text) for ob
= [co-co,,]= 2.1 A
electronic excited state will be performed starting with the initial ~O,O)~A,state, while the l 0 , O ) l ~will serve as the initial state for the propagation on the 3A1 and 3E electronic excited states, in an exploratory study of the molecular reaction dynamics following intersystem crossing processes subsequent to IVR:
a, CASSCF CASSCFl CASSCF2 CASSCF3
1.556
2.0
2.5
3.0
50.0
0.624 30 0.631 20 0.673 91
0.635 94 0.638 16 0.676 99
0.612 63 0.625 08 0.660 70
0.608 55 0.622 28 0.634 28
0.612 92 0.624 75 0.655 06
and u* orbitals (CASSCFI). Excluding the 3d, and 4dz orbitals from the active set represent a reasonable approximation. Indeed, the results obtained for the ground-state energy (see ref 11) and for several points toward the Co-hydrogen bond homolysis reaction path on the 3E potential energy surface agree well with those obtained from a more demanding CASSCF reference wave function including 10 electrons in 10 active orbitals (CASSCF2). In an additional test, we improved our calculations for selected key parts of the 3E potential energy surface (energy barrier on the Co-hydrogen bond homolysis reaction path), by carrying out a CASSCF calculation for the 3E state itself with the principal configuration being (3d,)4(3&)3( O ) ~ ( U * ) '(CASSCF3) followed by a multireference CCI. The results of the different methods are summarized in Table 1. The results obtained from the different methods appear to be similar, indicating reasonable convergence of the CASSCFI/CCI method with respect to some important qualitative or semiquantitative aspects of the molecular reaction dynamics. In particular, the presence of an energy barrier in the H Co(C0)d exit channel is not dramatically influenced by the level of calculation.
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SCHEME 1
The wave packet \v1A,,O(C&qb) is adapted from ref 30, whereas \YIE,o(qa,qb) is evaluated by the Chebychev relaxation method3' applied to the 'E electronic excited state potential energy surface VIE. The mechanism of the 'E 3A1and 'E 3E radiationless transitions will be considered elsewhere.32 The time evolutions of the wave packets qe(qa,qb,t) (with e corresponding to 'E, 3A1,or 3E) are evaluated by solving the time-dependent Schrodinger equation:
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ihye(qa&t)
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= Heye(qa,qb,t)
by the fast Fourier transform (FFT)m e t h ~ d , subject ~ ~ - ~ to~ the initial conditions (8). The propagations are based on representations of ve(qa,qb,t) on grids qat = 4% iAqa, qb, = qbo -I-jAqb, t k = kAt with the parameters 4% = qbo = 2aO, Aqa = h q b = O . l a ~ ,At = O.lh/Ehfor 1 5 i, j 5 128 and 0 5 k 5 ke, with upper bounds ke = 50 000,20 000, and 210 000 for e being 'E, 3A1, and 3E respectively. Several techniques are used to analyze the resulting wave packets ve(qa,qb,t) and to derive various properties useful for comparisons with experimental data and predictions for future gas-phase experiments. Movies of the Densities. The time evolution Y?e(qa,qb,t) is illustrated by corresponding movies of the densities:
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@e(qa,qbgt) = 1y(qa&t)12
The potential energy surfaces used for the simulation reported in the present paper are obtained by means of the most economical method (CASSCFI/CCI). From these ab initio points, the overall potential energy surfaces are generated by interpolation with additional smoothing in order to avoid any obvious artifacts such as shallow minima in the asymptotic domains. It has been assumed that the C3" symmetry is retained along the reaction path corresponding to the metal-hydrogen bond homolysis and to the dissociation of an axial carbonyl ligand (Scheme I). During the dissociation of the Co-H bond, the angle 6 was kept equal to 100" for all points along the reaction path (see ref 11). The photoabsorption and the subsequent competing bond breakings (3) and (4) of HCo(C0)4* are simulated by propagations of selected wave packets:
(10)
a few snapshots of which being shown in Figures 2, 5, and 6. Branching Ratio. When two competing product channels a and p are open, the analysis of the snapshots at sufficiently long times T gives an indication of the branching of the original rather compact wave packet ve(qa,qb,t=O) into two representative parts a and p with negligible overlap:19 = yea(qa?qbrT)
ye(qa,qb,n
+ yei?(qarqb,T)
(l la>
(vea(nIve,&n) = jjdqa
d q b y*ea(qa,qb,T)
y.$(qa,qb?n
0 (1 1b)
Backward and forward propagations of q e a ( q a , q b , T ) and veB(qa,qb,r) then yield the overall time evolutions of orthogonal parts of ve(qa,qb,t):
with
on the different excited potential energy surfaces. In the applications presented in section 3, we restrict the propagation to two different initial wave packets \Ve(qa,qb,t=O), namely, either the 10,O)lAl vibrational ground state of the 'A1 electronic ground state or the 10,o)lE vibrational ground sate of the 'E electronic excited state. The propagation on the 'E
(9)
Wea(l)Ivep(t))
=0
(1 1 4
for all times 2 0 due to the unitarity of the time evolution operator. The overall branching ratio into different product channels a and p is then determined as the ratio of integrated ("time-dependent") densities:
[al/Ul= s j d q a dqblye,(qa,qb,t)12/sjdqa
d q b Iye~(qa&9t)12
(12)
Daniel et al.
9826 J. Phys. Chem., Vol. 98, No. 39, 1994 This branching ratio (12) should be observed in a femtochemical pump-and-pulse experiment33 where an ultrafast pump pulse prepares essentially the initial wave packet @a), whereas sufficiently delayed prope pulses would monitor the ratio of populations of the product channels (12). In contrast, continuous-wave (cw) laser excitations would induce different branching ratios, depending on the absorption frequencies o;se eq 14c. Autocorrelation Functions. The autocorrelation functions
SCHEME 2
depending on the cw absorption frequency o,is given by [a,w]/[p,w] = I('A,-'E,a,w)/Z('A,-'E,P,o) or their absolute values serve as diagnostic tools of the molecular reaction dynamics, indicating the time scale of flux out of the initial state IYe(0)). To account for even more efficient fluxes out of the qa, q b modes into other vibrational modes, we also employ an empirical modification of (13a):
As a consequence, the decay of the autocorrelation function (Ye(0)\Ye(t)) is accelerated by an exponential decay function, with "autocorrelation decay acceleration time" Tad* In practice, we choose rather small values of Tada so that the resulting absorption spectra appear to be smooth; see eq 14. The decay of the autocorrelation function is much faster than z m ,because the flux out of the initial state IYe(0))should be much faster than complete intramolecular vibrational energy redistribution into all vibrational modes. The separation (1 la) of \Ve(qa,qb,f) suggests the corresponding separation of the autocorrelation function:
for analysis of the contributions from two different product channels a and p. The overlap integrals (13a) and (13c) are evaluated using the FFT grid representations of wave packets and the trapezoid rule. Absorption Spectrum. For the 'E d o* electronic excited state the Fourier transform of the autocorrelation function (13a) yields the absorption spectrum34for cw excitation:
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where E I A ~ is , Othe energy of the initial state, and o is the absorption frequency. Expression (14) is equivalent to the golden rule expression for absorption using the Franck-Condon approximation and assuming that the 'A1 'E transition dipole function is approximately constant in the Franck-Condon regime. With the separations ( l l c ) and (13c), the spectrum given by expression (14) may also be decomposed into two parts for different product channels a, p:
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z('Al
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'E,o) = Z('Al
-
'E,a,w)
+ Z('A, - 'E,P,w)
(144 where
(14c)
Similar branching ratios based on separated absorption spectra for competing dissociative product channels, e.g., a = HOD* H OD, /3 E HOD* HO D, have been evaluated by Imre et al.I9 Here, we extend their method for competing dissociative versus nondissociative (IVR) channels. All expressions (11)-( 14) may be generalized to situations where wave packets branch into more than two, dissociative or nondissociative channels a, p, y... The integrals (14) are evaluated by the FFT method in the finite domain where the autocorrelation functions (YlE(t=O)IY1E(t)) have nonnegligible values.
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3. Results The results are presented in the order of subsequent processes of the overall photodissociation. These proceed on the potential energy surfaces of the different electronic states, from the 'A1 electronic ground state via the 'E lowest singlet excited state to the 3A1 and/or 3E excited states. For a coherent presentation, we also include a few important results from refs 9- 11, and we recall some aspects of section 2, when appropriate. Before studying the reactive dynamics, let us consider some important properties of the potential energy surfaces represented by their equipotential contours in Figure 2 ('E), Figure 5 (3A1), and Figure 6 (3E). If the 3A1potential energy surface does show an exclusive dissociative character along the Co-H bond elongation, in contrast, the 1-3Epotential energy surfaces indicate two dissociative channels corresponding to the co-H bond homolysis and to the loss of a carbonyl ligand. The valley leading to the metal-hydrogen bond homolysis products is characterized by an energy barrier, slightly affected by more sophisticated calculation^,^^ of the order of 72.0 and 20.0 kT mol-' for the triplet and the singlet states, respectively. This energy barrier located at a distance Co-H around 2.9 8, for the 3E for the 3E state and around 2.4 8, for the 'E originates from a weak coupling between "valence" (1*3t,E)and "ionic" (1,3aE)states, the latter leading to the H- and +Co(CO)4 products (Scheme
2). The previous results, based on one-dimensional potential energy curves," did not indicate this exit channel toward the metal-hydrogen bond homolysis, because the calculated points were not located sufficiently close to this reaction path. This points to a shortcoming of the discrete geometry optimization procedure. lA1 Electronic Ground State of HCo(C0)d. The 'A1 'E photoabsorption starts from the l0,0)1~,vibrational ground state of the 'A' electronic ground state of HCo(C0)4. The representative wave packet ~IAl,O(qa.qb) is adapted from ref 30. According to the Franck-Condon principle and assuming that the 'A1 'E transition dipole function is approximately constant in the Franck-Condon regime, the wave packet ylAl,O(ql,qb) is lifted by a vertical transition from the potential energy surface
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J. Phys. Chem., Vol. 98, No. 39, 1994 9827
Photodissociation Dynamics of Organometallic Complexes 10
10
a/%
qb/%
a
8
6
6
4
4
2
2 2
4
5
8
1
0
2
4%
4
6
8
1
0
%/a0
0
20
40
60
80
100
120
timelfs
Figure 3. Absolute values of the total ( t ) autocorrelation funchons I(Y1 ~ ( t =I0Y) 1E(t))l with contnbutions j (Y1~(t=0) IYIE y ( t ) )I for product channels y = a = dissociation toward the products H Co(CO)4 ('E) and y = p IVR of HCo(C0)4 (IE), versus time. The effect of an accelerated decay, as modeled by a factor exp(-r/2tda), tada = 500W E h = 12 fs, is also shown.
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2
4
6
8
1
0
2
4%
4
6
8
1
0
%/% I
Figure 2. Time evolution of the wave packet y l & a , q b , t ) on the potential energy surface VIE of HCo(C0)4 ('E), from the initial state vI&a,qb,t=O) = YlA1,0(qa,qb) prepared by photoabsorption 'A1 'E to competing product channels H Co(CO)4 vs IVR of HCo(C0)d ('E). The coordinates qa and q b represent the Co-H and Co-CO, bonds elongations, respectively. The wave packet is shown by equidensity contours, IY(q,,qb,t)l* = Imax Y(qa,qb,t=0)1* x 0.02 i x 0.12, i = 0-5. The potential energy surface is shown by equipotential contours, V'E(qa,qb) = -1832. + 0.6 + i x O.O17Eh, i =
I
I
I
-
+
+
t
0-5.
of the 'A1 electronic ground state to the potential VIEof the 'E excited state. This wave packet will serve as initial state for the subsequent reaction dynamics: ylE(qa,qb$t=o) = yl,41,0(qa?qb)
(15)
The wave packet \V'E(qa,qb,t=O) embedded in the potential energy surface VIE is shown in the first panel Figure 2. Results for the 'E d 8* Electronic Excited State. The time evolution of the wave packet yY'E(qa.qb,t) is illustrated in Figure 2 by snapshots of the density @'E(qa,qb.t) (eq 10). The initial rather compact wave function \ViE(qa,qb,t=O) is split into two parts a and p, representing dissociation into the primary products H Co(CO)4 in the 'E excited state and IVR of the nondissociative HCo(C0)4 'E molecules. The "femtochemical" branching ratio (eq 12) gives [H Co(C0)4]/[HCo(C0)4 (IE, IVR)] c- 0.3Y0.65. Close inspection of the wave packet's time evolution shows that-within the present model-IVR consists of two contributions: most of the vibrational energy is redistributed into the Co-CO, vibration, but the Co-H stretch of nondissociative HCo(C0)4 ('E) molecules is also excited to a small extent. The corresponding autocorrelation function (13a) is shown in Figure 3 with the contributions from channels a = dissociation to the primary products H Co(C0)4 in the 'E electronic excited state, and p = IVR of HCo(C0)d ('E) (eq 13c). One notices the ultrafast (ca. 5 fs) initial decay, which is caused by about equal contributions from both channels a and p. At later times t 2 10 fs, one observes oscillatory recurrences, mainly due to the Co-CO vibrations which are the dominant accepting mode of the IVR channel p. In a more realistic model with more than two degrees of freedom, these recurrences should be dampled by flux of energy out of the initial modes qa, qb into additional spectator modes. The accelerated decay of the autocorrelation function is modeled by eq 13b, with parameter rads c- 12 fs. Similar values of t a d a yield similar results.
d
-
+
+
+
160
220
190
h /nm
-
-
250
-
Figure 4. Simulated total (t) absorption spectra I ('AI IE,o) = I (IA1 'E,a,o)+ I ('AI *E,B,w), with contributions from two competing product channels a dissociation toward the products H IVR of HCo(C0)d ('E). + C O ( C O )('E) ~ and p Finally, the resulting absorption spectrum, eq 14, is shown in Figure 4, with the contributions 14a and 14b from product channels a corresponding to the formation of the primary products H -t Co(C0)4 in the 'E electronic excited state plus p corresponding to the IVR process of HCo(C0)4 ('E). The absorption spectrum is characterized by three important features: (i) a very strong peak at 1 = 229.8 nm; (ii) a medium peak at 210.9 nm, and (iii) a broad, small peak, which appears as a shoulder of the medium peak, extending until 13. c- 185 nm. The decomposition of the spectrum into two parts related to channels a and p shows that the two peaks at 13. = 229.8 nm and A = 210.9 nm are due to excitation of IVR of HCo(C0)4 (lE); more specifically, the strong peak at 13. = 229.8 nm arises from the dominant vibrational excitation Co-Co,, which causes the prominent oscillations of the autocorrelation function at times t > 10 fs; the medium peak is due to excitation of Co-H vibrations. Finally, the long tail of the spectrum, extending to 13. = 185 nm, is due to the channel a corresponding to the
9828 J. Phys. Chem., Vol. 98, No. 39, 1994
Daniel et al.
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formation of H cO(c0)4 in the 'E electronic excited state. The dissociative wave packet for the a channel originates mainly from the steep repulsive wall of the potential energy surface VIE in the domain qa < 3Uo (see Figure 2) following FranckCondon transition from VIA^ by means of photons with rather large energies hw = hc/;l. Comparison with the experimental absorption spectrum3b yields good agreement for the intense band at A = 227 nm (calculated at 229.8 nm), for which there is no doubt e~perimentally.~~ The other details do not allow conclusive correlations. Indeed, the comparison with the experimental spectrum at higher energies is featureless since the detection is close to the lower wavelength limit of the spectrometer. Moreover, more accurate ab initio calculations (large basis sets with f functions on the metal center, better reference wave function for describing the 'E excited state, electron correlation of the metal-carbonyl bonds, geometries optimizations at a correlated level) around the minimum of the V I Epotential could modify the shape of the PES and as a consequence, some details (dissociating part, band width) of the theoretical absorption spectrum may be slightly different. Apparently, the molecular reaction dynamics of photoexcited HCo(C0)d ('E) do not yield primary products HCo(C0)3 CO corresponding to the dissociation of an axial carbonyl ligand. The path toward this exit channel is prohibited by two obstacles: (i) the potential energy barrier evaluated at 20.0 kJ/ mol in the exit valley toward the products HCo(C0)3 CO and (ii) the Franck-Condon region of HCo(C0)4 ('A') which prepares excited HCo(C0)4 ('E)molecules with rather compressed H-Co bonds at the repulsive wall of the potential energy surface VIE, in contrast with relaxed CO-COa, bonds (see Figure 2). As a consequence, the initial energy is released mainly as kinetic energy of the H-Co bond, not of the CoCO,, inducing exclusively dissociation to the primary products H Co(C0)4 ('E) or IVR of HCo(C0)4 ('E). Figure 2 also shows that the IVR produces mainly HCo(C0)4 ('E) molecules with zero vibrational quanta in the qa,qb modes, represented by the 10,O)l~vibrational ground state wavepacket vIE,O(q&b). Parts of this wave packet may be transferred, by intersystem crossings 'E 3A1 and 'E 3E, to the V3A1 and V ~ Epotential energy surfaces. The study of these radiationless transitions is the subject of a detailed investigation including the spin-orbit coupling in the calculations and which are published elsewhere.32 The most important finding is that the spin-orbit coupling between the 'E and the 3Estates is about 10 times as large as the coupling between the 'E and 3A1 states. But the presence of topologic peculiarities such as surface crossings between the 'E and 3A1PES,may favor efficient 'E 3A1 intersystem crossing in a very short time scale (
