19364
J. Phys. Chem. 1996, 100, 19364-19366
Do Photochemical Ring-Openings Occur in the Spectroscopic State? Cyclohexadiene/Hexatriene Photochemical Interconversion
1B
2
Pathways for the
Paolo Celani,† Fernando Bernardi,‡ Michael A. Robb,*,† and Massimo Olivucci*,‡ King’s College London, London WC2R 2LS, U.K., and Dipartimento di Chimica “G. Ciamician” dell’ UniVersita` di Bologna, Via Selmi 2, 40126 Bologna, Italy ReceiVed: July 22, 1996; In Final Form: September 30, 1996X
The potential energy paths (i.e., the minimum energy paths) that control the excited-state evolution of cyclohexadiene (CHD) and cZc-hexatriene (cZc-HT) from the Franck-Condon (FC) region are documented. These paths indicate that both CHD and cZc-HT undergo a barrierless motion in the 1B2 spectroscopic state until decay to the lower lying 2A1 state occurs via two distinct 1B2/2A1 conical intersections. Remarkably, it is found that both conical intersections correspond to open (i.e., acyclic) molecular structures. On the basis of these results and of the available experimental data, it is concluded that although the photochemical ringopening of CHD occurs (in femtoseconds) in the spectroscopic state, the associated photochemical ringclosure reaction of cZc-HT is initiated upon decay of 2A1 cZc-HT to the ground state picoseconds after the initial excitation.
The dynamics of the photochemical ring-opening of cyclohexa-1,3-diene (CHD) to cZc-hexa-1,3,5-triene (cZc-HT) has recently been studied via time-resolved resonance Raman (RR) spectroscopy.1
The RR intensities show that motion in the spectroscopic (1B2) state involves CHD ring-opening. However, the femtosecond lifetime of this state suggests that there is only a limited progression along the ring-opening coordinate. Accordingly, it has been proposed1b that CHD opens on a lower excited state of A1 symmetry populated via fast (∼10 fs) radiationless decay through a 1B2/2A1 conical intersection2 (i.e., a real crossing) at a point where the C1C6 σ-bond is only +0.05 Å stretched. Although ab initio computations have shown the existence of an “early” 1B2/2A1 crossing,3 these computations were carried out along an “ideal” ring-opening coordinate. Thus, in this report we rigorously document the 1B2 relaxation coordinate (i.e., the minimum energy path MEP5) from the CHD FranckCondon (FC) structure. We show that progression along this coordinate leads to a 1B2/2A1 crossing where the C1C6 σ-bond is +0.5 Å stretched, which suggests that decay to the 2A1 state occurs after the CHD ring has opened. To gain insight in the complementary photochemical ring-closure reaction, we have also investigated the path controlling the 1B2 relaxation of cZcHT. This path enters a 1B2/2A1 crossing with an acyclic structure. Thus, in contrast with CHD, cZc-HT decays to the 2A state before the C C σ-bond formation is initiated. 1 1 6 The 1B2 MEP starting at the FC (i.e., ground state) structures (see the CHD and cZc-HT structures above; the corresponding geometrical parameters are given in Figure 1) has been computed using complete active space self consistent field (CAS-SCF) theory,6 and the energies have been corrected for †
King’s College. Universita` di Bologna. X Abstract published in AdVance ACS Abstracts, November 1, 1996. ‡
S0022-3654(96)02206-X CCC: $12.00
dynamic correlation effects by multireference perturbation theory (PT2F7). The molecular orbitals defining the active space are linear combinations of the atomic orbitals illustrated below for the reactant and product species.
This active space gives a correct description of the σ-bond and π-bonds breaking/forming processes occurring during the ringopening and ring-closure reactions. The computed values8 for the 1B2 vertical excitation energies are 4.75 eV (experimental value is 4.90 eV3) for CHD and 5.19 eV for cZc-HT (this value must be >4.92 eV, which is the experimental value for tZt-HT). The 2A1 vertical excitation energies for CHD and cZc-HT are 6.03 and 5.74 eV, respectively. Although an experimental estimate for these quantities is lacking, the computed 0-0 2A1 excitation energy is 3.72 eV (experimental value is ∼3.72 eV1a) and the 0-0 1B2/2A1 energy gap is 0.49 eV (experimental value is ∼0.621a). We have limited our investigation to the 19-dimensional cross section spanning the C2 configuration space, since CAS-SCF computations of the 1B2 state do not converge at asymmetric nuclear configurations. In parts a and b of Figure 1 we show the 1B2 energy profile (together with the energy profile for the 2A1 state) along the computed MEP9 for CHD and cZc-HT 1B2 states, respectively. Both energy profiles show a 1B2/2A1 crossing that occurs along a barrierless relaxation path (see arrows). The quality of the computed energy surface has been assessed by comparison with the experimentally derived potential energy surface for CHD.1b In Table 1 we show that the observed CHD frequencies and ∆i factors agree with the initial, “harmonic” part of the MEP.10 The experimentally derived excited-state structure is close to structure MEPCHD(6), which is located at the beginning of the energy plateau shown in Figure 1a (see Table 2). The C1C6 bond length and axial C-Ha bond rotation angle of MEPCHD(6) agree with the 10 fs evolved (10% reacted) structure reported by Mathies et al.1b However, the computed 1B2/2A1 crossing is located near the 1B2 equilibrium structure of cZc© 1996 American Chemical Society
Cyclohexadiene/Hexatriene
J. Phys. Chem., Vol. 100, No. 50, 1996 19365
Figure 1. 1B2 and 2A1 energy profiles along the MEP coordinate describing the relaxation from the FC structures of (a) CHD and (b) cZc-HT. The distance along the MEP coordinate is given in au ) (amu-1/2 bohr). The bond lengths reported on the structures are given in Å. The labels on the top of the two plots indicate the MEP structures whose energy has been recomputed at the PT2F level of theory.
TABLE 1: CAS-SCF/6-31G* Ground State Vibrational Frequencies (ωi) and 1B2 Displacement Parameters (∆i) for CHD Totally Symmetric Modesd |∆i|
ωi (cm-1) calcda
exptlb
192 504 592 767 871 996 970 1105 1141 1128 1225 1329 1408 1473 1552 2859 2915 3011 3035
203 507 562 750 850 948 1014 1057 1150 1178 1237 1321 1404 1435 1578 2838 2939 3050 3050
gradc
geom
exptlb
assignment
0.27 0.03 0.90 0.56 0.06 0.07 0.38 0.30 1.18 0.29 0.16 1.73 0.07 0.03 0.03 0.06
0.03 0.25 0.25 0.36 0.11 0.55 0.50 0.09 0.30 0.20 0.14 1.38 0.17 0.12 1.51 0.00 0.02 0.05 0.10
