Reaction Path of a sub-200 fs Photochemical Electrocyclic Reaction

Abstract

Ab initio multistate second-order perturbation theory (MS−CASPT2) calculations are used to map the reaction path for the ultrafast photochemical electrocyclic ring-opening of cyclohexa-1,3-diene (CHD). This path is characterized by evolution along a complex reaction coordinate extending over two barrierless excited state potential energy surfaces and ultimately leading to deactivation through a S1/S0 conical intersection. The observed excited-state dynamics involve three sequential phases with lifetimes (traveling times) of 10, 43, and 77 fs, respectively. In this work we associate each phase to the evolution of the CHD molecular structure along a different mode. In particular, we show that (a) the decay of CHD from its spectroscopic (1B2) state to a lower lying dark (2A1) excited state involves motion along a highly curved coordinate corresponding to a mixture of σ bond expansion and symmetry breaking skeletal bending, (b) the evolution on the 2A1 (S1) state and the final 2A1→1A1 (i.e., S1→S0) decay involve a large amplitude displacement along the same asymmetric bending mode which ultimately leads to a S1/S0 conical intersection, and (c) the application of a novel strategy for mapping the multidimensional S1/S0 intersection space indicates that the ultrashort 77 fs lifetime of the 2A1 excited state is due to the existence of an extensive set of S1/S0 conical intersection points spanning the low-lying part of the 2A1 energy surface. Points (a) and (b) are validated by discussing the results of previously reported and new femtosecond time-resolved spectroscopic data on CHD and on the two dialkyl derivatives α-terpinene and α-phellandrene. An interpretation in terms of driving forces is also given.

Fundamental Limits on Spatial Resolution in Ultrafast X-ray Diffraction

“…This 50 fs time-scale is consistent with Figure 6, which indicates that the dynamics is comparatively uniform at early times (the so-called 'ballistic' nature of this particular reaction [35]), but diverges at later times (t > 50 fs in Figure 6). (c) Figure 7.…”

“…While in some cases, such as nonradiative transitions in intermediate or statistical limit scenarios, the wave packet is presumed to spread over all receiving modes [32][33][34]; in other cases, well-defined wave packets persist for at least part of the chemical reaction pathway. In 1,3-cyclohexadiene, for example, the propagation of the wavepacket has been described as 'ballistic', implying that it retains a well-localized form even as it propagates [35]. However, it is inevitable that the nonclassical nature and propagation of the wavepacket leads to a delocalization.…”

Ultrafast Dynamics of 1,3-Cyclohexadiene in Highly Excited States

“…Upon excitation to the 1B valence state, the reaction path carries the molecule within about 140 fs along the ring-opening coordinate through the 1B/2A and 2A/1A conical intersections back to the ground state [3][4][5][6][7]. But even while these curve-crossing transitions are now well understood, numerous questions remain.…”

“…Comparison with absorption spectra shows that excitation at the wavelength of the Dω 4 photon should yield a 1A 1 → 3p transition, identifying the intense band around 2.3 eV in the (4 + 2)-spectrum as the 3p Rydberg state [3]. While the admixture of valence states to 3p is not yet known, it has been suggested that 3s mixes with the doubly excited 2A state that plays an important role in the ring-opening mechanisms [4]. The 2.3 eV band is also seen in the (3+2)-spectrum, along with a progression of Rydberg peaks at lower binding energies.…”

“…The 4.63 eV photon places CHD on a steeply repulsive part of the 1B potential energy surface, which accelerates it along the ring-opening coordinate, yielding 1,3,5-hexatriene (HT). Along this reaction path the wave packet bypasses a cusp in a symmetry-breaking direction and enters the 2A 1 surface, from where it decays to the electronic ground state [4][5][6][7]. Ionization out of 2A 1 by two Dω 2 probe photons passes through the Rydberg states that are seen in the spectrum [7].…”