The photochemistry of cyclopentadiene: theory and simulations Lauren Bertram 1 , Peter M. Weber, 2 Adam Kirrander 1 1 University of Oxford, UK, 2 Brown University, USA Cyclopentadiene (CP) is an interesting target for studies of ultrafast photochemical reactions in conjugated polyenes. It forms two highly ring strained and energetically unfavourable photoproducts, bicyclo[2.1.0]pentene (BP) and tricyclo[2.1.0.0 2,5 ]pentane (TP), via electrocyclic ring closure. In addition, previous experiments also suggest that it can undergo a 1,3-sigmatropic hydrogen shift 1 and that there is a thermal back reaction from BP to CP on long time scales. To investigate this further, we have simulated the nonadiabatic photochemistry of CP 2 using semiclassical surface-hopping with XMS(3)-CASPT2(4,4) electronic structure theory, continuing the simulations on the ground state using DFT/PBE0 until 10 ps after excitation. A mixture of hot CP and BP is observed, on a short-time scale, and on a longer-time scale a slow conversion from BP to CP, with the latter further modelled using RRKM theory. Finally, observables for ultrafast x-ray scattering experiments are calculated in anticipation of future experiments that map the structural dynamics of the reaction. 3 The prospect of retrieving the nature of the electronic states from such experiments is also discussed. 4 References 1. W. Fuß, W. E. Schmid and S. A. Trushin, Chem. Phys. , 2005, 316 , 225. 2. T. S. Kuhlman, W. J. Glover, T. Mori, K. B. Møller and T. J. Martinez, Faraday Disc. , 2012, 157 , 193. 3. H Yong, et al., Proc. Nat. Acad. Sci. , 2021, 118 , e2021714118. 4. H Yong, et al., Nat. Comm. , 2020, 11 , 1-6.
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