The photochemistry of aldehydes in the gas phase has been the topic of extensive studies over the years. However, for all but the smallest aldehydes the dynamics of the processes is largely unknown, and key issues of the mechanisms are open. The photochemistry of pentanal1, pentanal cluster, pinonic acid and hydrated pinonic acid is studied by molecular dynamics simulation using a semiempirical MRCI code for the singlet and triplet potential energy surfaces involved. The simulations explore the processes on the triplet state following intersystem crossing from the initially excited singlet state.     The simulations reveal that aldehyde photochemistry includes Norrish type I, type II reactions and H detachment. In the case of pinonic acid an additional channel, namely ring opening, is observed. For the pentanal cluster the same reaction channels are found, but involve more then one pentanal molecule. The photochemistry of the pentanal cluster is therefore more complex and leads to an enrichment of the products. The simulations yield the timescales of the observed reactions. Bond orders and Mulliken partial charges are computed along the trajectories, and provide mechanistic insights. The results throw light on the timescales and mechanisms and competition between different channels in aldehyde photochemistry. It is suggested that direct dynamics simulations using semiempirical potentials can be a very useful tool for exploring the photochemistry of large aldehydes, ketones and related species.Reference: 1.    Shemesh, D. ; Lan, Z.G., Gerber, R.B. Dynamics of Triplet-State Photochemistry of Pentanal: Mechanisms of Norrish I, Norrish II and H Abstraction Reactions, Journal of Physical Chemistry A, Curt Wittig Festschrift , http://pubs.acs.org/doi/pdf/10.1021/jp401309b, in press

Dynamics of triplet state photochemistry of aldehydes

247th National Meeting (2014)

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