5-diazo Meldrum’s acid (DMA) undergoes the photo-induced Wolff rearrangement (WR). Recent gas-phase experiments have identified three photochemical products formed in a sub-ps scale after irradiation, a carbene formed after nitrogen loss, the ketene formed after WR and a second carbene formed after nitrogen and CO elimination (Steinbacher, A., et al. Phys. Chem. Chem. Phys., 2014, 16, 7290-7298). In this work the ground- and excited-state potential energy surfaces (PES) have been investigated at the MS-CASPT2//CASSCF level. The key element of the PES is an extended S0/S1 conical intersection seam along the C-N dissociation coordinate. The C-N predissociated region of the seam is accessed after excitation to the bright S2 state, and decay paths from the seam to the three primary products have been characterized. For the ketene and carbene II products, we suggest two possible formation pathways, a direct and a stepwise one, which suggests that these products may be formed in a bimodal fashion. We have also characterized two possible mechanisms for triplet formation, one ocurring before C-N dissociation involving a (S1/T2/T1) crossing region, and another one through the carbene. In contrast, excitation to S1 leads to a C-N bound region of the seam from where DMA regeneration or diazirine formation are possible, with a preference for the first case. The results are in good agreement with experimental data. Together with our previous work on diazonaphthoquinone, they show the importance of an extended seam in the photochemistry of α-diazoketones

This work is financially supported by the National Natural Science Foundation of China (21303007); the Excellent Young Scholars Research Fund of Beijing Institute of Technology (2013YR1917); the Spanish Ministerio de Economía y Competitividad (RYC-2011-09582,CTQ2015-69363-P, FIS2012-37549-C05-02, and UNGI-10-4E-801) together with the European Fonds for Regional Development (FEDER); and the Generalitat de Catalunya (2014-SGR-301,2014-SGR-1202, and Xarxa de Referència en Química Teòrica i Computacional)