Early events in the photochemistry of 5-diazo Meldrum's acid: formation of a product manifold in C-N bound and pre-dissociated intersection seam regions.

5-Diazo Meldrum's acid (DMA) undergoes a 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, a ketene formed after WR and a second carbene formed after nitrogen and CO elimination (A. Steinbacher, et al. Phys. Chem. Chem. Phys., 2014, 16, 7290-7298). In this work, ground- and excited-state potential energy surfaces (PESs) have been investigated at the MS-CASPT2//CASSCF level. The key element of the PESs 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 show two possible formation pathways, a direct and a stepwise one, which suggests that these products may be formed in a bi-modal fashion. We have also characterized two possible mechanisms for triplet formation, one occurring 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 is 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.