The excited state intramolecular hydrogen transfer mechanism of ortho-Nitrobenzaldehyde : a quantum chemical and molecular dynamics study

Leyva Novoa, Veronica GND

This thesis is a theoretical analysis of the photorelaxation mechanism of ortho-Nitrobenzaldehyde o-NBA. O-NBA is a photolabile protecting group, in which an excited state intramolecular H-transfer takes place. Recent experiments suggest that this photoreaction might proceed via ketene intermediate. Yet, little is known about the mechanism of this photoreaction. For this, first highly accurate multi-state complete active space second order perturbation over complete active space self-consistent field MSCASPT2/CASSCF calculations of o-NBA UV-spectrum in gas phase have been performed. The excited state S5 has been assigned as the spectroscopic state from where the experimental wavelength at 260 nm initiates the H-transfer photoreaction. The effects of different positional isomers and of solutesolvent interactions are also addressed along this thesis, without significant changes, merely blue-shift with solvatation. Once the UV-spectrum has been characterized, the deactivation mechanism of o-NBA has been deciphered by MS-CASPT2/CASSCF calculations. The location of different stationary points has suggested 2 deactivation pathways: The non-Kasha path relaxes from the S5 state to the ketene through a cascade of conical intersections along which a H-atom from the CHO is progressively transferred to the NO2. The Kasha path relaxes by means of internal conversion from the S5 to the S1 state, in which the H-atom is transferred. In order to discern which of the 2 photoreaction mechanisms is preferred, mixed quantum-classical dynamics MQCD simulations have been performed. The MQCD simulations from the high-lying S5 spectroscopic state are done with the time-dependent density functional theory method. None of the trajectories launched from the S5 show a H-transfer along deactivation. Thus, it is assumed that the Kasha path is the operative one in the deactivation of o-NBA from the S5. Further MQCD simulations performed on the S1 state with CASSCF confirm this.

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Leyva Novoa, Veronica: The excited state intramolecular hydrogen transfer mechanism of ortho-Nitrobenzaldehyde. a quantum chemical and molecular dynamics study. 2012.

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