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