Electronic excited states and photodissociation pathways of iso-propylamine: Experimental and theoretical investigations

We report for the first time absorption cross sections of iso-propylamine in the energy range of 40 000 to 80 000 cm−1 using synchrotron radiation. Quantum chemical calculations based on time-dependent density functional theory are used to conduct a detailed spectral analysis. The observed intensity profile of the room temperature absorption spectrum is well reproduced by a theoretical simulation that includes vertical excitation energies and oscillator strengths of its two stable conformers, gauche (G) and anti (A). Rydberg series that converge to the first ionization limit of iso-propylamine are assigned via quantum defect analysis. Several vibronic progressions are observed along with the Rydberg series. The dominance of progressions involving the NH2 wagging mode observed along with the 3s Rydberg transition implies a molecular geometry change involving the NH2 group in the excited state. This is confirmed by geometry optimization calculations, which predict a planar geometry in the excited state vis-à-vis pyramidal geometry in the ground state. The measured absorption cross sections are used to estimate the lower limit of the UV photolysis lifetime of iso-propylamine in the upper atmosphere. In addition, the photodissociation dynamics of iso-propylamine probed using potential energy scans of S1, T1, and T2 excited states along each of the possible direct bond cleavage channels show that direct dissociation via S1 is unlikely, and intersystem crossings may play an important role. Molecular channels involving H atom migration leading to CH4 and NH3 formation in both iso-propylamine and n-propylamine are investigated and compared using intrinsic reaction coordinate calculations.