UV-VUV absorption spectroscopy and photodissociation dynamics of n-propylamine

Abstract

We report for the first time the ultraviolet-vacuum ultraviolet absorption spectrum of n-propylamine in the wavenumber range 40 000 to 75 000 cm⁻¹ (5.0–9.3 eV) using synchrotron radiation. A detailed spectral analysis is performed with the help of time dependent density functional theory based calculations. All the electronic excited states of n-propylamine are found to be Rydberg in nature, and quantum defect analysis is used to assign Rydberg series converging to the first ionization limit of n-propylamine. Theoretical simulation of the electronic absorption spectrum is done including vertical excitation energies of all five stable conformers which reproduces, quite accurately, the observed intensity profile of the room temperature absorption spectrum. Vibrational structures associated with the 3s, 3p and 4s transitions clearly show a dominant excitation of the NH2 wagging mode implying a transformation of the NH₂ group from the pyramidal structure in the ground state to planar in the excited state. Furthermore, relative intensities of vibronic bands in the 3s Rydberg are simulated using Franck–Condon factor calculations and show overall good agreement with experiments. Relaxed potential energy scans across dihedral angles using density functional theory reveal two stable conformers in the cationic ground state in contrast to five in the neutral ground state. Absorption cross section data in the ultraviolet region, a valuable input in atmospheric modelling, is used to simulate the variation of photolysis rate and lifetime of n-propylamine with terrestrial altitude. Additionally, detailed potential energy scans of excited states, both constrained and relaxed, along each of the possible bond cleavage channels are presented, revealing new insights into the photodissociation dynamics of the molecule.