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

Abstract

We report for the first time, the UV-VUV 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 TDDFT based calculations. All the electronic excited states of n-propylamine are found to be Rydberg in nature, quantum defect analysis is used to assign Rydberg series converging to the first IP of n-propylamine. Theoretical simulation of the electronic absorption spectrum is done including vertical excited states of all five stable conformers which reproduces quite accurately, the observed intensity profile of the room temperature absorption spectrum. Vibrational structure associated with the 3s, 3p and 4s transitions clearly shows a dominant excitation of the NH2 wagging mode implying a transformation of the NH2 group from pyramidal structure in the ground state to planar in the excited state. Further, relative intensities of vibronic bands in the 3s Rydberg are simulated using Franck-Condon factor calculations and show overall good agreement with experiment. Relaxed potential energy scans across dihedral angles using DFT method reveal two stable conformers in the cationic ground state in contrast to five in the neutral ground state, which may have important implications in astrophysical studies. Absorption cross section data in the UV 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.