Ab initio simulation of pyrene spectra in water matrices

Abstract

The absorption and emission spectra of free pyrene and pyrene in a water ice matrix were simulated ab initio. Water ice was mimicked by a large cluster of explicit water molecules. The optimum geometries of the ground and excited states, vibrational frequencies, and normal modes were calculated using DFT. The transition energies were calculated by XMCQDPT/CASSCF. Ab initio time-dependent Franck–Condon (FC) and time-independent Franck–Condon and Herzberg–Teller (FCHT) approximations were used for the vibronic profiles of the spectra. The absorption spectra of the free molecule and the molecule in the water cluster are well reproduced within the FC approximation. The fluorescence spectrum of the gas-phase pyrene cannot be satisfactorily reproduced within pure FC or within the FCHT approximation. However, the fluorescence spectrum of pyrene in the water cluster is satisfactorily reproduced within the FC approximation. Both absorption and emission spectra of pyrene in the water cluster are broadened due to the translations and torques of pyrene in the solvent cage even at low temperatures. This broad but well structured spectrum shape should be taken into account in the identification of PAHs in cometary ice.