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Decay dynamics of 6‐azauracil from the light absorbing S 2 (ππ*) state
Author(s) -
Li Pengli,
Xue Jiadan,
Zheng Xuming
Publication year - 2019
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.5518
Subject(s) - intersystem crossing , conical intersection , internal conversion , chemistry , raman spectroscopy , atomic physics , complete active space , wave packet , density functional theory , molecular physics , absorption spectroscopy , excited state , spectroscopy , spectral line , physics , computational chemistry , optics , quantum mechanics , singlet state , basis set
The initial decay dynamics of 6AU in the S 2 state was investigated by using resonance Raman spectroscopy, the time‐dependent wave‐packet theory in a Brownian oscillator model, and complete‐active space self‐consistent field (CASSCF) protocol. The vibrational spectra and the ultraviolet absorption bands were assigned on the basis of the Fourier transform (FT)‐Raman, FT‐infrared measurements, the density‐functional theory computations, and the normal mode analysis. The absorption cross section and the absolute resonance Raman cross sections were simulated simultaneously by using the time‐dependent wave‐packet theory in a Brownian oscillator model. The obtained normal mode displacements of the Franck–Condon active modes were then converted to the short‐time structural dynamics in easy‐to‐visualize internal coordinates. The roles of two pathways via the S 2 → S 1 internal conversion and the S 2 → T 3 intersystem crossing were evaluated through the comparison between the short‐time structural dynamics and the CASSCF/CASTP2 calculated structural changes between FC and S 2 S 1 or between FC and S 2 T 3 . The results indicate that the structural dynamics in the Franck–Condon region of the S 2 state is mostly toward the S 2 /S 1 conical intersection, which supports the conclusion that the S 2 → S 1 internal conversion dominants the initial decay pathway of 6AU as revealed by the broadband fs TA spectroscopy, whereas the barrierless S 2 → T 3 intersystem‐crossing process is negligible owing to weak spin‐orbital coupling and unfavorable subsequent T 3 → T 2 → T 1 decay. The reaction coordinates towards the planar S 2 (ππ*)/S 1 (nπ*) and severely distorted S 2 (ππ*)/S 0 conical intersection points are proposed respectively, and the solvent effect on the initial decay mechanisms of 6AU, uracil, and thymine is clarified.