Browsing by Author "Cao, Zexing"

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Intersystem crossing-branched excited-state intramolecular proton transfer for o-nitrophenol: An ab initio on-the-fly nonadiabatic molecular dynamic simulation
(2016-05-25)
Xu, Chao
;
Yu, Le
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Zhu, Chaoyuan
;
Yu, Jianguo
;
Cao, Zexing
The 6SA-CASSCF(10, 10)/6-31G (d, p) quantum chemistry method has been applied to perform on-the-fly trajectory surface hopping simulation with global switching algorithm and to explore excited-state intramolecular proton transfer reactions for the o-nitrophenol molecule within low-lying electronic singlet states (S-0 and S-1) and triplet states (T-1 and T-2). The decisive photoisomerization mechanisms of o-nitrophenol upon S-1 excitation are found by three intersystem crossings and one conical intersection between two triplet states, in which T-1 state plays an essential role. The present simulation shows branch ratios and timescales of three key processes via T-1 state, non-hydrogen transfer with ratio 48% and timescale 300 fs, the tunneling hydrogen transfer with ratios 36% and timescale 10 ps, and the direct hydrogen transfer with ratios 13% and timescale 40 fs. The present simulated timescales might be close to low limit of the recent experiment results.
Multi-state nonadiabatic deactivation mechanism of coumarin revealed by ab initio on-the-fly trajectory surface hopping dynamic simulation
(2017-05-21)
Gan, Yanzhen
;
Yue, Ling
;
Guo, Xugeng
;
Zhu, Chaoyuan
;
Cao, Zexing
An on-the-fly trajectory surface hopping dynamic simulation has been performed for revealing the multi-state nonadiabatic deactivation mechanism of coumarin. The mechanism involves three adiabatic excited states, S-3(pi pi* L-b), S-2(n pi*, pi pi* L-a) and S-1(pi pi* L-a, n pi*), and the ground state S-0 at the four stateaveraged complete active space self-consistent field, SA(4)-CASSCF(12,10)/6-31G* level of theory. Upon photoexcitation to the third excited state S-3(pi pi* L-b) in the Franck-Condon region, 80% sampling trajectories decay to the dark S-2(n pi*) state within an average of 5 fs via the conical intersection S-3(pi pi* L-b)/S-2(n pi*), while 20% decay to the S-2(pi pi* L-a) state within an average of 11 fs via the conical intersection S3(pi pi* L-b)/S-2(pi pi* L-a). Then, sampling trajectories via S-2(n pi*)/S-1(pi pi* L-a) continue with ultrafast decay processes to give a final distribution of quantum yields as follows: 42% stay on the dark S-1(n pi*) state, 43.3% go back to the ground S-0 state, 12% undergo a ring-opening reaction to the Z-form S-0(Z) state, and 2.7% go to the E-form S-0(E) state. The lifetimes of the excited states are estimated as follows: the S-3 state is about 12 fs on average, the S-2 state is about 80 fs, and the S-1 state has a fast component of about 160 fs and a slow component of 15 ps. The simulated ultrafast radiationless deactivation pathways of photoexcited coumarin immediately interpret the experimentally observed weak fluorescence emission.
TDDFT studies for electronic excitations of the intermediates and radicals in the pyrolysis of 2,5-dimethylfuran
(2018-06-01)
Li, Yuanyuan
;
Cao, Zexing
;
Zhu, Chaoyuan
;
應用化學系
;
應用化學系分子科學碩博班
A large amount of complicated reaction networks of intermediates and radicals taken place in the pyrolysis of 2,5-dimethylfuran have been investigated based on the density function theory (DFT) and the sophisticated wave function theory (WFT) methodologies. The preliminary focus is concentrated on those without the furan-ring opening reaction processes. Calculations are performed for electronic structures, stability and electronic spectra of ground states and excited states for those intermediates and radicals. It is found that both low-lying valence excited states and Rydberg states (3s, 3P(x), 3P(y) and 3p(z)) of the pyrolytic species might be involved in pyrolysis and combustion of 2,5-dimethylfuran and influence their chemical reaction kinetics. A generous tendency is also found that the vertical transition energies of the similar transitions become bigger with the removal of methyl in the furan ring. (C) 2018 Author(s).