Browsing by Author "Xu, Chao"

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An excited-state Wolff rearrangement reaction of 5-diazo Meldrum's acid: an ab initio on-the-fly nonadiabatic dynamics simulation
(2018-09-21)
Xu, Chao
;
Gu, Feng Long
;
Zhu, Chaoyuan
;
交大名義發表
;
應用化學系
A global switching on-the-fly trajectory surface hopping dynamics simulation at the 3SA-CASSCF(12,11)/6-31G* quantum level has been employed to probe the photo-induced Wolff rearrangement (WR) reaction of 5-diazo Meldrum's acid (DMA) within three low-lying electronic excited states. The present simulation predicted that the branching ratios for relaxing back to the ground state, isomerizing to diazirine, and reaction to ketene I via carbene I are 69% +/- 0.1, 3% +/- 0.4, and 28% +/- 0.1, which are in excellent agreement with those obtained by the femtosecond spectroscopy experiment, 67%, 3% and 30%, respectively. In particular, the present simulation revealed that the major WR reaction to ketene I pathway is stepwise via the excited-state to carbene I (17.8% +/- 0.2) and via the ground-state to carbene I (8.7% +/- 0.2), and the minor pathway is concerted synchronous (1.5% +/- 0.6). The photo-induced WR reaction of DMA has been quantitatively interpreted in terms of the distribution of extended seam surfaces as a function of CN dissociation bonds for two important conical intersections within three low-lying electronic excited states. Ultrafast dynamic time constants have been estimated to be about 500 fs +/- 120 fs and 180 fs +/- 80 fs for the stepwise and the concerted WR reaction to ketene I which are also in good agreement with those determined by the experiment. Therefore, the photo-induced excited-state WR reaction mechanism has been quantitatively revealed by the present real-time dynamics simulation.
Benchmark Performance of Global Switching versus Local Switching for Trajectory Surface Hopping Molecular Dynamics Simulation: Cis <-> Trans Azobenzene Photoisomerization
(2017-05-19)
Yue, Ling
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Yu, Le
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Xu, Chao
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Lei, Yibo
;
Liu, Yajun
A newly developed global switching algorithm that does not require calculation of nonadiabatic coupling vectors reduces computational costs significantly. However, the accuracy of this simplest nonadiabatic molecular dynamic method has not been extensively compared with the conventional Tully's fewest switches. It is necessary to demonstrate the accuracy of this global switching algorithm. An extensive comparison between local and global switching on-the-fly trajectory surface hopping molecular dynamics is performed for cis-to-trans (800 sampling trajectories) and trans-to-cis (600 sampling trajectories) azobenzene photoisomerization at the OM2/MRCI level. The global switching algorithm is coded into the Newton-X program package. Excellent agreement between the two switching algorithms is obtained not only for highly averaged quantities of quantum yields and lifetimes, but also for detailed contour patterns of product distributions, hopping spot distributions and hopping directions in terms of conical intersections between ground and the first excited states. Therefore, the global switching trajectory surface hopping method can be applied to larger complex systems in which nonadiabatic coupling is not available for excited-state molecular dynamic simulations.
Functional and Basis Set Dependence for Time-Dependent Density Functional Theory Trajectory Surface Hopping Molecular Dynamics: Cis-Azobenzene Photoisomerization
(1970-01-01)
Ye, Linfeng
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Xu, Chao
;
Gu, Feng Long
;
Zhu, Chaoyuan
;
交大名義發表
Within three functionals (TD-B3LYP, TD-BHandHLYP, and TD-CAM-B3LYP) in combination with four basis sets (3-21g, 6-31g, 6-31g(d), and cc-pvdz), global switching (GS) trajectory surface hopping molecular dynamics has been performed for cis-to-trans azobenzene photoisomerization up to the S-1(n pi*) excitation. Although all the combinations show artificial double-cone structure of conical intersection between ground and first excited states, simulated quantum yields and lifetimes are in good agreement with one another; 0.6 (+/- 5%) and 40.5 fs (+/- 10%) by TD-B3LYP, 0.5 (+/- 10%) and 35.5 fs (+/- 4%) by TD-BHandHLYP, and 0.44 (+/- 9%) and 35.2 fs (+/- 10%) by TD-CAM-B3LYP. By analyzing distributions of excited-state population decays, hopping spots, and typical trajectories with performance of 12 functional/basis set combinations, it has been concluded that functional dependence for given basis set is slightly more sensitive than basis set dependence for given functional. The present GS on-the-fly time-dependent density functional theory (TDDFT) trajectory surface hopping simulation can provide practical benchmark guidelines for conical intersection driven excited-state molecular dynamics simulation involving in large complex system within ordinary TDDFT framework. (c) 2019 Wiley Periodicals, Inc.
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
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Yu, Le
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Zhu, Chaoyuan
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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.
Photoisomerization Reaction Mechanisms of o-Nitrophenol Revealed by Analyzing Intersystem Crossing Network at the MRCI Level
(2015-10-22)
Xu, Chao
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Yu, Le
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Zhu, Chaoyuan
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Yu, Jianguo
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應用化學系分子科學碩博班
6SA-CASSCF(10, 10) /6-31G (d, p) and MRCl/cc-pVDZ methods were performed to probe photoisomerization reaction mechanisms of o-nitrophenol. Two low-lying singlet electronic states (S-0 and S-1) and two low-lying triplet electronic states (T-1 and T-2) were found to weave an intersystem crossing network in which a dominant stepwise photoisomerization provides a very efficient reaction pathway; the reaction takes place in the wide region of crossing seam-surface woven by S-1 and T-1 states first, followed by T-1 and S-0 states. Both intersystem crossing regions show strong spin orbital coupling in the order of 40 wavenumbers. All nitro and aci-nitro isomers and transition states on four electronic potential energy surfaces are calculated along with analysis of both dominant and subdominant relaxation pathways, especially weak spin-orbital coupling (similar to 10 wavenumbers) between T-2 and S-1 states and effective conical intersection between T-2 and T-1 states opening a new relaxation pathway S-1 -> T-2 -> T-1.
Probing the pi -> pi* photoisomerization mechanism of cis-azobenzene by multi-state ab initio on-the-fly trajectory dynamics simulation
(2015-01-01)
Yu, Le
;
Xu, Chao
;
Zhu, Chaoyuan
;
應用化學系分子科學碩博班
;
Institute of Molecular science
Based on a newly developed algorithm to compute global nonadiabatic switching probability by using only electronic adiabatic potential energy surfaces and gradients, we performed on-the-fly, trajectory-surface hopping simulations at the 5SA-CASSCF(6,6)/6-31G quantum level to probe the pi -> pi* photoisomerization mechanism of the azobenzene within four singlet low-lying electronic states (S-0, S-1, S-2, and S-3) coupled with a complicated conical intersection network. We found that four conical intersections between the S-1 and S-2 states (one is near the cis-isomer region, another near the trans-isomer region, and two others between cis and trans) play the most important roles for understanding the photoisomerization mechanism of azobenzene upon S-2 and S-3 pi pi* excitation. We studied six cases to demonstrate the photoisomerization mechanism in detail by choosing eight (six) typical reactive (nonreactive) trajectories, namely, two-step fast-fast processes having lifetimes of several tenths to one hundred femtoseconds and two-step, fast-slow and slow-slow processes having lifetimes of several hundred to one thousand femtoseconds. We found for the first time from simulation that once a trajectory visits the conical intersection near the trans-isomer after pi pi* excitation, it could rapidly go through the inversion pathway to trans-azobenzene, and confirms the most recent experimental observations. We performed 536 sampling trajectories (336 from S-2 and 200 from S-3), initially starting from the Franck-Condon region of cis-azobenzene, and obtained a total reactive quantum yield of 0.3-0.45 in very good agreement with recent experimental results of 0.24-0.50. Moreover, the current method can estimate overall nonadiabatic transition probability for each sampling trajectory from beginning to end. This can greatly accelerate convergence of nonadiabatic molecular dynamic simulation, and, for instance, results in a quantum yield of 0.53 estimated from only eight typical reactive trajectories.
Probing the pi -> pi* photoisomerization mechanism of trans-azobenzene by multi-state ab initio on-the-fly trajectory dynamics simulations
(2018-10-07)
Xu, Chao
;
Yu, Le
;
Gu, Feng Long
;
Zhu, Chaoyuan
;
交大名義發表
Global nonadiabatic switching on-the-fly trajectory surface hopping simulations at the 5SA-CASSCF(6,6)/6-31G quantum level have been employed to probe the photoisomerization mechanism of trans-azobenzene upon pi pi* excitation within four coupled singlet low-lying electronic states (S-0, S-1, S-2, and S-3). We have performed 586 sampling trajectories (331 starting from S-2 and 255 from S-3), and we found about half of the sampling trajectories staying on S-1 or S-2 states as resonances and the other half of them ending on the ground S-0 state as active trajectories. The present simulation has demonstrated that there are six distinct photoisomerization pathways which can be summarized as three categories; one is the newly opened inversion-inversion nonreactive isomerization pathway accounting for 40% (34%) of active trajectories at a time constant of 80 fs (320 fs), the other is the inversion-torsion reactive and nonreactive isomerization pathways accounting for 40% (20%) of active trajectories at a time constant of 880 fs (1700 fs), and the third is the torsion-torsion reactive and nonreactive isomerization pathways accounting for 20% (46%) of active trajectories at a time constant of 780 fs (1000 fs) upon S-2 (S-3) pi pi* excitation. The simulated total reactive quantum yield for trans-azobenzene photoisomerization upon S-2 (S-3) pi pi* excitation is about 0.11 (0.13) which is in good agreement with recent experimental results of 0.09-0.20. Furthermore, the newly opened inversion-inversion nonreactive isomerization pathway from the present simulation agrees well with cascade experimental measurements of the S-n -> S-1 -> S-0 relaxation mechanism in both branching ratio and time constant.
Quantum yields of singlet and triplet chemiexcitation of dimethyl 1,2-dioxetane: ab initio nonadiabatic molecular dynamic simulations
(2020-05-28)
Yue, Ling
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Yu, Le
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Xu, Chao
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Zhu, Chaoyuan
;
Liu, Yajun
The global nonadiabatic switching on-the-fly trajectory surface hopping simulation at the 8SA-CASSCF quantum level has been performed to estimate the quantum yield of chemiexcitation for the uncatalyzed decomposition reaction of the open-shell biradical trans-3,4-dimethyl-1,2-dioxetane system. The present ab initio nonadiabatic molecular dynamic simulation involving both conical intersection and intersystem crossing is to compute for the first time the population evolution of quantum yields at the four lowest singlet and four lowest triplet states. The simulated results demonstrate not only the stepwise dissociation of O-O and C-C bond breaking, but also confirm the existence of a biradical entropic trap which is responsible for chemiexcitation. The simulated quantum yield of the triplet chemiexcitation phi(T1) = 0.266 +/- 0.096 agrees with the experimental value of 0.20 +/- 0.04 very well. The present nonadiabatic molecular dynamic simulation of dimethyl 1,2-dioxetanes provides a further advanced understanding and stepping stone for future studies on chemi- and bio-luminescence.
Remarkable nonlinear optical response of excess electron compounds: theoretically designed alkali-doped aziridine M-(C2NH5)(n)
(2017-09-21)
Li, Bo
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Xu, Chao
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Xu, Xuan
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Zhu, Chaoyuan
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Gu, Feng Long
Theoretically designed alkali-doped aziridine M-(C2NH5)(n) (M = Li, Na and K; n = 1, 2, 3, and 4) are investigated by density functional theory (DFT) and time-dependent TD-DFT. The interaction energies at optimized electronic structures indicate that alkali-doped aziridine are quite stable. The natural population analysis of charges on alkali atoms show that all positive and electronic transitions to LUMO orbitals are large, so that the designed compounds not only have electride features, but also have large flexible ligands. This leads to a high-performance nonlinear optical response (NLO) and this remarkable NLO response mainly comes from alkali atoms. By calculating the first hyperpolarizabilities for M-(C2NH5)(4) with M@ Calix[4] pyrrole for comparison, we demonstrate that enhancements of the NLO response of M-(C2NH5)(4) are 10 to 100 times larger than those of M@ Calix[ 4] pyrrole, and in particular, the largest first hyperpolarizability values of Na-(C2NH5)(4) is up to 3.4 x 10(6) (a. u.).
Trajectory-based nonadiabatic molecular dynamics without calculating nonadiabatic coupling in the avoided crossing case: trans <-> cis photoisomerization in azobenzene
(2014-01-01)
Yu, Le
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Xu, Chao
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Lei, Yibo
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Zhu, Chaoyuan
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Wen, Zhenyi
We develop a novel method to simulate analytical nonadiabatic switching probability based on effective coupling and effective collision energy by using only electronic adiabatic potential energy surfaces and its gradients in the case of avoided crossing types of nonadiabatic transitions. In addition, the present method can keep the same time step for computing both on-the-fly trajectory and nonadiabatic transitions accurately. The present method is most useful for localized nonadiabatic transitions induced by conical intersection. We employ the on-the-fly surface hopping algorithm with an ab initio quantum chemistry calculation to demonstrate a dynamic simulation for photoisomerization in azobenzene. Simulated quantum yield and lifetime converge to 0.39 and 53 femtosecond, respectively (0.33 and 0.81 picosecond) for cis-to-trans (trans-to-cis) photoisomerization with up to 800 (600) sampling trajectories. The present results agree well with those of the experiment, as well as results simulated with use of nonadiabatic coupling within Tully\'s fewest switching method. The present trajectory-based nonadiabatic molecular dynamics free from nonadiabatic coupling greatly enhances the simulation power of molecular dynamics for large complex chemical systems.
Ultrafast intersystem crossing for nitrophenols: ab initio nonadiabatic molecular dynamics simulation
(2018-02-28)
Xu, Chao
;
Gu, Feng Long
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Zhu, Chaoyuan
;
應用化學系
;
應用化學系分子科學碩博班
Ultrafast intersystem crossing mechanisms for two p- and m-nitrophenol groups (PNP and MNP) have been investigated using ab initio nonadiabatic molecular dynamics simulations at the 6SA-CASSCF level of theory. Trajectory surface hopping simulation has been performed within an intersystem crossing network constructed from two low-lying singlets (S-0 and S-1) and two low-lying triplets (T-1 and T-2). It is found that the dominant relaxation S-1 -> T-2 pathway accounts for 65.4% (85.0%) of the quantum yield with a time constant of 13.4 fs (22 fs) and the S-1 -> T-2 -> S-0 pathway accounts for 33.1% (13.5%) with a time constant of 275 fs ( 375 fs) for PNP ( MNP). In comparison with the previously studied excited-state proton transfer process for ONP, the dominant relaxation S-1 -> T-2 -> T-1 pathway accounts for 49.3% with a time constant of 40 fs and the S-1 -> T-2 -> T-1 -> S-0 pathway accounts for 47.5% with a time constant of 300 fs. The relaxation mechanisms and electronic structures of the intersystem crossings are in close relation with the relative motion between the torsion motions of the nitro-group and the hydroxyl group. The present simulation provides new physical insight for understanding ultrafast photochemical intersystem crossing dynamics.