Ab initio based surface-hopping dynamics study on ultrafast internal conversion in cyclopropanone

Cyclopropanone exhibits an intriguing phenomenon that the fluorescence from the S(1) state disappears below 365 nm. This is ascribed to the ultrafast S(1) → S(0) internal conversion process via conical intersection, which deprives opportunity of the fluorescence emission. In this work, we have used ab initio based surface hopping dynamics method to study vibrational-mode-dependent S(1) → S(0) internal conversion of cyclopropanone. A new conical intersection between the S(1) and S(0) states is determined by the state-averaged CASSCF/cc-pVDZ calculations, which is confirmed to play a critical role in the ultrafast S(1) → S(0) internal conversion by the nonadiabatic dynamics simulations. It is found that the internal conversion occurs more efficiently when the initial kinetic energies are distributed in the four vibrational modes related to the C═O group, especially in the C-O stretching and the O-C-C-C out-of-plane torsional modes. Meanwhile, the S(1) lifetime and the time scale of the S(1) → S(0) internal conversion are estimated by the ab initio based dynamics simulations, which is consistent with the ultrafast S(1) → S(0) internal conversion and provides further evidence that the ultrafast internal conversion is responsible for the fluorescence disappearance of cyclopropanone.     

Ab initio nonadiabatic quantum dynamics of cyclohexadiene/hexatriene ultrafast photoisomerization

Reaction mechanisms of the ultrafast photoisomerization between cyclohexadiene and hexatriene have been elucidated by the quantum dynamics on the ab initio potential energy surfaces calculated by multireference configuration interaction method. In addition to the quantum wave-packet dynamics along the two-dimensional reaction coordinates, the semiclassical analyses have also been carried out to correctly estimate the nonadiabatic transition probabilities around conical intersections in the full-dimensional space. The reaction time durations of radiationless decays in the wave-packet dynamics are found to be generally consistent with the femtosecond time-resolution experimental observations. The nonadiabatic transition probabilities among the ground (S0), first (S1), and second (S2) excited states have been estimated by using the semiclassical Zhu-Nakamura formula considering the full-dimensional wave-packet density distributions in the vicinity of conical intersections under the harmonic normal mode approximation. The cyclohexadiene (CHD) ring-opening process proceeds descending on the S1(1 1B) potential after the photoexcitation. The major part of the wave-packet decays from S1(1 1B) to S1(2 1A) by the first seam line crossing along the C2-symmetry-breaking directions. The experimentally observed ultrafast S1-S0 decay can be explained by the dynamics through the S1-S0 conical intersection along the direction toward the five-membered ring. The CHD: hexatriene (HT) branching ratio is estimated to be approximately 5:5, which is in accordance with the experiment in solution. This branching ratio is found to be mainly governed by the location of the five-membered ring S1-S0 conical intersection along the ground state potential ridge between CHD and HT.     

Ab initio molecular dynamics study of the hydrolysis reaction of diborane

The hydrolysis reaction of the diborane molecule in aqueous solution has been studied by a series of Car-Parrinello Molecular Dynamics simulations in the Blue Moon Ensemble. The total reaction has been divided into two parts: one dealing with the breaking of B(2)H(6) molecule and the formation of a BH(4)(-) ion, a H(2)BOH molecule and a H(+) ion; the second leads to the formation of two hydrogen molecules and another H(2)BOH molecule, starting from BH(4)(-), two water molecules and a H(+) ion. The total reaction studied in this work has been B(2)H(6) + 2H(2)O --> 2H(2)BOH + 2H(2). We have described both structurally and electronically the reagents and the products through the radial distribution functions and the Wannier Function Center positions calculations, with attention to the solvent effects on the compounds. The free energy barrier value for the first part of the reaction and a detailed mechanisms for both parts have been reported. An interesting behavior of BH(3) and H(2) molecules in solution has been observed. They form a quite stable three center bond between the electron pair of the hydrogen molecule and the empty orbital of the boron atom in BH(3), which has been described from both a structural and electronic point of view.     

Ab initio study on the photochemical behavior of styrene

Ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations have been performed to examine the photochemical behavior of styrene upon the strong S₀‐S₂ electronic excitation in the low‐lying excited states. The optimized structure at the S₂/S₁ conical intersection (CIX) is characterized by a quinoid structure. The transition state (TS) in S₁ is in the vicinity of the S₂/S₁‐CIX. At the S₁‐TS, two reaction paths branch. One is the relaxation into the stable structure in S₁ and then emission into S₀. The other is the radiationless decay through the S₁/S₀‐CIX. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 950–956, 2002     

Ab initio dynamics study of poly-para-phenylene vinylene

We present an ab initio dynamics investigation within a density-functional perturbation theory framework of the properties of the conjugated polymer poly-para-phenylene vinylene (PPV) in both the isolated chain and crystalline states. The calculated results show that for an isolated chain, most of the vibrational modes correspond to experimentally observed modes in crystalline PPV. However, additional hitherto unidentified modes have been observed in experiment and our calculations on crystalline material have allowed us to assign these. We also present the results of calculations of the polarizability and permittivity tensors of the material, which are in reasonable agreement with the typical values for conjugated polymers. Dynamical Born effective charges [S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73, 515 (2001)] are calculated and compared with atomic charges obtained from Mulliken population analysis [M. D. Segall, C. J. Pickard, R. Shah, and M. C. Payne, Mol. Phys. 89, 571 (1996)] and we conclude that effective charges are more appropriate for use in the study of the dynamics of the system. Notable differences are found in the infrared-absorption spectra obtained for the isolated chain and crystalline states, which can be attributed to the differences in the crystalline packing effects, which clearly play a key role in influencing the lattice dynamics of PPV.     

Ab initio study on the photochemical isomerization of furan derivatives

The photochemical isomerization reactions of furan, 2-methylfuran, 2-trimethylsilylfuran, and furan-2-carbonitrile were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. In particular, the sensitized irradiation of furan derivatives populates the first excited triplet state of the furan, and this triplet state can evolve only through O-Ca cleavage. The selection of the bond to be broken can depend on energetic factors (furan-2-carbonitrile) or on kinetic factors (2-methylfuran, 2-trimethylsilylfuran). The direct irradiation of furan derivatives populates the singlet excited state and leads to a conversion into the Dewar isomer or into the corresponding triplet state through the usual intersystem crossing procedure. The efficiency of these processes determines the presence or the absence of isomerized furan derivatives in the reaction mixtures.     

Ab initio trajectory surface-hopping study on ultrafast deactivation process of thiophene

The ultrafast S(1)((1)ππ*) → S(0) deactivation process of thiophene in the gas phase has been simulated with the complete active space self-consistent field (CASSCF) based fewest switch surface hopping method. It was found that most of the calculated trajectories (～80%) decay to the ground state (S(0)) with an averaged time constant of 65 ± 5 fs. This is in good agreement with the experimental value of about 80 fs. Two conical intersections were determined to be responsible for the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. After thiophene is excited to the S(1)((1)ππ*) state in the Franck-Condon region, it quickly relaxes to the minimum of the S(1)((1)ππ*) state, then overcomes a small barrier near the conical intersection (CI((1)ππ*/(1)πσ*)), and eventually arrives at the minimum of one C-S bond fission (S(1)((1)πσ*)). In the vicinity of this minimum, the conical intersection (CI((1)πσ*/S(0))) funnels the electron population to the ground state (S(0)), completing the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. This decay mechanism matches well with previous experimental and theoretical studies.     

Ab initio Ehrenfest dynamics

We present an ab initio direct Ehrenfest dynamics scheme using a three time-step integrator. The three different time steps are implemented with nuclear velocity Verlet, nuclear-position-coupled midpoint Fock integrator, and time-dependent Hartree-Fock with a modified midpoint and unitary transformation algorithm. The computational cost of the ab initio direct Ehrenfest dynamics presented here is found to be only a factor of 2-4 larger than that of Born-Oppenheimer (BO) dynamics. As an example, we compute the vibration of the NaCl molecule and the intramolecular torsional motion of H2C=NH2+ by Ehrenfest dynamics compared with BO dynamics. For the vibration of NaCl with an initial kinetic energy of 1.16 eV, Ehrenfest dynamics converges to BO dynamics with the same vibrational frequency. The intramolecular rotation of H2C=NH2+ produces significant electronic excitation in the Ehrenfest trajectory. The amount of nonadiabaticity, suggested by the amplitude of the coherent progression of the excited and ground electronic states, is observed to be directly related to the strength of the electron-nuclear coupling. Such nonadiabaticity is seen to have a significant effect on the dynamics compared with the adiabatic approximation.     

Ab initio molecular dynamics study of liquid methanol

We present a density-functional theory based molecular-dynamics study of the structural, dynamical, and electronic properties of liquid methanol under ambient conditions. The calculated radial distribution functions involving the oxygen and hydroxyl hydrogen show a pronounced hydrogen bonding and compare well with recent neutron diffraction data, except for an underestimate of the oxygen–oxygen correlation. We observe that, in line with infrared spectroscopic data, the hydroxyl stretching mode is significantly red-shifted in the liquid. A substantial enhancement of the dipole moment is accompanied by significant fluctuations due to thermal motion. Our results provide valuable data for improvement of empirical potentials.     

Ab initio and direct dynamics study of the reaction of Cl atoms with HOCO

The reaction of Cl with HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the reaction produces the HCl and CO(2) products as experimentally observed. The reaction occurs via a HOC(O)Cl intermediate with an estimated heat of formation of -97.8+/-2.0 kcal/mol. A direct ab initio dynamics method has been used to provide insight into the reaction mechanisms and to determine the thermal rate coefficients in the temperature range of 200-600 K. At room temperature, the thermal rate coefficient is predicted to be 3.0x10(-11) cm(3) molecule(-1) s(-1) with an activation energy of -0.2 kcal/mol. Two kinds of reactive trajectories are found. One kind proceeds through short-lived HOC(O)Cl complexes with a lifetime of 310 fs while the other kind occurs via longer-lived intermediates with a lifetime of 1.9 ps.     

Ab initio molecular dynamics calculations to study catalysis

The modern versions of the density functional theory (DFT), especially those using the generalized gradient approximation (GGA), have reached (almost) chemical accuracy and thus can be applied to study problems of real chemical interest such as catalysis. The important equations for the DFT, the local density approximation (LDA), and GGA are given. The full-potential linearized augmented plane wave method (LAPW) is used to check the accuracy of GGA in solids. The basic concepts of the ab initio molecular dynamics (MD) method by Car and Parrinello and its recent implementation using the projector augmented Wave (PAW) method which use a similar augmentation as LAPW are described. PAW applications to ferrocene and beryllocene are summarized, which illustrate that vibrational or fluxional behavior are well described. Sodalite, an aluminosilicate, is discussed as a generic zeolite in comparison with gmelinite. A study of the dynamics of such a system allows the determination of, e.g., the proton stretch vibrations which can be related to infrared spectra. This is illustrated for the OH stretch vibration of the acid site in silicon-rich sodalite. With this methodology, we are able to study the interaction of methanol trapped inside the cage structure of silicon-rich sodalite and to gain new insight into crucial steps of catalytic reactions, namely, the hydrogen-bonding and the possible protonation in this system, or a proton-exchange reaction. The strategies for parallelizing the PAW code are outlined. © 1997 John Wiley & Sons, Inc.     

Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the alizarin-TiO2 interface

The observed 6-fs photoinduced electron transfer (ET) from the alizarin chromophore into the TiO2 surface is investigated by ab initio nonadiabatic (NA) molecular dynamics in real time and at the atomistic level of detail. The system derives from the dye-sensitized semiconductor Gr?tzel cell and addresses the problems of an organic/inorganic interface that are commonly encountered in photovoltaics, photochemistry, and molecular electronics. In contrast to the typical Gr?tzel cell systems, where molecular donors are in resonance with a high density of semiconductor acceptor states, TiO2 sensitized with alizarin presents a novel case in which the molecular photoexcited state is at the edge of the conduction band (CB). The high level ab initio analysis of the optical absorption spectrum supports this observation. Thermal fluctuations of atomic coordinates are particularly important both in generating a nonuniform distribution of photoexcited states and in driving the ET process. The NA simulation resolves the controversy regarding the origin of the ultrafast ET by showing that although ultrafast transfer is possible with the NA mechanism, it proceeds mostly adiabatically in the alizarin-TiO2 system. The simulation indicates that the electron is injected into a localized surface state within 8 fs and spreads into the bulk on a 100-fs or longer time scale. The molecular architecture seen in the alizarin-TiO2 system permits efficient electron injection into the edge of the CB by an adiabatic mechanism without the energy loss associated with injection high into the CB by a NA process.     

Ab initio molecular dynamics study of formate ion hydration

We perform ab initio molecular dynamics simulations of the aqueous formate ion. The mean number of water molecules in the first solvation shell, or the hydration number, of each formate oxygen is found to be consistent with recent experiments. Our ab initio pair correlation functions, however, differ significantly from many classical force field results and hybrid quantum mechanics/molecular mechanics predictions. They yield roughly one less hydrogen bond between each formate oxygen and water than force field or hybrid methods predict. Both the BLYP and PW91 exchange correlation functionals give qualitatively similar results. The time dependence of the hydration numbers are examined, and Wannier function techniques are used to analyze electronic configurations along the molecular dynamics trajectory.     

Ab initio direct dynamics study of cyclopropyl radical ring-opening

Quasiclassical direct dynamics simulations, at the CASSCF(3,3)/6-31G(d) level of theory, are used to study the stereochemistry of the electrocyclic ring-opening reaction of the cyclopropyl radical. The trajectories are initiated at the reaction's transition state (TS), with their initial conditions sampled from the TS's 174 degrees C Boltzmann distribution. Intrinsic reaction coordinate calculations predict the overall reaction to have disrotatory stereochemistry. Though this is the preferred initial reaction stereochemistry in the trajectories, 43% of the trajectories follow the conrotatory path. Four unique trajectory types are observed during 200 fs dynamics of the product allyl radical. Intramolecular vibrational energy redistribution and internal rotation are incomplete on this time scale, and a statistical distribution of the allyl isomers is not observed.     

用从头算研究Shilov反应的机理

Shilov反应在CH~4活化中占有中心地位,它有氧化加成和σ迁移两种可能的机理。本文用较大基组的从头算研究了这两种机理的反应过程,认为Shilov反应应按氧化加成机理进行。     

Ab initio nonadiabatic dynamics study of ultrafast radiationless decay over conical intersections illustrated on the Na3F cluster

We present a theoretical approach for the ultrafast nonadiabatic dynamics based on the ab initio molecular dynamics carried out "on the fly" in the framework of the configuration interaction method combined with Tully's surface hopping algorithm for nonadiabatic transitions. This approach combined with our Wigner distribution approach allows us to perform accurate simulations of femtosecond pump-probe spectra in the systems where radiationless transitions among electronic states take place. In this paper we illustrate this by theoretical simulation of ultrafast processes and nonradiative relaxation in the Na(3)F cluster, involving three excited states and the ground electronic state. Furthermore, we show that our accurate simulation of the photoionization pump-probe spectrum is in full agreement with the experimental signal. Based on the nonadiabatic dynamics at high level of accuracy and taking into account all degrees of freedom, the nonradiative lifetime for the 1 (1)B(1) excited state of Na(3)F has been determined to be approximately 900 fs.     

Ab initio ONIOM-molecular dynamics (MD) study on the deamination reaction by cytidine deaminase

We applied the ONIOM-molecular dynamics (MD) method to the hydrolytic deamination of cytidine by cytidine deaminase, which is an essential step of the activation process of the anticancer drug inside the human body. The direct MD simulations were performed for the realistic model of cytidine deaminase by calculating the energy and its gradient by the ab initio ONIOM method on the fly. The ONIOM-MD calculations including the thermal motion show that the neighboring amino acid residue is an important factor of the environmental effects and significantly affects not only the geometry and energy of the substrate trapped in the pocket of the active site but also the elementary step of the catalytic reaction. We successfully simulate the second half of the catalytic cycle, which has been considered to involve the rate-determining step, and reveal that the rate-determining step is the release of the NH3 molecule.     

Ab initio photoionization dynamics of beta-alanine

Photoionization dynamics of beta-alanine is studied by the trajectory simulations using the ab initio potential energy surface. Vertical photoionization in the spirit of the Franck-Condon principle is assumed both for the adiabatic and thermostatic simulations. Both intramolecular proton transfer and fragmentation while only the proton transfer are found in the thermostatic and adiabatic simulations, respectively, for the conformer having the intramolecular hydrogen bond N...H-O. The theoretical predictions are in line with the experimental observations available in the literatures. It is reported for the first time that the thermostatic temperatures strongly affect the fragmentation processes induced by photoionization.     

Ab initio study on the reaction of uranium with oxygen

The primary reaction products and reaction mechanism of uranium with oxygen were discussed from MP2 method with the relativistic core potential of SDD basis set for U and 6-311+G* for O. The molecular geometries, electronic structure and energies of uranium oxides were obtained. The inspection on the three-dimensional potential energy surfaces of the U–O₂ interaction suggested that the abstraction and insertion mechanism were responsible for the studied reactions. The abstraction reaction channel resulting in the formation of UO and O is favored because the energy barrier is remarkably smaller than the one of the insertion channel resulting in the linear OUO product directly.