Nonadiabatic interactions in wave packet dynamics of the bromoacetyl chloride photodissociation

The competitive photodissociation of bromoacetyl chloride BrCH2COCl in the first 1A" state (S1) by 248 nm photons is investigated by nonadiabatic wave packet simulations. We show that the preferential breaking of the stronger C-Cl bond (alpha to the excited carbonyl) over the weaker C-Br bond (beta) could be explained by a diabatic trapping or nonadiabatic recrossing as previously proposed. Our energy resolved flux analysis agrees fairly well with the experimental branching ratio (C-Cl:C-Br=1.0:0.4). Even if this does not prove the mechanism, this at least prevents to discard it. A reduced dimensionality approach based on constrained Hamiltonian is used. The nonadiabatic dissociation is studied in the two C-O/C-X (X=Br, Cl) subspaces to emphasize the role of the C-O vibration upon [nO-->piCO*] excitation. The internal torsion and wagging dihedral angles are frozen at their Franck-Condon value, according to preliminary dynamical tests. The other inactive coordinates are optimized at the trans and Cs constrained geometry in the first excited state. Corresponding 2D cuts in the potential energy surfaces have been computed at the CASSCF level. The nonadiabatic kinetic couplings are highly peaked along an avoided crossing seam in both cases. A two-state diabatic model with a constant potential coupling is proposed in the two C-O/C-X subspaces. The inclusion of the C-O stretching in the active coordinates improves the value of the branching ratio over our previous 1D computation.     

Nonadiabatic effects in C-Br bond scission in the photodissociation of bromoacetyl chloride

Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and co-workers [J. Chem. Phys. 95, 3848 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH2C(O)Cl was prepared in its ground electronic state (S0) and excited with a laser at 248 nm to its first excited singlet state (S1). The two main ensuing photoreactions are the ruptures of the C-Cl bond and of the C-Br bond. A nonadiabatic model was proposed in which the C-Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S1 and S2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C-Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S0 and S1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C-Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C-Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations.     

Combined CASSCF and MR-CI study on photoinduced dissociation and isomerization of acryloyl chloride

The potential energy surfaces of isomerization and dissociation reactions for CH2CHCOCl in the S0, T1, T2, and S1 states have been mapped with DFT, CASSCF, MP2, and MR-CI calculations. Rate constants for adiabatic and nonadiabatic processes have been calculated with the RRKM rate theory, in conjugation with the vibronic interaction method. Mechanistic photochemistry of CH2CHCOCl at 230-310 nm has been characterized through the computed potential energy surfaces and rate constants. Upon photoexcitation of CH2CHCOCl at 310 nm, the S1-->T1 intersystem crossing is the dominant primary process, which is followed by the 1,3-Cl migration along the T1 pathway. Meanwhile, the S1-->S0 internal conversion occurs with considerable probability and the subsequent trans-cis isomerization proceeds in the ground state. The C-Cl bond cleavage is an exclusive primary channel upon photoexcitation of gaseous CH2CHCOCl at 230 nm. The direct C-Cl bond cleavage is partially blocked by effects of the matrix, and the internal conversion from S1 to S0 becomes an important process for the excited molecule to deactivate in the condensed phase. The present calculations not only provide a reasonable explanation of the experimental findings, but also give new insight into the mechanistic photochemistry of CH2CHCOCl.     

Adiabatic and nonadiabatic bond cleavages in Norrish type I reaction

One of the fundamental photoreactions for ketones is Norrish type I reaction, which has been extensively studied both experimentally and theoretically. Its α bond-cleavage mechanisms are usually explained in an adiabatic picture based on the involved excited-state potential energy surfaces, but scarcely investigated in terms of a nonadiabatic picture. In this work, the S(1) α bond-cleavage reactions of CH(3)OC(O)Cl have been investigated by using the CASSCF and MRCI-SD calculations, and the ab initio based time-dependent quantum wavepacket simulation. The numerical results indicate that the photoinduced dissociation dynamics of CH(3)OC(O)Cl could exhibit strong nonadiabatic bond-fission characteristics for the S(1) α C-Cl bond cleavage, while the dynamics of the S(1) α C-O bond cleavage is mainly of adiabatic characteristics. This nonadiabatic mechanism for Norrish type I reaction of CH(3)OC(O)Cl is uncovered for the first time. The quantum wavepacket dynamics, based on the reduced-dimensional coupled potential energy surfaces, to some extent illustrates the significance of the nonadiabatic effect in the transition-state region on the dynamics of Norrish type I reaction.     

Direct observation of the primary and secondary C-Br bond cleavages from the 1,2-dibromopropane photodissociation at 234 and 265 nm using the velocity map ion imaging technique

Photodissociation dynamics of 1,2-dibromopropane has been investigated at 234 and 265 nm by using the velocity map ion imaging method. At both pump energies, a single Gaussian-shaped speed distribution is observed for the Br*((2)P(1/2)) fragment, whereas at least three velocity components are found to be existent for the Br((2)P(3/2)) product. The secondary C-Br bond cleavage of the bromopropyl radical which is energized from the ultrafast primary C-Br bond rupture should be responsible for the multicomponent translational energy distribution at the low kinetic energy region of Br((2)P(3/2)). The recoil anisotropy parameter (beta) of the fragment from the primary C-Br bond dissociation is measured to be 0.53 (0.49) and 1.26 (1.73) for Br((2)P(3/2)) and Br*((2)P(1/2)), respectively, at 234 (265) nm. The beta value of Br((2)P(3/2)) from the secondary C-Br bond dissociation event at 265 nm is found to be 0.87, reflecting the fact that the corresponding Br((2)P(3/2)) fragment carried the initial vector component of the bromopropyl radical produced from the primary bond dissociation event. Density functional theory has been used to calculate energetics involved both in the primary and in the secondary C-Br bond dissociation dynamics.     

A two-dimensional wavepacket study of the nonadiabatic dynamics of CH2BrCl

The nonadiabatic photodissociation dynamics of CH2BrCl into CH2Br + Cl or CH2Cl + Br is studied using two-dimensional wavepacket propagations on ab initio multiconfigurational MS-CASPT2 potential energy surfaces. Using a three-state diabatic model, we investigate the electronic states responsible for the two competing fragmentation channels and how the conical intersection present between the two lowest excited states affects the dissociation rate. Within this model, we find that the Br/Cl branching ratio depends on the irradiation wavelength. Predominant C-Br fragmentation occurs for wavelengths longer than 200 nm, while nonadiabatic C-Cl dissociation with a constant branching ratio of 0.4 is predicted upon absorption of photons in the range of 170-180 nm. Additionally, we observe complete nonadiabatic population transfer in less than 100 fs, that is, before the wavepacket can reach the conical intersection. As a consequence, there is no three-body CH2 + Br + Cl dissociation.     

Multireference theoretical investigation on selectivity of the bond fissions in photodissociation of acetyl cyanide

The selectivity of the C-CH(3) and C-CN bond fissions upon excitation of acetyl cyanide at 193 nm has been investigated at the theoretical level of multistate complete active space self-consistent field second order perturbation. The calculated results indicated that the initially excited S(3) state relaxes to S(2) via ultrafast internal conversion. The S(2) state could dissociate via two pathways. One, adiabatically dissociates to CH(3)CO(X)+CN(A). The other one internally converts to S(1) before S(1) intersystem crossing to T(1). The T(1) state subsequently dissociates to two groups of products: CH(3)(X)+OCCN(X) and CH(3)CO(X)+CN(X). The experimentally observed preference branching of CN elimination over CH(3) one and bond selectivity are the results of the competition between the adiabatic and nonadiabatic dynamics of the S(2) state.     

Density functional and spin-orbit ab initio study of CF3Br: molecular properties and electronic curve crossing

Quantum chemical calculations of CF(3)Br and the CF(3) radical are performed using density functional theory (DFT) and time-dependent DFT (TDDFT). Molecular structures, vibrational frequencies, dipole moment, bond dissociation energy, and vertical excitation energies of CF(3)Br are calculated and compared with available experimental results. The performance of six hybrid and five hybrid meta functionals in DFT and TDDFT calculations are evaluated. The ωB97X, B3PW91, and M05-2X functionals give very good results for molecular structures, vibrational frequencies, and vertical excitation energies, respectively. The ωB97X functional calculates well the dipole moment of CF(3)Br. B3LYP, one of the most widely used functionals, does not perform well for calculations of the C-Br bond length, bond dissociation energy, and vertical excitation energies. Potential energy curves of the low-lying excited states of CF(3)Br are obtained using the multiconfigurational spin-orbit ab initio method. The crossing point between 2A(1) and 3E states is located near the C-Br bond length of 2.45 ?. Comparison with CH(3)Br shows that fluorination does not alter the location of the crossing point. The relation between the calculated potential energy curves and recent experimental result is briefly discussed.     

Four-dimensional quantum study on exothermic complex-forming reactions: Cl- + CH3Br-->ClCH3+Br-

The exothermic gas-phase bimolecular nucleophilic substitution (S(N)2) reaction Cl(-)+CH(3)Br (upsilon1',upsilon2',upsilon3')-->ClCH(3) (upsilon1,upsilon2,upsilon3)+Br- and the corresponding endothermic reverse reaction have been studied by time-independent quantum scattering calculations in hyperspherical coordinates on a coupled-cluster potential-energy surface. The dimensionality-reduced model takes four degrees of freedom into account [Cl-C and C-Br stretching modes (quantum numbers upsilon3' and upsilon3); totally symmetric modes of the methyl group, i.e., C-H stretching (upsilon1' and upsilon1) and umbrella bending vibrations (upsilon2' and upsilon2)]. Diagonalization of the Hamiltonian was performed employing the Lanczos algorithm with a variation of partial reorthogonalization. A narrow grid in the total energy was employed so that long-living resonance states could be resolved and extracted. While excitation of the reactant umbrella bending mode already leads to a considerable enhancement of the reaction probability, its combination with vibrational excitation of the broken C-Br bond, (0, 1, 1), results in a strong synergic effect that can be rationalized by the similarity with the classical transitional normal mode. Exciting the C-H stretch has a non-negligible effect on the reaction probability, while for larger translational energies this mode follows the expected spectatorlike behavior. Combination of C-Br stretch and symmetric C-H, (1,0,1), stretch does not show a cooperative effect. Contrary to the spectator mode concept, energy originally stored in the C-H stretching mode is by no means conserved, but almost completely released in other modes of the reaction products. Products are most likely formed in states with a high degree of excitation in the new C-Cl bond, while the internal modes of the methyl group are less important. Reactants with combined umbrella/C-Br stretch excitation, (0, 1, 1), may yield products with two quanta in the umbrella mode.     

Theoretical study of structure, stability and infrared spectra of CH3X-SO3 (X = F, Cl, Br) complexes

Ab initio computational study of the electronic structure and infrared spectra of donor-acceptor complexes formed between SO3 and CH3X (X = F, Cl, Br) molecules was carried out at the MP2(full)/6-31G(d) level of theory. The calculated complexation energy at G2MP2 level shows that stability of complexes decrease, as CH3Cl-SO3 > CH3Br-SO3 > CH3F-SO3. The NBO partitioning scheme show that the lengthening of the C-F, C-Cl, and C-Br bond lengths, upon complexation, is due to an decreasing "s" character in these bonds.     

Highly asymmetric coordination in alkenes: gas-phase structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene

The molecular structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene have been determined by gas-phase electron diffraction (GED) and ab initio molecular orbital calculations (MP2/6-311G). Both compounds were found to have highly asymmetric coordination around the carbon atoms with [ab initio (r(e))/GED (r(a))] C=C-Cl [117.0/117.0(2) degrees] and C=C-Si [126.2/128.1(1) degrees] in the C(2)(h) structure of trans-1,2-dichloro-1,2-disilylethene and C=C-Br [119.2/120.7(4) degrees] and C=C-Si [125.0/125.0(4) degrees] in the C(s) structure of 1-bromo-1-silylethene. Other important structural parameters for trans-1,2-dichloro-1,2-disilylethene are C=C [135.2/134.5(3) pm], C-Si [189.4/187.9(2) pm], and C-Cl [175.1/174.9(1) pm], and C=C [134.2/133.4(2) pm], C-Si [187.8/187.2(3) pm], and C-Br [191.3/191.0(3) pm] for 1-bromo-1-silylethene. Further ab initio calculations were carried out on CH(2)CRX and trans-(CRX)(2) (R = SiH(3), CH(3), or H; X = H, F, Cl, or Br) to gauge the effects of electron-withdrawing and electron-donating groups on the structures. They reveal some even more distorted structures. The asymmetric appearance of these molecules can largely be accounted for by valence shell electron pair repulsion theory.     

An application of HPLC and ion chromatography to study intramolecular charge transfer in polyhalogenated systems

Reaction of hydrated electrons with aromatics substituted with both bromine and chlorine results in the production of significant yields of chloride ion and the complementary bromine substituted phenyl radical. The total yields show that in all cases the reduction is essentially quantitative. For the dihalogenated benzenes and phenols the relative yields for C-Br and C-Cl bond rupture (0.86:0.14) reflect the relative rates for electron attachment at the Br and Cl positions, suggesting that there is little intramolecular charge transfer on the time scale of dissociation of the initial anion. In the case of dihalogenated benzoates about 40% of the reduction results in C-Cl bond rupture. In this case the added charge initially localized on the carboxyl group is transferred preferentially to the chlorine atom indicating that solvation of the intermediate radical anion must play an extremely important role in controlling the overall reduction process.     

Nonadiabatic proton-coupled electron transfer reactions: impact of donor-acceptor vibrations, reorganization energies, and couplings on dynamics and rates

Fundamental aspects of proton-coupled electron transfer (PCET) reactions in solution are analyzed with molecular dynamics simulations for a series of model systems. The analysis addresses the impact of the solvent reorganization energy, the proton donor-acceptor mode vibrational frequency, and the distance dependence of the nonadiabatic coupling on the dynamics of the reaction and the magnitude of the rate. The rate for nonadiabatic PCET is expressed in terms of a time-dependent probability flux correlation function. The time dependence of the probability flux correlation function is determined mainly by the solvent reorganization energy and is not significantly influenced by the proton donor-acceptor frequency or the distance dependence of the nonadiabatic coupling. The magnitude of the PCET rate becomes greater as the solvent reorganization energy decreases, the proton donor-acceptor frequency decreases, and the distance dependence of the nonadiabatic coupling increases. The approximations underlying a previously derived analytical PCET rate expression are also investigated. The short-time approximation for the solvent is valid for these types of systems. In addition, solvent damping effects on the proton donor-acceptor motion are not significant on the time scale of the probability flux. The rates calculated from the molecular dynamics simulations agree well with those calculated from the analytical rate expression.     

利用基于直接动力学的轨线面跳跃方法处理非绝热过程

势能面交叉引起的非绝热过程广泛存在于光化学和光物理中。对这一过程进行描述是理论化学的重要挑战之一。非绝热过程涉及原子核与电子之间的耦合运动,因此量子化学的基本假设之一"玻恩-奥本海默"近似被打破,所以对其进行描述需要发展新的动力学理论方法。在这些方法中,Tully发展的最少轨线面跳跃方法凭借易于程序化、便于计算等优点已经发展成为处理非绝热问题的主要动力学方法之一。其中原子核以经典的方式在单一势能面上进行演化,电子以量子的方式沿着同一轨线进行演化。在整个演化过程中,非绝热跃迁通过轨线在不同势能面间的跃迁来描述,其中跳跃发生的几率与电子的演化有关。如果将该方法与从头算直接动力学相结合,可以在全原子水平上研究实际分子体系的非绝热动力学,给出其激发态寿命、非绝热动力学中分子的主要运动方式、反应通道以及分支比等重要信息。本文旨在讨论最少面跳跃直接动力学方法研究非绝热问题的一些进展,包括动力学基本理论,特别关注将最少面跳跃方法和直接动力学结合的数值实现细节,同时讨论该方法在研究实际体系当中的一些应用,并对轨线面跳跃方法下一步发展的一些方向进行合理的展望。     

Quantum Chemistry Study on Dissociation of Oxalyl Bromide

1 INTRODUCTION In experiment, it is difficult to reveal the pheno- menon that molecules decompose into several frag- ments by UV light mainly due to the insufficient en- ergy of illuminating source. But in oxalyl halides, their bond energies are relatively lower. As a typical system for the study of multi-channel dissociation, oxalyl chloride can be dissociated into four frag- ments: Cl˙, Cl˙, CO and CO, under proper UV light[1, 2], which is the chief way to obtain free radi- cals. …     

On the location of conical intersections in CH2BrCl using MS-CASPT2 methods

Multiconfigurational second-order perturbation theory has been employed to calculate two-dimensional potential energy surfaces for the lowest low-lying singlet electronic states of CH2BrCl as a function of the two carbon-halogen bonds. The photochemistry of the system is controlled by a nonadiabatic crossing occurring between the A and B bands, attributed to the b1A' and c1A' states, which are found almost degenerate and forming a near-degeneracy line of almost equidistant C-Br and C-Cl bonds. A crossing point in the near-degeneracy line is identified as a conical intersection in this reduced two-dimensional space. The positions of the conical intersection located at CASSCF, single-state (SS)-CASPT2, and multistate (MS)-CASPT2 levels of theory are compared, also paying attention to the nonorthogonality problem of perturbative approaches. To validate the presence of the conical intersection versus an avoided crossing, the geometrical phase effect has been checked using the multiconfigurational MS-CASPT2 wave function.     

Insights into photodissociation dynamics of propionyl chloride from ab initio calculations and molecular dynamics simulations

The potential energy surfaces of isomerization, dissociation, and elimination reactions for CH3CH2COCl in the S0 and S1 states have been mapped with the different ab initio calculations. Mechanistic photodissociation of CH3CH2COCl at 266 nm has been characterized through the computed potential energy surfaces, the optimized surface crossing structure, intrinsic reaction coordinate, and ab initio molecular dynamics calculations. Photoexcitation at 266 nm leads to the CH3CH2COCl molecules in the S1 state. From this state, the C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase. The barrier to the C-Cl bond cleavage on the S1 surface is significantly increased by effects of the matrix and the internal conversion to the ground state prevails in the condensed phase. The HCl eliminations as a result of internal conversion to the ground state become the dominant channel upon photodissociation of CH3CH2COCl in the argon matrix at 10 K.     

Communication: Competition between π···π interaction and halogen bond in solution: a combined 13C NMR and density functional theory study

Competition between π···π interaction and halogen bond in solution has been investigated by using carbon nuclear magnetic resonance spectroscopy ((13)C NMR) combined with density functional theory calculation. Both experimental and theoretical results clearly show that there are no C-Cl···π or C-Br···π halogen bonds and only the π···π interactions exist in the binary liquid mixtures of C(6)D(6) with C(6)F(5)Cl and C(6)F(5)Br, respectively. The case is totally different for the binary liquid mixtures of C(6)D(6) with C(6)F(5)I in which the C-I···π halogen bonds not the π···π interactions are present. The important role of entropy in the competition between π···π interaction and halogen bond in solution was also discussed.     

Elucidating ultrafast nonradiative decay of photoexcited uracil in aqueous solution by ab initio molecular dynamics

Nonradiative decay of the photoexcited RNA base uracil has been studied in fully explicit aqueous solution using nonadiabatic ab initio molecular dynamics. Detailed comparison of the time-dependent nonadiabatic transition probability with specific molecular vibrational motions provides insight into the mechanism of the ultrafast internal conversion. From a monoexponential fit to the excited state ensemble population, the lifetime of the first electronically excited ππ^* singlet state has been determined to be 359 fs. Additional, reference, nonadiabatic simulations have been carried out in the gas phase, pinpointing the effects of the solvent on the photophysics of uracil. The gas phase excited state lifetime is calculated to be 608 fs, somewhat longer than in solution. In terms of excitation energies and geometrical parameters, the differences between gas phase and aqueous solution are found to be generally small. A notable exception is the excited state out-of-plane torsional motion about the CC double bond, which appears severely damped by the solvent. Moreover, hydrogen bond interactions between the uracil oxygens and the solvent hydrogens are seen to enhance internal conversion.     

Effects of Quantum Nuclear Delocalisation on NMR Parameters from Path Integral Molecular Dynamics

The influence of nuclear delocalisation on NMR chemical shifts in molecular organic solids is explored using path integral molecular dynamics (PIMD) and density functional theory calculations of shielding tensors. Nuclear quantum effects are shown to explain previously observed systematic deviations in correlations between calculated and experimental chemical shifts, with particularly large PIMD‐induced changes (up to 23 ppm) observed for carbon atoms in methyl groups. The PIMD approach also enables isotope substitution effects on chemical shifts and J couplings to be predicted in excellent agreement with experiment for both isolated molecules and molecular crystals. An approach based on convoluting calculated shielding or coupling surfaces with probability distributions of selected bond distances and valence angles obtained from PIMD simulations is used to calculate isotope effects.