Chemically-induced geometrical isomerization of stilbenes during peroxyoxalate chemiluminescence reaction: revisit to ‘photochemistry without light’

The chemically-induced isomerization of stilbenes during the peroxyoxalate chemiluminescence (PO-CL) reactions was reinvestigated. The PO-CL reactions using bis(2,4,6-trichlorophenyl) oxalate in the presence of several stilbenes (type A reaction) produced cis-stilbenes in 0–4% yields, which was dependent on the singlet excitation energy of the stilbenes. On the other hand, the PO-CL reactions of the oxalates, containing the stilbene moieties in the molecules (type B reaction), produced cis-stilbenes 0–9.3% yields, some of which were much more effective than the type A reactions considering the amount of the oxalate moiety as the energy supplier.     

Nonadiabatic molecular dynamics study of the cis-trans photoisomerization of azobenzene excited to the S1 state

Ab initio nonadiabatic dynamics simulations of cis-to-trans isomerization of azobenzene upon S(1) (n-π*) excitation are carried out employing the fewest-switches surface hopping method. Azobenzene photoisomerization occurs purely as a rotational motion of the central CNNC moiety. Two nonequivalent rotational pathways corresponding to clockwise or counterclockwise rotation are available. The course of the rotational motion is strongly dependent on the initial conditions. The internal conversion occurs via an S(0)/S(1) crossing seam located near the midpoint of both of these rotational pathways. Based on statistical analysis, it is shown that the occurrence of one or other pathway can be completely controlled by selecting adequate initial conditions.     

Full-dimensional quantum dynamics of vibrationally highly excited NHD2

We report on full-dimensional vibrational quantum dynamics of the highly excited ammonia isotopologue NHD(2) using a newly developed potential energy surface and the MCTDH program package. The calculations allow to realistically simulate an infrared laser induced stereomutation reaction at the pyramidal nitrogen atom in the femtosecond time domain. Our results allow for a thorough qualitative and quantitative understanding of infrared photoinduced stereomutation kinetics, the underlying quantum dynamics, and the reaction mechanisms. Comparison is made with a previous, reduced dimensionality study of the same reaction [R. Marquardt, M. Quack, I. Thanopulos, and D. Luckhaus, J. Chem. Phys. 118, 643 (2003)], and it is shown that slight variances of reduced spaces lead to significantly different kinetics. Because the quantum dynamics depends subtly on variances of reduced spaces, reduced dimensionality treatments are not reliable even for qualitative predictions of the stereomutation kinetics. The first direct comparison between the Multiconfigurational Time Dependent Hartree [M. H. Beck, A. Ja?ckle, G. A. Worth et al., Phys. Rep. 324, 1 (2000)] and Unimolecular Reactions Induced by Monochromatic Infrared Radiation [M. Quack and E. Sutcliffe, QCPE Bulletin 6, 98 (1986)] program packages on a specific, four dimensional quantum dynamical problem allows for their full validation in the present work.     

Ring-closing and dehydrogenation reactions of highly excited cis-stilbene: ultrafast spectroscopy and structural dynamics

The ultrafast dynamics of highly excited cis-stilbene (CS) in a molecular beam is explored using femtosecond time-resolved mass spectrometry and structure-sensitive photoelectron spectroscopy. cis-Stilbene is initially pumped by a 6 eV photon to the 7(1)B state and the reaction is followed by ionization with a time-delayed 3 eV probe pulse. Upon excitation, cis-stilbene rapidly decays to the 3(1)B state, where it undergoes a ring-closing reaction to form 4a,4b-dihydrophenanthrene (DHP). Whereas 14% of the ionized CS molecules dissociate one hydrogen atom to form hydrophenanthrene, the ionized DHP molecules completely dehydrogenate in the ion state to produce hydrophenanthrene and phenanthrene with a 1:1 ratio. We determined the lifetimes of the 7(1)B state and the 3(1)B state of CS to be 167 and 395 fs, respectively.     

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)相似文献   

Mechanism and dynamics of azobenzene photoisomerization

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate pipi* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.     

Quantum chemical reaction dynamics on a highly parallel supercomputer

Summary In this paper we describe the solution of the quantum mechanical equation for the scattering of an atom by a diatomic molecule on a high-performance distributed-memory parallel supercomputer, using the method of symmetrized hyperspherical coordinates and local hyperspherical surface functions. We first cast the problem in a format whose inherent parallelism can be exploited effectively. We next discuss the practical implementation of the parallel programs that were used to solve the problem. The benchmark results and timing obtained from the Caltech/JPL Mark IIIfp hypercube are competitive with the CRAY X-MP, CRAY 2 and CRAY Y-MP supercomputers. These results demonstrate that such highly parallel architectures permit quantum scattering calculations with high efficiency in parallel fashion and should allow us to study larger, more complicated chemical systems. Future extensions to this approach are discussed.Work performed in partial fulfillment of the requirements for the Ph.D. degree in Chemistry at the California Institute of Technology.Contribution number 8209     

The geometry of hindered stilbenes in their ground and excited states

The torsional angles about the 1-α single bond and about the α-α double bond in hindered stilbene derivatives were calculated for the ground and for the excited states. The calculated ground state geometries form a basis for a π-electron-SCF-LCAO-CI-MO calculation of the singlet excitation energies which agree accurately with the experimental values. The calculated potential curves for torsion about the 1-α bonds provide a qualitative explanation for the significant red shift and increase in intensity of the first singlet absorption band observed in these compounds at low temperatures.     

Wave packet simulation of nonadiabatic dynamics in highly excited 1,3-dibromopropane

We have conducted wave packet simulations of excited-state dynamics of 1,3-dibromopropane (DBP) with the aim of reproducing the experimental results of the gas-phase pump-probe experiment by Kotting et al. [ Kotting, C. ; Diau, E. W.-G. ; S?lling, T. I. ; Zewail, A. H. J. Phys. Chem. A 2002, 106, 7530 ]. In the experiment, DBP is excited to a Rydberg state 8 eV above the ground state. The interpretation of the results is that a torsional motion of the bromomethylene groups with a vibrational period of 680 fs is activated upon excitation. The Rydberg state decays to a valence state, causing a dissociation of one of the carbon bromine bonds on a time scale of 2.5 ps. Building the theoretical framework for the wave packet propagation around this model of the reaction dynamics, the simulations reproduce, to a good extent, the time scales observed in the experiment. Furthermore, the simulations provide insight into how the torsion motion influences the bond breakage, and we can conclude that the mechanism that delays the dissociation is solely the electronic transition from the Rydberg state to the valence state and does not involve, for example, intramolecular vibrational energy redistribution (IVR).     

Electronic spectroscopy and excited state dynamics of the Al-H2/D2 complex

The electronic spectra of the Al-H2 and Al-D2 complexes are investigated in a collaborative experimental and theoretical study. The complexes were prepared in a pulsed supersonic beam and detected with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the excited-state Al(3d, 4p, 4d) + H2/D2 asymptotes were observed by monitoring emission from lower excited Al atomic levels, formed in the non-radiative decay of the excited complex. Fluorescence depletion has also been used to verify that the observed Al-H2 bands all involve the same molecular carrier. The bands have been assigned to the more strongly bound Al-oH2 and Al-pD2 nuclear spin modifications. In contrast to our previous observations for Al(5s)-H2 [X. Yang and P. J. Dagdigian, J. Chem. Phys., 1998, 109, 8920], for which only one potential energy surface (PES) emanates from the dissociation asymptote, the Lorentzian widths of the different vibrational bands in the 3d, 4p, 4d<--3p transitions vary widely, in some cases allowing resolution of the rotational structure of the bands. With the help of the calculated Al(3p)-oH2/pD2 dissociation energies, binding energies of the observed excited vibronic levels are reported. The mechanism of predissociation is investigated theoretically through ab initio calculation of C2 nu cuts of the excited PESs. It is concluded that predissociation occurs through coupling with the repulsive Al(4s)-H2 PES. With these calculations, a qualitative interpretation of the observed bands could be made.     

Reactivity of low energy excited states: Mechanistic and exploratory organic photochemistry

The photochemistry of 3 - methyl - 3 - (1' - naphthyl) -1 - butene was investigated. Direct irradiation led to 1,1 dimethyl - 2 - (1' - naphthyl)cyclopropane as a primary photoproduct and 2 - methyl - 4 - (1' - naphthyl) - 1 - butene as -a secoandary product. The quantum yield for the formation of the cyclopropane was 0.037. The corresponding triple reaction was less efficient, with a quantum yield of 0.012, but still afforded the same product. The excited single rearragement rate was determined by single photon counting; this proved to be ¹k_XXX = 5.9 × 10⁵ sec^-1.The total rate of S₁ decay was determined as 1.59 × 10⁷ sec^-1 with a lifetime of 62.9 nsec.Thus, the lifetime of this rearranging system is quite simiar to that of simple 1-alkylnaphthalenes (Ca 65 nsec); and, the rate of di-π-methane rearrangement is the slowest known. Finally, the rate of radiationless decay of the singlet was found to be almost temperature independent between room temperature and 77 K.     

Electronic confinement effects on the reaction field type calculations of solvent effects

We analyze some procedures to introduce the effect of confining the electrons of the hydrogen atoms in cavitation spheres like those used in the self‐consistent reaction field models for studying the solvent influence on molecular properties [as polarizable continuum model (PCM), or conductor screening model (COSMO)]. We have found that the boundary conditions to be applied have an important effect on the system energy that by no means should be neglected in this type of calculations. We have found as well that “‐nG” expansion technique could be applicable in this kind of calculations (even at the very simple “‐3G” level) and lead us to a relatively simple form of applying the theory. Moreover, we have found a way to define the cavitation radius of PCM calculations, by minimizing the system energy with respect to this parameter, which could be a more satisfactory procedure—at least from a theoretical point of view—than the use of empirical values characteristic of most of the PCM or COSMO standard calculations. © 2013 Wiley Periodicals, Inc.     

Electronic transitions of polysilanes and their photochemistry

The photolytic fragmentation of trisilane to disilane and silylene has been studied by means of a pseudopotential method. The lower-lying electronically excited states in several of the silanes have been calculated and analysed with respect to their Rydberg character, with the inclusion of d orbitals. The effect of methylation on the stability of the silicon-silicon bond, as well as on the above photochemical model reaction, is discussed. An orbital correlation diagram with defined energy levels is presented.     

Substituent effects on the photochemistry of o-tolualdehydes

The effect of 4-oxy and 4,5-dioxy substituents on the photochemical conversion of 2-methylbenzaldehydes to o-quinodimethanes (o-QDMs) has been studied. The presence of a 4-methoxy, or a 4 and 5-methoxy substituent prevented the photochemical formation of the o-QDM whereas 4-acetoxy and 4,5-diacetoxy-2-methylbenzaldehyde and the corresponding mesylates and tosylates were successfully converted to the o-QDMs.     

Ground and excited electronic states of azobenzene: a quantum Monte Carlo study

Large-scale quantum Monte Carlo (QMC) calculations of ground and excited singlet states of both conformers of azobenzene are presented. Remarkable accuracy is achieved by combining medium accuracy quantum chemistry methods with QMC. The results not only reproduce measured values with chemical accuracy but the accuracy is sufficient to identify part of experimental results which appear to be biased. Novel analysis of nodal surface structure yields new insights and control over their convergence, providing boost to the chemical accuracy electronic structure methods of large molecular systems.     

Theoretical study of the reaction of H atoms with vibrationally highly excited HF molecules

Vibrationally highly excited molecules react extremely fast with atoms and probably with radicals. The phenomenon can be utilized for selectively enhancing the rate of reactions of specific bonds. On the basis of quasiclassical trajectory calculations, the paper analyzes mechanistic details of a prototype reaction, H + HF(v). At vibrational quantum numbers v above 2, the reaction exhibits capture-type behavior, that is, the reactive cross section diverges as the relative translational energy of the partners decreases, both for the abstraction and for the exchange channel. The mechanism of the reaction for both channels is different at low and at high translational energy. At low vibrational energy, the reaction is activated, which is switched to capture-type at high excitation. The reason is an attractive potential that acts on the attacking H atom when the HF molecule is stretched. In contrast to the 6-SEC potential surface of Mielke et al., the switch cannot be observed on the Stark-Werner potential surface, due to a small artificial barrier at high H-HF separation, preventing the reactants from obeying the attractive potential and also proving the importance of the latter. The exchange reaction can be observed even when the total energy available for the partners is below the exchange barrier, because at low translational energies the product F atom of a successful abstraction step can re-abstract that H atom from the intermediate product H2 molecule that was originally the attacker.     

Avoided crossing of molecular excited states and photochemistry: Butadiene and unprotonated Schiff base

Consequences of the twisting motion around the C?C bond of butadiene and around C?C, C?N, and C? C bonds of the small unprotonated Schiff base (allylideneimine) on low-lying singlet and triplet states have been investigated employing large scale CI treatments. Characterization of the important features of the electronic wave functions in terms of VB-like ionic and covalent contributions in different twist intervals has been carried out. Importance of the zwitterionic singlet states with large charge separation in two different parts of molecule attached to the relaxed bond versus low-lying covalent excited state has been discussed. Photochemical implications of different minima on the energy hypersurfaces of the excited states with different features of electronic wave functions have been proposed.