Bernhard Sellner

Dynamics starting at a conical intersection: Application to the photochemistry of pyrrole

The photochemical ring opening process in pyrrole has been investigated by performing classical on-the-fly dynamics using the multiconfiguration self-consistent field method for the computation of energies and energy gradients. As starting point for the dynamics the conical intersection corresponding to the ring-puckered ring-opened structure, determined previously [Barbatti et al., J. Chem. Phys. 125, 164323 (2006)], has been chosen. Two sets of initial conditions for the nuclear velocities were constructed: (i) nuclear velocities in the branching (g,h) plane of the conical intersection and (ii) statistical distribution for all atoms. Both sets of initial conditions show very similar results. Reactive trajectories are only found in a very limited sector in the (g,h) plane and reaction products are very similar. Within the simulation time of 1 ps, ring opening of pyrrole to the biradical NH=CH-CH(*)-CH=CH(*) chain followed by ring closure to a substituted cyclopropene structure (NH=CH-C(3)H(3)) is observed. The computed structural data correlate well with the experimentally observed dissociation products.

Azomethane: Nonadiabatic Photodynamical Simulations in Solution

The nonadiabatic deactivation of trans-azomethane starting from the nπ* state has been investigated in gas phase, water, and n-hexane using an on-the-fly surface-hopping method. A quantum mechanical/molecular mechanics (QM/MM) approach was used employing a flexible quantum chemical level for the description of electronically excited states and bond dissociation (generalized valence bond perfect-pairing complete active space). The solvent effect on the lifetime and structural parameters of azomethane was investigated in detail. The calculations show that the nonadiabatic deactivation is characterized by a CNNC torsion, mainly impeded by mechanic interaction with the solvent molecules. The similar characteristics of the dynamics in polar and nonpolar solvent indicate that solvent effects based on electrostatic interactions do not play a major role. Lifetimes increase by about 20 fs for both solvents with respect to the 113 fs found for the gas phase. The present subpicosecond dynamics also nicely show an example of the suppression of C-N dissociation by the solvent cage.

Ultrafast non-adiabatic dynamics of ethylene including Rydberg states

The photodynamics of ethylene has been studied by means of ab initio surface-hopping dynamics using extended multireference configuration interaction wavefunctions. At the highest level, the explicit possibility of excited-state CH dissociation and consideration of the Rydberg π−3s state was included into the electronic wavefunction. The initial dynamics is characterised by the torsional motion and the crossing between the bright π−π * state with S 1, the latter having primarily Rydberg character with only a minor contribution of the repulsive valence π−σ * state. Due to back-rotation to planar structures after 17 fs, part of the population flows into the Rydberg states. The lifetime for this fraction of trajectories is significantly longer than that for the valence population. An analysis of the latter population shows that the decay to the ground state proceeds mainly at the pyramidalised conical intersection. Thus, no major qualitative mechanistic changes as compared to previous dynamics simulations are observed for the valence population. In the present work, a decay time of 62 fs was found for the valence population. Simulations performed for ethylene-d4 show a slowdown of the torsional mode. However, since the crossing seam is reached in a more direct way with less torsional oscillations, the excited-state lifetime is almost unchanged as compared to ethylene.

Laser pulse trains for controlling excited state dynamics of adenine in water

We investigate theoretically the control of the ultrafast excited state dynamics of adenine in water by laser pulse trains, with the aim to extend the excited state lifetime and to suppress nonradiative relaxation processes. For this purpose, we introduce the combination of our field-induced surface hopping method (FISH) with the quantum mechanical-molecular mechanical (QM/MM) technique for simulating the laser-driven dynamics in the condensed phase under explicit inclusion of the solvent environment. Moreover, we employ parametric pulse shaping in the frequency domain in order to design simplified laser pulse trains allowing to establish a direct link between the pulse parameters and the controlled dynamics. We construct pulse trains which achieve a high excitation efficiency and at the same time keep a high excited state population for a significantly extended time period compared to the uncontrolled dynamics. The control mechanism involves a sequential cycling of the population between the lowest and higher excited states, thereby utilizing the properties of the corresponding potential energy surfaces to avoid conical intersections and thus to suppress the nonradiative decay to the ground state. Our findings provide a means to increase the fluorescence yield of molecules with an intrinsically very short excited state lifetime, which can lead to novel applications of shaped laser fields in the context of biosensing.

Photodynamics of Azomethane: A Nonadiabatic Surface-Hopping Study†

The internal conversion and hot ground-state dynamics of trans- and cis-azomethane starting in the S(1) state have been investigated by nonadiabatic ab initio surface hopping dynamics using MCSCF-GVB-CAS and MRCISD methods and by determining energy minima and saddle points, minima on the crossing seam, and minimum energy pathways on the ground and first excited-state surfaces. The lifetimes and photoproducts from the dynamics simulations, geometric properties, excitation energies of selected stationary points and minimum energy pathways between them are reported. Our results favor a statistical model with trans-AZM moving to the ground-state minima before the first CN dissociation takes place. A detailed discussion in comparison to recent experimental and theoretical data is presented.

O(3P) + C2H4 Potential Energy Surface: Study at the Multireference Level

The O((3)P) + C(2)H(4) reaction provides a crucial, initial understanding of hydrocarbon combustion. In this work, the lowest-lying triplet potential energy surface is extensively explored at the multiconfiguration self-consistent field (MCSCF) and MRMP2 levels with a preliminary surface crossing investigation; and in cases that additional dynamical correlation is necessary, MR-AQCC stationary points are also determined. In particular, a careful determination of the active space along the intrinsic reaction pathway is necessary; and in some cases, more than one active space must be explored for computational feasibility. The resulting triplet potential energy surface geometries mostly agree with geometries from methods using single determinant references. However, although the selected multireference methods lead to energetics that agree well, only qualitative agreement was found with the energetics from the single determinant reference methods. Challenges and areas of further exploration are discussed.

Singlet and triplet potential surfaces for the O2+C2H4 reaction.

Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing.

Nonadiabatic Excited-State Dynamics of Aromatic Heterocycles: Toward the Time-Resolved Simulation of Nucleobases

Ab inito molecular dynamics, although still challenge, is becoming an available tool for the investigation of the photodynamics of aromatic heterocyclic systems. Potential energy surfaces and dynamics simulations for three particular examples and different aspects of the excited and ground state dynamics are presented and discussed. Aminopyrimidine is investigated as a model for adenine. It shows ultrafast S1-S0 decay in about 400 fs. The inclusion of mass-restrictions to emulate the imidizole group increases the lifetime to about 950 fs, a value similar to the lifetime of adenine. The S2-S1 deactivation, typical in the fast component of the decay of nucleobases, is investigated in pyridone. In this case, the S2-state lifetime is 52 fs. The hot ground-state dynamics of pyrrole starting at the puckered conical intersection is shown to produce ring-opened structures consistent with the experimental results

Model Systems for Dynamics of π-Conjugated Biomolecules in Excited States

Mixed-quantum classical dynamics simulations have recently become an important tool for investigations of time-dependent properties of electronically excited molecules, including non-adiabatic effects occurring during internal conversion processes. The high computational costs involved in such simulations have often led to simulation of model compounds instead of the full biochemical system. This chapter reviews recent dynamics results obtained for models of three classes of biologically relevant systems: protonated Schiff base chains as models for the chromophore of Rhodopsin proteins ; nucleobases and heteroaromatic rings as models for UV-excited nucleic acids ; and formamide as a model for photoexcited peptide bonds