Jörg Schroeder

Pressure dependence of solvent-induced barrier shifts in the photoisomerization of trans-stilbene

Abstract The photoisomerization of trans-stilbene is liquid n -hexane solution was studied over wide pressure and temperature ranges. Measurements of the pressure and temperature dependence of the rate, as in earlier studies from our laboratory, allow for the separation of specific solute—solvent interactions from purely frictional effects in the liquid phase reaction dynamics. The viscosity dependences along isotherms of the photoisomerization rate reveal strong changes of the activation energy with solvent density. From the analysis of the isotherms we conclude that specific solute—solvent interactions lead to a substantial lowering of the reaction barrier in the S 1 state with increasing density. As observed earlier ethane as a solvent, also in n -hexane manifestations of the multidimensionality of the barrier crossing process are found. The viscosity as well as the temperature dependence of the isomerization rate coefficient over the entire range investigated can surprisingly well be represented by a combination of standard unimolecular rate theory with Kramers frictional model.

Photoisomerization of diphenylbutadiene in low‐viscosity nonpolar solvents: Experimental manifestations of multidimensional Kramers behavior and cluster effects

The photoisomerization of diphenylbutadiene was studied by picosecond absorption spectroscopy over wide pressure and temperature ranges in liquid and supercritical alkanes, CO2, SF6, and He. The reaction shows typical features of a thermal unimolecular reaction on the S1 potential energy surface. The rate can be expressed by a combination of standard unimolecular rate theory and Kramers–Smoluchowski theory. However, multidimensional behavior manifests itself in the transition to the gas phase low pressure range as well as to the high density Kramers–Smoluchowski range: in the former case, the low pressure limit of a unimolecular reaction of the polyatomic molecule is approached; in the latter case, the effective imaginary barrier frequency shows a marked apparent temperature dependence. The experiments also suggest contributions of reactant–solvent cluster interactions, which modify the barrier height even in nonpolar solvents.

FEMTOSECOND DYNAMICS OF INTRAMOLECULAR CHARGE TRANSFER IN 4-DIMETHYLAMINO-4'-CYANOSTILBENE IN POLAR SOLVENTS

Abstract The femtosecond dynamics of 4-(N,N-dimethylamino)-4′-cyanostilbene (DCS) in acetonitrile and methanol solvent was studied by fluorescence upconversion and pump-probe absorption spectroscopy. The spectral evolution observed provides strong evidence for a fast internal charge-transfer (CT) process dominating the dynamics in the first few picoseconds, while Stokes shift dynamics seem to play only a minor role. The initially excited state seems to have already a large amount of CT character, in line with observations in jet-cooled DCS-solvent clusters. The non-exponential CT-state formation contains components significantly faster than longitudinal dielectric relaxation rates in the solvents studied, indicating a possible contribution by high-frequency vibrational modes.

Molecular dynamics simulation of vibrational relaxation of highly excited molecules in fluids. II. Nonequilibrium simulation of azulene in CO2 and Xe

Results of nonequilibrium molecular dynamics simulations of vibrational energy relaxation of azulene in carbon dioxide and xenon at low and high pressure are presented and analyzed. Simulated relaxation times are in good agreement with experimental data for all systems considered. The contribution of vibration–rotation coupling to vibrational energy relaxation is shown to be negligible. A normal mode analysis of solute-to-solvent energy flux reveals an important role of high-frequency modes in the process of vibrational energy relaxation. Under all thermodynamic conditions considered they take part in solvent-assisted intramolecular energy redistribution and, moreover, at high pressure they considerably contribute to azulene-to-carbon dioxide energy flux. Solvent-assisted (or collision-induced) intermode energy exchange seems to be the main channel, ensuring fast intramolecular energy redistribution. For isolated azulene intramolecular energy redistribution is characterized by time scales from several to ...

Nonequilibrium molecular dynamics simulations of vibrational energy relaxation of HOD in D2O

Vibrational energy relaxation of HOD in deuterated water is investigated performing classical nonequilibrium molecular dynamics simulations. A flexible SPC/E model is employed to describe the intermolecular interactions and the intramolecular potential of the D(2)O solvent. A more accurate intramolecular potential is used for HOD. Our results for the OH stretch, OD stretch, and HOD bend vibrational relaxation times are 2.7, 0.9, and 0.57 ps, respectively. Exciting the OH stretching mode the main relaxation pathway involves a transition to the bending vibration. These results are in agreement with recent semiclassical Landau-Teller calculations. Contrary to this previous work, however, we observe a strong coupling of bending and OH stretching mode to the HOD rotation. As a result almost half of the total vibrational energy is transferred through the HOD rotation to the bath. At the same time the most efficient acceptor mode is the D(2)O rotation indicating the importance of resonant libration-to-libration energy transfer. We also find significant vibrational excitation of the D(2)O bending mode of the D(2)O solvent by V-V energy transfer from the HOD bending mode.

Fluorescence excitation spectra of jet-cooled 4-(diisopropylamino)benzonitrile and related compounds

Abstract Fluorescence excitation spectra of 4-(diisopropylamino)benzonitrile (DIABN) and 4-(dimethylamino)benzonitrile (DMABN) in thermal vapour and seeded jet expansions are compared. The spectrum of jet-cooled DIABN shows an intense 0–0-transition at 31751.8 cm −1 . The spectrum collapses at excess excitation energies above 800 cm −1 , indicating the presence of an efficient non-radiative decay channel. In the gas phase, fluorescence emission of DIABN occurs from the intramolecular charge transfer (ICT) state. The non-radiative decay channel, therefore, is attributed to rapid ICT in the isolated molecule. The related compounds 4-(methylamino)-3,5-dimethylbenzonitrile (MHD), 4-(azetidinyl)-3,5-dimethylbenzonitrile (M4D), and 4-(dimethylamino)-3,5-dimethylbenzonitrile (MMD) in the jet show extremely weak and structureless emission.

Molecular-dynamics simulation of collisional energy transfer from vibrationally highly excited azulene in compressed CO2.

Results from nonequilibrium molecular-dynamics simulations of collisional energy transfer from vibrationally highly excited azulene in compressed CO2 are compared with experimental results from our laboratory obtained under comparable physical conditions. As observed in the experiment, the cooling rates show a purely monoexponential decay of the excess energy. The influence of the microscopic solvent shell structure on these processes is investigated using the full three-dimensional anisotropic CO2 structure around azulene obtained from the simulation. The analysis shows that local heating effects of any kind do not play a role in our model system. Predictions of the pressure dependence of the energy transfer rates by the isolated binary collision model are compared with results from the simulations using two different definitions of the collision frequency in dense fluids.

From barrier crossing to barrierless relaxation dynamics. Photoisomerization of trans stilbene in compressed n-alkanols

We report the first observation of a reaction which, in a single solvent, starts as a barrier crossing process at low pressure and turns into a relaxation on a barrierless potential at high pressure. The pressure and temperature dependences of trans-stilbene photoisomerization in n-alkanols were investigated by picosecond transient absorption spectroscopy. As in n-alkane solvents, at constant temperature, the rate coefficients k for rotation about the central double bond in each solvent exhibit a fractional power dependence on solvent viscosity η, k≈η−α, with 0 < α ⩽ 1. α varies little with solvent, but increases with temperature. This observation is discussed in terms of a solvent shift effect which causes with increasing solvent density a lowering of the barrier height. In n-propanol at high pressure the barrier height approaches zero. Even under these conditions, however, the observed decay dynamics remain monoexponential.

Photoisomerization dynamics of diphenylbutadiene in compressed liquid alkanes and in solid environment

Abstract The investigation of the pressure dependence of the S1 photoisomerization of diphenylbutadiene in n -alkanes from ethane to n -dodecane allows us to differentiate between various models proposed to explain the observed viscosity dependence of the rate coefficient k iso for twisting about one of the double bonds. For each solvent we observe a linear dependence of k iso on the inverse of the solvent viscosity with a slope that increases with solvent size. Comparing this result with models-for microscopic friction we find that the observed solvent size, effect is significantly stronger than predicted. We conclude that a hydrodynamic description of frictional forces in terms of the zero frequency shear viscosity of the solvent is in agreement With the observed pressure and solvent dependence. We suggest, therefore, that the solvent size reflects variations in the shape of the reaction path with solvent that are possibly associated with the multidimensionality of the barrier crossing process. We also report nonradiative rate coefficients for DPB in solid solution.

Molecular dynamics simulation of vibrational energy relaxation of highly excited molecules in fluids. I. General considerations

Methods of implementation of classical molecular dynamics simulations of moderate size molecule vibrational energy relaxation and analysis of their results are proposed. Two different approaches are considered. The first is concerned with modeling a real nonequilibrium cooling process for the excited molecule in a solvent initially at equilibrium. In addition to the solute total, kinetic, and potential energy evolution, that define the character of the process and the rate constant or relaxation time, a great deal of important information is provided by a normal mode specific analysis of the process. Expressions for the decay of the normal mode energies, the work done by particular modes, and the vibration–rotation interaction are presented. The second approach is based on a simulation of a solute–solvent system under equilibrium conditions. In the framework of linear nonequilibrium statistical thermodynamics and normal mode representation of the solute several expressions for the rate constant are derived. In initial form, they are represented by integrals of the time correlation functions of the capacities of the solute–solvent interaction atomic or normal mode forces and include the solute heat capacity. After some approximations, which are adequate for specific cases, these expressions are transformed to combinations of those for individual oscillators with force–force time correlation functions. As an attempt to consider a strongly nonequilibrium situation we consider a two-temperature model and discuss the reason why the rate constant can be independent on the solute energy or temperature. Expressions for investigation of the energy redistribution in the solvent are derived in two forms. One of them is given in the usual form of a heat transfer equation with the source term describing the energy flux from the excited solute. The other form describes the energy redistribution in the solvent in terms of capacity time correlation functions and can be more convenient if memory effects and spatial dispersion play an important role in energy redistribution in the solvent.