Band 225, Heft 5 - Special Issue Prof. Brutschy

Using the combination of a linear 22-pole ion trap (22 PT) and a coaxial beam of hydrogen atoms, H abstraction from CH x + ( x =1, 4 and 5) has been studied. The temperature of the trap, T 22 PT , can be varied between 10 K and 300 K. The velocity distribution of the neutral target beam can be changed by cooling the accommodator ( T ACC =10–300 K) and using the focusing features of one or two hexapole magnets. The resulting velocity distributions are characterized by time-of-flight measurements. With the same setup, reactions of mass selected stored ions with a cold effusive beam of H 2 molecules have been measured with the discharge turned off. At temperatures of interstellar space, H-abstraction from CH + is efficient. CH 4 + reacts five times faster with H than with H 2 at 50 K. In contradiction to ab initio calculations and to the established proton affinity of methane, CH 5 + is slowly destroyed in collisions with H in our trap, even at 10 K. Some first results for collisions between CH x + and D atoms are reported. For x =1, H–D exchange is quite efficient, even though it is in competition with the exothermic abstraction reaction. For x =4, H-abstraction, i.e. formation of HD molecules, dominates. Deuteration of CH 5 + is measured to be very slow.

Electron impact to trifluoropropene (CF 3 CH=CH 2 ) and trifluoropropyne (CF 3 C≡CH) leads to rich resonant structures observed in the ion yields of various negatively charged fragment ions which are formed via resonant dissociative electron attachment (DEA). In the range between 1.8 and 3 eV the transient negative ions (TNIs) formed by electron attachment are assigned to one particle shape resonances of π * character, above 5 eV as two particle core excited resonances and in the energy range below 1 eV as diffuse transient negative ion states formed by polarisation and dipole forces. These transient parent anions decompose into various negatively charged fragments by the cleavage of single bonds (formation of F − and CF 3 − ) but also via surprisingly complex unimolecular processes involving multiple bond cleavages, geometrical and electronic rearrangement in the transient anion and formation of new bonds.

In this work we show that photodetachment photoelectron (PD-PE) spectroscopy on radical anions allows direct access to dark states, such as dipole-forbidden n π ∗ triplet and singlet states. p -benzoquinone (BQ) was chosen as model molecule because it is experimentally and theoretically well investigated and its lowest electronic states are electric dipole-forbidden n π ∗ triplet and singlet states. Because of the high density of electronic states we observe a dense structure with many transitions in our PD-PE spectrum. We identify the first peak at 2.3 eV above the S 0 onset as the T 1 state origin, in good agreement with previous data. We are not able to resolve the splitting between the states T 1 (1 3 B 1 g ) and T 2 (1 3 A u ) as well as the corresponding S 1 (1 1 A u ) and S 2 (1 1 B 1 g ) states, but perform a tentative assignment of the singlet-triplet splitting on the basis of literature data. The fact that the spectral features in our PD-PE spectrum cover a broad energy range is taken as a strong experimental evidence for the presence of the T 3 π π ∗ state, which has been predicted to lie in this energy range. Our investigations on BQ show that in principle, by starting spectroscopy in radical anions most of the selection rules, valid in neutral BQ molecules, can be circumvented and n π ∗ states, triplet states and other dipole-forbidden states become directly accessible with bright intensity. Much higher electron energy resolution is required to allow unambiguous assignments and make PD-PES of radical anions a valuable method for the spectroscopy dark states of neutral closed-shell molecules.

The experimental realization of accurately defined source conditions, combined with an undisturbed flight path of precisely known length, permits the generation of supersonic molecular beams with an extremely well characterized axial velocity distribution. This achievement can be used for a quantitative assessment of theoretical models of condensation during the jet expansion, based on high accuracy equations of state. For helium-4, which is commonly put on a level with the ideal gas, we contrast two empirical equations of state with respect to enthalpy changes. These can be validated through a comparison with experimentally determined mean terminal flow velocities. At cryogenic temperatures, strong deviations from the ideal gas behavior are observed. Most notably, even the differences between the predictions of the two fundamental equations are large enough to be distinguished by high-resolution time-of-flight measurements.

The ring-puckering motion in cyclopentene as well as the hindered internal rotation around the central C–C bond in 1-butene have been studied by high-level quantum chemical calculations. Relevant potential energy surfaces of these molecules along these large-amplitude motions are provided as well as rotational constants are given allowing for thorough comparison with recent results from time-resolved femtosecond degenerate four-wave mixing (fs DFWM) spectroscopy. Emphasis is put on the performance of various post-Hartree Fock methods, the required level of electron correlation as well as the basis set quality.

A model Hamiltonian is proposed for systems of poly-phenylene-vinylene (PPV) type, which describes excitation energy transfer (EET) in the presence of torsional and bond-length-alternation coordinates that couple neighboring fragments. This model is employed to describe coherent EET, starting from a partially delocalized exciton state which is initially separated from the rest of the chain by a defect due to a pronounced torsional displacement at one of the inter-fragment junctions. EET is found to be driven by the restoring force of the torsions towards the planar geometry. Quantum dynamical calculations are carried out for a minimal five-site model, using the multiconfiguration time-dependent Hartree (MCTDH) method.

The cooperativity effects have been evaluated on three model systems, the formamide, (formylamino)acetamide and amyloidic-layer oligomers with an increasing size of the monomer units (6, 13 and 214 atoms). In the last model, each layer is a dimer of the amino-acid sequence GNNQQNY in one-letter amino-acid abbreviations. The series of oligomers for each model system of up to six monomers have been constructed. For the calculation of the strength of a particular H-bond formed between various sub-oligomers within an oligomer, different wave function, density functional and semi-empirical quantum mechanical methods as well as empirical force fields have been used. Semi-empirical methods are found to be a reasonable compromise between accuracy and computational cost. These methods are able to describe the cooperativity effects with an accuracy almost comparable to that of the ab initio methods. On the contrary, the empirical force-field methods for all of the model systems mostly failed to describe the H-bonding cooperativity effects properly. Based on the results obtained in this work, we recommend using semi-empirical methods. For the systems where this is impossible, we agree to use polarizable force fields with some reservations. Generally, the more flexible the oligomer chain is (the less steric the repulsion or rigid motifs are), the larger the cooperativity that can be achieved. With the increasing number of monomers in a sequence connected via H-bonds, the cooperativity effects appear to be growing, but relatively soon (at 3–4 monomer units) they tend to become saturated.

Photoinduced charge transfer dynamics between CdSe semiconductor quantum dots (QDs) and the molecular acceptor methylviologen (MV 2+ ) were investigated by time resolved absorption spectroscopy. Biexponential behavior of the charge separation is observed and explained by different donor states involved in the electron transfer reaction. Furthermore, we found a linear dependence of the amplitude weighted average electron transfer rate < k ET > on the electron acceptor concentration until a methylviologen saturation limit is reached.

The feasibility of measuring circular dichroism (CD) in ion yields is demonstrated in multiphoton ionization employing femtosecond laser radiation for the first time. CD values for (R)-propylene oxide are reported at 738 nm, 810 nm and 878 nm. The data suggest the possibility of resonance contributions at 810 nm. At the other wavelengths the CD appears to originate from non-resonant multiphoton ionization. Possible implications of measuring non-resonant CD are discussed.

The complexation of alkali metal ions and the proton X + (X=H, Li, Na, K, Rb, Cs) by β -alanyl-L-histidine (carnosine, Car) was investigated by electrospray ionization mass spectrometry (ESI-MS) and MS/MS fragmentation studies. Collision-induced dissociation (CID) breakdown curves were recorded and the resulting apparent activation energies were compared to DFT calculations of binding energies at the B3LYP/cc-pVDZ level of theory. Additionally, the influence of metal complexation on hydrogen/deuterium exchange of CarX + with D 2 O was tested. Our investigations characterize the structures of Car-alkali complexes and elucidate the interaction with the differently sized solvated metal cations.

The review introduces to time-resolved NMR spectroscopic investigations of the kinetics of protein and RNA folding. The description of the experimental investigations is discussed in the context of possible kinetic folding pathways showing the extent of information that can be gained from the various kinetic experiments. The review introduces to four different methods to initiate folding reactions in connection with time-resolved NMR experiments and discusses examples of refolding of the model protein α -lactalbumin and of bistable RNAs.