spatial-separation-of-molecular-conformers-and-clusters

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

CFEL, DESY

We illustrate, experimentally and theoretically, a laser-based method to control the rotations of polyatomic molecules in 3D space. A linearly polarized nanosecond pulse strongly aligns the most polarizable axis of an asymmetric top molecule along its polarization axis while an orthogonally polarized, femtosecond pulse sets the molecules into controlled rotation about the aligned axis. As a result, strong three-dimensional (3D) alignment occurs shortly after the femtosecond pulse and is repeated periodically, reflecting coherent revolution about the molecular axis. Our method opens new directions for research in orientationally confined complex molecules.

Holding and spinning molecules in space.

The dissociative recombination of the acetaldehyde cation, CH(3)CHO(+), has been investigated at the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm, Sweden. The dependence of the absolute cross section of the reaction on the relative kinetic energy has been determined and a thermal rate coefficient of k(T) = (1.5 ± 0.2) × 10(-6) (T/300)(-0.70±0.02) cm(3) s(-1) has been deduced, which is valid for electron temperatures between ∼10 and 1000 K. The branching fractions of the reaction were studied at ∼0 eV relative kinetic energy and we found that breaking one of the bonds between two of the heavy atoms occurs in 72 ± 2% of the reactions. In the remaining events the three heavy atoms stay in the same product fragment. While the branching fractions are fairly similar to the results from an earlier investigation into the dissociative recombination of the fully deuterated acetaldehyde cation, CD(3)CDO(+), the thermal rate coefficient is somewhat larger for CH(3)CHO(+). Astrochemical implications of the results are discussed.

Dissociative recombination of the acetaldehyde cation, CH(3)CHO(+).

The highly exoergic nucleophilic substitution reaction F(-) + CH3I shows reaction dynamics strikingly different from that of substitution reactions of larger halogen anions. Over a wide range of collision energies, a large fraction of indirect scattering via a long-lived hydrogen-bonded complex is found both in crossed-beam imaging experiments and in direct chemical dynamics simulations. Our measured differential scattering cross sections show large-angle scattering and low product velocities for all collision energies, resulting from efficient transfer of the collision energy to internal energy of the CH3F reaction product. Both findings are in strong contrast to the previously studied substitution reaction of Cl(-) + CH3I [Science 2008, 319, 183-186] at all but the lowest collision energies, a discrepancy that was not captured in a subsequent study at only a low collision energy [J. Phys. Chem. Lett. 2010, 1, 2747-2752]. Our direct chemical dynamics simulations at the DFT/B97-1 level of theory show that the reaction is dominated by three atomic-level mechanisms, an indirect reaction proceeding via an F(-)-HCH2I hydrogen-bonded complex, a direct rebound, and a direct stripping reaction. The indirect mechanism is found to contribute about one-half of the overall substitution reaction rate at both low and high collision energies. This large fraction of indirect scattering at high collision energy is particularly surprising, because the barrier for the F(-)-HCH2I complex to form products is only 0.10 eV. Overall, experiment and simulation agree very favorably in both the scattering angle and the product internal energy distributions.

Article	Year
Holding and spinning molecules in space. Physical review letters\| PubMed ID: 17930670	2007
Dissociative recombination of the acetaldehyde cation, CH(3)CHO(+). Physical chemistry chemical physics : PCCP\| PubMed ID: 20714489	2010
Indirect dynamics in a highly exoergic substitution reaction. Journal of the American Chemical Society\| PubMed ID: 23324058	2013

Sebastian Trippel

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