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

The Br2 elimination channel is probed for 1,2-C2H2Br2 in the B(3)Pi(+)ou-X(1)Sigma(+)g transition upon irradiation at 248 nm by using cavity ring-down absorption spectroscopy (CRDS). The nascent vibrational population ratio of Br2(v=1)/Br2(v=0) is obtained to be 0.7+/-0.2, thus indicating that the Br2 fragment is produced in hot vibrational states. The obtained Br2 products are anticipated to result primarily from photodissociation of the ground-state cis isomer via four-center elimination or from cis/trans isomers via three-center elimination, each mechanism involving a transition state that has a Br-Br distance much larger than that of ground state Br2. According to ab initio potential energy calculations, the pathways that lead to Br2 elimination may proceed either through the electronic ground state by internal conversion or through the triplet state by intersystem crossing. Temperature-dependence measurements are examined, thereby supporting the pathway that involves internal conversion--which was excluded previously by using product translational spectroscopy (PTS). The quantum yield for the Br2 elimination reaction is determined to be 0.120.1, being substantially contributed by the ground-state Br2 product. The discrepancy of this value from that (of 0.2) obtained by PTS may rise from the lack of measurements in probing the triplet-state Br2 product.

Photodissociation of 1,2-dibromoethylene at 248 nm: Br2 molecular elimination probed by cavity ring-down absorption spectroscopy.

By using a pump-probe technique, the nascent rotational and vibrational state distributions of NaH are obtained in the Na(4 (2)S,3 (2)D, and 6 (2)S) plus H(2) reactions. The rotational distributions for the Na(4 (2)S,3 (2)D) reactions yield a bimodal feature with a major component peaking at J=20-22, similar to that obtained previously in the 4 (2)P reaction, whereas the Na(6 (2)S) reaction gives rise to a distinct distribution with a much lower rotational temperature. The vibrational populations (v=0-4) for these 4 (2)S, 3 (2)D, and 6 (2)S reactions are characterized by corresponding temperatures of 1692+/-120, 819+/-35, and 5329+/-350 K. Due to a significant contribution of configurational mixing between different states with the same symmetry, the collision species initiated from the 4 (2)S and 3 (2)D states are anticipated to track along the entrance surface in a near C(2v) symmetry, then undergo nonadiabatic transition to the inner limb of the reactive 2A(') surface. In contrast, the reaction pathway for the Na(6 (2)S) state with a significantly reduced ionization energy is anticipated to follow a harpoon-type mechanism via a (near) collinear configuration. The increased atomic size of Na may hinder the insertion approach.

Rotational and vibrational state distributions of NaH in the reactions of Na(4 (2)S,3 (2)D, and 6 (2)S) with H2: Insertion versus harpoon-type mechanisms.

Many molecules exhibit multiple rotational isomers (conformers) that interconvert thermally and are difficult to isolate. Consequently, a precise characterization of their role in chemical reactions has proven challenging. We have probed the reactivity of specific conformers by using an experimental technique based on their spatial separation in a molecular beam by electrostatic deflection. The separated conformers react with a target of Coulomb-crystallized ions in a trap. In the reaction of Ca(+) with 3-aminophenol, we find a twofold larger rate constant for the cis compared with the trans conformer (differentiated by the O-H bond orientation). This result is explained by conformer-specific differences in the long-range ion-molecule interaction potentials. Our approach demonstrates the possibility of controlling reactivity through selection of conformational states.

Article	Year
Photodissociation of 1,2-dibromoethylene at 248 nm: Br2 molecular elimination probed by cavity ring-down absorption spectroscopy. Chemphyschem : a European journal of chemical physics and physical chemistry\| PubMed ID: 18481339	2008
Rotational and vibrational state distributions of NaH in the reactions of Na(4 (2)S,3 (2)D, and 6 (2)S) with H2: Insertion versus harpoon-type mechanisms. The Journal of chemical physics\| PubMed ID: 18570501	2008
Specific chemical reactivities of spatially separated 3-aminophenol conformers with cold Ca+ ions. Science (New York, N.Y.)\| PubMed ID: 24092740	2013

Yuan-Pin Chang

Biography

JoVE Journal Publications

Other Publications