Reaction Kinetics

Fluxionality of Pentaorganylsilicates

The basic structure of stable pentaorganylsilicates may contain a variety of biaryl moieties, as I have demonstrated with e.g. the N-phenylpyrrol-2,2'-diyl analogue shown below. In solution, its three-dimensional structure is fluxional due to Berry pseudorotation. The equilibration with a second stereoisomer could be confirmed by 2D 1H,29Si ge-HMQC and 1H NOESY NMR experiments. We performed variable-temperature NMR measurements followed by lineshape analyses to determine thermodynamic and kinetic parameters for the interchange process.

Dynamic Configurational Isomerism of a Stable Pentaorganosilicate

This research was published in Angewandte Chemie

Gas-phase Reaction Barriers

Threshold CID
My current research in the Chen group involves ESI-MS/MS studies on model systems of transition-metal catalysts. I helped to equip our Finnigan MAT TSQ-700 tandem mass spectrometer with a home-built 24-pole ion guide, which serves to thermalize the species of interest. This is required for our threshold collision-induced dissociation (T-CID) experiments, for which I also improved the acquisition method to afford results of superior quality in a much less laborious way than before (see right).

We determine absolute gas-phase reaction barriers from the threshold curves with L-CID, a program developed in our group. The rates of intramolecular rearrangements increase less quickly with excess energy than those of dissociations. When the barrier for rearrangement is slightly below the subsequent product dissociation energy, so-called transition-state switching occurs. Using RRKM theory I showed that, for such cases, the overall reaction kinetics can be approximated with a model for rearrangements but that this actually affords an upper limit to the dissociation energy.
Furthermore, we implemented a model in L-CID to fit the threshold curves for a sequential reaction, as observed for an (N-heterocyclic carbene) palladium complex.

This research was published in Chemistry − A European Journal

Host−Guest binding − The approach of L-CID to extract reaction barriers from CID threshold curves is particularly suited for large molecular ions. Thus, I collaborated with Zhongshu Li, who is studying the host−guest interactions between α-cyclodextrin and benzoates. The experimentally determined binding strengths served to validate a DFT method, which allowed the decomposition into contributions from hydrogen bonding and other non-covalent interactions.

A Quantitative Study of Intrinsic Non-covalent Interactions within Complexes of α-Cyclodextrin and Benzoate Derivatives

This research was published in Chemical Communications

This research was published in the Journal of Physical Chemistry B

Work in Progress

Customizing a new MS − We have purchased a new electrospray tandem mass spectrometer, a Thermo Fisher TSQ Quantum Ultra EMR, to extend our capabilities to perform gas-phase threshold experiments. I am currently coordinating the implementation of the required modifications. Between the ESI source and the main vacuum manifold of the instrument, we have added a vacuum stage as illustrated below. This enables us to replace the original transfer quadrupole by a longer thermalization ion guide, for which I performed ion trajectory simulations to determine the optimal multipole order and geometry. Furthermore, a regulating panel for the thermalization and CID gases is attached, as well as sensitive gauges to monitor their pressures. Of course, I have adapted my previously developed acquisition scripts that facilitate the T-CID experiments, too. Finally, I have worked on a curved octapole to replace the built-in collision quadrupole ion guide.

Ion guideCustomizationModified TSQ