Here, we demonstrate that ultrafast two dimensional infrared (2D-IR) spectroscopy provides evidence for the transition state involved in a simple thermal chemical reaction, the fluxionality of Fe(CO) 5 .Fluxionality refers to the rearrangement of a molecule between chemically indistinguishable structures.These reactions produce no net change in molecular structure, yet they are important for understanding the basic chemical behavior and reactivity of molecules in solution.Department of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 * These authors contributed equally and are listed alphabetically † corresponding author email address: cbharris@berkeley.edu 2 Fe(CO) 5 , an organometallic complex with five CO ligands arranged in a trigonal bipyramidal geometry, is a textbook example for fluxionality. In the late 1950s, nuclear magnetic resonance (NMR) spectroscopy revealed that this molecule rapidly exchanges its CO ligands between axial and equatorial sites. The 13 C NMR spectrum of 13 C labeled Fe(CO) 5 exhibits only a single peak at all accessibly measured solution temperatures, indicating that 13 C nuclei shift between axial and equatorial positions faster than NMR spectroscopy is able to distinguish these two chemical environments (2)(3)(4). Careful analysis of IR, Raman, and NMR spectra of Fe(CO) 5 and various derivatives suggests that the exchange process possesses a low barrier and occurs on a time-scale of picoseconds (4)(5)(6). Nevertheless, the dynamics in solution have not been quantified.From a general perspective, fluxional processes are simple chemical reactions in which a molecule briefly rearranges to a new symmetry and geometry as it crosses a transition state and then returns to its original geometry as it completes the reaction. We show for Fe(CO) 5 that during this process energy is exchanged between the vibrational modes of the reactive ligands. Quantification of this energy exchange provides direct information on the time-scale, transition state, and consequently, mechanism of the reaction.2D-IR spectroscopy has recently received much attention for its ability to monitor thermal reactions and chemical exchange on the femtosecond and picosecond time-scales. Conventionally, ultrafast timing of chemical reactions is achieved by photoinitiating the reactions with a short, intense laser pulse which electronically excites the molecules and typically leaves them with significant excess energy (1). In comparison, 2D-IR spectroscopy only requires vibrational excitation with an ultrafast IR laser pulse and allows the investigation of an entirely different class of thermally activated reactions in liquids at or close to equilibrium. Although similar to one-dimensional IR-pump, IR-probe experiments (1D-IR), 2D-IR spectroscopy separates the contributions to the 1D-IR spectrum into two frequency dimensions, which provides information on the correlations, anharmonicities, and exchange dynamics...

Improved materials for storing heat could save energy in applications such as heating and cooling and could enhance generation from solar thermal plants.

Phonons in low-dimensional structures with feature sizes on the order of the phonon wavelength may be coherently scattered by the boundary. This may give rise to a new regime of heat conduction, which can impact thermal energy transport and conversion. Traditional methods used to investigate phonon transport in one-dimensional structures suffer from uncertainty due to contact resistance, defects, and limited control over sample dimensions. We have developed a new batch-fabrication technique for suspended microdevices with integrated silicon nanowires from silicon-on-insulator (SOI) wafers. The nanowires are defect-free and have extremely high aspect ratios (length/critical dimension >2000). The nanowire dimensions (length and critical dimension) can be precisely controlled during fabrication. With these novel devices, phonon transport in silicon nanowires is systematically investigated. The room temperature thermal conductivity of nanowires with critical width around 80 nm is about 20 W/(m K) and much lower than that in smooth VLS wires. This suggests that the surface morphology of the structures has a significant effect on the thermal conductivity, but this phenomenon is not currently understood. This fabrication technique can also be used for thermal transport investigation in a wide-range of low-dimensional structures.

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