Steric Effects in the Chemisorption of Vibrationally Excited Methane on Ni(100)

Newly available, powerful infrared laser sources enable the preparation of intense molecular beams of quantum-state prepared and aligned molecules for gas/surface reaction dynamics experiments. We present a stereodynamics study of the chemisorption of vibrationally excited methane on the (100) surface of nickel. Using linearly polarized infrared excitation of the C-H stretch modes of two methane isotopologues [CH4(n3) and CD3H(n1)], we aligned methane’s angular momentum and vibrational transition dipole moment in the laboratory frame. An increase in methane reactivity of as much as 60% is observed when the laser polarization is parallel rather than normal to the surface. The dependence of the alignment effect on the rotational branch used for excitation indicates that alignment of the vibrational transition dipole moment of methane is responsible for the steric effect. Potential explanations for the steric effect in terms of an alignment-dependent reaction barrier height or electronically nonadiabatic effects are discussed.

Alignment dependent chemisorption of vibrationally excited CH4(3) on Ni(100), Ni(110), and Ni(111)

Rainer Beck, Régis Bisson, Phil Morten Hundt, Bruce Yoder

We present a stereodynamics study of the dissociative chemisorption of vibrationally excited methane on the (100), (110), and (111) planes of a nickel single crystal surface. Using linearly polarized infrared excitation of the antisymmetric C–H stretch normal mode vibration (ν 3 ), we aligned the angular momentum and C–H stretch amplitude of CH4 (ν 3 ) in the laboratory frame and measured the alignment dependence of state-resolved reactivity of CH4 for the ν3 = 1, J = 0–3 quantum states over a range of incident translational energies. For all three surfaces studied, in-plane alignment of the C–H stretch results in the highest dissociation probability and alignment along the surface normal in the lowest reactivity. The largest alignment contrast between the maximum and minimum reactivity is observed for Ni(110), which has its surface atoms arranged in close-packed rows separated by one layer deep troughs. For Ni(110), we also probed for alignment effects relative to the direction of the Ni rows. In-plane C–H stretch alignment perpendicular to the surface rows results in higher reactivity than parallel to the surface rows. The alignment effects on Ni(110) and Ni(100) are independent of incident translational energy between 10 and 50 kJ/mol. Quantum state-resolved reaction probabilities are reported for CH4 (ν 3 ) on Ni(110) for translational energies between 10 and 50 kJ/mol.

American Institute of Physics2011

Steric Effects in the Chemisorption of Vibrationally Excited Methane on Nickel

Bruce Yoder

In this thesis, steric effects in the dissociative chemisorption of quantum state-prepared methane on single crystal surfaces of nickel (Ni(100) and Ni(110)) are detected and quantified for the first time. Exploiting a new, continuous-wave, high-power, single-mode infrared optical parametric oscillator, I produced an intense, quantum state-prepared molecular beam by rapid adiabatic passage. During the infrared excitation of the antisymmetric (ν3) stretch of CH4 or the C-H (ν1) stretch of CD3H by linearly polarized radiation, the angular momentum and vibrational transition dipole moment of methane is aligned in the laboratory frame. The excited, aligned molecular beam is used to probe the stereodynamics of the chemisorption reaction of vibrationally excited methane. For the reaction on Ni(100), an increase in state-prepared methane reactivity of nearly 60% is observed when the laser polarization direction is changed from normal to parallel to the surface. The dependence of the alignment effect on the rotational branch used for excitation (P-, Q-, or R-branch) indicates that alignment of the vibrational dipole moment rather than the angular momentum is responsible for the alignment dependent reactivity. Dephasing of the initially prepared alignment due to hyperfine coupling is observed to be on the timescale of 5-15 microseconds which does not preclude the study of alignment dependent reactivity in our experimental setup. Reactivity decreased monotonically from parallel to perpendicular alignment for both methane isotopologues studied. The alignment effect is shown to be independent of incident velocity for CH4(ν3) and decreases with increasing velocity of CD3H(ν1). For the Ni(110) surface, which consists of parallel rows of closely spaced Ni atoms separated by one-layer deep troughs, I probed if the reactivity depends on the methane alignment relative to the direction of the surface rows. The CH4(ν3) reactivity increases a factor of two when the laser polarization direction is changed from normal to parallel to the surface. Alignment of the vibration perpendicular to the surface rows produced a ∼10% higher reactivity than alignment parallel to the surface rows. Steric effects in a chemical reaction reveal detailed information about the reactive potential energy surface, which makes experimental studies of stereochemistry a powerful probe of microscopic chemical dynamics. The results in this thesis demonstrate and quantify specific steric requirements for this benchmark gas-surface reaction and will serve as a stringent test of multi-dimensional dynamics calculations.

EPFL2010

Preparation and characterization of Poly(Ethylene Glycol)/gamma-CD Polyrotaxanes using beta-CD derivatives as a capping reagent

Blaise Tardy

As living systems understanding got better, as life could be modeled in a batch, using immortalized cells, researches in fundamental sciences found startling applications of advanced techniques on living systems. As the biomaterial field developed various molecules were found to have very interesting applications on living systems. Polyrotaxane is one of these particularly interesting molecules. It is a non covalently bonded molecular necklace produced by the threading of cyclic molecules onto a linear polymer in order to form a pseudopolyrotaxane, the capping of this pseudopolyrotaxane to avoid de-threading of the cyclic compounds forms a polyrotaxane. This compound has already proven its versatility through various application including drug delivery systems, DNA delivery agent, molecular machine, hydrogel building block, resin stabilizer and a lot more. Cyclodextrins (CDs) and Polyester are known to form a pseudopolyrotaxane. CDs are composed of 6 to 8 glucopyranose forming a toroid shaped molecule that can contain different molecules depending on the solvent, experimental conditions and the number of sugar subunits. In this project an attempt has been made to synthesize a special type of polyrotaxane that would allow the whole complex to bare a higher degree of freedom compared to the ones currently documented. In order to do so the widest CD has been used (Gamma-CD, GCD), the chosen host has been poly(ethylene glycol) (PEG) chains of varying molecular weight and the capping reagent used was beta-CD derivatives. Furthermore since current literature hasn't documented it so well and its properties are essential to this study, some more characterization of the PEG/GCD pseudopolyrotaxane has been made. The Macromolecules have been characterized by maldi tof mass spectroscopy(MALDI-T-MS), electro spray ionization (ESI) mass detection, X-ray diffraction(XRD), gel phase chromatography (GPC) and liquid Nuclear magnetic resonance (NMR). The beta CD and poly(ethylene glycol) derivatives have been characterized using Fourier transform infra red spectroscopy (FTIR), NMR, MALDI-Toff MS, ESI and thin layer Chromatography (TLC)