Vibrational enhancement

Chi Z H, Chen X G, Holtz J S W and Asher S A 1998 UV resonance Raman-selective amide vibrational enhancement quantitative methodology for determining protein secondary structure Biochemistry 27 2854-64... 

Hammes-Schiffer, S., Tully, J.C. Vibrationally Enhanced Proton Transfer. J. Phys. Chem. 99 (1995) 5793-5797. 

In contrast, for an endothermic reaction for which the saddle point lies in the exit channel (late energy barrier), the reactant vibration enhances the... 

Agrawal, N., Hong, B., Mihai, C. and Kohen, A. (2004). Vibrationally enhanced hydrogen tunneling in the Escherichia coli thymidylate synthase catalyzed reaction. Biochemistry 43, 1998-2006... 

Nonempirical LCAO-MO-SCP study for linear HeH surface demonstrates a late barrier, or potential-energy step characteristic for reactions showing vibrational enhancement (Fig. 61) 453... 

Collinear quantum-mechanical No vibrational enhancement observed 461... 

S. Hammes-Schiffer and J. C. Tully. Vibrationally enhanced proton transfer. J. Phys. Chem., 99(16) 5793-5797, 1995. 

In Fig. 5, additionally the calculated and measured vibrational temperatures [50] are plotted. In contrast to the rotational cooling, there is vibrational heating indicating that there should be enhanced dissociation for vibrating hydrogen molecules on Pd(l 00). Vibrationally enhanced dissociation has been known for years in the gas phase dynamics community [53]. Usually it is associated with strongly curved reaction paths in activated systems [4]. However, the most favorable path towards dissociative adsorption in the system H2/Pd(l 0 0) is purely attractive and has a rather small curvature (see Fig. 2a). Therefore one would not expect any substantial influence of the vibrational state of H2 on the sticking probability. 

This simple model would lead one to conclude that H2 dissociation on transition metals, where the unfilled d-states produce a low and early barrier (or even zero barrier), will show no vibrational enhancement, whereas dissociation on simple and noble metals, for which the barrier is high and late, will have vibrationally enhanced dissociation. This appears to be borne out in molecular beam experiments there is no observable increase in dissociation with internal state temperature for H2 on Ni(l 1 1), Ni(l 1 0), Pt(l 1 1) or Fe(l 1 0) [16-19], whereas dissociation on all surfaces of Cu shows an... 

Zhao W, Wright IC. Doubly vibrationally enhanced four-wave mixing-the optical analogue to 2D NMR. Science, Phys Rev Lett 2000 84 1411-1414. 

Bruno, W.J. and Bialek, W. (1992) Vibrational enhanced tunneling as a mechanism for enzymatic hydrogen transfer, Biophys. J. 63, 6890-699. 

The final physical method to be considered here, which allows further probing of an absorption band, is resonance Raman spectroscopy. The excitation laser wavelength is tuned into an absorption band and the vibrations enhanced in the Raman spectrum are detected. Only those vibrational modes associated with distortion of the excited electronic state relative to the ground-state geometry will be resonance enhanced. This method, therefore, not only allows observation of vibrations directly associated with the active site but also provides valuable information on the nature of the excited state. Usually, charge transfer transitions are probed due to the high intensity (e > 500 M"1 cm-1) required for resonance enhancement. These points are well illustrated by reso-... 

Figure 12 Vibrational enhancement selectivity available from resonance Raman spectroscopy. The UV-visible spectrum of a P. aeruginosa azurinis shown together with two different Raman spectra (frozen solution at 77 K) that derive from laser excitation within the S(Cys) — Cu(II) charge-transfer absorption band at 625run (647.1 nm) and away from the absorption (488.Onm). Excitation within resonance leads to dramatically increased Raman scattering from the Cu active site, whereas off-resonance excitation produces a spectrum dominated by bands of nonchromophoric ice...

Al-Akoum et al. [82] compared the bubbling. Dean flow, and vibrating-enhanced membrane processes in terms of the shear stress and the permeate fluxes obtained in filtration of yeast suspension. The filtration with two-phase flow was carried out using 15 mm ceramic mono tubular UF (permeability 250 L/m h bar) and MF (permeability 1500 L/m h bar) membranes with TMPs of 100 and 25 kPa for UF and MF, respectively. The yeast concentrations used in the two-phase experiments were 1... 

Because high fluxes and the abdity to process streams containing suspended solids and fibers are often wanted in the pulp and paper industry, high-shear modules have been developed. Currently existing high-shear modules, excluding tubular modules, are modified plate and frame constructions. Both a cross-rotational module from Metso Paper and a vibration enhanced module (VSEP) from New Logic Inc. have been industrially used or tested in pulp and paper industry applications [48-51]. 

The RBU model can be used to study the effect of exciting the vibrational modes treated within the model. For the reactions X (X=C1, 0 and H) + CH4 HX + CH3 we find that exciting a vibrational inode results in a lower threshold to reaction. It was also found that exciting the reactive C-H stretch enhances the reactivity more than exciting the CH4 umbrella mode. Vibrational enhancements for the umbrella and C-H stretch vibrations have also been found in other studies of the dynamics[75, 80] and in canonical variational transition state theory (C T) calculations [84]. Enhancement of the Cl + CH4 reaction due to vibrational excitation of the H-CH3 stretch has also been confirmed b experimental measurements by Zare and coworkers[85]. 

The idea that the vibrational enhancement of the rate is due to the attraetive potential for excited vibrational states of the reactant is closely related to the observation made long ago based on transition state theoiy [25,26]. Poliak [25] found that for vibrationally highly excited reactants the repulsive pods (periodic orbit dividing surface) is way out in die reactant valley, and the corresponding adiabatic barrier is shallow. Based on this theory one can explain why dynamical thresholds are observed in reactions with vibrationally excited reactants. The simplicity of the theory and its success for mostly collinear reactions has a real appeal. However, to reconcile the existence of a vibrationally adiabatic barrier with the capture-type behavior - which seems to be supported by the agreement of the calculated and experimental rate coefficients [23] -needs further study. 

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