Low Barrier

Nikowa L, Schwarzer D, Troe J and Schroeder J 1992 Viscosity and solvent dependence of low barrier processes photoisomerization of c/s-stilbene in compressed liquid solvents J. Chem. Phys. 97 4827... 

Ammonia is a two-state system [16], in which the two base states lie at a minimum energy. They are connected by the inversion reaction with a small baiiier. The process proceeds upon the spin re-pairing of four electrons (Fig. 15) and has a very low barrier. The system is analogous to the tetrahedral carbon one... 

Figure 8 shows a one-dimensional sketch of a small fraction of that energy landscape (bold line) including one conformational substate (minimum) as well as, to the right, one out of the typically huge number of barriers separating this local minimum from other ones. Keeping this picture in mind the conformational dynamics of a protein can be characterized as jumps between these local minima. At the MD time scale below nanoseconds only very low barriers can be overcome, so that the studied protein remains in or close to its initial conformational substate and no predictions of slower conformational transitions can be made. 

Oxirene is probably a true intermediate, but is separated from ketene by only a very low barrier. Since its instability results from unimolecular isomerization rather than from attack of other molecules, the only viable current technique for its direct observation seems to be generation and spectroscopic examination in an inert matrix at temperatures near absolute zero. 

The conformational distance does not have to be defined in Cartesian coordinates. Eor comparing polypeptide chains it is likely that similarity in dihedral angle space is more important than similarity in Cartesian space. Two conformations of a linear molecule separated by a single low barrier dihedral torsion in the middle of the molecule would still be considered similar on the basis of dihedral space distance but will probably be considered very different on the basis of their distance in Cartesian space. The RMS distance is dihedral angle space differs from Eq. (12) because it has to take into account the 2n periodicity of the torsion angle. 

CS indicated that the enolate of acetyl-CoA is significantly more stable than the enol or a proton-sharing enolic form and thus do not support the proposal that a low barrier hydrogen bond is involved in catalysis in CS. This study demonstrates the practial application of high level QM-MM studies to the elucidation of mechanistic details of an enzymatic reaction that are otherwise unclear. 

The multiple minima nature of the bending energy and the low barriers for interconversion resemble the torsional energy for organic molecules. An expansion of bend in tenns of cosine or sine functions of the angle is therefore more natural than a... 

Except for reactions with low barriers (i.e. <10 kcal/mol at 7 = 300 K), or at high temperatures, the quantity /k T is large, and the last series can be neglected. The tunnel correction is then given completely in terms of the magnitude of the imaginary frequency. For small values of the first term may be Taylor expanded to give... 

Blom et al. [85] stated that the l/V characteristics in LEDs based on ITO/di-alkoxy-PPVs/Ca are determined by the bulk conductivity and not by the charge carrier injection, which is attributed to the low barrier heights at the interface ITO/PPV and PPV/Ca. They observed that the current flow in so called hole-only devices [80], where the work function of electrodes are close to the valence band of the polymer, with 1TO and Au as the electrodes, depends quadratically on the voltage in a logl/logV plot and can be described with following equation, which is characteristic for a space-charge-limitcd current (SCL) flow (s. Fig. 9-26) ... 

Another early example is nitromethane which is special in two respects. First, the barrier has sixfold symmetry because of the threefold character of the methyl group and the twofold character of the nitro group. Secondly, the barrier turns out to be extremely low, only about 5 small calories. For such a low barrier, it is convenient to treat the coupling between free internal rotation and overall rotation exactly and consider the barrier as a small perturbation. 

The aryl C—O—C linkage has a lower rotation barrier, lower excluded volume, and decreased van der Waals interaction forces compared to the C—C bond. Therefore, the backbone containing C—O—C linkage is highly flexible. In addition, the low barrier to rotation about the aromatic ether bond provides a mechanism for energy dispersion which is believed to be the principal reason for the toughness or impact resistance observed for these materials.15 17... 

Fig. 3. Free-enthalpy diagram of the interoonversion 5< 7 and 8 at 20°C in FHSO3—SbFs (concentrations expressed in mole litre i). The low barrier between ions 11 and 12 has been omitted. Underlined numbers directly from experimental data.

Six members of this series could be isolated in modest yields as highly air-sensitive, dark blue or dark purple crystalline solids for which analytical, spectroscopic, and single-crystal X-ray analyses were fully consistent with the side-on-biidged N2 structures shown in Scheme 102. These complexes show unusual structural features as well as a unique reactivity. An extreme degree of N = N bond elongation was manifested in rf(N-N) values of up to 1.64 A, and low barriers for N-atom functionalization allowed functionalization such as hydrogenation, hydrosilylation, and, for the first time, alkylation with alkyl bromides at ambient temperature. ... 

Relative to the rigid tr-conjugated systems, the locahzed diradicals are comph-cated by various possible conformations due to low barriers in rotations of o bonds. 

When the selectivity of a reaction is controlled by differences in the way molecules are activated on different sites, the probability of the presence of different sites becomes important. An example again can be taken from the activation of CO. For methanation, activation of the CO bond is essential. This will proceed with low barriers at step-edge-type sites. If one is interested in the production of methanol, catalytic surfaces are preferred, which do not allow for easy CO dissociation. This will typically be the case for terrace sites. The selectivity of the reaction to produce methanol will then be given by an expression as in Eq. (1.29a) ... 

The Nonmetal Atom Sharing Rule of Low-Barrier Transition States... 

The next section introduces the topological concept of low-barrier transition states through the prevention of formation of shared bonds between reacting surface adsorbates and surface metal atoms. 

As can be seen from Figure 1.20 [22], those transition states that do not share binding to the same surface metal atom have low barriers. The fcc(lOO) surface has the unique property that the reaction can occur through motion over the square hollow with bonds that remain directed toward the corner atoms of the square atom arrangement on the surface. This is a unique and important feature of reactions that require in their transition states interactions with several surface atoms. 

As first noted by Neurock etal. [24], the Pt(lOO) surface provides sites for extremely low barriers of NO and N2 recombination. For NO, the energetics on... 

Figure 1.22 Structures of high-barrier and low-barrier transition states of surface bond cleavage reactions.

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