Potential energy barriers

Saltiel J and Sun Y-P 1989 Intrinsic potential energy barrier for twisting in the f/ a/rs-stilbene SI State in hydrocarbon solvents J. Phys. Chem. 93 6246-50... 

In the statistical description of ununolecular kinetics, known as Rice-Ramsperger-Kassel-Marcus (RRKM) theory [4,7,8], it is assumed that complete IVR occurs on a timescale much shorter than that for the unimolecular reaction [9]. Furdiemiore, to identify states of the system as those for the reactant, a dividing surface [10], called a transition state, is placed at the potential energy barrier region of the potential energy surface. The assumption implicit m RRKM theory is described in the next section. 

For reactions with well defined potential energy barriers, as in figure A3.12.1(a) and figure A3.12.1(b) the variational criterion places the transition state at or very near this barrier. The variational criterion is particularly important for a reaction where there is no barrier for the reverse association reaction see figure A3.12.1(c). There are two properties which gave rise to the minimum in [ - (q,)] for such a reaction. 

Hu X and Hase W L 1989 Properties of canonical variational transition state theory for association reactions without potential energy barriers J. Rhys. Chem. 93 6029-38... 

Since in in irn izalion calculations cannot cross or penetrate potential energy barriers, the molecular structure found during an opti-... 

Overcome potential energy barriers and force a molecule into a lower energy conformation than the one you might obtain using geometry optimization alone. 

Temperature is handled the same way in Langevin dynamics as it its in molecular dynamics. High temperature runs may be used to overcome potential energy barriers. Cooling a system to a low temperature in steps may result in a different stable conformation than would be found by direct geometry optimization. 

Potential energy barriers which are equal in height, on the order of 12 kJ mol , for ethane. 

Construction In tin dioxide semiconductor sensors, the sensing material is small sintered particles. For the sensor current flow, particle boundaries form potential energy barriers, which act as a random barrier netw ork. Different types t)f semiconductor gas sensors are shown in Fig. 13..54. 

Fig. 20.17 Potential energy-distance curves for a cathodic reaction showing how the potential energy barrier is lowered by when E < p,z.c. The barrier is assumed to be symmetrical so that /S => yi, where 5 is the distance of the O.H.P. from the surface of the electrode. Full curve—no field across double layer dashed curve-potential diflcrence is E and is negative...

In a real polymer chain, rotation around backbone bonds is likely to be hindered by a potential energy barrier of height AEr. If AEr < RT, the population of the... 

Figure 2.3. Tunnelling of a wave with kinetic energy E through a rectangular potential energy barrier, height V. The narrower the barrier, the smaller the mass of the particle and the smaller the difference between V and E, the greater the tunnelling probability. If the amplitude of the wave has not reached zero at the far side of the barrier, it will stop decaying and resume the oscillation it had on entering the barrier (but with smaller amplitude).

In the classical world (and biochemistry textbooks), transition state theory has been used extensively to model enzyme catalysis. The basic premise of transition state theory is that the reaction converting reactants (e.g. A-H + B) to products (e.g. A + B-H) is treated as a two-step reaction over a static potential energy barrier (Figure 2.1). In Figure 2.1, [A - H B] is the transition state, which can interconvert reversibly with the reactants (A-H-l-B). However, formation of the products (A + B-H) from the transition state is an irreversible step. 

Factors that enhance tunnelling are a small particle mass and a narrow potential energy barrier. In biology, electron transfer is known to occur over large distances (up to about 25 X 10 m). Given the mass of protium is 1840 times that of the electron, the same probability for protium... 

---