Atomization, activation energy for

The limitation of this approach is that when the activation energy for atom transport is in excess of kT there is little probability of an atom or vacancy migrating during a simulation run, and the diffusion constant will appear to be zero. Thus, constraints need to be introduced to force the atom to move. One approach for identifying the diffusion... 

The rate at which the nuclei grow will also depend on the frequency with which atoms join it, and this may be written as [v exp(-AGJkT)], where v is the characteristic frequency and AGm is the activation energy for atom migration. Putting V = kTIh, where h is Planck s constant, an approximate expression for the nucleation rate is (47) ... 

If we assume that the apparent activation energy for atomic motion in the NP interfacial region can be scaled by (n), that is, <3 (n) Eq, where Eq is the high-... 

Zhao et al. [16] proposed an atomistic analytical expression for reproducing the yield strength over the whole HPR and IHPR size range. They modified the T-independent HPR by introducing the activation energy for atomic dislocation to the HPR slope. The activation energy was related directly to the melting point suppression, (K) [Pg.578]

The IHPR at larger solid size converges to the normal HPR that holds its conventional meaning of the accumulation of atomic dislocations that resist further atomic displacements in plastic deformation. The slope in the traditional HPR is proportional to exp(TVT), which addresses the relationship between the hardness and the activation energy for atomic dislocations. The term in the conventional HPR represents the accumulation of atomic dislocations that resists further dislocations. 

Fig. XVIII-15. Oxygen atom diffusion on a W(IOO) surface (a) variation of the activation energy for diffusion with d and (b) variation of o- (From Ref. 136. Reprinted with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

One reason for the low reactivity of pyridine is that its nitrogen atom because it IS more electronegative than a CH in benzene causes the rr electrons to be held more tightly and raises the activation energy for attack by an electrophile Another is that the nitrogen of pyridine is protonated in sulfuric acid and the resulting pyndinium ion is even more deactivated than pyndine itself... 

What other factors might be responsible for difference in activation energies Compare atomic charges anc electrostatic potential maps for the Sn2 transition states Does the increase in steric crowding lead to enhanced o diminished charge delocalization Explain. How, if at all would this be expected to affect the energy barrier Why ... 

Carlo-simulations for LI2 superlattice including saddle-point energies for atomic jumps in fact yielded two-process kinetics with the ratio of the two relaxation times being correlated with the difference between the activation barriers of the two sorts of atom. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

Hindered rotation around the P-N bond has been observed at low temperature in tetrasubstituted phosphorus amides [28]. For PhjPNJSiMCjJj, two different Me3Si groups are observed below -65 °C, the calculated activation energy for P-N rotation being 10.2 Real moT [29]. Chiral phosphinous amides with stereogenic phosphorus atoms have been prepared [30,31 ]. 

The presence of the zeolite cavity dramatically lowers the activation energy for the protonation of toluene. It is mainly due to screening of the charges in the transition state due to the polarizable lattice oxygen atoms. In the transition state, a positive charge develops on protonated toluene. 

The recombination of two bromine atoms to form a Br2 molecule has zero activation energy for the same reason. 

The measured [ OH]/[ OH] branching ratio versus inverse temperature is plotted in Fig. 4. If the two species are produced by two parallel pathways, the total reaction rate is a simple sum of the two pathway-resolved rates. In this case, the data points in an Arrhenius plot should fall on a straight line with a slope proportional to the difference in activation energies for the two competing pathways. A fit to the data in Fig. 4 yields the result that the barrier to O atom abstraction is 1.0 0.4kcal mol larger than for H atom abstraction. Although... 

It is now assumed that the adsorption of NO on Rh(lOO) is associative and not activated. The sticking coefficients S(T) can be set to unity for all temperatures. Furthermore shall we assume that the activation energy for the desorption of NO from Rh(lOO) is 140 kj mol and that two Rh atoms constitute an adsorption site. 

---