Dissociation kinetic energy plots

Figure 3.29. CH4 dissociation probability SQ (or S in this chapter) on Ni(100) plotted logarithmically vs. the normal kinetic energy En. (a) as a function of vibrational temperature Tv as noted in the figure. The lines are the fits of eq. (3.2) to the experiments. From Refs. [267,268]. (b) is for vibrationally state-resolved dissociation measurements, with CH4 in the Vj (solid triangles facing upwards), 2v3(open squares), v3 (open downward facing traingles) and v = 0 (solid circles) vibrational states. From Ref. [117].

Figure 10 Potential energy surface of the dissociation of 02/Pt(l 11) determined by the ab initio derived tight-binding Hamiltonian. The coordinates in the figure are the O2 center-of-mass distance from the surface Z and the 0-0 interatomic distance d. The configurations of the remaining O2 degrees of freedom are illustrated in the insets. The contour spacing is 0.2 eV per O2 molecule. In (a) a trajectory of an O2 molecule with an initial kinetic energy of 0.6 eV scattered at Pt(l 11) is also plotted.

The absolute probability for the dissociative chemisorption of CH4 on Ni(lll) is plotted versus the normal component of the kinetic energy of the incident methane molecule in Fig. 2. The dissociation probability was measured by monitoring the amount of deposited carbon by Auger electron spectroscopy. Since methane does not adsorb molecularly at the surface temperature of 475 K at which these measurements were carried out (ref. 9), the carbon Auger feature results only from the methane that has dissociatively chemisorbed. The carbon Auger signal is calibrated for absolute carbon coverage and the absolute flux of the incident methane beam is determined from procedures outlined in detail previously (refs. 6b,10). The absolute dissociation probability plotted in Fig. 2 is the ratio of the absolute number of adsorbed carbon atoms per unit area to the absolute number of methane molecules per unit area incident on the surface. 

Figure 31. (a) Dissociation probability as a function of the initial kinetic energy for Hj/NiflOO). The solid curves are from the quantum calculation, and the circles and diamonds correspond to the quantum-weighted classical and quasiclassical approaches, respectively (b) same as (a) except the gas atom mass is 2 (c) same as (a) except the gas atom mass is 3 (d) same as (a) except the gas atom mass is 7. The plots are from Chiang and Jackson (1987). 

Figure 35, (a) Variation of the initial sticking coeiiicient with initial normal kinetic energy (circles represent experimental results from Hamza and Madix (1985) while triangles and squares represent classical GLE calculations at incident angles of 0° and 45°, respectively) (b) calculated number density of scattered Hj vs. final polar angle (c) calculated initial rotational state dependence of the dissociation probability. The plots are from Kara and DePristo (1989). 

In contrast to the vibrational effect, the rotational effect on hydrogen dissociation on Cu is much less understood, until very recently. Most 3D quantum calculations have used the plane rotor model, which is not appropriate for studying rotational effects. The studies of Refs. 113, 114, and 117 using the spherical rotor treatment have obtained important results on the effect of rotational orientation and the nuclear symmetry. The rotational orientational effect is clearly shown in Fig. 15, where reaction probabilities for different initial rotational orientation states are plotted as a function of kinetic energy. Significant enhancement of reaction probability is seen for the state with j = m ( helicopter mode) while the m = o ( cartwheel mode) is least effective for dissociation. 

Figure 16 Degeneracy-averaged dissociation probability P(j) f°r H2 on Cu(lll) plotted as a function of the rotation quantum number j at kinetic energies Ek - 0.44, 0.50, and 0.56 eV.

The difference in mechanism, D for ligand replacement except when the leaving group is water (/ mechanism), adumbrated above, is also indicated by the different isokinetic plots obtained for the two types of reaction and illustrated in connection with the investigation of the kinetics of dissociation and of formation of thiourea complexes.The slope of the linear free energy plot connecting rates and equilibrium constants for reaction of pentacyanoaquoferrate(II) with a series of cobalt(III) complexes also indicates a dissociative mechanism for these reactions. 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

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