Barrier of dissociation

CX +, H H F MP2/6-31G state structure and energy barriers of dissociation processes, proton affinity of CH2F+, GIAOMP2/tzp/dz NMR chemical shifts, isodesmic reactions ... 

In this section we discuss the accuracy of the BOC-MP projections concerning the activation barriers of dissociation and recombination surface reactions. We shall begin with diatomic adsorbates AB A + B. 

Bader analysis, 667 balance, kinetic, 132 band, conduction, 533-534 band gap, 537 band structure, 523, 527 band, valence, 537, 610 bandwidth, 532 barrier as shell opening, 948 barrier of dissociation, 801 barriers of reaction, 948 basis, biorthogonal, 513 basis set, atomic, 428, 431,el37... 

Ref. [107] and references therein—with retention of the symmetry plane all along the reaction path (see Sect. 1.3.4.3). The latest ab initio calculations (MP4 SDTQ, see Sect. 2.2) have produced the value of activation barrier of dissociation equal to 85.9kcal/mol [108], while the experimental data are by 5-6kcal/mol lower. Calculations of the rate of this reaction by means of the reaction path Hamiltonian have shown that it indeed proceeds at an energy 5-lOkcal/mol lower than the classical limit, which can be explained by the effect of tunnelling. A similar result has been obtained also in the case of the HNC HCN isomerization [106]. 

Schroeder J and Troe J 1993 Soivent effects in the dynamics of dissociation, recombination and isomerization reactions Activated Barrier Crossing ed G R Fieming and P Hanggi (Singapore Worid Scientific) p 206... 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

Although the thermodynamic aspects of acylotropy are well documented, there have been few kinetic studies of the process. The activation barrier is much higher than for prototropy and only Castells et al. (72CC709) have succeeded in observing a coalescence phenomenon in H NMR spectra. At 215 °C in 1-chloronaphthalene the methyl groups of N-phenyl-3,5-dimethylpyrazole-l-carboxamide coalesce. The mechanism of dissociation-combination explains the reversible evolution of the spectra (Scheme 9). 

The calculated barrier to dissociation of the [S3N2] dication into [SN]" and [S2N] in the gas phase is 10.9 kcal mof . However, lattice-stabilization effects allow the isolation of [MEg] salts (M = As, Sb) of this six r-electron system in the solid state from the cycloaddition of [SN] and [S2N] cations in SO2 (Eq. 5.11)."° The S-S and S-N bond distances in the planar, monomeric dication are shorter than those in the... 

Figure 1.5 Plot of computed reaction barriers for dissociation at Eaa. for N2 dissociation as a function of nitrogen atom adsorption energy on surface terrace and stepped surface [2]. The upper curve is for surface terrace of (111) type of fee crystals, and the lower curve presents data on the stepped surfaces.

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

Saue and Jensen used linear response theory within the random phase approximation (RPA) at the Dirac level to obtain static and dynamic dipole polarizabilities for Cu2, Ag2 and Au2 [167]. The isotropic static dipole polarizability shows a similar anomaly compared with atomic gold, that is, Saue and Jensen obtained (nonrelativ-istic values in parentheses) 14.2 for Cu2 (15.1 A ), 17.3 A for Ag2 (20.5 A ), and 12.1 A for Au2 (20.2 A ). They also pointed out that relativistic and nonrelativistic dispersion curves do not resemble one another for Auz [167]. We briefly mention that Au2 is metastable at 5 eV with respect to 2 Au with a barrier to dissociation of 0.3 eV [168, 169]. 

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