Barrier, triple

Figure 6.15 Degenerate triple-barrier triple hydron transfer involving two z A/itterionic intermediates. A complete transfer consists of a d issociation step, one or more cation or anion propagation steps, characterized by the forward and backward rate constants kf = kb, and a neutralization step. These isotope dependent rate constants depend on whether a cation or an anion is propagated.

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

Figure 13.1. Cross-section of 0.5 triple-level integrated circuit (IC) with spin-on-glass planarization and Ti/TiN diffusion barrier.

By contrast, addition-elimination mechanisms in their simplest form begin with formation of an addition complex resulting from a well on the PES, followed by dissociation of the complex, yielding products. Both the entrance to and exit from the well may be hindered by barriers on the PES. Addition mechanisms are uncommon in radical -b saturated closed-shell reactions due to the difficulty of bond formation with the saturated species (ion-molecule reactions are exceptions). By contrast, additions are more common in radical -b unsaturated closed-shell species, where the double or triple bond allows a low barrier or barrierless pathway for addition of the radical into the 7i-bond of the stable species, such as the reaction... 

In Table 5 the insertion barrier at levels of theory higher than MP2 are also reported (runs 10-13). The MP3 and MP4 insertion barriers are both remarkably higher than the MP2 barrier. The CCSD insertion barrier also is quite larger than the MP2 barrier (5.2 kcal/mol above), but the perturbative inclusion of triple excitations in the couple cluster calculations reduces considerably the CCSD barrier, which is 8.7 kcal/mol (3.1 kcal/mol above the MP2 insertion barrier). The insertion barriers reported in Table 5 can be used to obtain a further approximation of the insertion barrier. In fact, the CCSD(T) barrier of 8.7 kcal/mol should be lowered by roughly 3 kcal/mol if... 

The test set used for most comparisons in the present paper is Database/3 18), which was introduced elsewhere. It consists of 109 atomization energies (AEs), 44 forward and reverse reaction barrier heights (BHs) of 22 reactions, 13 electron affinities (EAs), and 13 ionization potentials (IPs). There are a total of 513 bonds among the 109 molecules used for AEs, where double or triple bonds are only counted as a single bond. Note that all ionization potentials and electron affinities are adiabatic (not vertical), i.e., the geometry is optimized for the ions... 

Subsequently, Backvall and coworkers developed triple-catalysis systems to enable the use of dioxygen as the stoichiometric oxidant (Scheme 3) [30-32]. Macrocyclic metal complexes (Chart 1) serve as cocatalysts to mediate the dioxygen-coupled oxidation of hydroquinone. Polyoxometallates have also been used as cocatalysts [33]. The researchers propose that the cocatalyst/BQ systems are effective because certain thermodynamically favored redox reactions between reagents in solution (including the reaction of Pd° with O2) possess high kinetic barriers, and the cocatalytic mixture exhibits highly selective kinetic control for the redox couples shown in Scheme 3 [27]. 

Figure 7.7 Renner-Teller PES for ethylenedione radical anion. Geometrical data for the Au equilibrium structure are provided for various levels of theory using an augmented polarized triple-f basis set (TZP+). Barriers to linearity (AE. kcal rnol ) are from CCSD(T) calculations using, from top to bottom, DZP, DZP+, TZP, and TZP+ basis sets. If the initial guess is for the Bu state instead of the Au state, what will happen ...

To further clarify the situation, the authors examined two other quantities dependent on the shape of the PES in the vicinity of the linear form. First, they computed the barrier to double-inversion through the linear form. The data are listed in Figure 1.1., and show some basis set dependence. Note that the CCSD(T)/TZP- - result is approximated to within 0.1 kcal mol by summing the CCSD(T)/TZP barrier with the difference between the CCSD(T)/DZP- - and CCSD(T)/DZP barriers. That is, the effect of diffuse functions evaluated with a double-t basis set can be treated as additive to the non-augmented triple- results, along the lines described in Section 7.7.3. 

Considerable attention has been paid to the dynamic behaviour of Mo2+ complexes in solution and lH NMR spectroscopy has been an essential probe. These properties have been reviewed710 and emphasis given to the point that the barrier to rotation about the triple bond is determined only by steric factors associated with ligand-ligand interactions.205 95MoNMR... 

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