Repeller robustness

The ease of hydrolysis can vary enormously in some cases hydrolysis is so easy that inadvertent loss of the protecting group during chromatography becomes a nuisance (such as acetals of a, f3-unsaturated ketones). On the other hand acetals may be so robust that forcing conditions (mineral acid and heat) are required. For example, substrates that contain a basic amino function — even if it is remote — can be quite resistant to hydrolysis, because protonation first takes place at the more basic amino function,18 The resultant positive charge repels the second Q-protonation required to set in motion the hydrolysis. An acid-catalysed hydrolysis of a basic acetal that required refluxing with 6 M HC1 in acetone for 6-10 h19 is illustrated in Scheme 2,3, Perhaps the mildest conditions employ pyridinium p-toluenesulfonate (PPTS, pKa 5.2) in methanol or aqueous acetone exemplified by the transacetalisation reaction taken from a synthesis of the vacuolar H+-ATPase inhibitor Bafilomycin Ai [Scheme 2.4, 20... 

It is well at this stage to recall the basic premise of the VSEPR approach the electron pairs are similar in energy and repel by either simple electrostatic forces or by the Pauli exclusion principle (75). Within the d-block transition metals, this implies that the 4s, 4p, and 3d electrons should be of similar energy if the model is to work. We know that this is not true, and certainly the energy differences will be greater for metals like Sc, Ti, Zr, Zn, and Hg (27). Worse and different problems exist for the elements Rh, Ir, Pd, Pt, and Au, which exhibit the robust square planar 16-electron structure and which participate in oxidative addition and r uctive elimination reactions. Similarly, the geometry of the common low-spin square planar d compounds of Ni(II) (like Ni(DMG)2 and [Ni(CN)4] "), which do not obey the EAN rule, cannot be deduced from the VSEPR approach. 

Solid particle impact is a common test that is performed to evaluate the mechanical robustness of surface finishes. In this test, sand is typically used which is a mixture of micrometer-sized oxide particles with the silicon dioxide being the most dominant. This test can provide information whether a coating is suitable for use in outdoor applications where eventually it has to withstand harsh weather conditions and contamination/ degradation of the liquid repellent properties by embedded particles on the surface that can disrupt the surface chemistry by their presence or can cause partial destruction of the surface texture due to the impacts. It is also a common test in aerospace applications (e.g. for helicopters) where they... 

Gehrke et al. [ 131 ] recently used a dip-coating process to deposit photocatalytic TiO nanoparticles (P25, Evonik) on a metallic filter material (micro-sieve). The fouling repellent and photocatalytic nanocoatings degraded the water impurities close to the micro-sieve surface before a dense cake layer was formed. This kind of surface activation is, however, restricted to chemically robust materials, excluding polymeric membranes that would be degraded by the induced oxidation process [125]. 

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