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Bond Strength Effects

Previtali and Scaiano have attempted to correlate rates of hydrogen abstraction with thermodynamic parameters 69 along the lines of Polanyi 70 . In this approach (AH proportional to AH) the exothermicity of hydrogen abstraction varies with the triplet excitation energy and the carbonyl 51-bond energy, at constant C—H and O—H bond energies. [Pg.18]

The near constancy of h values as drops some 10 kcal/mole from acetone to benzophenone is readily explained by the close parallel between E and En values. Conjugation of the carbonyl with a benzene ring lowers the C—O jr-bond energy since the resulting semi-pinacol radical is resonance stabilized. This parallel further supports a simple 1,2-diradical valence bond picture for carbonyl triplets, a picture which implies that the excitation has broken the 51-bond and that the -electron on oxygen is very weakly correlated with the electron in the -system. [Pg.18]

It is at least amusing and perhaps even revealing that the dipole moment of triplet benzophenone is that of a C—0 single bond 71 . The measured h values for the n,n triplets of a-diketones are 2—3 orders of magnitude lower than for monoketones 71a . It is likely that the low Ei values ( 55 kcal/mole) are not compensated for by lowered jr-bond energy. [Pg.18]


The effect of storage of 100% RH at 21°C on the bond strength of glass/ aluminium sandwich butt tensile joints, with and without silane, is shown in Fig. 5. It can be seen that without silane the bond strength effectively fell to zero in 150 days. With MPS on the surface, the bond strength equilibriated at 24 MPa at 220 days. [Pg.40]

The natural conclusion to be drawn from Table 12 is that steric effects play a very big part indeed in determining the rate and orientation of radical addition. We believe this conclusion to be correct, but, just as it is impossible to separate bond strength effects from polar effects, so it is impossible to separate steric effects in radical addition from polar effects. Any steric hindrance in the addition of a radical to an olefin will depend to a major extent on the shape of the radical. However, the shape of the radical is directly connected to the electron density at the trivalent carbon atom. [Pg.62]

FIGURE 3.3 A summary of periodic trends in relative acidity. Acidity increases from left to right across a given row (electronegativity effect) and from top to bottom in a given column (bond strength effect) of the periodic table. [Pg.122]

The data also show that the Rh(lII)-Y bond becomes relatively easier to break along the series I < Br < Cl when an iodide is in the trans-position. This suggests that the effect of incipient solvation of the leaving ligand outweighs the bond-strength effects and these complexes can be considered as kinetically of class (b) type (3), When a chloride is in the trans-position values show bromide to be more easily removed than chloride so bond-strength effects are here more important than solvation effects and the complexes are kinetically of class (a). [Pg.314]

It can be readily confirmed thaf by decreases as the number of bonds N increases and/or llieir length (r ) decreases. This relationship between the bond strength and the number of neighbours provides a useful way to rationalise the structure of solids. Thus the high coordination of metals suggests that it is more effective for them to form more bonds, even though each individual bond is weakened as a consequence. Materials such as silicon achieve the balance for an infermediate number of neighbours and molecular solids have the smallest atomic coordination numbers. [Pg.263]

Bond Strength The effect of bond strength is easy to see by comparing the acidities of the hydrogen halides... [Pg.38]

The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... [Pg.50]

Vessel heads can be made from explosion-bonded clads, either by conventional cold- or by hot-forming techniques. The latter involves thermal exposure and is equivalent in effect to a heat treatment. The backing metal properties, bond continuity, and bond strength are guaranteed to the same specifications as the composite from which the head is formed. AppHcations such as chemical-process vessels and transition joints represent approximately 90% of the industrial use of explosion cladding. [Pg.150]

The stability toward additional disproportionation is dependent on the increase in B—N bond strength as well as steric effects resulting from the R group. [Pg.262]

In acyclic structures, such effects are averaged by rotation, but in cyclic structures differences in C—H bond strengths based on the different alignments can be recognized. The C—H bonds that are in an anti orientation to the lone pair are weaker than the C—H bonds in other orientations. [Pg.57]

Effect of silanes on bond strength of epoxide coatings... [Pg.407]

Fig. 11. Effect of polyolefin primers on bond strength of ethyl cyanoacrylate to plastics. All assemblies tested in accordance with ASTM D 4501 (block shear method). ETFE = ethylene tetrafluoroethylene copolymer LDPE = low-density polyethylene PFA = polyper-fluoroalkoxycthylene PBT = polybutylene terephthalate, PMP = polymethylpentene PPS = polyphenylene sulfide PP = polypropylene PS = polystyrene PTFE = polytetrafluoroethylene PU = polyurethane. From ref. [73]. Fig. 11. Effect of polyolefin primers on bond strength of ethyl cyanoacrylate to plastics. All assemblies tested in accordance with ASTM D 4501 (block shear method). ETFE = ethylene tetrafluoroethylene copolymer LDPE = low-density polyethylene PFA = polyper-fluoroalkoxycthylene PBT = polybutylene terephthalate, PMP = polymethylpentene PPS = polyphenylene sulfide PP = polypropylene PS = polystyrene PTFE = polytetrafluoroethylene PU = polyurethane. From ref. [73].
Nitrile rubber/phenolic resin blends. Blends of equal parts by weight of a nitrile rubber and a phenolic resin in methyl ethyl ketone (at a 20-30 wt% total solids content) is suitable for many adhesive purposes. The more phenolic resin in the formulation, the greater the bond strength and brittleness of the NBR adhesive [67]. Table 10 shows the effect of phenolic resin on nitrile rubber properties. On the other hand, the higher the acrylonitrile content in the rubber. [Pg.659]


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