Cyclic strain effects

The tensile and cyclic strain effects on the resistivity, p(e,B), of high-purity aluminum were measured. Due to the plastic deformation of 0.6% tensile strain. 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

Strain effects also contribute to reactivity of cyclic carbonyl compounds. The normal bond angles around a carbonyl group are about 120° ... 

Bartlett PD, Banavali R. Spontaneous oxygenation of cyclic olefins effects of strain. J Org Chem 1991 56 6043-6050. 

Isolated keratinocytes subjected to cyclic strain exhibit a significant increase in cell proliferation, DNA synthesis, and protein synthesis compared to stationary or constantly loaded cells, which appear to involve changes in cyclic AMP. Takei et al. (1997) reported a strain-induced reduction in the levels of cyclic adenosine monophosphate, protein kinase A (PKA), and prostaglandin E2 (PGE2) as compared to stationary controls. Takei et al. (1997) also studied the effects of cyclic strain on protein kinase C (PKC) activation and translocation in cultured keratinocytes. 

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. 

In addition to the two alternative active methylene groups in the side chain ring, strain effects have to be considered when cyclic analogues of VII/7 and VII/10 are treated with base. Thus, compounds with the general formula VII/13 give, apart from some side products (e.g. retro-Michael and bicyclic compounds... 

Dynamic Mechanical Spectroscopy. As pointed out above, the heterogeneous morphology, observed in the present study, implies network flaws (weak connections between the primary and secondary microgels). However, the low cyclic strains applied in dynamic mechanical tests detect only the effect of the basic network structure and not the network flaws. Therefore, as observed experimentally (, ) dynamic mechanical spectroscopy should not indicate heterogeneity in the samples. 

In contrast to the protonation of the disubstituted alkenes mentioned above, high ktranJkcis ratios of 9 x 108 and 3 x 103, respectively have been observed in the acid-catalyzed addition of methanol to the trans- and cis-isomers of cycloheptene (39a and 38a) and cyclooctene (37a and 36a) (114a). The rate constants reflect partially the release of strain in the transformation of the cyclic olefins to the appropriate cycloalkyl cations. Comparison of the relative activation energies for these addition reactions with the difference of strain release leads to the estimate that the response to strain effects is about 60%. In a more recent study of the acid-catalyzed hydration of cis- (36b) and trans- 1-methylcyclooctene (37c), it was concluded that two conformationally different 1-methyl-carbocationic intermediates are... 

The rates of 1,3-dipolar addition of mesitonitrile oxide to cis- (36a) and frans-cyclooctene (37a) and some other cyclic olefins have been discussed with respect to strain effects (271). It was concluded that frontier orbital energies of the alkenes and differences in olefinic strain energies do not correlate with the observed reactivities. The relative rates for addition of phenyl azide to several alkenes and the olefinic strain energies are shown in Table 31 (272). 

Carbon-13 n.m.r. studies may be used to investigate strain effects in cyclic olefins.The values of the chemical shifts in some strained cyclic olefins are shown in Table II. 

Note that there was little effect up to 10 fatigue cycles. Between 10 and 10 cycles, however, a rapid wear out of the composite occurred. For example, when cycled to 700 MN/m, the wire fractured at about 10" cycles. When cycled to less than 500 MN/m, however, the wire never failed at 4 K. Figure 5 shows that, at higher temperatures, this endurance limit was lower. The endurance limit corresponds to a peak-to-peak cyclic strain amplitude of about 0.35% at 4 K. At liquid nitrogen and room temperatures, the endurance strain limit was smaller, approximately 0.2% in both cases. 

Some structures with the structural motif of interest are from cyclic compounds [17-19]. These are not appropriate for our purposes, since the disilacyclus causes additional ring strain effects. In addition to these existing structures we have synthesized the new amino(chloro)methyldisilanes 5-7 (Fig. 2). Nevertheless, there are not sufficient experimental data to obtain reliable parameters. Accordingly, theoretical methods provide the only practical means of deriving force field constants. The X-ray structural data of the available compounds 1-7 will be used to prove the quality of the new developed molecular modeling parameters. 

An additional point of interest in cyclic straining was the MuUins effect. It was helpful to have a quantitative measure of how close the materials come to exhibiting an ideal Mullins response, as defined for example by Ogden et al (1999) [175, 296]. To this end we defined a Mullins factor M, such that an ideal Mullins response was characterized by M = 1 [175] ... 

By doing this study, a quantitative correlation was found by us, between the magnitude of the Mullins effect and the fractional energy dissipation by hysteresis under cyclic straining, giving a common relation that was approached by all the materials studied. Stress relaxation in interrupted tests also increased with hysteretic energy loss [175]. 

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