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Mecanique des milieux continus. Schema by Salencon J.

By Salencon J.

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While the values of p^ for the C — H bonds exhibit a much smaller variation—they are all formally of order o n e — t h e r e are important trends. In the acyclic alkanes the value of Pb increases in the order CH4 < CH3 < CH2 < C H . T h e value of Pb is greater still for the C — H b o n d in cyclopropane and decreases in value as the ring size increases, exhibiting the acyclic value in cyclohexane. Still larger values are found for the C — H bonds in the more strained bicyclic structures, particularly for hydrogens bonded to bridgehead carbons.

The trend in the values of Pb parallel the increase in the percent s character of the C — H bond as discussed in a subsequent 1. Atomic a n d group properties in the alkanes 27 section. A C — H b o n d in cyclopropane has a greater percent s character, a shorter b o n d length, a larger stretch frequency and a larger dissociation energy than any C — H b o n d in an acyclic alkane, in agreement with the larger value of in the cyclic molecule. The decrease in with increase in ring size to the values found in cyclopentane a n d cyclohexane correlates with corresponding decreases in percent s character, C — H stretch frequency and b o n d dissociation energy.

A point on a boundary possesses a structure which is different from, but transitional to, the structures characteristic of either of the regions it separates. Since a b o u n d a r y is of dimension less than R^, arbitrary motions of the nuclei will carry a point on the b o u n d a r y into neighbouring stable structural regions a n d its structure will undergo corresponding changes. The boundaries are the loci of the structurally unstable configurations of a system. In general, the trajectory representing the motion of a system point in R^ will carry it from one stable structural region through a b o u n d a r y t o a neighbouring stable structural region.

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