Structural Topics in Organic Systems

The only major groupings under this heading (Table 13) involve X=S and X=N. The Csp —S distance for divalent sulphur [1.681(5) A] is very considerably shorter than for 

TABLE 12. Geometrical characteristics of C=C—X (X it C,H,D, metal) systems from X-ray data  

As with the C=C bond, there is an apparent elongation of C=N in some of the substructures of Table 13 over the values cited in Tables 3b and 8. However, none of these elongations is statistically significant. 

The valence-angle constraints imposed by cyclization lead to very high strain energies (Es) in the small- and medium-ring cycloalkynes. Members of the series below ring size 6 

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