Triple Bonds in Conjugated Systems

The geometrical characteristics, determined by X-ray diffraction, for systems —C=C—in which acetylene is directly bonded to R = C=C (ethylenic), C==C (aromatic), C=0 (ketones and carboxylic acids), C=N and to other C=C bonds are collected in Table 5. In Table 6, we summarize the results of accurate ED studies of dimethylacetylene , vinylacetylene, propynaP and diacetylene which were designed to study the effects of conjugation in these systems. 

The gas-phase results of Table 6 show that the C=C bonds in vinylacetylene and in propynal are very slightly elongated by comparison with C=C in parent acetylene (Tables 2 and 3). However, the C=C bond length in diacetylene [1.2176(6) ] is significantly 

TABLE 6. Bond lengths (ED, To values) determined by Kuchitsu and co-  

TABLE 7. The effects of conjugation and changes in hybridization on C—C bond lengths in C C—C C systems where C C represents a single or multiple bond  

TABLE 8. Cyano compounds Mean X-ray bond lengths in conjugated systems —C=N 

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