C sp2 —F bonds

TABLE 16. C—F bond distances (in pm) in ethylenes and other compounds with C(sp2)—F bonds 

In the carbonyl compounds FC(0)Y the C—F bond distances show a strong dependence on the substituent X and shorten from 136.2(2) pm in acetyl fluoride (Y = Me) to 131.6(1) pm carbonyl fluoride (Y = F). In formyl fluoride (Y = H) and carbonyl chloride fluoride (Y = Cl) the C—F bond lengths are intermediate. When CF2 groups are double-bonded to C, O, S or Se, the C—F bond distances are remarkably constant and range only from 131.5 to 131.9 pm. Such bond lengths, however, are shorter by ca 1.5 pm in compounds with C=N double bonds, such as methanimine or 2,3-diaza-l, 3-butadiene. In both compounds only mean values for the C—F bond distances can be derived from the ED experiments. Ab initio calculations for perfluoromethanimine predict that the C—F bond trans to N—F is longer by 0.4 pm than the cis bond. 

Although aromatic rings belong to the group of the C(sp2)—F distances, these compounds are discussed separately (Table 17). The C—F bond distance in fluorobenzene 

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