Valence perchloric acid hydrates

Decreasing the 0-H...0 angle exposes the H+ ion to other neighbouring anions with which it can form one or more additional very weak bonds (Ah < 0.05 vu, H... O > 250 pm with O-H... O angles < 130°). While such very weak bonds individually make only a small contribution to the valence sums, in compounds such as the perchloric acid hydrates they are so numerous that... 

For example, it was the systematic deviation of the bond valence sums of O from 2.0 V.U. (valence units) in the perchloric acid hydrates that first drew attention to the importance of the very weak H... O interactions, those with valences less than 0.05 V.U. corresponding to O-H distances between 240 and 320 pm (see Figure 10.1) [19. Including these interactions in the valence sums reduced the average discrepancy between the atomic valences and the bond valence sums from 0.17 to 0.05 v.u. Bond valences were also used to show the importance of the weak C-H... O hydrogen bonds formed by the methyl group of the acetate ion [20], a topic discussed more fully in Section 10.5. 

Fig. 10.13. The position of the acceptor O atom in O - H. .. O hydrogen bonds relative to the O - H bond, projected onto the O - H. .. O plane. The circles correspond to observed positions. The curved line passing through the densely populated regions indicates the positions predicted by the Bond Valence Model [50]. The acceptor atoms shown near 100° are those found in the weak bonds in perchloric acid hydrates...

Stetter and ReischP described the preparation of 2,6-diphonyl-P5Tylium perchlorate (57) from l,5-diphenyl-2-penten-4-yn-l-one (56a) and concentrated sulfuric acid in the cold, followed hy perchloric acid, in 60% 5rield. The process may involve hydration to a 1,5-enedione, or an ion 56b related to 52, and valence tautomerism. 

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