Hydrogen bonding in perchloric acid hydrates

A bond-valence analysis of five perchloric acid hydrates shows that normal hydrogen bonds account for only about one half of the bonding of the perchlorate ion. Each H atom also forms an average of four additional weak interactions (H ⋯ O acceptor ~ 2.3 to 3.1 Å; O-H ⋯ O ~ 80 to 120°) which account for the rest of the bonding. A distinction can be made between the normal and the very weak hydrogen bonds in the lower hydrates but this distinction is less clear in the higher hydrates where the distribution of H ⋯ O bond strengths tends to form a continuum.