Hydration/dehydration behavior of polyalcoholic compounds governed by development of intramolecular hydrogen bonds

Hydration/dehydration behavior of polyalcoholic compounds with a rigid cyclohexane-type aliphatic molecular frame, such as 1,4-cyclohexanediol (ch(OH)2), 1α,3α,5α-cyclohexanetriol (cis-phloroglucitol, ch(OH)3), and cis-1,2,3,5-trans-4,6-cyclohexanehexaol (myo-inositol, ch(OH)6) was investigated using extremely high-frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C. The temperature dependencies of hydration numbers per hydroxy group (mOH) in the polyalcoholic compounds were determined. Although the obtained mOH value was ca. 5 in a temperature range lower than 30 °C, it decreased and approached to ca. 3 with increasing temperature for ch(OH)2 and ch(OH)3, in which hydroxy groups hardly form intramolecular hydrogen bonds due to separations between them. This temperature-dependent hydration/dehydration behavior is characteristic of isolated hydroxy groups without intramolecular hydrogen bond formation. On the other hand, a constant mOH value of ca. 1 was observed irrespective of temperature for ch(OH)6. Because all the separations between adjacent hydroxy groups in ch(OH)6 are proper for (circular type) intramolecular hydrogen bond formation, the obtained temperature-independent small mOH value resulted from an inherent feature of intramolecularly hydrogen bonded hydroxy groups.