Spectroscopic identification of amyl alcohol hydrates through free OH observation

In this study, we identify the crystal structures of amyl alcohol + CH ₄ hydrates and demonstrate that the free OH observation of alcohol hydrates provides evidence of OH incorporation into the host framework occurring in some amyl alcohols. While two amyl alcohols, 3-methyl-2-butanol and 2-methyl-2-butanol, were identified as encaged in the 5¹²6 ⁸ large cage of structure-H hydrate, as expected from their molecular sizes above 7.5 Å, two other amyl alcohols, 3-methyl-1-butanol and 2,2-dimethyl-1-propanol, were identified to be abnormally included in the 5 ¹²6⁴ large cage of structure-II hydrate in spite of their too large sizes of 9.04 and 7.76 Å, respectively. The Raman spectra of two "normal" amyl alcohol hydrates evolved free OH peaks around 3600 cm^-1, implying that there is no strong hydrogen bonding interaction between alcohol guest and water host; however, for two "abnormal" amyl alcohol hydrates, the corresponding peaks were not detected, which indicates that the OH is incorporated into the host lattice in order to make the large alcohol guest fit into the relatively small 5¹²6⁴ cage of structure-II. The present findings are expected to provide useful information for a better understanding of alcohol guest dynamic behavior that might be significantly affected by structural dimensions and host-guest interactions.