Infrared and Raman Spectroscopy Study of Alkyl Hydroxamic Acid and Alkyl Hydroxamate Isomers

Abstract

The isomeric structures of alkyl hydroxamic acid, as well as its potassium salt, sodium salt, and an alcohol complex, have been characterized in the solid, liquid, and gaseous states by Fourier transform infrared (FT-IR) and FT-Raman spectroscopy. Raman spectroscopy provides insight into the long-standing debate over the isomeric composition of hydroxamates in the solid state and in an aqueous basic solution. IR and Raman results are not consistent with the enol isomer existing in the solid or liquid states of octyl or decyl hydroxamic acid, potassium hydroxamate, and sodium hydroxamate. The infrared and Raman spectra of these compounds provide clear and convincing evidence regarding their chemical structure, mainly from amide-type carbonyl, NH bending, and OH/NH stretching bands. Vibrational spectroscopy is sensitive to polar (FT-IR) and non-polar (FT-Raman) vibrations and the influence of ionic and hydrogen bonding on these vibrations, and these abilities are particularly useful for characterizing keto versus enol and trans versus cis conformations in alkyl hydroxamic acid and its salts. Evolved gas analysis (EGA) in a nitrogen gas environment of alkyl hydroxamic acid and its salts is also discussed. EGA data reveal that water is not incorporated into the solid-state crystal structure of alkyl hydroxamic acid or the potassium salt; however, the sodium salt form is found to have a stable hydrate conformer that is shown to affect the Z isomer (NH trans to carbonyl, OH cis to carbonyl) IR absorbance bands. EGA data also indicates results that could be of interest to bio-pharmaceutical applications involving nitric oxide donation.