Vibrational spectra of potassium 4-pentenoate and potassium 5-hexenoate and conformational change on micellization

Abstract

Potassium 4-pentenoate (P4P) and potassium 5-hexenoate (P5H) have been synthesized and their infrared and Raman spectra were measured in the crystalline state and aqueous solutions. The two bands of the crystalline P4P at 623–625 and 558–560 cm^–1 were assigned to the characteristic bands of skew and cis forms about the C—C single bond nearest to the CC double bond, respectively. The intensity of the 623–625 cm^–1 band in the infrared spectra increases at 170 °C, compared to that of the 558–560 cm^–1 band. For P5H the intensity of the 640–643 cm^–1 band characteristic of skew increases at low temperature. From these observations, it was concluded that the skew from in the crystalline P4P and P5H is stabilized at low temperature.

On the basis of these observations and X-ray diffraction and Raman studies of unsaturated fatty acids by other workers, the following assumptions were made; the infrared bands of the crystalline P4P increasing in the intensity at –170 °C and the corresponding Raman bands arise from the TS form, and those of P5H are due to the TTS form. On these assumptions, normal mode analysis was made for possible rotational isomers. The calculated results successfully explain the vibrational bands of P4P and P5H.

The Raman spectra of P4P and P5H in aqueous solutions were measured at various concentrations. At higher concentration the TS form of P4P and the TTS form of P5H were found to be stabilized. The inflection in the plots of the relative Raman peak heights in the 1000–800 cm^–1 region vs. the inverse concentrations is clearly observed. These observations show that micelle formation brings about the conformational change.