Vibrational analysis of α-cyanohydroxycinnamic acid

Abstract

In the present study, a comparative Raman vibrational analysis of alpha-cyano-4-hydroxycinnamic acid (4CHCA) and its derivative, alpha-cyano-3-hydroxycinnamic acid (3CHCA), was performed. The Raman spectra of the 4CHCA and 3CHCA in solid form were obtained and analyzed to determine differences between the two structurally similar derivatives. For comparison, the CHCA derivatives cyanocinnamic acid (CCA) and coumaric acid (CA) were also studied. The plausible vibrational assignments were made and matched with those obtained theoretically using density functional theory (DFT) based method employing a 6–31 g basis set. The computational wavenumbers obtained were in good agreement with the observed experimental results. This was the first reported Raman study of CCA, 3CHCA and 4CHCA.

Conformational differentiation of α‐cyanohydroxycinnamic acid isomers: a Raman spectroscopic study

“…Dimerization of carboxylic acid groups yields a distinctive –COOH stretch mode at ~1700 cm −1 , as observed for similar compounds like p‐ cyanobenzoic acid and 4‐butylbenzoic acid . We assigned the –COOH stretch modes for CHCA3 and CHCA4 at 1617 cm −1 and 1596 cm −1 respectively . These peaks are clearly depicted in the spectra of the recrystallized and solid samples of CHCA3 and CHCA4 (Figure ).…”

“…This was evident because the Raman spectra of CHCA4 at pH 1 and 7 are clearly very similar, indicating that the functional group is similarly protonated at pH 7 as at pH 1, while that for CHCA3 show an obvious difference. The Raman spectra of CHCA3 at pH 7 showed a decrease in intensity of the peak at 1617 cm −1 , the –COOH stretch mode, relative to the 1599 cm −1 band, ring 8a mode . The –COOH stretch mode is more intense in the pH 1 solution of CHCA3.…”

“…In the Raman spectra of CHCA4, a single intense peak in the region 2280–2200 cm −1 is attributed to the ‐CN stretch vibrational mode (Figures nd ) . This allowed us to interrogate the nature of the CN‐ring OH interaction for both CHCA isomers.…”

“…Density functional theory‐based simulation of the CHCA4 predicted that the ring 9a vibrational mode (Figure S7) should be at 1631 cm −1 . The experimental Raman spectra reveal peaks at ~1566 cm −1 for solid and recrystallized samples of CHCA4.…”

“…We measured the Raman spectra of the two isomers and assigned the observed peaks based on DFT calculations and by comparison to literature spectra of similarly structured compounds . The Raman assignments expose very distinct vibrational modes for each compound that betrays the different types of interactions that exist in CHCA3 and CHCA4.…”