DFT Study of the Molecular Structure, Conformational Preference, Spectroscopic and Vibrational Analysis of Cinnamic Acid and Cinnamaldehyde

B3YLP with the 6-311++G (d, p) basis set was used to investigate the conformational preference, geometrical structure, and spectroscopic properties of the conformational isomers of cinnamic acid and cinnamaldehyde in gas and in solvents. In the gas phase, the s-cis isomer of cinnamic acid was found to be more stable than the s-trans conformer, while for cinnamaldehyde the s-trans conformer was found to be the more stable conformer. The effects of solvents on the conformational preference of these molecules were investigated using the IEF-PCM model. For both cinnamic acid and cinnamaldehyde, the solvent has shown no significant effect on the stability preference. However, the stability of both conformational isomers of cinnamic acid and cinnamaldehyde increases as the dielectric constant of solvent increases, because solvation energies decrease as the dielectric constant of the solvent increases. The 13C and 1H NMR chemical shifts were calculated in DMSO and chloroform. The NBO charges and the UV-visible spectra have been computed in the gas phase, chloroform, methanol, and water.