Fig. (2) FT‒IR spectroscopy of 4-ethoxy-cinnamic acid at the region of a) 3500‒400 cm^-1 and b) 2000‒200 cm^-1. Asterisks: vibrations of carboxylic; Closed circle: C‒C virbations in phenyl; Open circle: C‒C vibrations in double bond; Open diamond: C‒H vibrations; Closed diamond: CH₂ and CH₃ vibrations; Open triangle: ether vibrations