Energetics and structural properties, in the gas phase, of trans -hydroxycinnamic acids

Abstract

We have studied the energetics and structural properties of trans-cinnamic acid (CA), o-, m-, and p-coumaric acids (2-, 3-, and 4-hydroxycinnamic acids), caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), iso-ferulic acid (3-hydroxy-4-methoxycinnamic acid), and sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid). The experimental values of ΔfHm°(g), determined (in kJ·mol–1) for CA (−229.8 ± 1.9), p-coumaric acid (−408.0 ± 4.4), caffeic acid (−580.0 ± 5.9), and ferulic acid (−566.4 ± 5.7), allowed us to derive ΔfHm°(g) of o-coumaric acid (−405.6 ± 4.4), m-coumaric acid (−406.4 ± 4.4), iso-ferulic acid (−565.2 ± 5.7), and sinapic acid (−698.8 ± 4.1). From these values and by use of isodesmic/homodesmotic reactions, we studied the energetic effects of π-donor substituents (-OH and -OCH3) in cinnamic acid derivatives and in the respective benzene analogues. Our results indicate that the interaction between -OCH3 and/or -OH groups in hydroxycinnamic acids takes place without significant influence of the propenoic fragment.