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 Delta H-f(m)degrees(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 Delta H-f(m)degrees(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 pi-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.