Energetics of neutral and deprotonated (Z)-cinnamic acid

• We determined ΔfHm0(g) of neutral and deprotonated (Z)-cinnamic acid.
• We determined the acidity, in gas and liquid phases, of (Z)-cinnamic acid.
• (Z)-cinnamic acid is less stable but more acid than the corresponding (E)-isomer.

We have performed a study of structural, thermochemical and thermophysical properties of the (Z)-cinnamic acid neutral molecule and its corresponding oxyanion (formed by deprotonation of the carboxylic group). The thermophysical study (heat capacities, temperature and enthalpy of fusion) was made by DSC. The following intrinsic (gas-phase) thermochemical magnitudes have been experimentally determined: (i) standard enthalpy of formation, at T = 298.15 K, of the neutral molecule, ΔfHm0(g) = (−215.5 ± 3.2) kJ · mol−1, by combustion calorimetry and by the Knudsen effusion technique, (ii) deprotonation enthalpy, ΔacidH0(g) = (1416.4 ± 8.8) kJ · mol−1 and acidity, GA = (1386.7 ± 8.8) kJ · mol−1, by the EKM method using ESI-TQ Mass Spectrometry. From these results we have also derived the enthalpy of formation of the oxyanion, ΔfHm0(oxyanion, g) = (−303.5 ± 9.4) kJ · mol−1. A computational study, through density functional calculations at the B3LYP/6-311++G(d,p) level of theory, was used to check the good consistency of the experimental results. The global results show that (Z)-cinnamic acid is significantly less stable than the corresponding (E)-isomer, which can be related to the greater acidity of the (Z)-form found in both the gas and aqueous solution phases.

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