Conformational Analysis, Structural and Vibrational Investigations of trans–2–Chlorocinnamic Acid and trans–4–Chlorocinnamic Acid

Abstract

The various conformers and the exact geometry of the more stable conformer of trans 2-chloro- and trans-4-chlorocinnamic acids have been determined by analysing the potential energy profile of the compounds. The s–cis orientation of the carbonyl group (C=O) and the C=C bond appears as the only stable structure in 2CCA and is more stable by 0.6648 kcal mol–1 than the corresponding s–trans conformation. In 4CCA, the s–cis conformer is more stable than the s–trans conformer by 0.8939 kcal mol–1. The structure and vibrational characteristic of the compounds have been investigated by using both experimental and quantum chemical methods. The bond distances, bond angles, dihedral angles, the kinetic and thermodynamic stability of the molecule, global and local reactivity parameters, the energy gap between the frontier molecular orbitals, the probable electronic transitions and bonding concepts of the compounds are elaborated in the present investigation. The frontier orbital energy gap of 2CCA and 4CCA are 4.5533 eV and 4.4366 eV, respectively shows that 4CCA is highly reactive than 2CCA molecule. The bond pair donor orbital, πCC → π*CC interaction from C3–C4 bond pair to C1–C2 and C5–C6 antibonding orbitals have the stabilisation energies 252.31 and 203.28 kcal mol–1, respectively. In the case of 4CCA, πCC → π*CC interaction from C1–C2 bond pair to C3–C4 and C5–C6 antibonding orbitals gives the strongest stabilisation of 258.75 and 203.58 kcal mol–1. The global and local reactivity descriptors clearly reveals that C9 is more favorable for nucleophilic attack while the ring carbon atoms are favorable for both electrophilic and nucleophilic attack.