Potential energy profile, structural, vibrational and reactivity descriptors of trans–2–methoxycinnamic acid by FTIR, FT-Raman and quantum chemical studies

• The trans–2–methoxycinnamic acid has two stable conformations s-cis and s-trans.
• The more stable structure has CO and CC bond in s–cis orientation.
• The s–cis conformation is more stable than s–trans conformer by 0.13 kcal mol−1.
• The MEP of s–cis is ±1.374e × 10−2 while for s–trans it is ±1.591e × 10−2.
• Spectral properties of trans–2–methoxycinnamic acid have been studied.

The stable conformers of trans–2–methoxycinnamic acid (trans–2MCA) are determined by potential energy profile analysis. The energies of the s–cis and s–trans conformers of trans–2MCA determined by B3LYP/cc–pVTZ method are −612.9788331 Hartrees and −612.9780953 Hartrees, respectively. The vibrational and electronic investigations of the stable s–cis and s–trans conformers of trans–2–methoxycinnamic acid have been carried out extensively with FTIR and FT–Raman spectral techniques. The s–cis conformer (I) with a (C16–C17–C18–O19) dihedral angle equal to 0° is found to be more favoured relative to the one s–trans (II) with (C16–C17–C18–O19) = 180°, possibly due to delocalization, hydrogen bonding and steric repulsion effects between the methoxy and acrylic acid groups. The DFT studies are performed with B3LYP method by utilizing 6–311++G** and cc–pVTZ basis sets to determine the structure, thermodynamic properties, vibrational characteristics and chemical shifts of the compound. The total dipole moments of the conformers determined by B3LYP/cc–pVTZ method are 3.35 D and 4.87 D for s–cis and s–trans, respectively. It reveals the higher polarity of s–trans conformer of trans–2MCA molecule. The electronic and steric influence of the methoxy group on the skeletal frequencies has been analysed. The energies of the frontier molecular orbitals and the LUMO-HOMO energy gap have been determined. The MEP of s–cis conformer lie in the range +1.374e × 10−2 to −1.374e × 10−2 while for s–trans it is +1.591e × 10−2 to −1.591e × 10−2. The total electron density of s–cis conformer lie in the range +5.273e × 10−2 to −5.273e × 10−2 while for s–trans it is +5.403e × 10−2 to −5.403e × 10−2. The MEP and total electron density shows that the s–cis conformer is less polar, less reactive and more stable than the s–trans conformer. All the reactivity descriptors of the molecule have been discussed. Intramolecular electronic interactions and their stabilisation energies have analysed by NBO method.

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