Evaluation of molecular assembly, spectroscopic interpretation, intra-/inter molecular hydrogen bonding and chemical reactivity of two pyrrole precursors

• The computational calculations were done using DFT/B3LYP/6-31G(d,p) and B3LYP/6-311+G(d,p) methods.
• FT-IR spectra of the studied compounds were recorded and compared with the theoretical results.
• Intra- and intermolecular charge transfer have been evaluated using NBO analysis.
• Chemical reactivity has been explained with the aid of global and local electronic descriptors.

This paper describes the evaluation of conformational, spectroscopic, hydrogen bonding and chemical reactivity of pyrrole precursor: ethyl 3,5 dimethyl-1H-pyrrole-2-carboxylate (EDPC) and ethyl 3,4-dimethyl-4-acetyl-1H-pyrrole-2-carboxylate (EDAPC) for the convenient characterization, synthetic usefulness and comparative evaluations. All experimental spectral values of 1H NMR, UV–Vis and FT-IR spectra coincide well with calculated values by DFT. The orbital interactions in EDPC and EDAPC are found to lengthen their NH and CO bonds and lowers their vibrational frequencies (red shift) resulting to dimer formation. The QTAIM and NBO analyses provide the strength of interactions and charge transfer in the hydrogen bonding unit and stability of dimers. The binding energy of EDPC and EDPAC dimer are found to be 9.92, 10.22 kcal/mol, respectively. In EDPAC and EDPC dimer, hyperconjugative interactions between monomer units is due to n1(O) → σ*(NH) that stabilize the molecule up to 9.7 and 9.3 kcal/mol, respectively. On evaluation of molecular electrostatic potential (MEP) and electronic descriptors for EDPC it has been found that it is a good precursor for synthesis of formyl and acetyl derivatives whereas EDAPC has been found to be a good precursor for synthesis of schiff base, hydrazones, hydrazide–hydrazones and chalcones.

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