Quantum Chemical NMR Spectroscopic Structural Analysis in Solution: The Investigation of 3-Indoleacetic Acid Dimer...

We present a DFT-PCM NMR study of 3-indoleacetic acid (3-IAA), used as a working example, including explicit solvent molecules, named PCM-nCHCl₃, PCM-nDMSO (n = 0, 2, 4, 8, 14, 20, and 25), to investigate the dimer formation in solution. Apart from well-known cyclic (I) and open (II) acetic acid (AA) dimers, two new structures were located on DFT-PCM potential energy surface (PES) for 3-IAA named quasicyclic A (III) and quasicyclic B (IV), the last one having N-H…O hydrogen bond (instead of O-H…O). In addition, four other structures having π-π type interactions named V, VI, VII, and VIII were also obtained completing the sample on the PES. Our theoretical results and experimental ¹H NMR data (CDCl₃) strongly indicate that 3-IAA should exist in a quasicyclic form (III) in a chloroform solution different from AA. Solute-solvent interactions play a key role in O-H and N-H chemical shifts. The strong H-bond formation between the S=O and O-H and N-H groups produces large chemical shift value THAT masquerades the identification of dimer formation in DMSO solution based on ¹H NMR chemical shift changes. However, analysis of ¹³C NMR and relative energy DFT-PCM-nDMSO results strongly indicate the presence of parallel ring interacting dimer having OH…benzene ring bond (VI). There can be a competition between solute-solute and solute-solvent interactions, and polar DMSO solvent can break the quasicyclic dimers (III and IV) intermolecular O-H…O and N-H…O bonds yielding two solvated monomeric species hydrogen bonded to O=S(CH₃)₂ groups, what may take place for other organic molecules in solution. However, it did not happen for the π-π interacting dimers and structure VI survived in DMSO solution.