Quantum Chemical NMR Spectroscopic Structural Analysis in Solution: The Investigation of 3‐Indoleacetic Acid Dimer Formation in Chloroform and DMSO Solution

ABSTRACT We present a DFT‐PCM NMR study of 3‐indoleacetic acid (3‐IAA), used as a working example, including explicit solvent molecules, named PCM‐ n CHCl 3 , PCM‐ n DMSO ( 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 1 H NMR data (CDCl 3 ) 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 1 H NMR chemical shift changes. However, analysis of 13 C NMR and relative energy DFT‐PCM‐ n DMSO 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(CH 3 ) 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.