Intramolecular O···H Hydrogen Bonding of Salicylic Acid: Further Insights from O 1s XPS and H NMR Spectra Using DFT Calculations.

Intramolecular hydrogen bonding (HB) is a complex phenomenon that extends beyond a simple valence event, affecting the core electrons of a molecule. Salicylic acid (SA) and its conformers provide an excellent model compound for studying intramolecular HB as the proton donor (H) and acceptor (O) can be toggled by rotating the C-O and C-C bonds to form up to seven potential conformers through various HB. In this study, we computationally investigated intramolecular interactions in SA conformers with and without such HB, by examining their calculated O 1s core electron-binding energy (CEBE) and H NMR chemical shifts validated using recent measurements. The quantum mechanically stable SA conformers are fully defined by three rotatable bonds in the compound, which are abstracted as SA(ηηη) digital structures, where η = 0 if the η angles match the most stable SA conformer (000) and η = 1 otherwise. Our findings suggest that the stability is dominated by the appearance of the intergroup intramolecular HB of H···O (where O is in the carboxylic acid functional group and H is the phenolic proton in -OH), and η serves as a switch of such HB. As a result, the (ηη0) SA conformers containing such H···O HB are more stable than other SA conformers (ηη1) without such the H···O HB. The present density functional theory calculations reveal that this H···O HB results in splitting of the O 1s CEBEs of two hydroxyl groups (-OH) by up to 1 eV and deshielding the H proton H NMR (δH) up to 11.68 ppm for the (ηη0) conformers. Without such H···O HB, the O 1s XPS binding energies of two -OH groups will be closely located in the same band, and the H NMR chemical shift of the H atom will be as small as an 4.09 ppm SA conformer [SA-G(101)]. The present study indicates that the O 1s CEBE splitting between two -OH groups serves as an indicator of the presence of the H···O HB in SA conformers, which is also supported by the H NMR results.