This study investigates the structural and energetic properties of hydrogen-bonded complexes between indole and a range of aliphatic, cyclic, and aromatic ketones using a combined vibrational spectroscopic and quantum-chemical approach. FTIR measurements in CCl₄ revealed redshifts in the N-H stretching vibration of indole upon complexation, with formation constants (K_a) ranging from 0.3 to 6.6 M⁻¹. Cyclohexanone displayed the strongest binding, while benzophenone exhibited the weakest interaction. Quantum-chemical calculations, employing CREST and MMFF94 conformational sampling, along with M06-2X/6-311++G(d,p) optimizations, confirmed the formation of hydrogen bonds and additional weak interactions that govern the stability of the complex. QTAIM analysis revealed moderate closed-shell hydrogen bonds with electron densities at the bond critical points (ρ) ranging from 0.010 to 0.019 a.u. and potential energy densities (V) from −18.4 to −36.4 kJ mol⁻¹. Multivariate regression analysis established strong correlations (R² = 0.928 and 0.957) between experimental binding constants and theoretical descriptors, including binding energy, NBO charge on oxygen atom, ionization potential, and electrophilicity index, highlighting the interplay between geometric, electronic, and global reactivity factors. This comprehensive study underlines the predictive power of spectroscopic and quantum descriptors for assessing hydrogen bonding in biologically relevant systems.

The molecular structures of lipids, including branching, double bonds and substitution within the hydrophobic chains, are key aspects in research fields such as biomarkers and signaling, and determine cell membrane properties. Although mass spectrometry has become the dominant method for lipidomics, determining fatty acid chain structures using widely available methods (MSn via CID) remains challenging. In this study, we present reactive ion deposition as a method for chemically modifying interfacial lipids in thin layers. Highly reactive [B12I11]− anions are mass-selected and deposited onto layers containing various fatty acids, phosphatidylserines, phosphatidylcholines and triacylglycerides. Using isotope-labelled compounds, we observed that [B12I11]− selectively binds to the terminal methyl groups of fatty acid hydrophobic n-alkyl chains (≥95%). The molecular structure of the lipid remains intact during covalent binding of the reactive ion on the surface, effectively charge-tagging the lipid at the alkyl chain. Therefore, ESI-MS analysis of the interfacially active layer components is enabled. Collision-induced dissociation of the anionic reaction products reveals structural information about the lipid via MSn spectra regarding both head group and fatty acid chain structure (i.e., branching, double bonds and substituent positions). This method enables not only analysis of lipids molecular structures, but also reveals information about the interfacial composition and molecular orientation of complex samples due to the binding of molecular groups located at the interface of the probed layer.