Probing the Competition among Different Coordination Motifs in Metal–Ciprofloxacin Complexes through IRMPD Spectroscopy and DFT Calculations

The vibrational spectra of ciprofloxacin complexes with monovalent (Li⁺, Na⁺, K⁺, Ag⁺) and polyvalent (Mg²⁺, Al³⁺) metal ions are recorded in the range 1000–1900 cm^–1 by means of infrared multiple-photon dissociation (IRMPD) spectroscopy. The IRMPD spectra are analyzed and interpreted in the light of density functional theory (DFT)-based quantum chemical calculations in order to identify the possible structures present under our experimental conditions. For each metal–ciprofloxacin complex, four isomers are predicted, considering different chelation patterns. A good agreement is found between the measured IRMPD spectrum and the calculated absorption spectrum of the most stable isomer for each complex. Metal ion size and charge are found to drive the competition among the different coordination motifs: small size and high charge density metal ions prefer to coordinate the quinolone between the two carbonyl oxygen atoms, whereas large-size metal ions prefer the carboxylate group as a coordination site. In the latter case, an intramolecular hydrogen bond compensates the weaker interaction established by these cations. The role of the metal cation on the stabilization of ionic and nonionic structures of ciprofloxacin is also investigated. It is found that large-size metal ions preferentially stabilize charge separated motifs and that the increase of metal ion charge has a stabilizing effect on the zwitterionic form of ciprofloxacin

Chlorine Para-Substitution of 1‑Phenylethanol: Resonant Photoionization Spectroscopy and Quantum Chemical Calculations of Hydrated and Diastereomeric Complexes

The conformational landscape of (<i>S</i>)-1-(4-chlorophenyl)­ethanol, its monohydrated complex, and its diastereomeric adducts with <i>R</i>- and <i>S</i>-butan-2-ol, have been investigated by resonant two-photon ionization (R2PI) spectroscopy coupled with time-of-flight mass spectrometry. Theoretical calculations at the D-B3LYP/6-31++G** level of theory have been performed to assist in the interpretation of the spectra and in the assignment of the structures. The R2PI spectra and the predicted structures have been compared with those obtained on the analogous non-halogenated and fluorinated systems, i.e., (<i>R</i>)-1-phenylethanol and (<i>S</i>)-1-(4-fluorophenyl)­ethanol, respectively. It appears that the presence of chlorine atom in the para position of the aromatic ring does not influence the overall geometry of bare molecule and its complexes with respect to the non-halogenated analogous systems. Anyway, it affects the electron density in the π system, and in turn the strength of OH···π and CH···π interactions. A spectral chiral discrimination is evident from the R2PI spectra of the diastereomeric adducts of (<i>S</i>)-1-(4-chlorophenyl)­ethanol with the two enantiomers of butan-2-ol

Cation−π Interactions in Protonated Phenylalkylamines

Phenylalkylamines of the general formula C₆H₅(CH₂)_<i>n</i>NH₂ (<i>n</i> = 1–4) have been delivered to the gas phase as protonated species using electrospray ionization. The ions thus formed have been assayed by IRMPD spectroscopy in two different spectroscopic domains, namely, the 600–1800 and the 3000–3500 cm^–1 regions using either an IR free electron laser or a tabletop OPO/OPA laser source. The interpretation of the experimental spectra is aided by density functional theory calculations of candidate species and vibrational frequency analyses. Protonated benzylamine presents a relatively straightforward instance of a single stable conformer, providing a trial case for the adopted approach. Turning to the higher homologues, C₆H₅(CH₂)_<i>n</i>NH₃⁺ (<i>n</i> = 2–4), more conformations become accessible. For each C₆H₅(CH₂)_<i>n</i>NH₃⁺ ion (<i>n </i>= 2–4), the most stable geometry is characterized by cation−π interactions between the positively charged ammonium group and the aromatic π-electronic system, permitted by the folding of the polymethylene chain. The IRMPD spectra of the sampled ions confirm the presence of the folded structures by comparison with the calculated IR spectra of the various possible conformers. An inspection of the NH stretching region is helpful in this regard

Adsorption and Dissociation of <i>R</i>‑Methyl <i>p</i>‑Tolyl Sulfoxide on Au(111)

Sulfur-based molecules producing self-assembled monolayers on gold surfaces have long since become relevant functional molecular materials with many applications in biosensing, electronics, and nanotechnology. Among the various sulfur-containing molecules, the possibility to anchor a chiral sulfoxide to a metal surface has been scarcely investigated, despite this class of molecules being of great importance as ligands and catalysts. In this work, (R)-(+)-methyl p-tolyl sulfoxide was deposited on Au(111) and investigated by means of photoelectron spectroscopy and density functional theory calculations. The interaction with Au(111) leads to a partial dissociation of the adsorbate due to S–CH3 bond cleavage. The observed kinetics support the hypotheses that (R)-(+)-methyl p-tolyl sulfoxide adsorbs on Au(111) in two different adsorption arrangements endowed with different adsorption and reaction activation energies. The kinetic parameters related to the adsorption/desorption and reaction of the molecule on the Au(111) surface have been estimated