Stepwise dissociation of ion pairs by water molecules: cation-dependent separation mechanisms between carboxylate and alkali-earth metal ions.

Microhydrated H-tagged ion pairs (Ca, AcO)(HO) and (Ba, AcO)(HO) are investigated by IR photodissociation laser spectroscopy and DFT-D frequency calculations. The detailed picture of the first steps of ion dissociation reveals two mechanisms, where water molecules promote dissociation either directly or indirectly depending on the nature of the cation.

Ion Pair Supramolecular Structure Identified by ATR-FTIR Spectroscopy and Simulations in Explicit Solvent*.

The present work uses ATR-FTIR spectroscopy assisted by simulations in explicit solvent and frequency calculations to investigate the supramolecular structure of carboxylate alkali-metal ion pairs in aqueous solutions. ATR-FTIR spectra in the 0.25-4.

Conformational analysis by UV spectroscopy: the decisive contribution of environment-induced electronic Stark effects.

UV chromophores are frequently used as probes of the molecular structure. In particular, they are sensitive to the electric field generated by the molecular environment, resulting in the observation of Stark effects on UV spectra. While these environment-induced electronic Stark effects (EI-ESE) are already used for conformational analysis in the condensed phase, this work explores the potential of such an approach when performed at much higher conformational resolution in the gas phase.

An intraresidue H-bonding motif in selenocysteine and cysteine, revealed by gas phase laser spectroscopy and quantum chemistry calculations.

Models of protein chains containing a seleno-cysteine (Sec) residue have been investigated by gas phase laser spectroscopy in order to document the effect of the H-bonding properties of the SeH group in the folding of the Sec side chain, by comparison with recent data on Ser- and Cys-containing sequences. Experimental data, complemented by quantum chemistry calculations and natural bonding orbital (NBO) analyses, are interpreted in terms of the formation of a so-called 5γ intra-residue motif, which bridges the acceptor chalcogen atom of the side chain to the NH bond of the same residue. This local structure, in which the O/S/Se atom is close to the plane of the N-terminal side amide, is constrained by local backbone-side chain hyperconjugation effects involving the S and Se atoms.

Electronic Stark Effect in Isolated Ion Pairs.

Stark spectral shifts of a molecular probe are commonly used to estimate the local electric field in condensed media. The very large fields reported, typically in the 0.1-10 GV m range, are, however, difficult to reproduce in a controlled manner, limiting the calibration of these molecular probes to ranges below 0.

Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations.

In a context where structure elucidation of ion pairs in solution remains a contemporary challenge, this work explores an original approach where accurate gas phase spectroscopic data are used to refine high level quantum chemistry calculations of ion pairs in solution, resulting in an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase studies focus on a series of isolated contact ion pairs (M, Ph-CH-COO, with M = Li, Na, K, Rb, Cs) for which conformer-selective IR spectra in the CO stretch region are recorded. These experiments reveal the interactions at play in isolated contact ion pairs, and provide vibrational frequencies enabling us to assess the accuracy of the theoretical approach used, i.