Conformational Analysis and Intramolecular Interactions of l‑Proline Methyl Ester and Its <i>N</i>‑Acetylated Derivative through Spectroscopic and Theoretical Studies

This work reports a detailed study regarding the conformational preferences of l-proline methyl ester (ProOMe) and its <i>N</i>-acetylated derivative (AcProOMe) to elucidate the effects that rule their behaviors, through nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies combined with theoretical calculations. These compounds do not present a zwitterionic form in solution, simulating properly amino acid residues in biological media, in a way closer than amino acids in the gas phase. Experimental ³<i>J</i>_HH coupling constants and infrared data showed excellent agreement with theoretical calculations, indicating no variations in conformer populations on changing solvents. Natural bond orbital (NBO) results showed that hyperconjugative interactions are responsible for the higher stability of the most populated conformer of ProOMe, whereas for AcProOMe both hyperconjugative and steric effects rule its conformational equilibrium

Experimental and Theoretical Studies of Intramolecular Hydrogen Bonding in 3‑Hydroxytetrahydropyran: Beyond AIM Analysis

The conformational preferences of 3-hydroxytetrahydropyran (<b>1</b>) were evaluated using infrared and nuclear magnetic resonance spectroscopic data in solvents of different polarities. Theoretical calculations in the isolated phase and including the solvent effect were performed, showing that the most stable conformations for compound <b>1</b> are those containing the substituent in the axial and equatorial orientations. The axial conformation is more stable in the isolated phase and in a nonpolar solvent, while the equatorial conformation is more stable than the axial in polar media. The occurrence of intramolecular hydrogen-bonded O–H···O in the axial conformer was detected from infrared spectra in a nonpolar solvent at different concentrations. Our attempt to evaluate this interaction using population natural bond orbital and topological quantum theory of atoms in molecules analyses failed, but topological noncovalent interaction analysis was capable of characterizing it

Gauche Preference of β‑Fluoroalkyl Ammonium Salts

The strong gauche preference along with the F–C–C–N⁺ fragment in 3-fluoropiperidinium cation and analogues, in the gas phase, is dictated by electrostatic interactions, which can be both hydrogen bond F···H­(N⁺) and F/N⁺ attraction. In aqueous solution, where most biochemical processes take place, electrostatic effects are strongly attenuated and hyperconjugation is calculated to be at least competitive with Lewis-type interactions