3 research outputs found
Conformational Analysis and Intramolecular Interactions of l鈥慞roline Methyl Ester and Its <i>N</i>鈥慉cetylated 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 <sup>3</sup><i>J</i><sub>HH</sub> 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鈥慔ydroxytetrahydropyran: 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鈥揌路路路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 尾鈥慒luoroalkyl Ammonium Salts
The strong gauche preference along
with the F鈥揅鈥揅鈥揘<sup>+</sup> fragment in 3-fluoropiperidinium
cation and analogues, in the gas phase, is dictated by electrostatic
interactions, which can be both hydrogen bond F路路路H颅(N<sup>+</sup>) and F/N<sup>+</sup> 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