Análise do efeito do grupo metila do 2-metil-1,3-propanodiol e de seus derivados na estabilização da ligação de hidrogênio intramolecular O-H...O usando RMN, infravermelho e cálculos teóricos

Abstract

The conformational analysis covers two aspects of study: the determination of the molecular geometry and the energy of the conformers from this geometry, which give different thermodynamic states to the same molecule. In organic chemistry, some effects influence the energetic stability of a conformation, and contribute to the conformational equilibrium of a molecule, with emphasis on steric repulsions, hyperconjugative interactions, and inter- (LH) or intramolecular (LHI) hydrogen bonds. In some organic compounds, these effects are easily observed. Among them are the diols, more specifically, 1,3-disubstituted compounds which can carry out LHI and form a 6-membered pseudo ring. To investigate the influence of this effect on 1,3-disubstituted compounds, the conformational preferences of 2-methyl-3-Rpropanol ´R = OH (1), OCH3 (2), OCH2CH3 (3) and OCH(CH3)2 (4)] were determined through theoretical calculations using the theory level LC-ωPBE/6-311+G(d,p) and utilizing the experimental analysis of infrared spectroscopy and nuclear magnetic resonance for the compounds of 1-5 (compound 5 [R = OCOCH3]. Compound 1 was obtained commercially, and the remaining were prepared by alkylation or acylation reactions of one of the hydroxyls of compound 1. Among all compounds evaluated, the most stable conformer was the one with O-H...O LHI. By studying the thermal population, it was observed that the sum of the conformers population that have LHI was always higher than the population of the remaining. The population of LHI conformers were 64.0, 62.5, 73.1 and 80.0% for compounds 1-4, respectively. Infrared analyzes showed that the Δv values increased with the addition of alkyl groups by 79, 93, 95, 105 and 98 cm-1 for compounds 1-5, respectively. Results indicate the addition of donor alkyl groups by inductive effect increases the strength of the LHI, although the increasing steric repulsion proved by the calculations of the natural bonding orbitals, due to the volume of the substituent group. The results of 3JHH obtained by 1H NMR and mole fraction showed that the population of the LHI-like conformers decreases with increasing solvent basicity and dielectric constant. The influence of carbon-2 chirality allowed to determine the 3JHH of all hydrogens, separately, of the C-1 and C-3 methylenes of these compounds, despite the high complexity of the signals.