Análise conformacional do 2-Aminoetanol e derivados através de cálculos teóricos e espectroscopia no infravermelho

Abstract

The conformation of a molecule is governed by its intramolecular interactions, such as steric, eletrostatic, hypercojugative and hydrogen bonding effects. In amino-alcohols, intramolecular hydrogen bond (IHB) can be formed either as O-H···N or N-H···O, since both O and N atoms can act as proton-donors and proton-receptors. 1,2-disubstituted compounds with electronegative groups are known to present the só-called gauche effect, as they are more stable in their gauche form compared to anti conformation, but their causes are variable and still widely discussed. The work presents a study on the compounds 2-R-ethanol [R = NH2 (1), NHCH3 (2), N(CH3)2 (3) e NH(C6H5) (4) ] and seeks to verify the possibility of IHB being formed, and its effects on the conformational stability and influence on the gauche effect, as well as analyzing the effects of the different substituents. Gaussian09 software was used for theoretical calculations. Geometry optimizations and frequency calculations were perfomed at M06-2X/ccPVTZ level of theory. AIMALL software for the application of the Quantum Theory of Atoms in Molecules (QTAIM), the NBO 6.0 software for the analysis of Natural Bonding Orbitals (NBO) and NCIPLOT software for Non-Covalent Interactions (NCI) analysis. Infrared spectroscopy analysis were perfomed for compounds 1-3. Results showed a clear preference for gauche conformations, with such arrangements presenting thermal populations greater than 90% for the four compounds. Through NBO analysis it was determined the existence of ηN →σ*OH and ηO →σ*NH charge transfer interactions, with the first being most common and intense, which can be interpreted as IHB O-H···N and N-H···O, respectively. Such interactions were also identified by NCI analysis, but were not obtained under the application of QTAIM. The presence of one or two methyl substituents to the nitrogen atom increased the intensity of the ηN →σ*OH interaction, although the addition of the second methyl destabilizes more than with just one group due to steric effects, indicating that the IHB is more intense in compound 2. This result was corroborated by the NCI analysis, which indicated a higher value of signal of λ2ρ(r) in this compound, which translates as a higher attractive force. However, the addition of methyl groups negatively impacted the formation of LHI when analyzing only structural parameters, causing a direct increase in the angle α(CC-N). The presence of an aromatic ring as a substituent in compound 4 decreased the intensity of the ηN →σ*OH interaction, due to its characteristic of acting as an electron withdrawnin group by ressonance. Stabilization by antiperiplanar hyperconjugative interactions of the type σH →σ*CN e σH →σ*CO was observed in gauche conformations, more stabilizing than the σN →σ*CO e σO →σ*CN interactions present in the anti conformations. Infrared analyzes confirmed the presence of IHB O-H···N in compounds 1-3.