Can 2-X-Ethanols Form Intramolecular Hydrogen Bonds?

Area of Science:

• Computational Chemistry
• Molecular Interactions
• Organic Chemistry

Background:

• The gauche conformer preference in 2-X-ethanols (X = F, OH, NH2) was initially attributed to intramolecular hydrogen bonds (IMHBs).
• This attribution has been debated, with arguments suggesting the gauche effect or the absence of bond critical points (BCPs) negates IMHB presence.

Purpose of the Study:

• To re-evaluate the existence and nature of intramolecular hydrogen bonds in 2-X-ethanols.
• To address conflicting arguments regarding the gauche effect and the quantum theory of atoms in molecules (QTAIM) BCP criterion.

Main Methods:

• Utilized five different computational methods to assess the gauche effect's contribution to conformational energy.
• Applied the quantum theory of atoms in molecules (QTAIM) and the theory of noncovalent interactions (NCI).
• Analyzed 2-X-ethanols at various fixed XCCO torsional angles.

Main Results:

• The gauche effect contributes less than 1 kcal/mol to the conformational preference, indicating it does not fully explain the observed energy difference.
• The bond critical point (BCP) criterion from QTAIM was found inconsistent with energy, bond lengths, and vibrational frequencies.
• Inconsistencies with the BCP criterion were resolved using the theory of noncovalent interactions, which identified IMHBs.
• Intramolecular hydrogen bonds in 2-X-ethanols were found to be similar in form but weaker than intermolecular hydrogen bonds.

Conclusions:

• 2-X-ethanols definitively form intramolecular hydrogen bonds.
• The presence of IMHBs is supported by multiple computational approaches, including energy, bond properties, and noncovalent interaction analysis.
• The study reconciles conflicting evidence, confirming IMHBs as a significant factor in the conformational preferences of 2-X-ethanols.