Alkyl radicals as hydrogen bond acceptors: computational evidence

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Hydrogen bonding is crucial in chemistry and biology.
• Alkyl radicals are typically considered poor hydrogen bond acceptors.
• Understanding interactions with radicals is key to reaction mechanisms.

Purpose of the Study:

• To investigate the hydrogen bonding interactions between charged proton donors and simple alkyl radicals.
• To quantify the strength and characteristics of these hydrogen bonds.
• To compare the acceptor abilities of alkyl radicals with conventional molecules.

Main Methods:

• Density Functional Theory (DFT) computational studies.
• G3-type computational methods.
• Analysis of spectroscopic (IR), energetic, and structural data.

Main Results:

• Charged proton donors form moderately strong hydrogen bonds with alkyl radicals.
• These hydrogen bonds stabilize the resulting adducts and modify their structures.
• Alkyl radicals exhibit significant IR red-shifts and increased absorption intensities upon hydrogen bonding.
• The hydrogen bond acceptor strength of alkyl radicals is comparable to molecules like formaldehyde and water.

Conclusions:

• Alkyl radicals can act as effective hydrogen bond acceptors.
• Hydrogen bond strength is influenced by proton affinity and ionization energy, not just the difference.
• Charge-transfer aspects are prominent in these interactions, especially with low-polarity, low-basicity acceptors.