Long-range bonding/nonbonding interactions: a donor-acceptor resonance studied by dynamic NMR

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Understanding long-range bonding interactions is crucial in molecular design.
• Phenylpyridine derivatives are important scaffolds in various chemical applications.

Purpose of the Study:

• To investigate the influence of substituents on arylpyridyl rotational barriers.
• To elucidate the nature of long-range bonding interactions in substituted phenylpyridines.

Main Methods:

• Variable-temperature Nuclear Magnetic Resonance (NMR) spectroscopy was employed.
• Computational calculations, specifically CCSD(T)/6-31+G(d), were utilized for theoretical support.
• Rotational barriers of analogous biphenyls were analyzed to exclude steric effects.

Main Results:

• Arylpyridyl rotational barriers were found to be lower with electronegative atoms (F, OMe, NMe2) at the α-carbon of the 2'-moiety.
• This reduction in barrier height was attributed to a stabilizing interaction between the heterocyclic nitrogen lone pair and the α-carbon in the transition state.
• Computational results supported the proposed stabilizing interaction mechanism.

Conclusions:

• The study demonstrates a novel stabilizing interaction influencing conformational preferences in phenylpyridines.
• Electronegative substituents can modulate rotational barriers through electronic effects, not just steric ones.
• This finding provides insights into controlling molecular conformation via substituent effects.