How Repulsive Are Lone Pairs? A Rotational Barrier-Based Experimental Study.

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Lone pair-lone pair (lp-lp) repulsion is often assumed to destabilize molecular structures.
• However, the quantitative impact of lp-lp repulsion is not well-defined in chemical literature.

Purpose of the Study:

• To experimentally quantify the magnitude of lp-lp repulsion.
• To compare rotational barriers in molecular rotors with and without through-space lp-lp interactions.

Main Methods:

• Comparison of rotational barriers in two families of molecular rotors (rotor 1 with lp-lp proximity, rotor 2 as steric control).
• Compilation of 23 rotational barriers per rotor from literature and new measurements.
• Analysis of substituents with varied steric and electronic properties.

Main Results:

• Rotors with through-space lp-lp interactions showed no statistically significant increase in rotational barrier height compared to steric controls.
• The average difference in barrier height was less than 0.5 kcal/mol.
• Results align with computational studies on lp-lp interactions.

Conclusions:

• lp-lp repulsion does not appear to exert significant additional repulsive forces beyond electrostatics and steric effects.
• Experimental findings challenge the conventional view of lp-lp repulsion as a major destabilizing factor.
• Further research may refine understanding of non-bonding electron pair interactions.