Intramolecular Nonbonded Attractive Interactions: 1-Substituted Propenes

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Typically, trans-alkenes are more stable than cis-alkenes due to reduced steric hindrance.
• However, specific 1-substituted propenes exhibit an inversion of this stability trend.

Purpose of the Study:

• To investigate the factors contributing to the unusual stability of certain cis-1-substituted propene isomers.
• To quantify the interplay of steric, electrostatic, and dispersive forces in determining alkene stability.

Main Methods:

• Computational chemistry methods were employed to calculate the structures and energies of cis- and trans-isomers.
• Second-order Møller–Plesset perturbation theory (MP2) was used to calculate pair energies, quantifying dispersive interactions.
• A comprehensive model summing steric, electrostatic, and dispersive interactions was developed.

Main Results:

• Calculations revealed steric repulsion in cis-isomers, which is counteracted by attractive forces.
• For fluoro (F) and methoxy (OMe) substituents, Coulombic interactions were found to stabilize cis-isomers.
• For chloro (Cl) and bromo (Br) substituents, attractive dispersive interactions were identified as the stabilizing factor.
• The summation of calculated steric, electrostatic, and dispersive interactions accurately reproduced observed energy differences.

Conclusions:

• The stability of cis-1-substituted propenes is governed by a delicate balance of repulsive and attractive forces.
• Electrostatic and dispersive interactions can overcome steric repulsion, leading to cis-isomer stability.
• Computational methods, including MP2 energy calculations, provide accurate insights into these molecular interactions.