pi bonding in second and third row molecules: testing the strength of Linus's blanket

Area of Science:

• Quantum Chemistry
• Chemical Bonding Theory

Background:

• Valence bond (VB) theory offers a flexible approach to chemical bonding.
• Calculating pi-bond energies in double-bonded systems (H(m)A=BH(n)) presents challenges in separating sigma and pi effects.
• Previous models assumed resonance energy (RE) varied significantly between homonuclear and heteronuclear bonds.

Purpose of the Study:

• To develop a novel method for calculating pi-bond energies using VB theory.
• To achieve a clear separation of sigma and pi bonding contributions.
• To investigate trends in bond strength across the Periodic Table and understand the nature of resonance energy.

Main Methods:

• Utilized the flexibility of valence bond theory to target pi-bond calculations.
• Kept all factors constant except for the pi bond to isolate its energy contribution.
• Analyzed pi-bond strengths in various double-bonded species including C, N, O, Si, P, and S.

Main Results:

• The new method successfully calculates pi-bond energies and separates sigma and pi effects.
• Intra-row pi bonds align with Pauling's predictions, but inter-row bonds deviate due to sigma bond constraints.
• Found significant resonance energies (REs) for all studied bonds, indicating RE depends on electronegativity difference and sum.

Conclusions:

• Charge shift (CS) bonding, a mix of covalent and ionic character, is a prevalent bonding form extending beyond single bonds.
• The study challenges the traditional view of resonance energy dependence solely on electronegativity differences.
• Optimal p(pi)-p(pi) overlap is crucial for bond strength, and sigma bonds can impose constraints on this.