The Intrinsic Barrier Width and Its Role in Chemical Reactivity

Area of Science:

• Chemical kinetics and reaction dynamics
• Physical organic chemistry
• Computational chemistry

Background:

• Chemical reactions are explained using molecular potential energy landscapes, where reaction barriers dictate reactivity.
• The Marcus theory dissects barrier height into intrinsic and thermodynamic contributions, explaining kinetic vs. thermodynamic control.
• An analogous concept for barrier width has been lacking.

Purpose of the Study:

• To define and explore the concepts of intrinsic barrier width and the effect of driving force on barrier width.
• To demonstrate the distinct roles of these barrier width components in chemical reactions.
• To provide a more complete description of chemical reactions by considering barrier width.

Main Methods:

• Theoretical and experimental studies were conducted.
• The concept of intrinsic barrier width and driving force effect was defined.
• Conformational isomerizations of aromatic carboxylic acids were modeled to alter barrier widths.
• Quantum mechanical tunneling (QMT) half-lives were used to measure changes in barrier width.

Main Results:

• The distinct roles of intrinsic and thermodynamic contributions to barrier width were demonstrated.
• The sensitivity of QMT half-lives to barrier width variations was confirmed.
• Experimental and theoretical data support the new concepts.

Conclusions:

• The study establishes the importance of intrinsic barrier width and driving force effects.
• These concepts help resolve conflicting trends in chemical reactivity where barrier width is significant.
• The findings contribute to a more comprehensive understanding of chemical reaction mechanisms.