Nuclear Quantum Effects in Proton or Hydrogen Transfer

Area of Science:

• Chemical kinetics
• Quantum chemistry
• Molecular dynamics

Background:

• Proton and hydrogen transfers are fundamental chemical reactions.
• These transfers can occur via thermal activation or quantum mechanical tunneling.
• Understanding these mechanisms is key to controlling chemical reactivity.

Purpose of the Study:

• To investigate the role of quantum tunneling in proton and hydrogen transfer reactions.
• To explore how molecular vibrations influence tunneling probability and reaction dynamics.
• To re-evaluate the relationship between calculated reaction pathways and experimental activation energies.

Main Methods:

• Studying single and double proton/hydrogen transfer in ground and excited electronic states.
• Analyzing the impact of specific vibrational mode excitation on tunneling.
• Considering reaction dynamics in solution at 293 K, comparing fast and slow vibrational relaxation environments.

Main Results:

• Quantum tunneling can dominate proton/hydrogen transfer even when thermal processes are expected to prevail.
• Excitation of specific vibrational modes significantly alters tunneling probability by modifying barriers and transfer distances.
• Mode selectivity is maintained in solution at 293 K if the reaction is faster than vibrational relaxation.
• The multidimensional nature of the reaction coordinate is highly dependent on the environment's relaxation properties.

Conclusions:

• Quantum tunneling plays a more significant role in hydrogen transfer reactions than previously assumed.
• Molecular vibrations are critical modulators of tunneling, affecting reaction energetics and kinetics.
• Experimental activation energies should be interpreted cautiously, considering the limitations of static minimum energy path calculations.
• The definition of hydrogen bonds should incorporate the dynamic influence of molecular vibrations.