Why urea eliminates ammonia rather than hydrolyzes in aqueous solution

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Biochemistry

Background:

• Urea is a key biological molecule and industrial chemical.
• Understanding urea decomposition pathways is essential for various chemical and biological processes.
• Previous studies have explored urea decomposition, but detailed mechanistic insights, especially concerning solvent effects, remain crucial.

Purpose of the Study:

• To investigate the mechanisms of urea decomposition in both gas and aqueous phases.
• To elucidate the role of solvent effects on urea decomposition pathways and kinetics.
• To compare the activation energies and transition states of different decomposition routes, including ammonia elimination and hydrolysis.

Main Methods:

• Combined Quantum Mechanics/Molecular Mechanics (QM/MM) and ab initio calculations.
• Free energy perturbation calculations using the TIP4P explicit water model.
• QM/MM Monte Carlo simulations to model solvent interactions.
• Natural Bond Order (NBO) analysis to examine electronic structure differences.

Main Results:

• Intramolecular ammonia (NH3) elimination assisted by a water molecule exhibits the lowest activation energy.
• Explicit solvent representation is critical for accurately determining reaction mechanisms, rate-determining steps, and energy barriers.
• The decomposition of the zwitterionic intermediate (H3NCONH) is the rate-limiting step, with an overall free energy of activation of ~37 kcal/mol for urea decomposition in water.
• Hydrolysis via an addition/elimination mechanism has a higher activation barrier (~40 kcal/mol).

Conclusions:

• Ammonia elimination is the preferred decomposition pathway for urea in aqueous solution.
• Water molecules play a vital role as hydrogen shuttles and in stabilizing intermediates.
• Differences in resonance stabilization and electronic structures of transition states dictate the favored reaction mechanism.