Ammonia-water cation and ammonia dimer cation

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Spectroscopy

Background:

• Ionized molecular complexes are crucial in various chemical processes.
• Understanding the structure and properties of protonated species is essential for reaction mechanisms.

Purpose of the Study:

• To investigate the structural, energetic, and electronic properties of the ammonia-water cation (NH(3)H(2)O)(+).
• To evaluate the accuracy of Density Functional Theory (DFT) methods for describing ionized molecular systems.

Main Methods:

• High-level ab initio theory
• Density Functional Theory (DFT) with various functionals
• Moller-Plesset second-order perturbation (MP2) theory
• Coupled cluster theory with single, double, and perturbative triple excitations (CCSD(T))

Main Results:

• Identified three minimum-energy structures for the ammonia-water cation: H(2)NH(+)...OH(2), H(3)N(+)...OH(2), and H(3)NH(+)...OH.
• The H(2)NH(+)...OH(2) structure was found to be the most stable.
• A low interconversion barrier of approximately 6 kcal/mol exists between the identified structures.
• Many DFT functionals overstabilize N...O and N...N binding, yielding results inconsistent with higher-level theories like MP2 and CCSD(T).

Conclusions:

• The study highlights the importance of carefully validating DFT functionals for accurate predictions of ionized molecular systems.
• Discrepancies between DFT and high-level ab initio methods underscore the need for reliable theoretical approaches in computational chemistry.