Donor-Acceptor Complexes between Ammonia and Sulfur Trioxide: An FTIR and Computational Study

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Ammonia (NH3) and sulfur trioxide (SO3) are key chemical species with potential for complex formation.
• Understanding these interactions is crucial for various chemical processes and atmospheric chemistry.
• Previous studies characterized the H3N·SO3 electron donor-acceptor complex.

Purpose of the Study:

• To investigate the complexes formed between ammonia and sulfur trioxide.
• To characterize novel complexes using experimental and computational methods.
• To elucidate the bonding and stability of different NH3/SO3 stoichiometries.

Main Methods:

• Fourier Transform Infrared (FTIR) matrix isolation spectroscopy.
• Density Functional Theory (DFT) calculations using the B3LYP functional and aug-cc-pVTZ basis set.
• Analysis of potential energy surface (PES), interaction energy decomposition, and electron localizability indicator (ELI-D).

Main Results:

• Formation and characterization of the H3N·SO3 complex via two different matrix preparation methods.
• Identification of a new complex, H3N-SO3···NH3 (II(D)), in matrices derived from sulfamic acid thermolysis.
• Calculations confirmed II(D) as a local minimum, featuring collinear N-S-N atoms and nonequivalent S-N bonds, with the stronger bond identified as dative.

Conclusions:

• The H3N-SO3···NH3 complex (II(D)) is experimentally observed and computationally supported.
• The II(D) complex is less stable than the global minimum (NH3)2SO3 complex (II(HB)).
• The study discusses the reasons for the observed presence of II(D) over II(HB) in the specific experimental conditions.