Trimethylamine/Sulfuric Acid/Water Clusters: A Matrix Isolation Infrared Study

In continuation of our studies of sulfuric acid H-bonded complexes of atmospheric relevance we report the infrared spectra of the matrix isolated complexes formed between trimethylamine and sulfuric acid. Evidence for proton transfer was anticipated for the present system, as trimethylamine ((CH₃)₃N) is of strong basic nature. However, the spectra of this system are complicated by the inevitable presence water in the vapor and in the matrix, resulting in matrix layers containing three species capable of forming H-bonded complexes. The complex formed between trimethylamine and sulfuric acid is of ionic character due to proton transfer of the H⁺ proton from sulfuric acid to (CH₃)₃N to form a new N–H bond and the replacement of the <i>intramolecular</i> O–H bond in H₂SO₄ by a strong <i>intermolecular</i> N–H···O hydrogen bond. The complex is further stabilized by hydration. The skeletal modes show clear bisulfate related bands and are only slightly affected by hydration. The ν­(OH) region shows a rich band scheme, best explained by a structure involving (at least) three H₂O molecules. A broad spectral feature spanning the 1700–500 cm^–1 is assigned, in analogy to previous studies to a double-well potential quasi-symmetric, Zundel-like, ionic species with a (CH₃)₃–N···H⁺···N–(CH₃)₃ configuration. A band in the skeletal SO stretch spectral region may be assigned to hydrated sulfate as its counterion