H-Bonding of Sulfuric Acid with Its Decomposition Products: An Infrared Matrix Isolation and Computational Study of the H₂SO₄·H₂O·SO₃ Complex

The FTIR matrix isolation spectra of H₂SO₄ vapors show a group of bands with synchronous growth of their relative intensities which is independent of the water species content of the matrix layer. Their frequency positions indicate that the species they represent is H-bonded and composed of all three components (H₂SO₄, H₂O, and SO₃) involved in the vapor decomposition equilibrium of the acid molecule. Structure, stabilization energies, and vibrational frequencies of several H-bonded complexes between these components were considered in B3LYP calculations employing Dunnings correlation-consistent aug-cc-pVTZ basis sets. Correlations between spectral shifts, bond lengths, and H-bond energies were also considered. The best fitting complex is a ring structured 1:1:1 H₂SO₄·H₂O·SO₃. The indications are that the complex is formed in the vapor phase and not after deposition. The atmospheric significance may be in its ability to serve as a H-bonding nucleation center even without the presence of additional contaminants.