Acetonitrile/sulfuric acid/water clusters: A matrix isolation infrared and computational 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 acetonitrile (CD₃CN) and sulfuric acid. Due to the “free” electron pair on its nitrogen, acetonitrile is of basic nature and a formation of the relatively strong intermolecular O–H⋯N hydrogen bond with sulfuric acid may be expected. However, the matrix isolation spectra of this system are complicated by the inevitable presence of water in the vapor and in the matrix. We identify the spectral signatures of the binary 1:1 CD₃CN*H₂SO₄ and triple 1:1:1 CD₃CN*H₂O*H₂SO₄ H-bonded complexes. The infrared matrix isolation spectra are accompanied by quantum mechanical studies of relevant model H-complexes. By its red shift from the position of the “free” v(OH) band of sulfuric acid and by the calculated H-bond length of the R(O–H⋯N) hydrogen bond it is concluded that the acetonitrile/sulfuric acid interaction energy is considerably higher than that of acetonitrile/water on the one hand and significantly lower than the sulfuric acid/water interaction, on the other.