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Download file# Donor–Acceptor Complexes between Ammonia and Sulfur Trioxide: An FTIR and Computational Study

journal contribution

posted on 29.10.2015, 00:00 by Karolina Haupa, Andrzej Bil, Zofia MielkeThe complexes of ammonia with sulfur
trioxide have been studied
using FTIR matrix isolation spectroscopy and DFT/B3LYP calculations
with the aug-cc-pVTZ basis set. The NH

_{3}/SO_{3}/Ar matrixes were prepared in two different ways. In one set of experiments the matrix was prepared by the simultaneous deposition of the NH_{3}/Ar mixture and SO_{3}vapor from the thermal decomposition of K_{2}S_{2}O_{7}. In the second set of experiments thermolysis products of sulfamic acid were trapped in an argon matrix. Both methods of matrix preparation led to the formation of the H_{3}N·SO_{3}electron donor–acceptor complex that was characterized earlier. In the matrixes comprising thermolysis products of sulfamic acid, in addition to H_{3}N·SO_{3}, the H_{3}N–SO_{3}···NH_{3}complex (II^{D}) was also identified. The identity of the complex was confirmed by comparison of the experimental and theoretical spectra of H_{3}N–SO_{3}···NH_{3}and D_{3}N–SO_{3}···ND_{3}. The performed calculations show that in H_{3}N–SO_{3}···NH_{3}the two N atoms and the S atom are collinear; the two S–N bonds are nonequivalent, one is much shorter (2.230 Å) than the other one (2.852 Å). In the AIM topological analysis, the interaction energy decomposition and topological properties of the electron localizability indicator (ELI-D) allowed us to categorize the stronger N–S bond in the II^{D}complex as a dative bond and to assume that the fragile N–S bond is a consequence of a weak electron-donor–acceptor interaction. The calculations indicate that the identified II^{D}complex corresponds to a local minimum on the PES of the NH_{3}/SO_{3}system of 2:1 stoichiometry. The (NH_{3})_{2}SO_{3}complex, II^{HB}, corresponding to a global minimum is 7.95 kcal mol^{–1}more stable than the II^{D}complex. The reason that the II^{D}complex is present in the matrix but not the II^{HB}complex is discussed.