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Ab Initio SCF and DFT Study of Configuration and Reactivity of Silacarbamic Acid

M. Remko

Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Comenius University, SK-832 32 Bratislava

 

Abstract: Ab initio and DFT methods have been used to study molecular structure and reactivity of silacarbamic acid. The basis set included the double-zeta 6-31G**, triple-zeta 6-311G**, and 6-311+G* basis. Complete geometry optimization was accomplished for silacarbamic acid and its protonated species using the 6-31G** basis. The proton affinities were calculated at the HF, MP4SDTQ, CCSD, CCSD(T), and Becke3LYP levels of theory and 6-311+G* basis set. The protonation energies were found to increase in the order PA(Si-O ... H) < PA(Si-N ... H) < PA(Si=O ... H). The absolute values of calculated gas phase proton affinities depend on the basis set used and way of evaluating the correlation energy. The decomposition of silacarbamic acid was investigated at HF and various correlated levels of theory. The decarboxylation reaction is at highest theoretical levels (MP4SDTQ/6-311+G*//MP2/6-311G** and CCSD(T)/6-311+G*//MP2/6-311G**) highly positive (226.8 kJ mol-1 and 248.9 kJ mol-1). It means that the equilibrium (at 0 K) for this reaction is shifted towards silacarbamic acid. On the other hand, dimerization reaction is for silacarbamic acid greatly exothermic with the energy of -390.2 kJ mol-1 (the Becke3LYP/6-311G** computation). Changes in the natural atomic charges and atom-atom net linear NLMO/NPA bond orders were examined with respect to their dependence on protonation site of silacarbamic acid.

Full paper in Portable Document Format: 514a185.pdf

 

Chemical Papers 51 (4) 185–192 (1997)

Monday, July 26, 2021

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