Neutral zinc(II) O,O-di-alkyldithiopho- sphates-variable temperature 31P NMR and quantum chemical study of the ZDDP monomer-dimer equilibrium.
A full line-shape analysis of the VT 31P NMR spectra was carried out for the monomer-dimer equilibrium of neutral ZDDP. The energy surface and the energetics of the monomer-dimer equilibrium (DeltaH degrees , DeltaG degrees , Ea, DeltaH(not equal), and DeltaG(not equal)) are reported for three variants wherein the alkyl groups in the ZDDP are 2-ethylhexyl, isopropyl, and isobutyl. We explored a reaction pathway between the monomer and dimer form by means of density functional theory (DFT). The linear combination of atomic orbitals (LCAO) code DMol3 was used together with a synchronous transient method to effectively locate transition states. Vibrational eigenmodes of all intermediates were computed to capture finite temperature effects. Methyl and ethyl were considered as alkyl groups. Two novel intermediates were located-a four-membered ring and a six-membered ring intermediate along the reaction coordinate. Comparison of the experimentally derived and computed energy surfaces was carried out.