Structures, vibrations, and force fields of dithiophosphate wear inhibitors from ab initio quantum chemistry

Zinc dithiophosphates (ZnDTPs) are ubiquitous lubricating oil additives in today's passenger car motor oils, providing the important functions of wear and oxidation inhibition. However, the molecular-level mechanism by which these materials reduce wear is not understood. As a first step in developing an understanding of this mechanism, we used ab initio quantum chemical methods to examine the structures, vibrations, and energetics of these systems. The results show that the two phosphorus-sulfur bonds of the dithiophosphate of ZnDTPs are equivalent and have character intermediate between single and double bonds. This contrasts with the paradigm of one double bond (P=S) and one single bond (P-S) often used. Vibrational studies of DTP systems lead to a strong IR transition at about 650 cm^-1 and a weak transition at about 530 cm^-1. We find modes in good agreement with experiment, where the high-frequency mode is antisymmetric PS stretch (not P=S), while the lower mode is symmetric PS stretch (not P-S). On the basis of the ab initio calculation results, we used the biased Hessian method to develop a vibrationally accurate force field (FF) for ZnDTPs. This FF can be used to examine the binding of DTPs to metal and metal oxide surfaces.