Conformational Dependence of Dipole Moments in Zinc Dialkyldithiophosphates: Unexpected Dominance of Alkyl Chain Configurations

Molecular dipole moments are required for empirical correlations used to estimate thermophysical properties of zinc dialkyldithiophosphates (ZDDPs), an important class of anti-wear additives. We calculated dipole moments for three ZDDPs and their precursor dialkyldithiophosphates using density functional theory and explicitly accounted for their dependence on molecular conformation. We unexpectedly found that dipole ∗Corresponding author. Email: karlj@pitt.edu moments are highly sensitive to conformational degrees of freedom. The dipole moments for each of these six molecules range from almost zero to almost 5 D across different conformers. Surprisingly, this variation appears to be dominated by the conformations of the alkyl chains rather than the polar inorganic core. Despite this wide range, the arithmetic mean and Boltzmann weighted average dipole moments fall within a narrow range of 2.2–2.4 D for all six molecules examined. This consistency arises because the dipole moment shows no correlation with the conformational energy. The robust average value significantly simplifies property estimation by eliminating the need for conformation-specific dipole calculations. We recommend that thermophysical property correlations use a constant dipole moment of 2.3 D for ZDDPs and their precursors. We predict that experimental measurements will yield values close to this average, since they inherently sample an ensemble of conformations.