Quantum Chemical Studies of Free-radical, Hindered Phenol Stabilizers

Quantum chemical investigations of the action of hindered phenol radical stabilizers in lubricant degradation chemistry have been completed. We are currently developing the first mechanistic models of lubricant degradation with the aim of including the chemistry of additives such as radical stabilizers that are found in commercial lubricant formulations. A key component of development of these models is obtaining reliable estimates of the kinetic parameters for all reactions. Specifically, rate constants and structure-reactivity correlations for the interaction of hindered phenol radical stabilizers with small peroxy radicals have been sought using quantum chemistry and transition state theory. Accurate thermodynamic properties of hindered phenol radical stabilizers and structure-reactivity relationships have been obtained. Such relationships provide researchers the ability to quickly estimate reactivity of hindered phenol stabilizers using their thermodynamic properties and therefore may be useful in additive design. INTRODUCTION Through more stringent environmental regulations and more demanding performance requirements, lubricant additive engineering is a field experiencing continued growth. Experimental studies of lubricant additives are numerous, and several volumes Address all correspondence to this author. 1 have been written discussing additive chemistry. [1, 2] A particularly important category of additives is free-radical stabilizers. These stabilizers work to impede thermal degradation, one of the main routes of lubricant degradation. The chief strategy employed to control this process is the addition of antioxidants which disrupt the free-radical mechanism and suppress propagation of radical intermediates. The research literature concerning antioxidants for use in lubricants is primarily experimental in nature. Research thus far has yielded important insights into the mechanism of suppression of radicals and, qualitatively, we now understand why radical stabilizers have been effective lubricant additives. However, there have not been any significant computational studies of radical stabilizers used for lubricant degradation. This is in spite of the enormous advances in computing technology over the past decade. Quantum chemistry and transition state theory have been used for many years in studying the reactivity of a wide range of systems. This work details our investigations into the reactivity of hindered phenol radical stabilizers using quantum chemistry. The main goal of this work is to develop correlations between the reactivity of such stabilizers and their structural or thermodynamic properties.