A complete method for quantifying the Born Oppenheimer barriers of radical-molecule abstraction reactions is derived from first principles with the dual objectives of analytical prediction and conceptual understanding. Expanding upon the work of Donahue et al. (J. Phys. Chem. A 1998, 102, 3923-3933) this treatment uses the strategic construction of reactant-like and product-like wave functions to evaluate the coupling between crossing diabatic states. The overall reaction coordinate is evaluated in a modular fashion, whereby each region is analyzed within the context of its governing physics.