Trimethyl-p-benzoquinone (TMQ) has been proposed to furnish an accurate thermochemical model for plastoquinones, key electron accepters in oxygenic photosynthetic electron transfer. Free energy perturbation/molecular dynamics simulations combined with hybrid Hartree-Fock/density functional (HF/DF) calculations confirm that TMQ and plastoquinone-l have approximately equal aqueous one-electron reduction potentials, within the accuracy of the calculations. HF/DF calculations using the B3LYP/6-31G(d) method also show that TMQ and its radical anion have (1) structures almost identical to those of PQ-a model for plastoquinone-1 without the isoprenoid chain’s methyl groups-and its radical anion, respectively, have (2) spin densities for TMQ(.-) and PQ(.-) which differ by 0.01 electrons at most, and have (3) key C=O and C=C stretching frequencies for the TMQ/PQ and TMQ(.-)/PQ(.-) pairs differing by only 1-8 cm(-1). Thus, TMQ and TMQ(.-) are excellent models for the structures, spin densities, and vibrational frequencies of plastoquinones and their radical anions, respectively.