A proposed dinitro device, Au-(2-nitro-4-ethynylphenyl-4'-ethynylphenyt-5'-nitro-1-benzene thiolate)-Au is analyzed using a combination of density functional and Green function theories complemented with information from theoretical and experimental studies of a similar nitroamino device, Au-(2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiolate)-Au. The dinitro compound might also perform as a molecular memory but with different characteristics than those of the nitroamino, showing well-defined charge states; however, the neutral charge state of the nitroamino presents well-defined resonant tunneling characteristics and a larger intrinsic dipole moment. Density of states, transmission functions, and current-voltage characteristics for the neutral, anion, and dianion of the two molecules are compared. The effect of the bias potential is explicitly considered in the calculations as well as the effect of the contacts and the spin states of the open shell systems. The theoretical results for the training molecule are in good agreement with experiment. It is concluded that observed negative differential resistance is due mainly to charge effects combined in less degree with resonant tunneling intrinsic to single molecules.