The concept of molecular rectifiers introduced by Aviram and Ratner in 1974 has been the starting point of the field of molecular electronics and the possibility of unimolecular rectification has been widely debated ever since. Despite the large amount of publications on this topic over the years, the physical mechanisms leading to this phenomenon have not yet been clarified to the point where a systematic route for enhancing the rectification ratio (RR) of molecules could be suggested. We present here a theoretical study of RR for a range of molecules with a carboxylic group as a bridge between pi-conjugated chains substituted by nitro and amino groups to improve the rectification. We estimate the RR in two distinct ways, namely: i) as the ratio of the threshold electric fields required to transfer one electron between the donor and acceptor units of the molecule and vice versa, using quantum-chemical calculations based on the semi-empirical Hartree-Fock Austin Model 1 (AMI) method, ii) as the ratio of the currents in forward versus reverse bias, as obtained with a non-equilibrium Green's function (NEGF) approach for charge transport through gold/molecule/gold junctions within the framework of density functional theory (DFT). The trends in RR as a function of the molecular structure agree very well when these two methods are compared and can be explained in terms of the relative position of the nitro group within the generated electrostatic potential. These findings allow us to derive some general conclusions about the physical mechanisms behind unimolecular rectification.