Recently, molecular electronics has attracted much attention as a "post-silicon technology" for future nanoscale electronic devices. One of the most important elements in molecular electronic devices, is the realization of a unimolecular rectifier. In the present study, the geometric and electronic structure of the alkyl derivative C37H50N 4O4 (PNX), (donor - spacer - acceptor), a leading candidate for a molecular rectifying device, has been investigated theoretically using ab initio quantum mechanical calculations. The results suggest that in such donor-acceptor molecular complexes, while the lowest unoccupied orbital is concentrated on the acceptor subunit, the highest occupied molecular orbital is localized on the donor subunit. The approximate potential differences for the optimized PNX molecule have been estimated at the HF/6-311g++(d,p) level of theory, which achieves quite good agreement with experimentally reported results.