Recently, field of molecular electronics has attracted strong attention as a " postsilicon technology" to enable future nanoscale electronic devices. To realize this molecular device, unimolecular rectifying function is one of the most fundamental requirements using nanotechnology. In the present study, the geometric and electronic structures of alkyl derivative C37H50N4O4 (PNX) molecule, (donor - spacer - acceptor), a 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 concentrates on the acceptor subunit, the highest occupied molecular orbital is localized on the donor subunit. After the optimization of the structure by B3LYP/6-31(d), the approximate potential differences for the optimized PNX molecule have been estimated at the B3LYP/ 6-311++G(d,p) level of theory, which achieves quite good agreement with experimentally reported results.