The conformational potential of the dimethyl phosphate (DMP) anion and the 2-ammonioethanol (AME) cation, which are substructures of the phosphoethanolamine head group of phospholipids, has been investigated at the Hartree-Fock (HF) level using the 3-21G, 3-21G(*), 6-31G*, and 6-31+G* basis set. For this purpose, both the DMP anion and the AME cation were considered as geminal double rotors with the two rotor groups OCH3 in the case of DMP and OH and CH2NH3+ in the case;of AME. Extensive scans (17 points including eight stationary points for DMP and 30 points including seven stationary points for AME) of the conformational energy surface were carried out by complete geometry optimizations at the HF/3-21G and HF/3-21G(*) levels, respectively, and subsequent single point calculations with larger basis sets. The most stable DMP form has the two OCH3 groups in syn-clinal (+sc) positions (both dihedral angles alpha(2) and alpha(3) = 75.3 degrees) while the AME cation prefers an anti-periplanar (ap) (alpha(4) = 173.3 degrees), syn-clinal (+sc) (alpha(5) = 48.5 degrees) conformation with regard to the OH and CH2NH3+ groups. The DMP anion is a rather flexible rotor that can undergo various flip-flop rotations (barriers 1 and 6 kcal/mol) that indicate strong coupling between the rotor groups. The AME cation, on the other hand, is conformationally not flexible, which has to do with;the fact that the two rotor groups OH and CH2NH3+ are electronically very different. The preferred rotational processes of the AME cation involve inwardly or outwardly directed rotations at the CC bond (barriers of 5.1 and 9.3 kcal/mol) with the OH group kept essentially in an ap position. Calculations reveal that semiempirical methods such as PM3 are not able to describe the conformational tendencies of either DMP anion or AME cation correctly.