Structural parameters, originating from x-ray crystallographic data, have been compiled for 13 derivatives of amino acids, peptides and related compounds, which contain a total of 14 Fmoc-NH- moieties. For these moieties, molecular geometries and conformations--described by the omegao, theta1, theta2 and theta3' torsion angles--were analysed and compared with the corresponding parameters for the Z-NH- and Boc-NH-moieties (290 and 553, respectively). To gain a deeper insight into the conformational features of the Fmoc-NH- moiety, ab initio free molecule calculations were performed for fully relaxed minima. Also the potential energy surface as a function of the torsion angles (theta3', theta2) was generated. The conformational features of the Fmoc-NH- moiety: (i) two possible values for the angle omegao (approximately 180 degrees or, rarely, approximately theta degrees) and (ii) the angle theta1 = 180 degrees +/- 15 degrees, are common to the Z-NH- and Boc-NH- systems. By contrast, the theta2 and theta3 angles in the Fmoc, Z and Boc groups differ essentially. In the Fmoc groups theta2 mostly has values of 180 degrees +/- 30 degrees and values up [115 degrees] seem to be forbidden, whereas fewer than half of the Z groups adopt theta2 approximately 180 degrees and the remainder have theta2 in the range of [90 degrees +/- 20 degrees]. On the other hand, the Boc methyl groups are staggered. The theta3 values observed for Fmoc are limited to the regions of 180 degrees +/- 20 degrees and 160 degrees +/- 20 degrees], while for the Z group a variety of theta3 occurs. The orientation of the fluorenyl vs the urethane function is mostly trans. Our results suggest a lower conformational flexibility for the Fmoc group compared with that of the Z group. Our calculations confirm that the observed conformational features for the Fmoc-NH- moiety are inherent properties. The Fmoc-NH-moiety in crystals involves the participation of its O=C-NH functionality in hydrogen bonds.