(5)-1-(Methylaminocarbonyl)-3-phenylpropanaminium chloride (52·HCl), C 10 H 15 N 2 O + ·Cl - , crystallizes in the orthorhombic space group P2 1 2 1 2 1 with a single formula unit per asymmetric unit. (5R/S)-5-Benzyl-2,2,3-trimethyl-4-oxoimidazolidin-1-ium chloride (R3 and S3), C 13 H 19 N 2 O + ·Cl - , crystallize in the same space group as S2-HCl but contain three symmetry-independent formula units. (R/S)-5-Benzyl-2,2,3-trimethyl-4- oxoimidazolidin-1-ium chloride monohydrate (R4 and S4), C 13 H 19 N 2 O + -Cl - ·H 2 O, crystallize in the space group P2 1 with a single formula unit per asymmetric unit. Calculations at the B3LYP/6-31G(d,p) and B3LYP/6-311G(d,p) levels of the conformational energies of the cation in R3, S3, R4 and S4 indicate that the ideal gas-phase global energy minimum conformation is not observed in the solid state. Rather, the effects of hydrogen-bonding and van der Waals interactions in the crystal structure cause the molecules to adopt higher-energy conformations, which correspond to local minima in the molecular potential energy surface. © 2008 International Union of Crystallography.