Structural diversity in imidazolidinone organocatalysts: a synchrotron and computational study.

(S)-1-(Methylaminocarbonyl)-3-phenylpropanaminium chloride (S2 x HCl), C10H15N2O+ x 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), C13H19N2O+ x Cl-, crystallize in the same space group as S2 x 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), C13H19N2O+ x Cl- x H2O, 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.