Theoretical Study of Urea. I. Monomers and Dimers

Ab initio (HF, MP2, and B3PW91 with basis sets up to D95++**) and semiempirical (AM1, PM3, and SAM1) molecular-orbital calculations are presented for urea and four different general classes of urea dimeric structures that correspond to interactions between nearest-neighbor molecular pairs in the crystal structures of urea and thiourea. While the urea monomers have nonplanar minima on all ab initio surfaces, on the HF and MP2/6-311+G(3df,2p) surfaces they are planar after vibrational and thermal corrections. Urea chain and ribbon dimers are calculated to be planar after counterpoise and thermal corrections for all HF calculations and nonplanar for MP2 calculations (D95** and D95++**). The DFT calculations predict planar chain but nonplanar ribbon dimers. The ribbon dimer is the most stable, as it uses both H-bond acceptors, while the chain dimer uses only one and the herringbone dimer has one H bond. Stacking interaction is much less stabilizing. The PM3 method fails in both predicting molecular conformations and H bonds, while AM1 gives reasonable results.