Theoretical study of urea. I. Monomers and dimers

Ab initio (HF, MP2, and B3PW91 with basis sets up to D95++**) and semiempir ical (AM1, PM3, and SAM1) molecular-orbital calculations are presented fur urea and four different general classes of urea dimeric structures that cor respond to interactions between nearest-neighbor molecular pairs in the cry stal structures of urea and thiourea. While the urea monomers have nonplana r minima on all ab initio surfaces, on the HF and MP2/6-311 + G(3df,2p) sur faces they are planar after vibrational and thermal corrections. Urea chain and ribbon dimers are calculated to be planar after counterpoise and therm al 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 I-I-bo nd accepters, while the chain dimer uses only one and the herringbone dimer has one H bond. Stacking interaction is much less stabilizing. The PM3 met hod fails in both predicting molecular conformations and H bonds, while AMI gives reasonable results.