Molecular modeling of the H-bonds in polyurethane with multiple donors and acceptors

The molecular mechanics (MM) method with COMPASS force field was used to study the H-bonds in polyurethane with carboxyl (PUc), which has multiple donors and acceptors. 2-Methyl-3-{[(methylamino)carbonyl]oxy}propanoic acid was used as the model molecule. It was found that the model PUc possesses four conformers with lowest energy. Considering six possible H-bond types such as OH⋯OC(OH) (Type I), OH⋯OH(CO) (Type II), OH⋯OC(NH) (Type III), NH⋯OC(OH) (Type IV), NH⋯OH(CO) (Type V), NH⋯OC(NH) (Type VI), in such system 192 H-bond complexes are simply expected. All the complexes were simulated in this modeling. Obtained total energies of the complexes were used to analyze the existence probability of each H-bonding configuration. The results show that for the six types of H-bonds, Types I (61%) and VI (37%) are the main H-bonding configurations in PUc, Types III and IV have the low probability (2%) and mostly coexist with other H-bond types, and Type II and V hardly exist.