Evaluation of the individual hydrogen bonding energies in N-methylacetamide chains

The individual hydrogen bonding energies in N-methylacetamide chains were evaluated at the MP2/6-31+G** level including BSSE correction and at the B3LYP/6-311++G(3df,2pd) level including BSSE and van der Weals correction. The calculation results indicate that compared with MP2 results, B3LYP calculations without van der Weals correction underestimate the individual hydrogen bonding energies about 5.4 kJ mol(-1) for both the terminal and central hydrogen bonds, whereas B3LYP calculations with van der Waals correction produce almost the same individual hydrogen bonding energies as MP2 does for those terminal hydrogen bonds, but still underestimate the individual hydrogen bonding energies about 2.5 kJ mol(-1) for the hydrogen bonds near the center. Our calculation results show that the individual hydrogen bonding energy becomes more negative (more attractive) as the chain becomes longer and that the hydrogen bonds close to the interior of the chain are stronger than those near the ends. The weakest individual hydrogen bonding energy is about -29.0 kJ mol(-1) found in the dimer, whereas with the growth of the N-methylacetamide chain the individual hydrogen bonding energy was estimated to be as large as -62.5 kJ mol(-1) found in the N-methylacetamide decamer, showing that there is a significant hydrogen bond cooperative effect in N-methylacetamide chains. The natural bond orbital analysis indicates that a stronger hydrogen bond corresponds to a larger positive charge for the H atom and a larger negative charge for the O atom in the N-H center dot center dot center dot O=C bond, corresponds to a stronger second-order stabilization energy between the oxygen lone pair and the N-H antibonding orbital, and corresponds to more charge transfer between the hydrogen bonded donor and acceptor molecules.