The role of charge transfer in the hydrogen bond cooperative effect of cis-N-methylformamide oligomers

Two accumulating molecular systems have been designed to investigate the cooperative effect of hydrogen bonding in theory. The first system included a series of linear oligomers of cis-N-methylformamide (c-NMF) molecules. Substantial cooperative effect has been confirmed in the electronic structures and energies of the hydrogen bonds in them as shown by the results obtained using the B3LYP method at the level of cc-pVTZ basis sets. Such a cooperative effect gradually increases with the growth of the c-NMF oligomer. The second system included a series of modified c-NMF trimers whose central c-NMF molecules contained insertion fragments of varying structural and electrical compositions. On the basis of an examination of the structures and charge populations of the c-NMF oligomers in these two systems, a mechanism of the cooperative effect of hydrogen bonding in these systems based on charge flow in the c-NMF molecules is proposed. The results from the second system of c-NMF trimers were particularly instrumental in formulating this mechanism, because the charge flows between the C=O and N-H groups in the modified c-NMF molecule of these trimers were dampened by the various molecular insertions. A clear correlation between the degree of charge flow dampening from each inserted fragment and the magnitude of the cooperative effect of hydrogen bonding was observed. On the basis of an analysis of the electronic structural characteristics of the molecular fragments, we conclude that the charge flow between the hydrogen bond donor and acceptor groups in the c-NMF molecule is the most important factor inducing the cooperative effect of hydrogen bonding.