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| - The Role of Charge Transfer in the Hydrogen Bond Cooperative Effect ofcis-N-Methylformamide Oligomers
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| - Two accumulating molecular systems have been designed to investigate the cooperative effect of hydrogenbonding 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 thehydrogen bonds in them as shown by the results obtained using the B3LYP method at the level of cc-pVTZbasis sets. Such a cooperative effect gradually increases with the growth of the c-NMF oligomer. The secondsystem included a series of modified c-NMF trimers whose central c-NMF molecules contained insertionfragments of varying structural and electrical compositions. On the basis of an examination of the structuresand charge populations of the c-NMF oligomers in these two systems, a mechanism of the cooperative effectof hydrogen bonding in these systems based on charge flow in the c-NMF molecules is proposed. The resultsfrom the second system of c-NMF trimers were particularly instrumental in formulating this mechanism,because the charge flows between the CO and NH groups in the modified c-NMF molecule of thesetrimers were dampened by the various molecular insertions. A clear correlation between the degree of chargeflow dampening from each inserted fragment and the magnitude of the cooperative effect of hydrogen bondingwas 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-NMFmolecule is the most important factor inducing the cooperative effect of hydrogen bonding.
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