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An attempt was made to calculate the energetical stability of the double-stranded helix of poly(ethylenimine) oligomers, a model compound for polymer double-stranded helix, by using the ab
initio molecular orbital method. Calculated total energy of double-stranded helix for pentamer is stabilized, ΔE=−1.83 kcal mol−1 of monomer unit, with N-H···N hydrogen bonding between the
twisted two pentamer chains in the double-strand, in comparison with twice of the isolated single helix. Moreover, it was found that the calculated helix radii for pentamer are about 4–10%
larger than those of polymer helix determined by X-ray analysis. Tight double-stranded helix structure of polymer may originate from the long range intermolecular-interactions along helix
direction.
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