Molecular Orbital Study on a Chlorine Anion Cryptate

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Closed shell LCAO-SCF-MO calculations were carried out for the chloride anion cryptate, which is of interest in connection with organic anion receptors and carriers. In the Cl- cryptate of [(C₉H₁₈)₃(NH)₂Cl]⁺Cl^-, Cl^- is encapsulated in the host molecule. Since the positions of the two protons bonded to the two nitrogens of the host molecule were not determined in the reported X-ray diffraction analyses, the positions of the two protons in the inclusion complex were decided by calculation using the ab initio molecular orbital method. A structure N⁺-H…Cl^-…H-N⁺ in which Cl^- is in between the two nitrogens was most stable at values of γ(NN) less than 7.0 Å. The structure N⁺-H…Cl^-…H-N⁺ was most stable at γ(NN)=6.023 Å. In the most stable structure, the two protons are covalently bonded to the two nitrogens. The structure obtained by geometry optimization for the two ammonium ions and Cl^- was similar to that determined by X-ray diffraction analysis.