Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe+ Affinities of Pyridines

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• 作者：Shuguang Ma ; Philip Wong ; Sheng Sheng Yang ; and R. Graham Cooks
• 刊名：Journal of the American Chemical Society
• 出版年：1996
• 出版时间：June 26, 1996
• 年：1996
• 卷：118
• 期：25
• 页码：6010 - 6019
• 全文大小：350K
• 年卷期：v.118,no.25(June 26, 1996)
• ISSN：1520-5126

文摘

The relative molecular, molecular pair, and molecular tripletFe⁺ affinities of pyridines are determinedbythe kinetic method through examination of the competitive dissociationof Fe⁺-bound dimer, trimer, and tetramercluster ions. The molecular Fe⁺ affinities varystrongly with the electron donating power of the substituent onthepyridine. Similar behavior is seen in the molecular pair andmolecular triplet affinities, the first triplet affinityvalues determined by the kinetic method. The absoluteFe⁺ affinity of pyridine itself is estimated to be 49 ±3kcal/mol. A linear correlation (slope = 0.38) is demonstratedbetween the relative Fe⁺ affinities and thecorrespondingproton affinities of the meta- and para-substituted pyridines. Theslope together with data from previous studiesfalls in the order of OCNCO⁺SiCl₃⁺ > Cl⁺ >CN⁺ > SiCl⁺ > Fe⁺, whichindicates that the strength of bindingto pyridines falls in the same order. The relative molecular pairand molecular triplet Fe⁺ affinities alsocorrelatelinearly with the number averaged proton affinities of the pyridines.Lower than expected affinities are observed forsystems containing ortho-substituted pyridines due to intramolecularsteric effects between the ortho-substituted alkylgroup(s) and the central Fe⁺ cation. A set ofgas-phase steric parameters (S^k), measured bythe deviation of theexperimental data from the regression line, shows much larger stericeffects in the crowded trimers and tetramers.