No abstract

The relative molecular, molecular pair, and molecular triplet Fe + affinities of pyridines are determined by the kinetic method through examination of the competitive dissociation of Fe + -bound dimer, trimer, and tetramer cluster ions. The molecular Fe + affinities vary strongly with the electron donating power of the substituent on the pyridine. Similar behavior is seen in the molecular pair and molecular triplet affinities, the first triplet affinity values determined by the kinetic method. The absolute Fe + affinity of pyridine itself is estimated to be 49 ( 3 kcal/mol. A linear correlation (slope ) 0.38) is demonstrated between the relative Fe + affinities and the corresponding proton affinities of the meta-and para-substituted pyridines. The slope together with data from previous studies falls in the order of OCNCO + ≈ SiCl 3 + > Cl + > CN + > SiCl + > Fe + , which indicates that the strength of binding to pyridines falls in the same order. The relative molecular pair and molecular triplet Fe + affinities also correlate linearly with the number averaged proton affinities of the pyridines. Lower than expected affinities are observed for systems containing ortho-substituted pyridines due to intramolecular steric effects between the ortho-substituted alkyl group(s) and the central Fe + cation. A set of gas-phase steric parameters (S k ), measured by the deviation of the experimental data from the regression line, shows much larger steric effects in the crowded trimers and tetramers.