SiCl 3 + and SiCl+ affinities for pyridines determined by using the kinetic method with multiple stage mass spectrometry: Agostic effects in the gas phase
- 8 Downloads
Cluster ions, Py1SiCl 3 + Py2 and Py1SiCl+Py2, where Py1 and Py2 represent substituted pyridines, formed upon reactive collisions of mass-selected SiCl 3 + or SiCl+ cations with a mixture of pyridines, are shown to have loosely bound structures by multiple stage mass spectrometry experiments in a pentaquadrupole mass spectrometer. The fragment ion abundance ratio, ln([Py1SiCl n + ]/[Py2SiCl n + ]) (n=1 or 3) is used to estimate the relative SiCl 3 + or SiCl+ affinities of the constituent pyridines by the kinetic method. In the case of clusters comprised of meta- and/or para-substituted pyridines (unhindered pyridines), the SiCl 3 + and SiCl+ affinities are shown to display excellent linear correlations with the proton affinities (PAs). On the assumption that the effective temperatures of the SiCl 3 + - and SiCl+-bound dimers are 555 K (i. e., the same as those of the corresponding Cl+-bound dimers), SiCl 3 + and SiCl+ affinities of the substituted pyridines, relative to pyridine, are estimated to be 3-MePy (2.1 kcal/mol), 4-MePy (3.2 kcal/mol), 3-EtPy (3.7 kcal/mol), 4-EtPy (4.2 kcal/mol), 3,5-diMePy (4.8 kcal/mol), and 3,4-diMePy (5.4 kcal/mol). The SiCl 3 + and SiCl+ cation affinities are related to the proton affinities by the expressions: relative (SiCl 3 + ) affinity = 0.95 ΔPA and relative (SiCl+) affinity = 0.60 ΔPA. The smaller constant in the relationship between the relative SiCl affinity and the relative proton affinity is the result of weaker bonding.
Steric effects between the ortho-substituted alkyl group and the central SiCl 3 + cation reduce the SiCl 3 + affinities of dimers that contain ortho-substituted pyridines. The magnitude of the steric acceleration of fragmentation is used to measure a set of gas-phase steric parameters (S k). The steric effects in the SiCl 3 + dimers are similar in magnitude to those in the corresponding Cl+-bound dimers but weaker than those produced by the bulky [OCNCO]+ group. An inverted steric effect is observed in those SiCl+-bound dimers that incorporate ortho-substituted pyridines and is ascribed to auxiliary Si-H-C bonding, which stabilizes the ortho-substituted pyridine-SiCl+ bond. This auxiliary bonding appears to correspond to agostic bonding, which is well characterized in solution and occurs in competition with steric effects that weaken the pyridine-SiCl+ interaction.
Ion-molecule reactions of pyridines with halosilicon radical cations SiCl 2 + and SiCl 4 + as well as alkylated halosilicon cations Si(CH3)2Cl+ and Si(CH3)Cl 2 + also are investigated. In these cases, charge exchange and associated reactions are the main reaction channels, and clustering is not observed.
Unable to display preview. Download preview PDF.
- 1.(a)Oppenstein, A.; Lampe, F. W. In Review of Chemical Intermediates; Strsusz, O. P., Ed.; Elsevier: Amsterdam, 1986; Vol. 6, p 275;Google Scholar
- 5.(b)Meot-ner (Mautner), M. In Gas Phase Ion Chemistry Bower, M. T., Ed.; Academic: New York, 1979; Vol. 2;Google Scholar
- 6.(a)Foucaud, A. In The Chemistry of Functional Groups; Patai, S.; Rappoport, Z.; Eds.; Wiley: New York 1983; Suppl. D, Chap. 11, p 441;Google Scholar
- 6.(b)Lwowski, W., Ed. Nitrenes; Interscience: New York, 1970;Google Scholar
- 6.(c)Jones, M., Jr.; Moss, R. A., Eds. Carbenes; Wiley-Interscience: New York, 1973; Vol. 1;Google Scholar
- 6.(f)Cox, R. A. Org. React. Mech. 1989, 283;Google Scholar
- 6.(g)Creary, X., Ed. Advances in Carboncation Chemistry; JAI Press: Greenwich, CT, 1989.Google Scholar
- 8.(a)Patrick, J. S.; Kotiaho, T.; McLuckey, S. A.; Cooks, R. G. Mass Spectrom. Rev. 1994, 33, 287;Google Scholar
- 8.(b)Cooks, R. G.; Kruger, T. L. J. Am. Chem. Soc. 1981, 203, 3274;Google Scholar
- 14.Schwartz, J. C.; Schey, K. L.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1990, 1, 101.Google Scholar
- 15.(b)Cooks, R. G.; Amy, J.; Bier, M.; Schwartz, J. C.; Schey, K. L. Adv. Mass Spectrom. 1989, 11A, 33.Google Scholar
- 16.Cooks, R. G.; Rockwood, A. L. Rapid Commun. Mass Spectrom. 1991, 5, 93.Google Scholar
- 17.Cedra B. A.; Wesdemiotis C. Presented at the 42nd ASMS Conference on Mass Spectrometry and Allied Topics, Chicago, IL., June 1994.Google Scholar
- 18.(a)Gallo, R.; Roussel, C.; Berg, U. In Advances in Heterocyclic Chemistry; Katritzky, A. R., Ed.; Academic Press: New York, 1988, Vol. 43, p 173;Google Scholar
- 18.(b)Berg, U.; Gallo, R.; Klatte, G.; Metzger, J. J. Chem. Soc., Perkin Trans. 1980, 2, 1350;Google Scholar
- 18.(h)Splitter; Tureček, F., Eds. Application of Mass Spectrometry to Organic Stereochemistry; VCH Publishers: New York, 1994.Google Scholar