SICL3-SPECTROMETRY - AGOSTIC EFFECTS IN THE GAS-PHASE( AND SICL+ AFFINITIES FOR PYRIDINES DETERMINED BY USING THE KINETIC METHOD WITH MULTIPLE STAGE MASS)
Ss. Yang et al., SICL3-SPECTROMETRY - AGOSTIC EFFECTS IN THE GAS-PHASE( AND SICL+ AFFINITIES FOR PYRIDINES DETERMINED BY USING THE KINETIC METHOD WITH MULTIPLE STAGE MASS), Journal of the American Society for Mass Spectrometry, 7(2), 1996, pp. 198-204
Cluster ions, Py(1)SiCl(3)(+)Py(2) and Py(1)SiCl(+)Py(2), where Py(1)
and Sy(2) represent substituted pyridines, formed upon reactive collis
ions of mass-selected SiCl3+ or SiCl+ cations with a mixture of pyridi
nes, are shown to have loosely bound structures by multiple stage mass
spectrometry experiments in a pentaquadrupole mass spectrometer. The
fragment ion abundance ratio, In([Py(1)SiCl(n)(+)]/[Py(2) SiCln+]) (n
= 1 or 3) is used to estimate the relative SiCln+ or SiCl+ affinities
of the constituent pyridines by the kinetic method. In the case of clu
sters comprised of meta- and/or para-substituted pyridines (unhindered
pyridines), the SiCl3+ and SiCl+ affinities are shown to display exce
llent linear correlations with the proton affinities (PAs). On the ass
umption that the effective temperatures of the SiCl3+- and SiCl+-bound
dimers are 555 K (i.e., the same as those of the corresponding Cl+-bo
und dimers), SiCl3+ and SiCl+ affinities of the substituted pyridines,
relative to pyridine, are estimated to be 3-MePy (2.1 kcal/mol), 4-Me
Py (3.2 kcal/mol), 3-EtPy (3.7 kcal/mol), 4-EtPy (4.2 kcal/mol), 3,5-d
iMePy (4.8 kcal/mol), and 3,4-diMePy (5.4 kcal/mol). The SiCl3+ and Si
Cl+ cation affinities are related to the proton affinities by the expr
essions: relative (SiCl3+) affinity = 0.95 Delta PA and relative (SiCl
+) affinity = 0.60 Delta PA. The smaller constant in the relationship
between the relative SiCl+ affinity and the relative proton affinity i
s the result of weaker bonding. Steric effects between the ortho-subst
ituted alkyl group and the central SiCl3+ cation reduce the SiCl3+ aff
inities of dimers that contain ortho-substituted pyridines. The magnit
ude of the steric acceleration of fragmentation is used to measure a s
et of gas-phase steric parameters (S-k). The steric effects in the SiC
l3+ dimers are similar in magnitude to those in the corresponding Cl+-
bound dimers but weaker than those produced by the bulky [OCNCO](+) gr
oup. An inverted steric effect is observed in those SiCl+-bound dimers
that incorporate ortho-substituted pyridines and is ascribed to auxil
iary Si-H-C bonding, which stabilizes the ortho-substituted pyridine-S
iCl+ bond. This auxiliary bonding appears to correspond to agostic bon
ding, which is well characterized in solution and occurs in competitio
n with steric effects that weaken the pyridine-SiCl- interaction. Ion-
molecule reactions of pyridines with halosilicon radical cations SiCl2
(+.) and SiCl4+. as well as alkylated halosilicon cations Si(CH3Cl+ an
d Si(CH3)Cl-2(+) also are investigated. In these cases, charge exchang
e and associated reactions are the main reaction channels, and cluster
ing is not observed.