“…The 13 C NMR chemical shifts and coupling constants J C-H of C 3 to C 8 alkyl cations 1-13 are shown in Tables 3.2 and 3.3. Also subsequently, Myhre and Yannoni 50 have obtained 13 C NMR spectrum of tert-butyl cation 1 in the solid state, which agrees very well with the solution data. Data in Tables 3.2 and 3.3 are characterized by substantial chemical shift deshieldings and coupling constants (J C-H ) that indicate sp 2 hybridization.…”
Section: Preparation From Alkyl Fluorides In Antimony Pentafluoridesupporting
confidence: 72%
“…This methodology has been further improved. 49 Myhre and Yannoni 50 have successfully utilized the above method to generate carbocations in an SbF 5 matrix at very low temperatures for their solid-state 13 C NMR spectroscopic work. It uses deposition of the starting reagents from the gas phase (using high vacuum) on a surface cooled to liquid nitrogen temperature to produce stable solutions of carbocations.…”
Section: Methods Of Generating Carbocations In Superacids Systemsmentioning
“…The 13 C NMR chemical shifts and coupling constants J C-H of C 3 to C 8 alkyl cations 1-13 are shown in Tables 3.2 and 3.3. Also subsequently, Myhre and Yannoni 50 have obtained 13 C NMR spectrum of tert-butyl cation 1 in the solid state, which agrees very well with the solution data. Data in Tables 3.2 and 3.3 are characterized by substantial chemical shift deshieldings and coupling constants (J C-H ) that indicate sp 2 hybridization.…”
Section: Preparation From Alkyl Fluorides In Antimony Pentafluoridesupporting
confidence: 72%
“…This methodology has been further improved. 49 Myhre and Yannoni 50 have successfully utilized the above method to generate carbocations in an SbF 5 matrix at very low temperatures for their solid-state 13 C NMR spectroscopic work. It uses deposition of the starting reagents from the gas phase (using high vacuum) on a surface cooled to liquid nitrogen temperature to produce stable solutions of carbocations.…”
Section: Methods Of Generating Carbocations In Superacids Systemsmentioning
“…These species are known to rearrange with some facility but differ from their carbenium analogues in that isomeric silylenium ions can often be distinguished by reactivity or collisional activation studies. By contrast, carbon scrambling and branching rearrangements in butyl cations occur even at very low temperatures and in the solid state . The collisionally activated dissociation (CAD) mass spectra of gas-phase carbenium ions formed from isomeric precursors are frequently indistinguishable …”
Section: Introductionmentioning
confidence: 99%
“…By contrast, carbon scrambling and branching rearrangements in butyl cations occur even at very low temperatures and in the solid state. 3 The collisionally activated dissociation (CAD) mass spectra of gas-phase carbenium ions formed from isomeric precursors are frequently indistinguishable. 4 In an early study of silylenium ion chemistry, Allen and Lampe found evidence for formation of persistent collision complexes in the reaction of SiH 3 + with C 2 H 4 and proposed that the complex is a nascent β-silylcarbenium ion that rearranges by a reversible β-hydrogen transfer to form H 2 Si(C 2 H 5 ) + (eq 1).…”
The slow unimolecular dissociations (k ≈ 105 s-1) of gas-phase silylenium ions, SiC
n
H2
n
+3
+ ions (n = 2−4)
have been studied by mass-analyzed ion kinetic energy spectroscopy. On the microsecond time scale, the
SiC2H7
+ isomers undergo dissociations corresponding to H2 (72%) and C2H4 loss (28%). The product ion
translational energy distributions and product ratios are the same for HSi(CH3)2
+ and H2Si(C2H5)+, indicating
that these isomers equilibrate prior to dissociation. For the SiC3H9
+ isomers, elimination of C2H4 is the
dominant reaction pathway, comprising 97% of the products for Si(CH3)3
+ and 89% for HSi(CH3)(C2H5)+.
The translational energy distributions for elimination of ethylene from these two ions are different, indicating
that equilibration of Si(CH3)3
+ and HSi(CH3)(C2H5)+ does not occur prior to dissociation. The mechanisms
for C2H4 loss from the Si(CH3)3
+ and HSi(CH3)(C2H5)+ ions were characterized by ab initio computational
methods, and the results were used for statistical phase space modeling of the experimental translational
energy distributions. Excellent agreement between theory and experiment was obtained for ethylene loss
from the SiC2H7
+ isomers and from HSi(CH3)(C2H5)+. The calculated distribution was broader than the
experimental distribution for Si(CH3)3
+. Possible reasons for this result are discussed. The microsecond
unimolecular dissociation of Si(CH3)2(C2H5)+ proceeds exclusively by elimination of C2H4, which arises from
the ethyl group and not from the two methyl groups.
“…Other NMR evidence including dipolar dephasing experiments (not shown) and observation at higher temperature of a scalar doublet ( 1 J C-H = 165 Hz) proves that the species we observed is a secondary carbenium ion and not the tertiary cations that we obtain at higher temperatures by oligomerization and rearrangement (not shown) or directly from other precursors. Myhre and Yannoni used somewhat different methodology to observe the isotropic shifts of the sec -butyl cation in frozen SbF 5 . Those workers also concluded that the frozen SbF 5 matrix greatly restricts the dynamics of this cation, even at 213 K, and our experience with the isopropyl cation and other cations in this medium near 77 K is that large amplitude motions of the carbons can be discounted as a possible mechanism for averaging the chemical shift tensor. 1 75.4 MHz 13 C CP/MAS spectrum of the isopropyl cation in SbF 5 acquired at 83 K. The spinning speed for this experiment was 3681 Hz.…”
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