Deuterium isotope effects as a probe of structure and conformation in cyclic carbocations
Deuterium isotope effects on the {sup 13}C NMR chemical shifts were determined for several isotopomers of the 2-methyl-2-bicyclo(2.2.1)heptyl cation and 2-methyl-2-bicyclo(2.1.1)hexyl cation. The results indicate that the isotope shifts are not equilibrium isotope effects but rather an anomalous intrinsic isotope effect which derives from the presence of a shallow potential surface for the bending motion along the direction associated with {sigma}-bridging. The chemical shifts at C{sub 2} and the temperature dependent chemical shifts at C{sub 1}, C{sub 2}, and C{sub 3} help define shapes of potential surfaces. {beta}-Deuterium isotope effects as a function of electron demand in series of 2-aryl-2-bicyclo(2.2.2)octyl cations and 2-aryl-2-bicyclo(2.1.1)hexyl cations were also measured. These data are compared to the previously reported data for 2-aryl-2-bicyclo(2.2.1)heptyl cations and the reference system, 2-aryl-2-propyl cations. The {sup 13}C isotope shifts at the cation center in 2-aryl-2-bicyclo (2.2.2)octyl cations are quite similar to those in the reference system indicating the increased C{sub 3}-H(D) hyperconjugation with increased electron demand. The isotope shift behavior of the 2-aryl-2-bicyclo(2.1.1)hexyl cations is somewhat between the behavior of the 3-oxo-d and 3-endo-d 2-aryl-2-bicyclo(2.2.1)heptyl systems for nonbridging structures. The isotope shifts in 2-aryl-2-bicyclo(2.2.2)octyl cations and 2-aryl-2-bicyclo (2.1.1)hexyl cations do not give any evidence for signalling the onset of {sigma}-bridging.
- Research Organization:
- Northeastern Univ., Boston, MA (USA)
- OSTI ID:
- 6409951
- Country of Publication:
- United States
- Language:
- English
Similar Records
Long-range intrinsic NMR isotope shifts as a probe of hyperconjugation: demonstration of the angular dependence in bicyclic carbocations
Deuterium isotope effects on the carbon-13 chemical shifts in 2-substituted 2-nonbornyl cations
Related Subjects
400101 -- Activation
Nuclear Reaction
Radiometric & Radiochemical Procedures
400202* -- Isotope Effects
Isotope Exchange
& Isotope Separation
ALKYL RADICALS
CARBON 13
CARBON COMPOUNDS
CARBON ISOTOPES
CATIONS
CHARGED PARTICLES
CHEMICAL SHIFT
DEUTERIUM
EVEN-ODD NUCLEI
HEPTYL RADICALS
HEXYL RADICALS
HYDROGEN ISOTOPES
IONS
ISOTOPE EFFECTS
ISOTOPES
LIGHT NUCLEI
MAGNETIC RESONANCE
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
RADICALS
RESONANCE
STABLE ISOTOPES
TEMPERATURE DEPENDENCE