title: Ab initio/GIAO-CCSD(T) (13)C NMR study of the rearrangement and dynamic aspects of rapidly equilibrating tertiary carbocations, C6H13(+) and C7H15(+).
abstract: The rearrangement pathways of the equilibrating tertiary carbocations, 2,3-dimethyl-2-butyl cation (C6H13(+), 1), 2,3,3-trimethyl-2-butyl cation (C7H15(+), 5) and 2,3-dimethyl-2-pentyl cation (C7H15(+), 8 and 9) were investigated using the ab initio/GIAO-CCSD(T) (13)C NMR method. Comparing the calculated and experimental (13)C NMR chemical shifts of a series of carbocations indicates that excellent prediction of δ(13)C could be achieved through scaling. In the case of symmetrical equilibrating cations (1 and 5) the Wagner-Meerwein 1,2-hydride and 1,2-methide shifts, respectively, produce the same structure. This indicates that the overall (13)C NMR chemical shifts are conserved and independent of temperature. However, in the case of unsymmetrical equilibrating cations (8 and 9) the Wagner-Meerwein shift produces different tertiary structures, which have slightly different thermodynamic stabilities and, thus, different spectra. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level structure 8 is only 90 calories/mol more stable than structure 9. Based on computed (13)C NMR chemical shift calculations, mole fractions of these isomers were determined by assuming the observed chemical shifts are due to the weighted average of the chemical shifts of the static ions.
authors.affiliation: Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, 90089-1661, California.
authors.forename: George A
authors.initials: GA
authors.lastname: Olah
authors.affiliation: Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, 90089-1661, California.
authors.forename: G K Surya
authors.initials: GK
authors.lastname: Prakash
authors.affiliation: Department of Chemistry, Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, 90089-1661, California.
authors.forename: Golam
authors.initials: G
authors.lastname: Rasul
keywords: C-13 NMR; GIAO-CCSD(T); ab Initio; carbocation; equilibrating; tertiary