Ab Initio/GIAO-CCSD(T) C-13 NMR Study of the Rearrangement and Dynamic Aspects of Rapidly Equilibrating Tertiary Carbocations, C6H13+ and C7H15+

作者：Olah George A^*; Prakash G K Surya; Rasul Golam

来源：Journal of Computational Chemistry, 2016, 37(1): 70-77.

DOI：10.1002/jcc.24019

• carbocation
• tertiary
• C-13 NMR
• ab Initio
• GIAO-CCSD(T)
• equilibrating

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) C-13 NMR method. Comparing the calculated and experimental C-13 NMR chemical shifts of a series of carbocations indicates that excellent prediction of delta C-13 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 C-13 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 C-13 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.

• 出版日期2016-1-5