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Ab initio /GIAO‐CCSD(T) 13 C NMR study of the rearrangement and dynamic aspects of rapidly equilibrating tertiary carbocations, C 6 H 13 + and C 7 H 15 +
Author(s) -
Olah George A.,
Prakash G. K. Surya,
Rasul Golam
Publication year - 2016
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.24019
Subject(s) - carbocation , ab initio , computational chemistry , chemistry , physics , medicinal chemistry , organic chemistry
The rearrangement pathways of the equilibrating tertiary carbocations, 2,3‐dimethyl‐2‐butyl cation (C 6H 13 + , 1 ), 2,3,3‐trimethyl‐2‐butyl cation (C 7H 15 + , 5 ) and 2,3‐dimethyl‐2‐pentyl cation (C 7H 15 + , 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. © 2015 Wiley Periodicals, Inc.