Naslov
NMR Isotope Shift as a Probe of Conformation in Carbocations. Experimental 13C NMR Spectroscopy and GIAO-DFT Study of 1-(2-Propyl)Cyclopentyl Cation

Autori
Šakić, Davor ; Kronja, Olga ; Vrček, Valerije

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
The 10th Central European NMR Symposium & The 10th Central European Bruker NMR Users Meeting (CEUM 2008) : Book of abstracts / Plavšić, Dejan ; Smrečki, Vilko ; Roca, Sunčica ; Vinković, Marijana - Zagreb : Institut Ruđer Bošković, 2008, 18-18

Skup
Central European NMR Symposium (10 ; 2008)

Mjesto i datum
Zagreb, Hrvatska, 29.09.2008

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
carbocations; NMR; deuterium isotope effect

Sažetak
NMR isotope shifts due to β-deuteration (deuterium substitution at a carbon atom adjacent to the carbocation center) have been postulated to be related to the hyperconjugative interaction. C-D bond behaves as a poorer electron donor than a C-H bond in cationic hyperconjugation, resulting in decreased electron density in the π system of the cation and hence NMR deshielding. Typical NMR isotope shifts are additive, but nonadditivity of NMR isotope shifts in carbocations may occur when C-H(D) bonds are involved in hyperconjugation and are also conformationally mobile, because of the imposition of an equilibrium isotope effect on the population of conformers that may have different intrinsic NMR isotope shifts. In the present work we report the determination of the conformation of the 1-(2-propyl)cyclopentyl cation by use of the additivity characteristics of the 2ΔC+(D) NMR isotope shifts at C+ due to β-deuteration. The nonadditivity of isotope shifts indicates that the cyclopentyl cation is nonplanar, and isotopomers of the cation reveal the preferred conformation to be the twist structure. In addition to isotopic perturbation of hyperconjugation, the 1-(2-propyl)cyclopentyl cation undergoes fast reversible nondegenerate hydride shift which results in the averaged 13C NMR chemical shifts of carbocation center. A theoretical model is developed that provides a possible explanation for the relative magnitude of isotope shifts in this cation, based on DFT-GIAO NMR calculations.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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