engleski

On the Long Range Deuterium Isotope Effects in 13C NMR Spectra

Deuteriation of organic molecules often generates long range deuterium isotope effects in 13C NMR spectra. In extended  -systems such effects have been detected even through 12 bonds. Substitution by deuterium induces shielding of nearby carbons, i.e. upfield chemical shifts. However, in the case of long range deuterium isotope effects (LRDIE) both upfield and downfield shifts of carbon signals have been observed, with the sign alternation which parallels that of  -polarization. Theoretically isotope effects on chemical shifts are interpreted in terms of vibrational and rotational averaging of nuclear shielding. The subtle changes in average bond lengths and average bond angles accompany deuteration, due to the differences in zero-point vibrational motion and anharmonicity of C-H and C-D bonds. In calculations of LRDIE two contributions have to be taken into account: (i) the secondary change in shielding at remote C-atom due the shorter C-D than C-H mean bond length at the site of deuteration and (ii) the primary change in shielding at remote C-atom due to change in mean bond length at remote site. Low magnitude of isotope effects (1-300 ppb) make calculations of LRDIE still challenging.1, 2 In our treatment we assume that LRDIE predominantly arise from shorter C-D than C-H mean bond length at the site of deuteration (i) and that this change is felt throughout the whole  -system.  -system We investigated several isotopomers of binuclear aromatic molecules (stilbenes, azobenzenes, benzophenones, etc.) and detected a number of LRDIE. Since experimental data revealed shorter C-D than C-H bond we modeled C-D bond by reduction of the corresponding C-H bond in the range 0.003-0.018 &Aring ; ; . Fully optimized geometry, nuclear shieldings (GIAO) and atomic charges (Löwdin, Mulliken) were calculated at various ab initio levels. Differences in shieldings and charges between H/D molecules correlate well with experimentally measured LRDIE. Therefore, by using proposed approach we fairly calculated LRDIE on 13C NMR chemical shifts of investigated  -molecules. 1 D. Vikic-Topic, M. Hodoscek, A. Graovac, and E. D. Becker, Croat. Chem. Acta 1999, 68, 193-203. 2 D. Vikic-Topic and Lj. Pejov, J. Chem. Inf. Comput. Sci. 2001, 41, 1478-1487.

long range deuterium isotope effects

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