engleski

Deuterium isotope effects in C-13 NMR spectra

Deuterium is the most often studied isotope due to its easy incorporation into molecules and because of the large fractional H/D mass change. Deuterium substitution causes some peculiar effects in molecules, e. g. optical activity in CXYHD, slight dipole moment (10^-2 - 10^-4 D) in monodeuterated benzene and acetylene, which enables the measuring of pure rotational spectra of these compounds and deuterium isotope effects on C-13 NMR chemical shifts over many chemical bonds. Long range deuterium isotope effects (LRDIE) on C-13 NMR chemical shifts have been found in extended pi-electron molecules even up to twelve bonds (ca. 0.1 Hz). Deuterium substitution causes shielding of the directly deuterated carbon and the nearby carbon atoms, giving rise to upfield chemical shifts. In contrast, for LRDIE, both upfield and downfield shifts of carbon signals are observed. The LRDIE have a pattern of sign alternation which resembles that of the pi-polarization effects of substituents. The calculations of LRDIE are still challenging because of their low magnitude (0.5 - 300 ppb) and incompletely developed theoretical concepts on isotope effects through many bonds. 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, which accompany deuteration, are due to differences in zero-point vibrational motion and anharmonicity between C-H and C-D bonds. In calculations of LRDIE at least two contributions have to be taken into account: the secondary change in shielding at the remote C-atom, due to a shorter C-D than C-H mean bond length, and the primary change in remote C-atom shielding, due to a change in the mean bond length at that remote site. We assumed that LRDIE predominantly arise from primary change of bond lengths at the site of deuteration, since vibrational changes at remote C-atoms due to deuteration are very minute. The C-D bond was modeled by reduction of the corresponding C-H bond in the range 0.003-0.018 A. Nuclear shieldings (GIAO, TEXAS) and atomic charges (Lőwdin and Mulliken, GAMESS) were calculated at different ab initio levels. Experimental LRDIE on C-13 chemical shifts and their correlations with differences in shielding and charges between protonated and deuterated molecules will be discussed.

deuterium; long-range isotope effect; C-13 NMR spectroscopy

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