Abstract: MATH/CHEM/COMP 2002, Dubrovnik, June 24-29, 2002

 

 

DEUTERIUM ISOTOPE EFFECTS IN 13C NMR SPECTRA

 

Drazen Vikic-Topic and Zeljko Marinic

 

Rudjer Boskovic Institute, NMR Center, POB 180, HR-1002 Zagreb,Croatia

 

 

 

Deuterium is the most often studied isotope due to easy of its incorporation into the molecules as well as because of 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 13C NMR chemical shifts over many chemical bonds.

Long range deuterium isotope effects (LRDIE) on 13C NMR chemical shifts have been found in extended p-electron molecules even up through twelve bonds (ca. 0.1 Hz). Deuterium substitution causes shielding of directly deuterated carbon as well as nearby carbon atoms, giving rise to upfield chemical shifts. Contrary to that, for LRDIE both upfield and downfield shifts of carbon signals are observed. The LRDIE have a pattern of sign alternation which resembles that of p-polarization effects of substituents.

The calculations of LRDIE are still challenging because of their low magnitude (0.5-300 ppb) and not completely developed theory of 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 accompany deuteration 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 remote
C-atom due to shorter C-D than C-H mean bond length and the primary change in remote C-atom shielding, due to change in mean bond length at this remote site. We assumed that LRDIE predominantly arise from primary change of bond length 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 . Nuclear shieldings (GIAO, TEXAS) and atomic charges (Lwdin and Mulliken, GAMESS) were calculated at different ab initio levels.

Experimental LRDIE on 13C chemical shifts and their correlations with differences in shielding and charges between protonated and deuterated molecules will be discussed.