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DEUTERIUM ISOTOPE EFFECTS IN 13C NMR SPECTRA OF THE PHENYL GROUP

The role of deuterium isotope effects (DIE) in assigning 13C NMR spectra and structural consideration is well established[1]. The dependance of isotope effects in vibrational and rotational modes upon isotope substitution of a molecule is also well known [2]. However, it is noticed that some effects in deuterated phenyl group display variations both in their magnitude and sign [1]. A number of DIEs through two (2Δ ) and three bond (3Δ ) in 13C NMR spectra of phenyl group with various substitents will be presented. Thus in the o-deuterated (C-2) phenyl group 2Δ upon m-carbon (C-3) takes a usual shielding value between 100 and 200 ppb, wheras 2Δ upon i-carbon (C-1) varies from 51.7 to 107.6 ppb. In both the m- and p-deuterated phenyl group there are no deviations from the expected range. Variations in 3Δ values are more pronnounced. Beside changes in magnitude, in some cases the sign is also changed, i.e deshielding effects were observed. Thus 3Δ is positive (3.0 - 17.5 ppb) if the first substituent atom is carbon and negative (-14.8 to - 8.2 ppb) if the first atom next to the phenyl group is nitrogen. There are also several long range deshielding effects upon deuteration. Resons for ˝unusual˝ behavior of deuterium effcts will be discussed. [1] P.Novak, D.Vikić-Topić, V.Smrečki and Z.Meić, Isotope Effects in NMR specra as a Structural Tool for Organic Molecules, Recent Advances in Analytical techniques, Science Publishers : Amsterdam, 2000. [2] C.J.Jameson, Isotope effects on Chemical Shifts and Coupling Constants, Encyclopedia of Nuclear Magnetic Resonance, John Wiley: London, 1995.

NMR; isotope effects; phenyl group

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