Naslov
Deuterium Isotope Effects in 13C NMR Spectra of the Phenyl Group

Autori
Biljan, Tomislav ; Meić, Zlatko ; Vikić-Topić, Dražen ; Smrečki, Vilko ; Novak, Predrag

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

Izvornik
The 2nd International DU NMR Course and Conference: Program & Book of Abstracts / Vikić-Topić, Dražen - Zagreb : Institut Ruđer Bošković, 2003, 35-36

Skup
Opening of NMR Center and FT EPR Facility at Ruđer Bošković Institute & The 2nd International DU NMR Course and Conference

Mjesto i datum
Zagreb, Hrvatska, 20.10.2003. - 22.10.2003

Vrsta sudjelovanja
Poster

Vrsta recenzije
Domaća recenzija

Ključne riječi
-

Sažetak
The role of deuterium isotope effects (DIEs) in assigning 13C NMR spectra and structural considerations is well established[1]. The dependence of isotope effect on overall changes in vibrational and rotational modes upon isotope substitution of a molecule is also well know[2]. However, it is noticed that some effects in a deuterated phenyl group display variations both in their magnitude and sign1. A number of DIEs through two (2Δ ) and three bonds (3Δ ) in 13C NMR spectra of the phenyl group with various substituents will be presented. Thus in the ortho-deuterated (C-2) phenyl group 2Δ upon the meta-carbon (C-3) takes a usual shielding value between 100 and 120 ppb, whereas 2Δ upon i-carbon (C-1) varies from 51.7 to 19.7 ppb. In both the meta- and para-deuterated phenyl group there are no deviations from the expected range. Variations in 3Δ 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 next atom to the phenyl group is nitrogen. There are also several long range deshieliding effects upon deuteration. Theoretical calculations of nuclear shielding constants and NMR shifts effects needs to include four main issues: rotovibrational effects, electron correlation effects, basic-set convergence, accurate equilibrium geometries.The theoretical research[3] demonstrates that for achieving agreement between theory and experiment there is a need for large basis set on CCSD(T) calculations that take into account rotovibrational effects. In principle, it is now possible to compute anharmonic force fields and corresponding vibrational corrections to molecular properties employing analytic second derivative techniques[4]. However, calculations are not feasible beyond rather small molecules. Calculations of deuterium isotope effects from first principles is the only way of explaining origin and properties of DIEs in the phenyl group and all other systems. These calculations with the growth of computing power will be feasible at regular basis in few years. Reasons for ˝unusual˝ behaviour of deuterium effects will be discussed. REFERENCES: [1] P. Novak, D. Vikić-Topić, V. Smrečki and Z. Meić, Isotope Effects in NMR Spectra 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. [3] A. A. Auer and J. Gauss, J. Chem. Phys., 118 (2003) 10470. [4] J. F. Stanton and J. Gauss, Int. Rev. Phys. Chem., 19 (2000) 61.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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