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Deuterium Isotope Effects in 13C NMR Spectra of trans-N-Salicylideneaniline Isotopomers

Isotope effects on NMR spectral parameters have gained a considerable attention in natural sciences due to their relation to molecular structure and dynamics [1, 2]. Deuterium is particularly interesting because of a large fractional change in mass and relative ease of its substitution. trans-N-Salicylideneaniline (SAN) was chosen as a prototype aromatic Schiff base with intramolecular hydrogen bond. A series of isotopically labelled species was prepared (Scheme 1) and their 13C NMR spectra recorded with a 500 MHz spectrometer. Scheme 1 In 4'-D-SAN the longest deuterium isotope effect (DIE) observed is the one over six bonds: 6DELTA(C-alpha) = 5.3 ppb, which is similar to 6DELTA observed in 4’ D trans-N- benzylideneaniline (7.0 ppb) [3]. In the corresponding para deuterium labelled isotopomers of cis-stilbene [4], trans-stilbene [5] and trans-azobenzene [6] DIE range over ten bonds, which reflects the overall molecular geometry and conjugational effects of pi- electrons. DIE over six bonds (6DELTA) observed in 4’-D-SAN isotopomer can be used in conformational analysis for an estimation of the N-Ph torsional angle in solution [5, 7]. There is a considerable deviation of the additivity rule observed in 2’, 3’, 4', 5’, 6’-D5- SAN due to the intramolecular H-bond dynamics. DIE in 13C NMR spectrum of isotopomer with perdeuterium labelled salicylidene ring is compared to those of analogous isotopomers in a series of binuclear aromatic molecules. REFERENCES [1] P. E. Hansen, Isotope Effects on Chemical Shifts as a Tool in Structural Studies, Roskilde University Press, Roskilde, 1996. [2] C. J. Jameson, The Dynamic and Electronic Factors in Isotope Effects on NMR Parameters, in Isotopes in the Physical and Biomedical Science, E. Buncel, J. R. Jones, (eds), Vol. 2, Elsevier, Amsterdam, 1991, 1 54. [3] V. Smrečki, N. Müller, D. Vikić-Topić, P. Vujanić, Z. Meić, J. Mol. Struct., 348 (1995) 69 72. [4] Z. Meić, P. Novak, D. Vikić Topić, V. Smrečki, Magn. Reson. Chem., 34 (1996) 36 41. [5] D. Vikić Topić, P. Novak, Z. Meić, J. Plavec, D. Kovaček, Croat. Chem. Acta, 72 (1999) 967 974. [6] D. Vikić Topić, P. Novak, V. Smrečki, Z. Meić, J. Mol. Struct., 410 411 (1997) 5 7. [7] P. Novak, D. Vikić Topić, V. Smrečki, Z. Meić, Isotope Effects in NMR Spectra as a Structural Tool for Organic Molecules, in New Advances in Analytical Chemistry, Atta ur Rahman (ed), Harwood Academic Publishers, Amsterdam, 2000, 135 168.

deuterium isotope effects ; salicylideneaniline ; hydrogen bond

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