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Interpretation of substituent effects on 13C and 15N NMR chemical shifts in 6-substituted purines

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STANDARA Stanislav BOUZKOVÁ Kateřina STRAKA Michal ZACHAROVÁ Zuzana HOCEK Michal MAREK Jaromír MAREK Radek

Year of publication 2011
Type Article in Periodical
Magazine / Source Physical Chemistry Chemical Physics
MU Faculty or unit

Central European Institute of Technology

Web DOI: 10.1039/c1cp20680j
Field Physical chemistry and theoretical chemistry
Keywords Purine; Nuclear magnetic shielding; Localized molecular orbitals; Conformational dependence
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Description The present work aims to analyze the effects of substituents on the electron distribution in the purine core as reflected by NMR chemical shifts. We collected a comprehensive set of experimental NMR data for a variety of 6-substituted purines and determined the molecular and crystal structures of three derivatives by X-ray diffraction. The density-functional methods have been employed to enable understanding of the substituent-induced changes in the NMR chemical shifts of the atoms in the purine skeleton. Analyses of the nuclear shielding using localized molecular orbitals (LMOs) were used to break down the values of the isotropic 13C and 15N NMR chemical shifts and the chemical shift tensors into the contributions of the individual LMOs. The experimental and calculated trends in the chemical shift of the N-3 atom correlate nicely with the Hammett constants and the calculated natural charges on N-3, whereas the contributions of the LMOs to the N-1 and C-6 chemical shifts are found to be more complex.
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