Publication details

Origin of the Conformational Modulation of the 13C NMR Chemical Shift of Methoxy Groups in Aromatic Natural Compounds

Authors

TOUŠEK Jaromír STRAKA Michal SKLENÁŘ Vladimír MAREK Radek

Year of publication 2013
Type Article in Periodical
Magazine / Source The Journal of Physical Chemistry A
MU Faculty or unit

Central European Institute of Technology

Citation
Web DOI: 10.1021/jp310470f
Doi http://dx.doi.org/10.1021/jp310470f
Field Physical chemistry and theoretical chemistry
Keywords Nuclear magnetic schielding; DFT calculations; molecular orbitals; excitation analysis; orbital rotation model
Attached files
Description The interpretation of nuclear magnetic resonance (NMR) parameters is essential to understanding experimental observations at the molecular and supramolecular levels and to designing new and more efficient molecular probes. In many aromatic natural compounds, unusual 13C NMR chemical shifts have been reported for out-of-plane methoxy groups bonded to the aromatic ring (~62 ppm as compared to the typical value of ~56 ppm for an aromatic methoxy group). Here, we analyzed this phenomenon for a series of aromatic natural compounds using Density Functional Theory (DFT) calculations. First, we checked the methodology used to optimize the structure and calculate the NMR chemical shifts in aromatic compounds. The conformational effects of the methoxy group on the 13C NMR chemical shift then were interpreted by the Natural Bond Orbital (NBO) and Natural Chemical Shift (NCS) approaches, and by excitation analysis of the chemical shifts, breaking down the total nuclear shielding tensor into the contributions from the different occupied orbitals and their magnetic interactions with virtual orbitals. We discovered that the atypical 13C NMR chemical shifts observed are not directly related to a different conjugation of the lone pair of electrons of the methoxy oxygen with the aromatic ring, as has been suggested. Our analysis indicates that rotation of the methoxy group induces changes in the virtual molecular orbital space, which, in turn, correlate with the predominant part of the contribution of the paramagnetic deshielding connected with the magnetic interactions of the BD(CMet–H)---BD*(CMet–OMet) orbitals, resulting in the experimentally observed deshielding of the 13C NMR resonance of the out-of-plane methoxy group.
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