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

J 2013

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

TOUŠEK, Jaromír, Michal STRAKA, Vladimír SKLENÁŘ and Radek MAREK

Basic information

Original name

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

Authors

TOUŠEK, Jaromír (203 Czech Republic, belonging to the institution), Michal STRAKA (203 Czech Republic, belonging to the institution), Vladimír SKLENÁŘ (203 Czech Republic, belonging to the institution) and Radek MAREK (203 Czech Republic, guarantor, belonging to the institution)

Edition

The Journal of Physical Chemistry A, American Chemical Society, 2013, 1089-5639

Other information

Language

English

Type of outcome

Článek v odborném periodiku

Field of Study

10403 Physical chemistry

Country of publisher

United States of America

Confidentiality degree

není předmětem státního či obchodního tajemství

References:

DOI: 10.1021/jp310470f

Impact factor

Impact factor: 2.775

RIV identification code

RIV/00216224:14740/13:00067650

Organization unit

Central European Institute of Technology

DOI

http://dx.doi.org/10.1021/jp310470f

UT WoS

000314205300015

Keywords in English

Nuclear magnetic schielding; DFT calculations; molecular orbitals; excitation analysis; orbital rotation model

Abstract

V originále

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.

Links

ED1.1.00/02.0068, research and development project

Name: CEITEC - central european institute of technology

Files attached

JPCA13_117_661.pdf

Request the author's version of the file

Citovat

TOUŠEK, Jaromír, Michal STRAKA, Vladimír SKLENÁŘ and Radek MAREK. Origin of the Conformational Modulation of the 13C NMR Chemical Shift of Methoxy Groups in Aromatic Natural Compounds. The Journal of Physical Chemistry A. American Chemical Society, 2013, vol. 117, No 3, p. 661-669. ISSN 1089-5639. Available from: https://dx.doi.org/10.1021/jp310470f.

@article{1083034,
   author = {Toušek, Jaromír and Straka, Michal and Sklenář, Vladimír and Marek, Radek},
   article_number = {3},
   doi = {http://dx.doi.org/10.1021/jp310470f},
   keywords = {Nuclear magnetic schielding; DFT calculations; molecular orbitals; excitation analysis; orbital rotation model},
   language = {eng},
   issn = {1089-5639},
   journal = {The Journal of Physical Chemistry A},
   title = {Origin of the Conformational Modulation of the 13C NMR Chemical Shift of Methoxy Groups in Aromatic Natural Compounds},
   url = {http://dx.doi.org/10.1021/jp310470f},
   volume = {117},
   year = {2013}
}

TY  - JOUR
ID  - 1083034
AU  - Toušek, Jaromír - Straka, Michal - Sklenář, Vladimír - Marek, Radek
PY  - 2013
TI  - Origin of the Conformational Modulation of the 13C NMR Chemical Shift of Methoxy Groups in Aromatic Natural Compounds
JF  - The Journal of Physical Chemistry A
VL  - 117
IS  - 3
SP  - 661-669
EP  - 661-669
PB  - American Chemical Society
SN  - 10895639
KW  - Nuclear magnetic schielding
KW  - DFT calculations
KW  - molecular orbitals
KW  - excitation analysis
KW  - orbital rotation model
UR  - http://dx.doi.org/10.1021/jp310470f
L2  - http://dx.doi.org/10.1021/jp310470f
N2  - 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.
ER  -

TOUŠEK, Jaromír, Michal STRAKA, Vladimír SKLENÁŘ and Radek MAREK. Origin of the Conformational Modulation of the 13C NMR Chemical Shift of Methoxy Groups in Aromatic Natural Compounds. \textit{The Journal of Physical Chemistry A}. American Chemical Society, 2013, vol.~117, No~3, p.~661-669. ISSN~1089-5639. Available from: https://dx.doi.org/10.1021/jp310470f.

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