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Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate

The gas-phase structures of the fruit ester methyl hexanoate, CH(3)-O-(C=O)-C(5)H(11), have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers w...

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Autores principales: Dang, Nhu-Ngoc, Pham, Hoang-Nam, Kleiner, Isabelle, Schwell, Martin, Grabow, Jens-Uwe, Nguyen, Ha Vinh Lam
Formato: Online Artículo Texto
Lenguaje:English
Publicado: MDPI 2022
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9105109/
https://www.ncbi.nlm.nih.gov/pubmed/35565991
http://dx.doi.org/10.3390/molecules27092639
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author Dang, Nhu-Ngoc
Pham, Hoang-Nam
Kleiner, Isabelle
Schwell, Martin
Grabow, Jens-Uwe
Nguyen, Ha Vinh Lam
author_facet Dang, Nhu-Ngoc
Pham, Hoang-Nam
Kleiner, Isabelle
Schwell, Martin
Grabow, Jens-Uwe
Nguyen, Ha Vinh Lam
author_sort Dang, Nhu-Ngoc
collection PubMed
description The gas-phase structures of the fruit ester methyl hexanoate, CH(3)-O-(C=O)-C(5)H(11), have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with C(s) symmetry and the other with C(1) symmetry where the γ-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH(3)-O, from which torsional barriers of 417 cm(−1) and 415 cm(−1), respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C(1) conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH(3)-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm(−1), it decreases in molecules with longer R.
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spelling pubmed-91051092022-05-14 Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate Dang, Nhu-Ngoc Pham, Hoang-Nam Kleiner, Isabelle Schwell, Martin Grabow, Jens-Uwe Nguyen, Ha Vinh Lam Molecules Article The gas-phase structures of the fruit ester methyl hexanoate, CH(3)-O-(C=O)-C(5)H(11), have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with C(s) symmetry and the other with C(1) symmetry where the γ-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH(3)-O, from which torsional barriers of 417 cm(−1) and 415 cm(−1), respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C(1) conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH(3)-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm(−1), it decreases in molecules with longer R. MDPI 2022-04-20 /pmc/articles/PMC9105109/ /pubmed/35565991 http://dx.doi.org/10.3390/molecules27092639 Text en © 2022 by the authors. https://creativecommons.org/licenses/by/4.0/Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
spellingShingle Article
Dang, Nhu-Ngoc
Pham, Hoang-Nam
Kleiner, Isabelle
Schwell, Martin
Grabow, Jens-Uwe
Nguyen, Ha Vinh Lam
Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate
title Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate
title_full Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate
title_fullStr Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate
title_full_unstemmed Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate
title_short Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate
title_sort methyl internal rotation in fruit esters: chain-length effect observed in the microwave spectrum of methyl hexanoate
topic Article
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9105109/
https://www.ncbi.nlm.nih.gov/pubmed/35565991
http://dx.doi.org/10.3390/molecules27092639
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