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<sub>3</sub>-O-(C=O)-C<sub>5</sub>H<sub>11</sub>, have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was m...
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MDPI AG
2022-04-01
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| Series: | Molecules |
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| Online Access: | https://www.mdpi.com/1420-3049/27/9/2639 |
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| author | Nhu-Ngoc Dang Hoang-Nam Pham Isabelle Kleiner Martin Schwell Jens-Uwe Grabow Ha Vinh Lam Nguyen |
| author_facet | Nhu-Ngoc Dang Hoang-Nam Pham Isabelle Kleiner Martin Schwell Jens-Uwe Grabow Ha Vinh Lam Nguyen |
| author_sort | Nhu-Ngoc Dang |
| collection | DOAJ |
| description | The gas-phase structures of the fruit ester methyl hexanoate, CH<sub>3</sub>-O-(C=O)-C<sub>5</sub>H<sub>11</sub>, 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<sub>s</sub> symmetry and the other with C<sub>1</sub> symmetry where the γ-carbon atom of the hexyl chain is in a <i>gauche</i> 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 <b>CH<sub>3</sub></b>-O, from which torsional barriers of 417 cm<sup>−1</sup> and 415 cm<sup>−1</sup>, 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<sub>1</sub> 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<sub>3</sub>-O-(C=O)-<b>R</b> molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm<sup>−1</sup>, it decreases in molecules with longer <b>R</b>. |
| first_indexed | 2024-03-10T03:55:38Z |
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| id | doaj.art-701d0ba3c50e4225877a28b16bc27051 |
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| last_indexed | 2024-03-10T03:55:38Z |
| publishDate | 2022-04-01 |
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| series | Molecules |
| spelling | doaj.art-701d0ba3c50e4225877a28b16bc270512023-11-23T08:47:27ZengMDPI AGMolecules1420-30492022-04-01279263910.3390/molecules27092639Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl HexanoateNhu-Ngoc Dang0Hoang-Nam Pham1Isabelle Kleiner2Martin Schwell3Jens-Uwe Grabow4Ha Vinh Lam Nguyen5Department of Life Sciences, University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, VietnamDepartment of Life Sciences, University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, VietnamUniversité Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, FranceUniv Paris Est Creteil and Université Paris Cité, CNRS, LISA, 94010 Créteil, FranceInstitut für Physikalische Chemie und Elektrochemie, Gottfried-Wilhelm-Leibniz-Universität Hannover, Callinstraße 3A, 30167 Hannover, GermanyUniv Paris Est Creteil and Université Paris Cité, CNRS, LISA, 94010 Créteil, FranceThe gas-phase structures of the fruit ester methyl hexanoate, CH<sub>3</sub>-O-(C=O)-C<sub>5</sub>H<sub>11</sub>, 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<sub>s</sub> symmetry and the other with C<sub>1</sub> symmetry where the γ-carbon atom of the hexyl chain is in a <i>gauche</i> 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 <b>CH<sub>3</sub></b>-O, from which torsional barriers of 417 cm<sup>−1</sup> and 415 cm<sup>−1</sup>, 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<sub>1</sub> 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<sub>3</sub>-O-(C=O)-<b>R</b> molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm<sup>−1</sup>, it decreases in molecules with longer <b>R</b>.https://www.mdpi.com/1420-3049/27/9/2639microwave spectroscopyrotational spectroscopyinternal rotationlarge amplitude motion |
| spellingShingle | Nhu-Ngoc Dang Hoang-Nam Pham Isabelle Kleiner Martin Schwell Jens-Uwe Grabow Ha Vinh Lam Nguyen Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate Molecules microwave spectroscopy rotational spectroscopy internal rotation large amplitude motion |
| 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 | microwave spectroscopy rotational spectroscopy internal rotation large amplitude motion |
| url | https://www.mdpi.com/1420-3049/27/9/2639 |
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