Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study
For decades one has strived to synthesize a compound with the longest covalent C−C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation...
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MDPI AG
2021-02-01
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| Series: | Molecules |
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| Online Access: | https://www.mdpi.com/1420-3049/26/4/950 |
| _version_ | 1797411455098683392 |
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| author | Alexis Antoinette Ann Delgado Alan Humason Robert Kalescky Marek Freindorf Elfi Kraka |
| author_facet | Alexis Antoinette Ann Delgado Alan Humason Robert Kalescky Marek Freindorf Elfi Kraka |
| author_sort | Alexis Antoinette Ann Delgado |
| collection | DOAJ |
| description | For decades one has strived to synthesize a compound with the longest covalent C−C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation in its D<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>3</mn><mi>d</mi></mrow></msub></semantics></math></inline-formula> form. Recently, negative hyperconjugation effects occurring in diamino-o-carborane analogs such as di-N,N-dimethylamino-o-carborane have been held responsible for their long C−C bonds. In this work we systematically analyzed CC bonding in a diverse set of 53 molecules including clamped bonds, highly sterically strained complexes such as diamondoid dimers, electron deficient species, and di-N,N-dimethylamino-o-carborane to cover the whole spectrum of possibilities for elongating a covalent C−C bond to the limit. As a quantitative intrinsic bond strength measure, we utilized local vibrational CC stretching force constants <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mi>k</mi><mi>a</mi></msup></semantics></math></inline-formula>(CC) and related bond strength orders BSO <i>n</i>(CC), computed at the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>ω</mi></semantics></math></inline-formula>B97X-D/aug-cc-pVTZ level of theory. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C−C bond, electronic effects can lead to even longer and weaker C−C bonds. Within our set of molecules the electron deficient ethane radical cation, in D<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>3</mn><mi>d</mi></mrow></msub></semantics></math></inline-formula> symmetry, acquires the longest C−C bond with a length of 1.935 Å followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 Å. However, the C−C bond in di-N,N-dimethylamino-o-carborane is the weakest with a BSO <i>n</i> value of 0.209 compared to 0.286 for the ethane radical cation; another example that the longer bond is not always the weaker bond. Based on our findings we provide new guidelines for the general characterization of CC bonds based on local vibrational CC stretching force constants and for future design of compounds with long C−C bonds. |
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| spelling | doaj.art-cbe1e3038c294b178874ef64dbd0ee932023-12-03T13:16:14ZengMDPI AGMolecules1420-30492021-02-0126495010.3390/molecules26040950Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode StudyAlexis Antoinette Ann Delgado0Alan Humason1Robert Kalescky2Marek Freindorf3Elfi Kraka4Computational and Theoretical Chemistry Group, Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275-0314, USAComputational and Theoretical Chemistry Group, Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275-0314, USAComputational and Theoretical Chemistry Group, Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275-0314, USAComputational and Theoretical Chemistry Group, Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275-0314, USAComputational and Theoretical Chemistry Group, Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275-0314, USAFor decades one has strived to synthesize a compound with the longest covalent C−C bond applying predominantly steric hindrance and/or strain to achieve this goal. On the other hand electronic effects have been added to the repertoire, such as realized in the electron deficient ethane radical cation in its D<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>3</mn><mi>d</mi></mrow></msub></semantics></math></inline-formula> form. Recently, negative hyperconjugation effects occurring in diamino-o-carborane analogs such as di-N,N-dimethylamino-o-carborane have been held responsible for their long C−C bonds. In this work we systematically analyzed CC bonding in a diverse set of 53 molecules including clamped bonds, highly sterically strained complexes such as diamondoid dimers, electron deficient species, and di-N,N-dimethylamino-o-carborane to cover the whole spectrum of possibilities for elongating a covalent C−C bond to the limit. As a quantitative intrinsic bond strength measure, we utilized local vibrational CC stretching force constants <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mi>k</mi><mi>a</mi></msup></semantics></math></inline-formula>(CC) and related bond strength orders BSO <i>n</i>(CC), computed at the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>ω</mi></semantics></math></inline-formula>B97X-D/aug-cc-pVTZ level of theory. Our systematic study quantifies for the first time that whereas steric hindrance and/or strain definitely elongate a C−C bond, electronic effects can lead to even longer and weaker C−C bonds. Within our set of molecules the electron deficient ethane radical cation, in D<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>3</mn><mi>d</mi></mrow></msub></semantics></math></inline-formula> symmetry, acquires the longest C−C bond with a length of 1.935 Å followed by di-N,N-dimethylamino-o-carborane with a bond length of 1.930 Å. However, the C−C bond in di-N,N-dimethylamino-o-carborane is the weakest with a BSO <i>n</i> value of 0.209 compared to 0.286 for the ethane radical cation; another example that the longer bond is not always the weaker bond. Based on our findings we provide new guidelines for the general characterization of CC bonds based on local vibrational CC stretching force constants and for future design of compounds with long C−C bonds.https://www.mdpi.com/1420-3049/26/4/950longest CC bondsvibrational spectroscopylocal mode theorylocal mode force constantssteric versus electronic effects |
| spellingShingle | Alexis Antoinette Ann Delgado Alan Humason Robert Kalescky Marek Freindorf Elfi Kraka Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study Molecules longest CC bonds vibrational spectroscopy local mode theory local mode force constants steric versus electronic effects |
| title | Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study |
| title_full | Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study |
| title_fullStr | Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study |
| title_full_unstemmed | Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study |
| title_short | Exceptionally Long Covalent CC Bonds—A Local Vibrational Mode Study |
| title_sort | exceptionally long covalent cc bonds a local vibrational mode study |
| topic | longest CC bonds vibrational spectroscopy local mode theory local mode force constants steric versus electronic effects |
| url | https://www.mdpi.com/1420-3049/26/4/950 |
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