{"title":"Aromaticity of tropylium derivatives: When and why might captodative structures be preferred over the isomeric push-pull structures?","authors":"Bagrat A. Shainyan","doi":"10.1002/qua.27449","DOIUrl":null,"url":null,"abstract":"<p>An intriguing question in the general problem of aromaticity is whether captodative aromatic systems with the donor and acceptor substituents at the same carbon of the CC bond can be more stable than the π-conjugated push-pull counterparts? The analysis of electronic, magnetic, and structural criteria of aromaticity showed that for conventional organic substituents XO, TfN, (NC)<sub>2</sub>C, (NO<sub>2</sub>)<sub>2</sub>C, Tf<sub>2</sub>C, the push-pull tropylidene derivatives [tropylium]<sup>+</sup><span></span>CHCH<span></span>X<sup>−</sup> are expectedly more stable than their captodative isomers [tropylium]<sup>+</sup>C(X<sup>−</sup>)CH<sub>2</sub>, with the lowest Δ<i>E</i> for the most strong acceptor Tf<sub>2</sub>C. A different behavior is observed for XMHlg<sub>3</sub> (MB, Al; HlgF, Cl). They are not only structurally and magnetically most aromatic in both series but show the inverse stability of the push-pull and captodative isomers, the latter being more stable by up to 10 kcal/mol (in gas).The difference between the MHlg<sub>3</sub> groups and conventional organic groups is that in the latter the electron density is transferred to the π-system of the substituent, while the former can accept it only to the σ*(C<span></span>M) orbital. Thus, when the electron donor and acceptor effects are separated between the σ and π systems, captodative isomers can be more stable than their push-pull isomers with more extended conjugation.</p>","PeriodicalId":182,"journal":{"name":"International Journal of Quantum Chemistry","volume":null,"pages":null},"PeriodicalIF":2.3000,"publicationDate":"2024-07-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"International Journal of Quantum Chemistry","FirstCategoryId":"92","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1002/qua.27449","RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
Abstract
An intriguing question in the general problem of aromaticity is whether captodative aromatic systems with the donor and acceptor substituents at the same carbon of the CC bond can be more stable than the π-conjugated push-pull counterparts? The analysis of electronic, magnetic, and structural criteria of aromaticity showed that for conventional organic substituents XO, TfN, (NC)2C, (NO2)2C, Tf2C, the push-pull tropylidene derivatives [tropylium]+CHCHX− are expectedly more stable than their captodative isomers [tropylium]+C(X−)CH2, with the lowest ΔE for the most strong acceptor Tf2C. A different behavior is observed for XMHlg3 (MB, Al; HlgF, Cl). They are not only structurally and magnetically most aromatic in both series but show the inverse stability of the push-pull and captodative isomers, the latter being more stable by up to 10 kcal/mol (in gas).The difference between the MHlg3 groups and conventional organic groups is that in the latter the electron density is transferred to the π-system of the substituent, while the former can accept it only to the σ*(CM) orbital. Thus, when the electron donor and acceptor effects are separated between the σ and π systems, captodative isomers can be more stable than their push-pull isomers with more extended conjugation.
期刊介绍:
Since its first formulation quantum chemistry has provided the conceptual and terminological framework necessary to understand atoms, molecules and the condensed matter. Over the past decades synergistic advances in the methodological developments, software and hardware have transformed quantum chemistry in a truly interdisciplinary science that has expanded beyond its traditional core of molecular sciences to fields as diverse as chemistry and catalysis, biophysics, nanotechnology and material science.