Ginny Karir, Enrique Mendez-Vega, Adrian Portela-Gonzalez, Mayank Saraswat, Wolfram Sander and Patrick Hemberger
{"title":"The elusive phenylethynyl radical and its cation: synthesis, electronic structure, and reactivity†","authors":"Ginny Karir, Enrique Mendez-Vega, Adrian Portela-Gonzalez, Mayank Saraswat, Wolfram Sander and Patrick Hemberger","doi":"10.1039/D4CP02129K","DOIUrl":null,"url":null,"abstract":"<p >Alkynyl radicals and cations are crucial reactive intermediates in chemistry, but often evade direct detection. Herein, we report the direct observation of the phenylethynyl radical (C<small><sub>6</sub></small>H<small><sub>5</sub></small>C<img>C˙) and its cation (C<small><sub>6</sub></small>H<small><sub>5</sub></small>C<img>C<small><sup>+</sup></small>), which are two of the most reactive intermediates in organic chemistry. The radical is generated <em>via</em> pyrolysis of (bromoethynyl)benzene at temperatures above 1500 K and is characterized by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). Photoionization of the phenylethynyl radical yields the phenylethynyl cation, which has never been synthesized due to its extreme electrophilicity. Vibrationally-resolved ms-TPES assisted by <em>ab initio</em> calculations unveiled the complex electronic structure of the phenylethynyl cation, which appears at an adiabatic ionization energy (AIE) of 8.90 ± 0.05 eV and exhibits an uncommon triplet (<small><sup>3</sup></small>B<small><sub>1</sub></small>) ground state, while the closed-shell singlet (<small><sup>1</sup></small>A<small><sub>1</sub></small>) state lies just 2.8 kcal mol<small><sup>−1</sup></small> (0.12 eV) higher in energy. The reactive phenylethynyl radical abstracts hydrogen to form ethynylbenzene (C<small><sub>6</sub></small>H<small><sub>5</sub></small>C<img>CH) but also isomerizes <em>via</em> H-shift to the <em>o</em>-, <em>m</em>-, and <em>p</em>-ethynylphenyl isomers (C<small><sub>6</sub></small>H<small><sub>4</sub></small>C<img>CH). These radicals are very reactive and undergo ring-opening followed by H-loss to form a mixture of C<small><sub>8</sub></small>H<small><sub>4</sub></small> triynes, along with low yields of cyclic 3- and 4-ethynylbenzynes (C<small><sub>6</sub></small>H<small><sub>3</sub></small>C<img>CH). At higher temperatures, dehydrogenation from the unbranched C<small><sub>8</sub></small>H<small><sub>4</sub></small> triynes forms the linear tetraacetylene (C<small><sub>8</sub></small>H<small><sub>2</sub></small>), an astrochemically relevant polyyne.</p>","PeriodicalId":99,"journal":{"name":"Physical Chemistry Chemical Physics","volume":null,"pages":null},"PeriodicalIF":2.9000,"publicationDate":"2024-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://pubs.rsc.org/en/content/articlepdf/2024/cp/d4cp02129k?page=search","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Physical Chemistry Chemical Physics","FirstCategoryId":"92","ListUrlMain":"https://pubs.rsc.org/en/content/articlelanding/2024/cp/d4cp02129k","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
Alkynyl radicals and cations are crucial reactive intermediates in chemistry, but often evade direct detection. Herein, we report the direct observation of the phenylethynyl radical (C6H5CC˙) and its cation (C6H5CC+), which are two of the most reactive intermediates in organic chemistry. The radical is generated via pyrolysis of (bromoethynyl)benzene at temperatures above 1500 K and is characterized by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). Photoionization of the phenylethynyl radical yields the phenylethynyl cation, which has never been synthesized due to its extreme electrophilicity. Vibrationally-resolved ms-TPES assisted by ab initio calculations unveiled the complex electronic structure of the phenylethynyl cation, which appears at an adiabatic ionization energy (AIE) of 8.90 ± 0.05 eV and exhibits an uncommon triplet (3B1) ground state, while the closed-shell singlet (1A1) state lies just 2.8 kcal mol−1 (0.12 eV) higher in energy. The reactive phenylethynyl radical abstracts hydrogen to form ethynylbenzene (C6H5CCH) but also isomerizes via H-shift to the o-, m-, and p-ethynylphenyl isomers (C6H4CCH). These radicals are very reactive and undergo ring-opening followed by H-loss to form a mixture of C8H4 triynes, along with low yields of cyclic 3- and 4-ethynylbenzynes (C6H3CCH). At higher temperatures, dehydrogenation from the unbranched C8H4 triynes forms the linear tetraacetylene (C8H2), an astrochemically relevant polyyne.
期刊介绍:
Physical Chemistry Chemical Physics (PCCP) is an international journal co-owned by 19 physical chemistry and physics societies from around the world. This journal publishes original, cutting-edge research in physical chemistry, chemical physics and biophysical chemistry. To be suitable for publication in PCCP, articles must include significant innovation and/or insight into physical chemistry; this is the most important criterion that reviewers and Editors will judge against when evaluating submissions.
The journal has a broad scope and welcomes contributions spanning experiment, theory, computation and data science. Topical coverage includes spectroscopy, dynamics, kinetics, statistical mechanics, thermodynamics, electrochemistry, catalysis, surface science, quantum mechanics, quantum computing and machine learning. Interdisciplinary research areas such as polymers and soft matter, materials, nanoscience, energy, surfaces/interfaces, and biophysical chemistry are welcomed if they demonstrate significant innovation and/or insight into physical chemistry. Joined experimental/theoretical studies are particularly appreciated when complementary and based on up-to-date approaches.