Infrared Spectrum of the Si3H8+ Cation: Evidence for a Bridged Isomer with an Asymmetric Three‐Center Two‐Electron SiHSi Bond. Issue 45 (17th September 2013)

• Record Type: Journal Article
• Authors: George, Martin Andreas Robert; Savoca, Marco; Dopfer, Otto
• Is Part Of: Chemistry. Volume 19:Issue 45(2013)
• Page Start: 15315
• Page End: 15328
• Publication Date: 2013-09-17
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201302189
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The IR spectrum of Si<sub>3</sub>H<sub>8</sub><sup>+</sup> ions produced in a supersonic plasma molecular beam expansion of SiH<sub>4</sub>, He, and Ar is inferred from photodissociation of cold Si<sub>3</sub>H<sub>8</sub><sup>+</sup>–Ar complexes. Vibrational analysis of the spectrum is consistent with a Si<sub>3</sub>H<sub>8</sub><sup>+</sup> structure (<bold>2<sup>+</sup></bold>) obtained by a barrierless addition reaction of SiH<sub>4</sub> to the disilene ion (H<sub>2</sub>SiSiH<sub>2</sub><sup>+</sup>) in the silane plasma. In this structure, one of the electronegative H atoms of SiH<sub>4</sub> donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric SiHSi bridge of the <bold>2<sup>+</sup></bold> isomer with a bond energy of approximately 60 kJ mol<sup>−1</sup> is characteristic for a weak three‐center two‐electron bond, which is identified by its strongly IR active asymmetric SiHSi stretching fundamental at about 1765 cm<sup>−1</sup>. The observed <bold>2<sup>+</sup></bold> isomer is calculated to be only a few kJ mol<sup>−1</sup> less stable than the global minimum structure of Si<sub>3</sub>H<sub>8</sub><sup>+</sup> (<bold>1<sup>+</sup></bold>), which is derived from vertical ionization of trisilane. Although more stable, <bold>1<sup>+</sup></bold> is not detected in the measured IR spectrum of Si<sub>3</sub>H<sub>8</sub><sup>+</sup>–Ar, and its lower abundance in the supersonic plasma is rationalized by the production mechanism of Si<sub>3</sub>H<sub>8</sub><sup>+</sup> in the silane plasma, in which a high barrier between <bold>2<sup>+</sup></bold> and <bold>1<sup>+</sup></bold> prevents the efficient formation of <bold>1<sup>+</sup></bold>. The potential energy surface of Si<sub>3</sub>H<sub>8</sub><sup>+</sup> is characterized in some detail by quantum chemical calculations. The structural, vibrational, electronic and energetic properties as well as the chemical bonding mechanism are investigated for a variety of low‐energy Si<sub>3</sub>H<sub>8</sub><sup>+</sup> isomers and their fragments. The weak intermolecular bonds of the Ar ligands in the Si<sub>3</sub>H<sub>8</sub><sup>+</sup>–Ar isomers arise from dispersion and induction forces and induce only a minor perturbation of the bare Si<sub>3</sub>H<sub>8</sub><sup>+</sup> ions. Comparison with the potential energy surface of C<sub>3</sub>H<sub>8</sub><sup>+</sup> reveals the differences between the silicon and carbon species.</p> </abstract>