Infrared Spectrum of the Si₃H₈⁺ Cation: Evidence for a Bridged Isomer with an Asymmetric Three‐Center Two‐Electron SiHSi Bond

Abstract

The IR spectrum of Si3H8(+) ions produced in a supersonic plasma molecular beam expansion of SiH4, He, and Ar is inferred from photodissociation of cold Si3H8(+)-Ar complexes. Vibrational analysis of the spectrum is consistent with a Si3H8(+) structure (2(+)) obtained by a barrierless addition reaction of SiH4 to the disilene ion (H2Si=SiH2(+)) in the silane plasma. In this structure, one of the electronegative H atoms of SiH4 donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric Si-H-Si bridge of the 2(+) isomer with a bond energy of approximately 60 kJ mol(-1) 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(-1). The observed 2(+) isomer is calculated to be only a few kJ mol(-1) less stable than the global minimum structure of Si3H8(+) (1(+)), which is derived from vertical ionization of trisilane. Although more stable, 1(+) is not detected in the measured IR spectrum of Si3H8(+)-Ar, and its lower abundance in the supersonic plasma is rationalized by the production mechanism of Si3H8(+) in the silane plasma, in which a high barrier between 2(+) and 1(+) prevents the efficient formation of 1(+). The potential energy surface of Si3H8(+) 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 Si3H8(+) isomers and their fragments. The weak intermolecular bonds of the Ar ligands in the Si3H8(+)-Ar isomers arise from dispersion and induction forces and induce only a minor perturbation of the bare Si3H8(+) ions. Comparison with the potential energy surface of C3H8(+) reveals the differences between the silicon and carbon species.

IR spectrum of the protonated neurotransmitter 2-phenylethylamine: dispersion and anharmonicity of the NH3+–π interaction

“…For this purpose, the octopole is filled with N 2 up to 10 À5 mbar, which results in collisions with B10 eV collision energy in the laboratory frame. Recent applications of this IRPD approach of tagged ions in our laboratory include hydrocarbon ions, [48][49][50][51][52][53][54][55][56][57] silicon-containing ions, [58][59][60] and biological molecules and their hydrates. [61][62][63][64][65][66] DFT calculations 67 are carried out at the B3LYP-D3/aug-cc-pVTZ level to determine the geometric, vibrational, and energetic properties of the H + PEA conformers and their H + PEA-Rg n clusters.…”

Vibrational spectra and structures of bare and Xe-tagged cationic SinOm+ clusters

“…Hydrogen termination is an effective means of passivation and stabilization of silicon clusters. [12][13][14][15][16][17] There is also significant interest in SiO 2 matrices with embedded nanocrystalline silicon. [18][19][20] For small isolated Si n clusters, dissociative adsorption of dioxygen molecules has a significant impact on their geometry and electronic structure.…”

Identification of higher order silanes during monosilane pyrolysis using gas chromatography-mass spectrometry

“…George et al [69] identified a hydrogen bridged trisilane cation structure H 3 SiAHASiH 2 ASiH 3 + whose energy is only a few kJ/mol higher than the classical trisilane cation SiH 3 ASiH 2 ASiH 3 + . The barrier between the two isomers is only 0.8 eV [69].…”

“…George et al [69] identified a hydrogen bridged trisilane cation structure H 3 SiAHASiH 2 ASiH 3 + whose energy is only a few kJ/mol higher than the classical trisilane cation SiH 3 ASiH 2 ASiH 3 + . The barrier between the two isomers is only 0.8 eV [69]. Tarczay and co-authors [55] have therefore recently suggested that the fragmentation of silanes during vacuum ultra-violet (VUV) photoionization goes via the hydrogen bridged structure.…”