Infrared spectrum of the Si3H8+ cation: evidence for a bridged isomer with an asymmetric three-center two-electron Si-H-Si bond.
Infrared spectrum of the Si3H8+ cation: evidence for a bridged isomer with an asymmetric three-center two-electron Si-H-Si bond.
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Si3H8 阳离子的红外光谱：具有不对称三中心二电子 Si-H-Si 键的桥联异构体的证据
作者：
M.A.R. George;M. Savoca;O. Dopfer
Si3H8+离子在SiH4、He和Ar的超声速等离子体分子束膨胀过程中产生的红外光谱是由Si3H8+ -Ar配合物的光解作用推断出来的。光谱的振动分析与硅烷等离子体中sih4与二乙烯离子（H2Si <s:1> SiH2+）无阻碍加成反应得到的Si3H8+结构（2+）一致。在这种结构中，sih4的一个电负性H原子将电子密度提供给部分填满的亲电性二烯阳离子的π轨道。2+异构体的不对称Si - H - Si桥键键能约为60 kJ mol - 1，具有弱三中心二电子键的特征，其不对称Si - H - Si桥键在1765 cm - 1左右具有强红外活性。计算结果表明，d2+同分异构体的稳定性仅比Si3H8+(1+)的全局最小结构低几kJ mol−1，Si3H8+(1+)是由三硅烷垂直电离得到的。Si3H8+ -Ar虽然更稳定，但在测量的红外光谱中没有检测到1+，并且Si3H8+在超音速等离子体中的低丰度是由Si3H8+在硅烷等离子体中的产生机制合理的，其中2+和1+之间的高屏障阻止了1+的有效形成。用量子化学方法对Si3H8+的势能面进行了较为详细的表征。研究了各种低能Si3H8+异构体及其碎片的结构、振动、电子和能量性质以及化学键机制。Si3H8+ -Ar同分异构体中Ar配体的弱分子间键是由分散力和诱导力产生的，仅引起裸Si3H8+离子的轻微扰动。与C3H8+的势能面比较，揭示了硅和碳两种物质的差异。
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 SiH4to the disilene ion (H2SiSiH2+) in the silane plasma. In this structure, one of the electronegative H atoms of SiH4donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric SiHSi bridge of the2+isomer with a bond energy of approximately 60 kJ mol−1is characteristic for a weak three‐center two‐electron bond, which is identified by its strongly IR active asymmetric SiHSi stretching fundamental at about 1765 cm−1. The observed2+isomer is calculated to be only a few kJ mol−1less 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 between2+and1+prevents the efficient formation of1+. 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.
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