Protonated silanoic acid HSi(OH)2+ and its neutral counterpart: a tandem mass spectrometric and CBS-QB3 computational study

✍ Scribed by R. Srikanth; K. Bhanuprakash; R. Srinivas; Cathy Y. Wong; Johan K. Terlouw

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 310 KB
• Volume: 39
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.583

✦ Synopsis

Protonated silanoic acid, HSi.OH/ 2 + , 1a + , is cleanly generated by the dissociative electron ionization of triethoxysilane, HSi.OC 2 H 5 / 3 , and tetraethoxysilane, Si.OC 2 H 5 / 4 . This follows from tandem mass spectrometric experiments and CBS-QB3 model chemistry calculations. The calculations predict that 1a + (1H f .298 K/ = 205 kJ mol -1 / is separated by high barriers from its isomers HOSiOH 2 + , 1b + and HSi. O/OH 2 + , 1c + . Low-energy (metastable) ions 1a + dissociate by loss of H 2 O via the pathway 1a + → 1b + → SiOH + + H 2 O. Analysis of the metastable peak for this process confirms that the isomerization step 1a + → 1b + is rate determining. The calculations further predict that the incipient ions 1b + communicate via a low barrier with the proton-bound dimer Si O• • •H• • •OH 2 + , 1d + . This dimer ion is much lower in energy than its counterpart O Si• • •H• • •OH 2 + , 1e + , which is calculated to be only marginally stable. A comparison of the potential energy diagram for the silicon-containing ions 1a + -1e + with that of their carbon analogues reveals that the dissociation chemistries of HSi.OH/ 2 + and HC.OH/ 2 + are only superficially similar. Neutralization-reionization experiments confirm the theoretical prediction that the HSi.OH/ 2 ž radical (1H f .298 K/ = -455 kJ mol -1 / is a stable species in the rarefied gas phase. However, owing to a mismatch of Franck-Condon factors a large fraction of the neutralized ions dissociates by loss of H ž yielding Si.OH/ 2 .