Tautomerism of monochalcogenosilanoic acids CH₃Si(O)XH (X = S, Se, Te) in the gas phase and in the polar and aprotic solution: An ab initio computational investigation

Abstract

ABSTRACT:Computational investigations by an ab initio molecular orbital method (HF and MP2) with the 6-311ϩG(d,p) and 6-311ϩϩG(2df, 2pd) basis sets on the tautomerism of three monochalcogenosilanoic acids CH 3 Si(AO)XH (X ϭ S, Se, and Te) in the gas phase and a polar and aprotic solution tetrahydrofuran (THF) was undertaken. Calculated results show that the silanol forms CH 3 Si(AX)OH are much more stable than the silanone forms CH 3 Si(AO)XH in the gas-phase, which is different from the monochalcogenocarboxylic acids, where the keto forms CH 3 C(AO)XH are dominant. This situation may be attributed to the fact that the SiOO and OOH single bonds in the silanol forms are stronger than the SiOX and XOH single bonds in the silanone forms, respectively, even though the SiAX (X ϭ S, Se, and Te) double bonds are much weaker than the SiAO double bond. These results indicate that the stability of the monochalcogenosilanoic acid tautomers is not determined by the double bond energies, contrary to the earlier explanation based on the incorrect assumption that the SiAS double bond is stronger than the SAO double bond for the tautomeric equilibrium of RSi(AO)SH (R ϭ H, F, Cl, CH 3 , OH, NH 2 ) to shift towards the thione forms [RSi(AS)OH]. The binding with CH 3 OCH 3 enhances the preference of the silanol form in the tautomeric equilibrium, and meanwhile significantly lowers the tautomeric barriers by more than 34 kJ/mol in THF solution.

Treatment of Silylene–Phosphinidene with Chalcogens Resulted Exclusively in the Formation of Silicon‐Bonded Chalcogens

“…Hence the formation of the P−E bond is less favorable than the formation of a Si−E bond. Note that our calculated reaction energies are comparable to the previously reported bond‐dissociation energies for Si=S (112.8), Si=Se (95.5), and Si=Te (77.2 kcal mol −1 ) bonds in CH 3 Si(=E)OH compounds (E=S, Se, Te) at the MP2/6–311++G(2df, 2pd)//MP2/6–311+G(d, p) level of theory . The Si−S bond is the strongest among Si−E bonds in 1 – 3 , which is in accordance with the experimental observation that the regeneration of parent molecule A could not be achieved when 1 is treated with the L′Al (L′=HC{(CMe)(2,6‐ i Pr 2 C 6 H 3 N)} 2 ) (Scheme ).…”

Ein Bis(silaselenon) mit zwei donorstabilisierten SiSe‐Bindungen, erhalten durch eine unerwartet stereokonvergente Hydrolyse eines Diselenadisiletans

“…Ein zweiter Hydrolyseschritt liefert den monomeren Silaheterocyclus 9 mit einer OH‐ und einer SeH‐Gruppe. Hierauf folgt eine 1,3‐H‐Wanderung vom Selen‐ zu einem Stickstoffatom unter Bildung der Selensilacarbaminsäure 10 7. Zum Schluss wird durch Kondensation zweier Moleküle 10 das sauerstoffverbrückte 7 gebildet.…”

A Bis(silaselenone) with Two Donor‐Stabilized SiSe Bonds from an Unexpected Stereoconvergent Hydrolysis of a Diselenadisiletane

“…The hydrolysis of both the racemic and the meso forms of 6 involves an initial ring opening of the central Si/Se heterocycle by protonation of the selenium atom and nucleophilic attack at the second silicon atom to give compound 8 . A second hydrolysis step affords the monomeric OH/SeH‐ligated silicon‐containing heterocycle 9 , followed by 1,3‐hydrogen transfer from the selenium atom to one nitrogen atom to give the selenosilacarbamic acid 10 7. In the final step, condensation of two molecules of 10 affords the oxygen‐bridged compound 7 .…”