Isomeric C5H11Si+ Ions from the Trimethylsilylation of Acetylene: an Experimental and Theoretical Study

The gas phase offers a unique medium to conduct the electrophilic addition reaction of (CH3)3Si+ (trimethylsilylium ion) to acetylene. However, this deceptively simple reaction displays a remarkable dependence on the gas phase pressure, revealing the interplay of competitive pathways. In FT-ICR mass spectrometry at ca 10-8 mbar, the nascent (CH3)3Si+-acetylene complex undergoes a rearrangement process yielding the CH2=C(CH3)-Si(CH3)2+ ion. This structure has been assigned on the basis of the ion-molecule reactivity displayed by the sampled C5H11Si+ adducts, matching the one of the model ion obtained from 2-(trimethylsilyl)propene. Whereas the absolute values of kinetic rate constants could not discriminate between isomeric species, the branching ratios for competitive addition-elimination channels in the reaction with i-C3H7OH and t-C4H9OCH3 were found to be diagnostic of different structures. The pathways leading from the (CH3)3Si+-acetylene complex primarily formed to the candidate C5H11Si+ isomers have been investigated by ab initio quantum chemical calculations at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-31G(d,p) level. The energy profiles show that the path to the CH2=C(CH3)-Si(CH3)2+ isomer is associated to the lowest activation energy barrier, below the reactants energy level. The energy released in the (CH3)3Si+-acetylene association process, remaining stored in the complex formed at low pressure, thus allows the isomerization to a species holding the positive charge on electropositive silicon. Interestingly, the most stable of the conceivable isomers, (E)-(CH3)-CH=CH-Si(CH3)2+, is not accessed because of an activation energy barrier protruding above the reactants energy level. The combined information of ion-molecule reactivity and ab initio calculations of potential isomers and rearrangement pathways has thus afforded a comprehensive view of the (CH3)3Si+ addition reaction to acetylene under various pressure regimes.