Elimination of Ethylene from Metastable Isomeric Silylenium Ions in the Gas Phase:  Experiment and Theory

Abstract

The slow unimolecular dissociations (k ≈ 105 s-1) of gas-phase silylenium ions, SiC n H2 n +3 + ions (n = 2−4) have been studied by mass-analyzed ion kinetic energy spectroscopy. On the microsecond time scale, the SiC2H7 + isomers undergo dissociations corresponding to H2 (72%) and C2H4 loss (28%). The product ion translational energy distributions and product ratios are the same for HSi(CH3)2 + and H2Si(C2H5)+, indicating that these isomers equilibrate prior to dissociation. For the SiC3H9 + isomers, elimination of C2H4 is the dominant reaction pathway, comprising 97% of the products for Si(CH3)3 + and 89% for HSi(CH3)(C2H5)+. The translational energy distributions for elimination of ethylene from these two ions are different, indicating that equilibration of Si(CH3)3 + and HSi(CH3)(C2H5)+ does not occur prior to dissociation. The mechanisms for C2H4 loss from the Si(CH3)3 + and HSi(CH3)(C2H5)+ ions were characterized by ab initio computational methods, and the results were used for statistical phase space modeling of the experimental translational energy distributions. Excellent agreement between theory and experiment was obtained for ethylene loss from the SiC2H7 + isomers and from HSi(CH3)(C2H5)+. The calculated distribution was broader than the experimental distribution for Si(CH3)3 +. Possible reasons for this result are discussed. The microsecond unimolecular dissociation of Si(CH3)2(C2H5)+ proceeds exclusively by elimination of C2H4, which arises from the ethyl group and not from the two methyl groups.

Metastable and collision-induced fragmentation studies and thermochemistry of isomeric C4H11Si+ ions and their adducts with C4H12Si silanes

“…For example, [(Me) 3 Si] + and [(Me) 2 SiH] + lose C 2 H 4 after isomerization into [EtMeSiH] + and [EtSiH 2 ] + , respectively 10. 16 By analogy, we suggest that similar isomerization reactions might occur for the isomeric [C 4 H 11 Si] + ions examined here, in particular for those isomers which cannot lose C 2 H 4 directly ([ i ‐PrMeSiH] + , [ i ‐BuSiH 2 ] + and [ t ‐BuSiH 2 ] + ). The sums of the heats of formation of the various possible products of [C 4 H 11 Si] + dissociations are as follows: …”

“…[(Et) 2 SiH] + could also be obtained by collisions of [SiH 3 ] + with C 2 H 4 at a relatively high pressure 15. Most recently, Willard and Graul16 examined the kinetic energy release distributions (KERDs) for the unimolecular dissociations of isomeric [C 2 H 7 Si] + and [C 3 H 9 Si] + ions. In addition, they carried out ab initio calculations to model these dissociations.…”

Rearrangement and decomposition of (CH3)3M+ (M=Si, Ge, Sn) ions: A DFT study

“…The existence of side-on complexes, which was demonstrated for the SiH + 3 cation [2], was not taken into account in the analysis of the potential energy surfaces of silylium ions containing alkyl groups [10][11][12][13][14] before our previous communication [15] in which we found energy minima corresponding to methane and ethane complexes at the potential energy surface of H 5 CSi + and H 7 C 2 Si + systems. These complexes may be derived from the side-on complexes found by Kapp et al [2] in H 3 Si + and H 3 Ge + cations by the methyl substitution of the dihydrogen moiety atoms.…”

“…Earlier, in the studies in which the ethane complex was not located [11][12][13] it was proposed that the transition state with symmetric displacement of two hydrogens from silicon to the ethylene moiety (TS4 in Fig. 1) connects directly the ethylene complex 4 with the most stable tertiary isomer 1.…”

“…To the best of our knowledge there were no attempts to analyze the mechanisms of rearrangements and fragmentation for germylium or stannylium cations, although there exists numerous experimental [3][4][5][6][7][8][9]13] and theoretical [4,[10][11][12][13][14] studies of similar processes for silylium ions. However in these studies no attention was paid to the possibility of formation of the ''side-on" structures, which in the case of cations containing methyl groups manifest themselves as complexes with alkanes.…”