Isomeric C5H11Si+ ions from the trimethylsilylation of acetylene: An experimental and theoretical study

Article abstract of DOI:10.1016/j.ijms.2012.10.007

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 CH₂C(CH₃)Si(CH ₃)₂⁺ ion. This structure has been assigned on the basis of the ion-molecule reactivity displayed by the sampled C ₅H₁₁Si⁺ 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-C₄H₉OCH ₃ 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/6-311G(2d,p) level. The energy profiles show that the path to the CH₂C(CH₃)Si(CH₃) ₂⁺ isomer is associated to the lowest activation energy barrier, below the reactants energy level. The energy released in the (CH ₃)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)CHCHSi(CH ₃)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.

Full text of DOI:10.1016/j.ijms.2012.10.007

International Journal of Mass Spectrometry 334 (2013) 58–66
Contents lists available at SciVerse ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
Isomeric C5H11Si+ ions from the trimethylsilylation of acetylene: An
experimental and theoretical study
Hans-Ullrich Siehla,∗, Sandra Brixnera, Cecilia Colettib, Nazzareno Reb, Barbara Chiavarinoc,
Maria Elisa Crestonic, Alberto De Petrisc, Simonetta Fornarinic,∗∗
a Abteilung Organische Chemie I, Universität Ulm, Albert Einstein Allee 11, D-89069 Ulm, Germany
b Dipartimento di Scienze del Farmaco, Università G. D’Annunzio, Via dei Vestini 31, I-66100 Chieti, Italy
c Dipartimento di Chimica e Tecnologie del Farmaco, Università degli Studi di Roma La Sapienza, P.le A. Moro 5, I-00185 Roma, Italy
a r t i c l e i n f o
a b s t r a c t
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 remark-
able 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
Article history:
Received 8 October 2012
Accepted 8 October 2012
Available online 23 October 2012
+
rearrangement process yielding the CH2 C(CH3) Si(CH3)2 ion. This structure has been assigned on
Dedicated to the memory of
Detlef Schröder.
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 diag-
nostic 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 calcu-
lations at CCSD(T)/6-311 ++G(2d,2p)//B3LYP/6-311G(2d,p) level. The energy profiles show that the path
Keywords:
Ion-molecule reactions
Silacations
Electrophilic silylation
Ion rearrangements
Pressure effects
+
to the CH2 C(CH3) Si(CH3)2 isomer is associated to the lowest activation energy barrier, below the
FT-ICR mass spectrometry
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.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
have been surpassed by reagents based on carborane anions [13].
Although the R3Si (carborane) species do not contain free silylium
The electrophilic silylation of hydrocarbons can in principle be
obtained by delivering a formal R3Si+ species on an unsaturated
compound. However, the neat process is not easily accomplished
in solution. Indeed, trivalent R3Si+ silylium ions are hardly accessi-
ble in condensed phases due to their high reactivity and enormous
electrophilicity, though being thermodynamically more stable than
equally substituted carbenium ions [1–12]. In recent years consid-
erable efforts have been devoted to the development of powerful
sources of “R3Si+” and conventional trialkylsilyl triflate reagents
ions they may react like silylium ions. Still, the silylation of a weak
base such as benzene has yet to be established in the condensed
phase [13–15]. An ideal environment where R3Si+ ions can be easily
generated and studied is the gas phase [16,17] In this medium the
formation of trimethylsilylbenzene from the reaction of (CH3)3Si+
with benzene and neutralization by a strong nitrogen base was
obtained [18–22]. In this gas phase work it was recognized for
the first time that the formation of the intermediate silylated ␴-
complex was promptly achieved but the critical step to obtain
neutral phenylsilane was the deprotonation of the intermediate.
Following these gas phase studies demonstrating the key role of
deprotonation, the aromatic silylation of toluene by R3SiCl under
AlCl3 catalysis has been successfully accomplished in the presence
of hindered tertiary amines by Olah et al, although in only moderate
yield [23]. In the gas phase, not only benzene but also toluene and
∗
Corresponding author.
Corresponding author. Tel: +39 06 4991 3510; fax: +39 06 4991 3602.
E-mail addresses: ullrich.siehl@uni-ulm.de (H.-U. Siehl),
∗∗
simonetta.fornarini@uniroma1.it (S. Fornarini).
1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ijms.2012.10.007

H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
59
other aromatic and heteroaromatic compounds have been success-
fully silylated [24–28]. The silylium ions were formed in a gaseous
environment at atmospheric pressure using a radiolytic approach
[29]. Also a nucleogenic approach based on the ␤-decay of tritium
has been used to generate silylium ions in the gas phase and the
reaction with ␲-nucleophiles has been reported [30–33].
pressure, the adduct ions, [(CH3)3Si+–acetylene complex]*, primar-
ily excited by the exothermicity of the formation process (step (a) in
Eq. (2)), undergo an efficient cooling by collisions with unreactive
molecules (M) of the bath gas (step (b) in Eq. (2)), thus allowing
the formation of thermalyzed adduct ions, [(CH3)3Si+–acetylene
complex], that can subsequently be deprotonated to yield the sub-
stitution product (CH3)3SiC CH (step (c) in Eq. (2)) [34].
(b)
+M
(c)
+B
-BH+
(a)
[(CH3)3Si+ - acetilene complex]
(CH3)3Si+ + HC CH
[(CH3)3Si+ - acetylene complex]*
(CH3)3SiC CH
(d)
-M*
(e)
rearrangement
C5H11Si+
(2)
In the gas phase at atmospheric pressure (CH3)3Si+ ions are
found to react with acetylene yielding a substitution product,
namely (CH3)3SiC CH [34]. However, mass spectrometric studies
on the reaction of (CH3)3Si+ with acetylene have yielded apparently
contrasting results. In high pressure mass spectrometry (HPMS)
operating at 3–5 mbar, Me3Si+ appears to be unreactive with acety-
lene (i.e. it does not yield the expected adduct ion) and the
ion is ultimately captured by adventitious water [35], a potent
nucleophile toward silylium ions. Quite in contrast, operating at
10−8–10−7 mbar in the cell of an FT-ICR mass spectrometer, the
formation of an adduct ion at m/z 99 from the reaction of (CH3)3Si+
with acetylene has been reported to occur [36], albeit with low effi-
ciency (0.013, as measured by the ratio kexp/kcoll, where kexp is the
bimolecular rate constant for reaction (1) and kcoll is the calculated
collision rate constant) [37].
In the 3–5 mbar environment of the HPMS experiments the
[(CH3)3Si+–acetylene complex] is apparently a fleeting species,
undergoing fast desilylation by ubiquitous water. However, it is
only at the 10−8–10−7 mbar pressure in the FT-ICR cell that the
excess energy released in the adduct ion may not be dissipated
(unless by radiative emission, a poorly efficient process in a small
molecule) [58–60] and remains available to surmount the acti-
vation energy barriers leading to rearranged ions C5H11Si+ (step
(e) in Eq. (2)). The latter species, that reasonably lie in a deeper
energy well, become resistant to dissociation under the prevailing
environmental conditions. Probing these suggested rearrangement
processes and uncovering the structure of the end product ions was
the aim of the present contribution. The problem has been tack-
led with a combined approach based on bimolecular ion-molecule
reactions with appropriate neutrals as diagnostic tools to test ion
structures and on ab initio calculations to assist in assigning plau-
sible rearrangement pathways. The study of this system (namely
(CH3)3Si+ + HC CH) lends itself to uncover important issues in the
reactivity of silyl-substituted carbenium ions and silacations such
as the thermodynamic drive to the most stable species on the
C5H11Si+ potential energy surface, the electronic factors affecting
their stability, the kinetic barriers involved in the rearrangement
processes, representing a number of fundamental questions about
the chemistry of these reactive intermediates.
(CH3)3Si+ + HC ≡ CH → C5H11Si+
(1)
m/z 99
The nascent ionic addition product from the reaction of (CH3)3Si+
with acetylene may be assigned the structure of an unrearranged
species with the features of a ␤-silyl-substituted vinyl cation (1A),
possibly acquiring a cyclic geometry (1B) [38–41].
(CH3)3Si
(CH3)3Si
+
HC=CH+
HC = CH
1A
1B
2. Experimental details
␤-silyl hyperconjugative stabilization of vinly cations is well doc-
umented. In solution ␤-silyl-substituted vinyl cation can be gener-
ated by protonation of silyl substituted allenes [42] and silyl sub-
stituted alkynes [43–46] under carefully controlled experimental
conditions in non-nucleophilic solvents and by addition of inicip-
ient silylium ions to alkynes in the presence of inert counterions
[47–49]. The determination of the X-ray structure of a vinyl cation
stabilized by two ␤-silyl substituents has also been achieved [50].
The ␤-silyl effect, stabilizing a positively charged carbon, exerts
a strong influence on gaseous carbenium ions [51–57] and the for-
mation of an intermediate resembling 1A or 1B is inferred from the
formation of the (CH3)3SiC CH substitution product in the exper-
iments at atmospheric pressure, run in the presence of a strong
base [34]. However, the structure of the adduct at m/z 99 formed in
the FT-ICR cell does not respond to any kind of behavior expected
from a species like 1A or 1B. For example, the reaction with water,
2.1. Materials
The hydrocarbons CH4, C2H2, and C3H8 were high-purity
(99.9 mol%) gases from Matheson Gas Products Inc. The other
chemicals used as reagents or as precursors of desired ions were
purchased from Sigma–Aldrich. The silylated alkenes that were
not commercially available were prepared from the Wurtz–Fittig
reaction of (CH3)3SiCl with the appropriate alkenyl bromide [61]
and were purified by preparative GLC using a 4-m long, 4-mm i.d.
stainless steel column, packed with 5% DC 200 methylsilicone on
Supelcoport. Their identity was verified by NMR spectroscopy and
mass spectrometry.
2.2. FT-ICR mass spectrometry
expected to yield (CH3)3SiOH2 by (CH3)3Si+ transfer to the oxy-
The experiments were performed with a Bruker Spectrospin
Apex TM 47e mass spectrometer equipped with a cylindrical
“infinity” cell within a 4.7 T superconducting magnet and with an
external ion source. In this source, the chemical ionization (CI) of a
neutral admitted through a probe for volatile compounds is effected
by the ions of a chosen CI reagent, ionized by a 200 eV electron
beam. The ions formed in the external source are led into the FT-ICR
cell by an ion guide that should not impart any excess translational
+
gen nucleophile, leads rather to (CH3)2SiOH+ with concomitant
loss of C3H6, an indication that a rearrangement process has taken
place. The same C3H6 neutral is lost in collision induced disso-
ciation (CID) as the prevailing fragmentation channel. A unifying
justification for the observed reactivity pattern has been put for-
ward, based on the different pressure range prevailing in the three
experiments, as illustrated in Eq. (2) [36]. At nearly atmospheric

60
H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
energy to the ions. The ions can be further allowed to equilibrate
at the room temperature of the FT-ICR cell by unreactive collisions
with argon, admitted in the cell at 10−5 mbar by a pulsed valve. After
a delay time the ions of interest were selected by ejecting unwanted
ions using rf sweeps and single shots. The selected ions were then
let to react with a neutral reagent leaked in the cell by a needle valve
at a constant pressure, typically in the range of 10−8–10−7 mbar.
The pressure readings were obtained by a cold cathode gauge
calibrated using the rate constant value of 1.1 × 10−9 cm3 s−1 for
the reference reaction CH4•+ + CH4 → CH3• + CH5+, and corrected
using individual response factors [62,63]. The kinetics of the ion-
molecule reactions were monitored by recording 10–20 averaged
scans for each mass spectrum in series of runs corresponding to
increasing reaction time. In these conditions the kinetics are pseudo
first order and the rate constants (kobs) were obtained from the
slope of the semilogarithmic decrease of reactant ion abundance
versus reaction time. The pseudo first order rate constants divided
by the substrate concentration yield the bimolecular rate constants
(kexp). Their values are having an estimated error ( 30%) largely
due to the uncertainty affecting the pressure reading of the neu-
tral. The reported kexp values are the average of at least three values
for kinetic runs at different neutral pressure. The reaction efficien-
cies (˚) were calculated as % ratio of kexp relative to the collision
rate constant (kc) calculated by the parameterized trajectory theory
[37]. The elemental composition of the product ions was verified
by m/z measurements at high resolution.
CH3
Si
H3C
H3C
CH3
CH3
H
H3C
Si
H
Si
+
+
H3C
CH3
H
+
H
CH2
H
H
H
3
2
1
CH3
CH3
Si
CH3
H3C
H3C
+
H
H
+
Si
H
+
Si
H3C
H3C
H
CH3
CH2
H3C
6
5
4
Chart 1.
were sought whose reactivity behavior, compared with that of
the sample ion S, can yield diagnostic information. To this end
a number of Me3Si-substituted propenes have been synthesized,
namely 2-(trimethylsilyl)propene, allyltrimethylsilane, (E)- and
(Z)-1-(trimethylsilyl)propene. The chemical ionization (CI) using
+
C3H8 as neutral precursor of reagent C3H7 ions forms abundant
C5H11Si+ product ions that are expected to retain the structure
of the parent neutrals. The ensuing species (labeled 2,3,4,6) are
depicted in Chart 1 together with the trimethylsilyl cation com-
plex with acetylene (1) and a ␣-silylvinyl cation (5) that may
be involved as transient intermediate. Their reactivity behavior
toward selected reagents is demonstrated to be quite distinct for
each one of them (with the possible exception of the (E)- and (Z)-
isomers of 1-propenyl-dimethylsilyl cation, i. e. 2 and 4), supporting
2.3. Computational details
The quantum chemical calculations were performed using the
Gaussian 09 suites of programs [64]. All structures were fully opti-
mized using standard methods (density functional theory (DFT)
hybrid methods with the B3LYP functional). The 6-311G(2d,p)
basis set was used for geometry optimizations and frequency cal-
culations. Analytical vibrational analyses at the same level were
performed to determine the zero-point vibrational energy (ZPE)
and to ascertain each stationary point as a minimum or first-order
saddle point. Corrections for ZPE (not scaled) are included in the
calculated energies. All transition states were found by using QST2
and QST3 methods and their correct connection with the relative
reagents and products was verified by means of intrinsic reaction
coordinate calculations [65,66]. Extra valence functions and polar-
ization functions as in the 6-311++G(2d,2p) basis set were used
to allow CCSD(T)/6-311++G(2d,2p) energy calculations of B3LYP/6-
311G(2d,p) optimized geometries.
+
the idea that the mild CI by C3H7 ions, while able to abstract a
methide anion [16,67], does not confer any excess energy to the
incipient silyl cation that could activate subsequent rearrangement
processes. The notable stability of the silicon containing cations, in
which the charge is located on a silicon atom, is the driving force of
the reaction, as found responsible for example for the facile cleav-
age of a Si C bond in radical cations [17,68]. The distinct behavior of
so-formed species 2,3,4,6 (and therefore their distinct structure) is
+
retained also when they are formed by the more fierce CI by CH5
ions and, conversely, when the ions are submitted to collisional
relaxation by a pulse of argon at 10−5 bar.
3.2. Ion-molecule reactions of C5H11Si+ ions
In order to further check the reliability of the obtained geome-
tries, MP2 optimizations have also been performed with the
same basis set on few exemplary structures. MP2/6-311G(2d,p)
geometries display very small differences, showing that B3LYP/6-
311G(2d,p) can give a good description of the geometrical features
of the investigated system. Thus, the relative energies, intercon-
version pathways and transition state structures that are finally
reported were evaluated at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-
311G(2d,p) level of theory.
CID experiments on ion S have suggested the formation of
a covalent Si C bond in an addition product that subsequently
evolves by loss of C3H6. However, CID is not structurally diag-
nostic for simple alkylsilyl cations [69] and any characterization
using this tool was not further attempted. Ion-molecule reac-
tions appear as more selective probes for structural elucidation.
In particular, one may rely on the well established notion that
cations in which the positive charge is borne by the silicon
atom are powerful electrophiles and tend to form strong covalent
bonds with various compounds containing electronegative atoms
including water, alcohols and amines [70]. Accordingly, poten-
tially diagnostic reagents have been sought in the ROR’ class of
compounds (R,R’ = H,H; i-C3H7,H; t-C4H9,CH3), though preliminary
experiments were run to explore also the reactivity toward alkyl
chlorides such as C3H5Cl and t-C4H9Cl. The reactions of both the
sample ion S and the model ions (2,3,4,6) have been studied by
allowing the ions formed in the external ions source to react with
the neutral present in the FT-ICR cell at a controlled pressure value
in the range of 10−8–10−7 mbar.
3. Results and discussion
3.1. Sample ion(s) and model ions
The ion (or, possibly, ion mixture) of interest is the adduct
at m/z 99 formed from the addition of (CH3)3Si+ (obtained by
chemical ionization of (CH3)4Si using CH4 gas) to acetylene in
the cell of an FT-ICR mass spectrometer, henceforth named sam-
ple ion(s) S. In order to obtain experimental information that
could allow a structural assignment to this species, model ions

H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
61
100
100
80
C5H11Si+
80
C5H11Si+
C2H7SiO+
m/z 135
m/z 93
60
40
20
0
60
I%
I%
C2H7SiO+
40
20
0
C5H13SiO+
0
1
2
3
4
Reaction time (s)
0
2
4
6
8
10
Reaction time (s)
Fig. 2. Time dependence of the relative ion abundances (I%) when selected model
ions 2 (ꢂ) at m/z 99 from CI(C3H8) of (Z)-1-(trimethylsilyl)propene, are allowed to
react with i-C3H7OH at 4.3 × 10−8 mbar. Primary product ions are formed at m/z 75
(ꢁ) and m/z 117 (×). Secondary product ions are at m/z 93 (ꢀ) and m/z 135 (*).
Fig. 1. Time dependence of the relative ion abundances (I%) when selected model
ions 3 (ꢀ) at m/z 99 from CI(C3H8) of allyltrimethylsilane, are allowed to react with
H2O at 1.5 × 10−8 mbar. Product ions are formed at m/z 75 (ꢁ).
Table 2
The reaction with water leads to a unique product ion of
C2H7SiO+ composition, likely corresponding to (CH3)2SiOH+ [Eq.
(3)].
Product ratios for the gas phase reaction of C5H11Si+ ions with ROR’ nucleophiles.
b
Precursor of reactant iona
ROR’
P1/P2
(Z)-1-(Trimethylsilyl)propene (2)
(E)-1-(Trimethylsilyl)propene (4)
2-(Trimethylsilyl)propene (6)
Allyltrimethylsilane (3)
S
i-C3H7OH
i-C3H7OH
i-C3H7OH
i-C3H7OH
i-C3H7OH
8.5 1.7c
11 1.8c
20 5c
C5H11Si+ + H2O → C2H7SiO+ + [C3H6]
(3)
4.3 0.5c
21 2c
An exemplary plot of the time dependence of ion abundances is
reported in Fig. 1, illustrating the reaction of model ion 3, from
CI(C3H8) of allyltrimethylsilane. The second order rate constants
were obtained from kinetics run at three or more different val-
ues of the H2O pressure and found to yield the same value within
experimental error. The results are listed in Table 1 showing that
C5H11Si+ ions display the same reactivity with water, irrespective
of their origin. This reagent is therefore unsuitable to discriminate
among the different structures of the model ions.
(Z)-1-(Trimethylsilyl)propene (2)
(E)-1-(Trimethylsilyl)propene (4)
2-(Trimethylsilyl)propene (6)
Allyltrimethylsilane (3)
S
t-C4H9OCH3
t-C4H9OCH3
t-C4H9OCH3
t-C4H9OCH3
t-C4H9OCH3
2.6 0.3d
2.5 0.3d
4.6 0.7d
24 5d
4.1 0.7d
a
Reactant ion designated in parentheses.
b
Ratio of the yields of product ions P1 and P2 obtained from the ion-molecule
reactions run at 300 K.
The reaction of C5H11Si+ ions with i-C3H7OH proceeds along
two competing pathways leading to C2H7SiO+ (likely correspond-
ing to (CH3)2SiOH+) and to C5H13SiO+ (which may be assigned to
i-C3H7OSi(CH3)2+) [Eq. (4a) and (4b)]. In the presence of i-C3H7OH,
(CH3)2SiOH+ ions react further to give (CH3)2Si(OH)(OH2)+ and
i-C3H7OSi(CH3)2(OH2)+ as the plausible products at m/z 93 and
m/z 135 from two subsequent reaction steps. An exemplary
c
P1 = C2H7SiO+ and P2 = C5H13SiO+.
d
P1 = C3H9SiO+ and P2 = C6H15SiO+.
C2H7SiO+ + [C6H12]
C5H13SiO+ + [C3H6]
(4a)
(4b)
C5H11Si+ + i-C3H7OH
kinetic run is shown in Fig.
2 illustrating the relative ion
abundances for the reaction of model ion 2, from CI(C3H8) of
(Z)-1-(trimethylsilyl)propene, with i-C3H7OH. The kinetic results
obtained at different values of the neutral pressure were averaged
and are listed in Table 1.
As expected from a nucleophile stronger than water, the reaction
efficiencies are higher, their values comprised within 26–40%, and
do not lend a useful tool to distinguish among isomeric struc-
tures. The branching between the two reaction channels 4a and
4b appears more informative and is extracted by extrapolating
the relative ion yields to initial time. The formation of C2H7SiO+
is always favored with respect to C5H13SiO+ but the branching
ratio is different for the various isomers ranging from 4.3 to 21
(Table 2). Thus, while the C5H11Si+ ions deriving from (Z)- and
(E)-1-(trimethylsilyl)propene showing ratios of 8.5 and 11, respec-
tively, are not clearly discriminated, the observed value of 21 for
the sample ion S is quite close to the value for the model ion 6 from
CI(C3H8) of 2-(trimethylsilyl)propene and is far from the ratio of
4.3 displayed by the model ion 3 from allyltrimethylsilane.
Table 1
Kinetic data for the gas phase reaction of C5H11Si+ ions with ROR’ nucleophiles.
b
c
Precursor of reactant iona
ROR’
kexp
˚
(Z)-1-(Trimethylsilyl)propene (2)
(E)-1-(Trimethylsilyl)propene (4)
2-(Trimethylsilyl)propene (6)
Allyltrimethylsilane (3)
S
H2O
H2O
H2O
H2O
H2O
2.2 0.8
2.1 1.2
2.0 1.3
2.3 0.5
2.0 0.6
12
12
11
13
11
4
6
7
3
3
(Z)-1-(Trimethylsilyl)propene (2)
(E)-1-(Trimethylsilyl)propene (4)
2-(Trimethylsilyl)propene (6)
Allyltrimethylsilane (3)
S
i-C3H7OH
i-C3H7OH
i-C3H7OH
i-C3H7OH
i-C3H7OH
4.5 1.5
5.3 0.5
6.0 1.5
4.1 0.2
5.3 1.2
29 10
34
40 10
In a further effort to obtain safer support for a structural
assignment, the reactivity pattern toward tert-butyl methyl ether
(t-C4H9OCH3) has been examined. When C5H11Si+ ions are allowed
to react with t-C4H9OCH3 three primary product ions are observed,
namely C6H15SiO+, C3H9SiO+, and C4H9+, as shown in Eq. (5). The
3
26
34
2
4
S
t-C4H9OCH3
7.6 0.8
49
5
plausible composition of C6H15SiO+, C3H9SiO+, and C4H9 is likely
+
a
Reactant ion designated in parentheses.
+
corresponding to t-C4H9OSi(CH3)2+, CH3OSi(CH3)2+, and t-C4H9
,
b
c
Second order rate constant in units of 10−10 cm3 molecule−1 s−1, at 300 K.
Reaction efficiency = kexp/kcoll, expressed as percentage.
respectively.

62
H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
C6H15SiO+ + [C4H8]
(5a)
(5b)
(5c)
100
C5H11Si+
90
80
70
60
50
40
30
20
10
C5H11Si+ + t-C4H9OCH3
C3H9SiO+ + [C7H14]
+
C4H9 + [C5H11SiOCH3]
The exemplary plot of Fig. 3 shows the kinetic progress of the reac-
tion of the sample ion S from the addition of (CH3)3Si+ to acetylene.
As in the case of the nucleophiles H2O and i-C3H7OH, the evalua-
tion of the efficiency for the reactions of the model ions does not
appear to yield distinct values that could be used for any reliable
structural assignment and further efforts to collect these data were
not engaged. Given the higher nucleophilicity of t-C4H9OCH3 also
the reaction efficiency is higher, on the order of 50%, and the value
for the reaction of S is reported in Table 1.
I%
C5H13O+
+
C4H9
C6H15SiO+
C3H9SiO+
0
0
2
4
6
8
10
12
As observed for the reaction with i-C3H7OH as probe nucle-
ophile, it is the branching ratio into the different product channels
that is most revealing about structural differences in the model
ions. The branching ratio into the C3H9SiO+ and C6H15SiO+ product
ions is found to be the most informative.
Reaction time (s)
Fig. 3. Time dependence of the relative ion abundances (I%) when selected sample
ions S at m/z 99 from the addition of (CH3)3Si+ to acetylene (ꢂ) are allowed to react
with t-C4H9OCH3 at 1.9 × 10−8 mbar. Primary product ions are formed at m/z 131
(ꢃ) (C6H15SiO+), m/z 89 (ꢀ) (C3H9SiO+), and m/z 57 (ꢁ) (C4H9+). Secondary product
ions are at m/z 89 (᭹) (C5H13O+), and m/z 121 (+).
The third product ion, C4H9+, is however interfering due to
a proton transfer reaction to t-C4H9OCH3 which forms a cation
(C5H13O+) formally isobaric with C3H9SiO+. The high resolving
power of the FT-ICR mass spectrometer has permitted to solve the
problem by the distinct separation of the two ion signals at m/z
89.0961(C5H13O+) and m/z 89.0417 (C3H9SiO+). The values of the
branching ratios gained by the correct evaluation of the relative
abundance of the C3H9SiO+ species is reported in Table 2.
The relative abundances of C3H9SiO+ and C6H15SiO+ product
ions are fairly constant with respect to reaction time (though only
the early part of the kinetic progress of the reaction was considered)
and neutral pressure. The data collected in Table 2 indicate that the
reactivity behavior of the sampled ion population S conforms to the
one displayed by the model ion 6 from 2-(trimethylsilyl)propene.
The partition into the C2H7SiO+ and C5H13SiO+ products of
the reaction with i-C3H7OH is 20 5 from the model ion and
21 2 from S. The two values are equal within experimental
error and so are the corresponding ratios for the formation of
C3H9SiO+ and C6H15SiO+ product ions, 4.6 0.7 and 4.1 0.7,
respectively, from the reaction with t-C4H9OCH3. The results of
the ion-molecule reactions clearly suggest that the C5H11Si+ addi-
tion product from the (CH3)3Si+ reaction with acetylene should
be ascribed the structure of 2-propenyl-dimethylsilyl cation 6
[Eq. (6)].
calculations
at
the
CCSD(T)/6-311 + +G(2d,2p)//B3LYP/6-
311G(2d,p) level of theory were undertaken whose results
are summarized. The significant structures that were investigated
include the isomers depicted in Chart 1 together with the nascent
(CH3)3Si+-acetylene complex 1, formerly described by the limiting
geometries 1A and 1B.
Fig. 4 illustrates the optimized geometries of the stable isomers
and the transition states for the rearrangement paths obtained by
B3LYP/6-311G(2d,p) calculations. Table S1 in the supplementary
data summarizes the computational data obtained at both B3LYP/6-
311G(2d,p) and CCSD(T)/6-311 + +G(2d,2p)//B3LYP/6-311G(2d,p)
level about all relevant species that have been examined and
Table S2 provide the Cartesian coordinates of the optimized struc-
tures.
Because silyl substituted vinyl cations are known to be quite
flexible and the obtained structures may be dependent on the cho-
sen method, also MP2 computations have been applied to optimize
few significant species and check whether significantly different
structures were obtained. To this purpose geometry optimization
and frequency analysis at MP2/6-311G(2d,p) level of theory have
been performed on species 1,2,5,6, TS12 and TS15. The so-obtained
thermodynamic data are listed in Table S3. Table S4 provides the
Cartesian coordinates of the optimized structures. Table S5 presents
a comparison of selected geometrical parameters evaluated by the
B3LYP and MP2 methods. The MP2 results do not present sig-
nificant differences with respect to the corresponding structures
optimized at B3LYP/6-311G(2d,p) level. In view of the substantial
consistency of B3LYP/6-311G(2d,p) and MP2/6-311G(2d,p) com-
putational results, the forthcoming discussion will refer to the final
data computed at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311G(2d,p)
level.
CH3
FT-ICR
10-8 mbar
+
Si
(CH3)3Si+ + HC≡CH
H3C
CH2
H3C
(6)
3.3. C5H11Si+ isomers and rearrangement pathways: a quantum
chemical assay
As shown by the results collected in Fig. 4, ion 4 is the most sta-
ble species among all C5H11Si+ ions that were examined, while the
(CH3)3Si+–acetylene complex 1, showing an optimized geometry in
between the limiting structures 1A and 1B, is 95 kJ mol−1 higher in
energy. Complex 1 is endowed with a highly fluxional structure. In
fact, the displacement of the silyl group between the two C-atoms
in two equivalent geometries occurs via a Cs transition state that is
about 1 kJ mol−1 less stable than 1.
To explore the relevant sections of the potential energy sur-
face whereby the conceivable rearrangements of C5H11Si+ ions can
take place, several paths have been explored. Eq. (7) shows the
rearrangement of 1 to ion 2 involving the migration of a methyl
The rearrangement reaction depicted in Eq. (6) involves a
complex pathway that is conceivably initiated after the formation
of a nascent (CH3)3Si+–acetylene complex. This adduct seemingly
undergoes at least a 1,2-methyl migration from Si to C and a 1,2-H
shift process. The driving force of the reaction lies in the formation
of an ion where the positive charge is formally placed on silicon but
the involvement of activation barriers is underlined by the reactiv-
ity behavior observed at higher pressure (3 mbar to 1 atmosphere),
where the isomerization does not take place. In order to elucidate
the relative energies of the various ionic intermediates and the
energy profile of the plausible paths linking them, ab initio

H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
63
Fig. 4. Geometries of C5H11Si+ isomers and of the transition states linking them, obtained by B3LYP/6-311G(2d,p) computations. Relative energies are reported in kJ mol−1
(in parentheses, see Table S1).
group from silicon to carbon (transition state TS12). The arrows
in Eq. (7) point out the motion of the CH3 group and the rota-
tion around the double bond. The product ion 2 is the second
most stable species but the isomerization to 4, representing the
most stable among the investigated structures, requires a (Z) → (E)
rearrangement (transition state TS24).
may then undergo electrocyclic ring opening to give 4. However,
the candidate 2-silacyclopropyl intermediate does not repre-
sent a local minimum evolving into structure 6 upon geometry
optimization.
H3C
H3C
CH3
H3C
H3C
CH3
H
H3C
H3C
H3C
CH3
H
Si
+
Si
+
1,2-H shift
H
H
1,3-CH3 shift
+
Si
+
H3C
CH3
H
Si
TS15
H
TS12
H
H
5
1
1
2
1,2-CH3 shift
(E)-/(Z)-
isomerization
TS56
TS24
CH3
+
CH3
+
H3C Si
Si
H3C
H
CH2
CH3
H
H3C
6
4
(7)
(8)
(9)
In a similar way, the formation of isomer 6 is depicted in Eq. (8).
The process starts from 1 and proceeds by a 1,2-H shift to form
an intermediate ␣-silyl vinyl cation 5 (transition state TS15). A
1,2-shift of a methyl group from Si to C leads from 5 to ion 6
holding the positive charge on silicon (transition state TS56). From
ion 6 a rearrangement process can be conceived involving both
a 1,2-shift of (CH3)2Si and a 1,2-shift of a hydrogen atom to give
the most stable isomer 4 (transition state TS46), as shown in Eq.
(9). In this equation, and in the following ones, the arrows point
out the migration of an atom or group. Alternatively, a stepwise
path for the 6 to 4 rearrangement has been taken into consid-
eration involving an intermediate 2-silacyclopropyl cation which
CH3
CH3
1,2-H shift
1,2-(CH3)2Si shift
+
+
Si
H3C
CH3
Si
H
H
H3C
H
H
H3C
TS46
6
4
A
pathway may be further envisioned leading to the
allyldimethylsilyl cation 3, whose formation derives from ions

64
H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
450
400
350
300
250
200
150
100
50
E
rel (kJ mol-1)
TS 34
TS23
TS 46
TS24
TS12
TS15
Si(CH3)3+ + C2H2
TS56
5
2
1
3
6
4
0
Fig. 5. Energy profiles of the proposed pathways following the (CH3)3Si+ addition to acetylene evaluated at CCSD(T)/6-311 ++G(2d,2p)//B3LYP/6-311G(2d,p) level of theory.
Values of Erel at 0 K are given in kJ mol−1
.
(by ca. 6 kJ mol−1) than the separated (CH3)3Si+-acetylene couple. It
could provide a route to isomer 4, if a (Z) → (E) isomerization were
further allowed. However, not only is the latter process inhibited
by a transition state placed above the reference level, but also the
formation of 2 in any significant amount is rather disproven by
the gathered experimental evidence. The lack of any evidence sug-
gesting the presence of the allyl-substituted silyl cation 3 is amply
justified by the high energy barrier associated to the 1,3-H shift link-
ing 3 to the (E)/(Z) isomers 2 and 4 by way of TS23 and TS34. The high
barriers computed for the 1,3-H shifts are indeed what is expected
for this rearrangement in allylic systems on the basis of FMO the-
ory. It may further be noted that the reference species 2,3,4,6 do
indeed retain the original structure from each individual precursor
because of the high activation barriers for their interconversion, as
testified by the computed energy profiles.
2/4 by a formal 1,3-H shift (transition states TS23 and TS34
respectively), as shown in Eq. (10).
,
CH3
+
Si
H3C
CH3
H
H
2
1,3-H shift
TS23
CH3
(E)-/(Z)-
isomerization
TS24
+
H3C
Si
H
CH2
H
CH3
TS34
1,3-H shift
H
3
+
Si
H3C
H
CH3
H
4. Conclusion
4
(10)
The neat process of (CH3)3Si+ addition to acetylene can only be
examined in the gas phase where the CI(CH4) of (CH3)4Si provides
an entry to naked (CH3)3Si+ ions in a dilute environment lacking
potential nucleophiles that tend to trap this highly electrophilic
species, masking its intrinsic reactivity. Though seemingly sim-
ple, the (CH3)3Si+ addition to acetylene is a complex process when
examined at low pressure in the 10−8–10−7 mbar range of FT-ICR
mass spectrometry. The formation of formal adduct ions, C5H11Si+,
reveals the occurrence of a rearrangement process allowed by the
excess internal energy released in the primary addition of (CH3)3Si+
to acetylene. The structure of C5H11Si+ ions has been probed by ion-
molecule reactions, taking advantage of the temporal resolution
allowing kinetic determinations and also of the mass resolution
which permitted to discriminate formally isobaric product ions.
When confronted with the reactivity pattern of model ions, the
sampled C5H11Si+ adducts, ions S, are found to behave in a quite
similar way as the 2-propenyl-dimethylsilyl cation 6. Addition-
ally, the partition in competing paths following the reaction with
selected neutrals (i-C3H7OH and t-C4H9OCH3) is different when
other stable isomers (2,3,4) are tested. The structural assignment
of the observed C5H11Si+ adducts, allowed by the distinct ion-
molecule reactivity, is further supported by the quantum chemical
calculations of the potential isomers and the related energy profiles
for their interconversion pathways. The computational results con-
firm the experimental findings. The nascent (CH3)3Si+-acetylene
complex 1 may rearrange to isomer 6 passing the lowest activa-
tion energy barrier associated to a 1,2-H shift from C˛ to Cˇ of
The energy profile of the various paths that were envisioned is
illustrated in Fig. 5, showing the relative energies of the sig-
nificant species evaluated at CCSD(T)/6-311 + +G(2d,2p)//B3LYP/6-
311G(2d,p) level of theory. Throughout the diagram a broken line
marks the energy content of the separated (CH3)3Si+–acetylene
molecules combining to form complex 1 that is stabilized by
91 kJ mol−1 relative to the free reagents. The reaction exothermic-
ity, however, remains stored in the adduct ion 1 unless dissipated by
radiative emission or energy transfer by unreactive collisions with a
bath gas. Both processes are inefficient for a relatively small species
in a highly dilute environment [58,59] so that the energy released
in the addition process contributes to form a ‘hot’ species that can
undergo rearrangement processes involving substantial activation
barriers [Eq. (2)]. The plot shows that the transition state lying low-
est in energy, TS15, allows the conversion of 1 to 5. Ion 5 is probably
a fleeting intermediate because it can proceed without any appre-
ciable activation barrier to form 6. Ion 6 is likely to represent a dead
end product under the prevailing experimental conditions.
A further rearrangement to the most stable species 4 requires
passing an activation barrier that protrudes above the energy con-
tent of the reactant pair so that the corresponding transition state
TS46 is hardly accessible. The TS46 transition state combines a 1,2-
H shift with a 1,2-(CH3)2Si shift involving the same two C-atoms,
though in opposite direction [Eq. (9)]. The reaction path allowing
the conversion of 1 into 2 could also be accessed because the cor-
responding transition state, TS12, lies only slightly higher in energy

H.-U. Siehl et al. / International Journal of Mass Spectrometry 334 (2013) 58–66
65
the former acetylene unit. The so-formed ␣-silyl vinyl cation 5 is
a transient intermediate that can proceed to 6 in a nearly barri-
erless process involving a Si to C methyl migration and forming
a stable species with a positive charge centered on silicon. Fur-
ther rearrangements to other isomers including the most stable
one, 4, is inhibited by activation energy barriers protruding above
the energy level of the reactant pair. An alternative path energeti-
cally accessible, namely 1 → 2, is associated to an activation energy
barrier of ca. 97 kJ mol−1. This rearrangement does not appear to
take place, though, suggesting that the differential energy barrier
of ca. 35 kJ mol−1 relative to 1 → 6 is effective in rendering the
reaction selective. At the same time, the 1 → 6 rearrangement is
hampered by the collisional relaxation occurring at 3–5 mbar in
HPMS where adventitious water is a final trap for (CH3)3Si+ ions
whereas 1 is efficiently stabilized at atmospheric pressure where
it ultimately undergoes deprotonation to the neutral substitution
product, (CH3)3SiC CH. In conclusion, the integrated approach
combining bimolecular ion-chemistry with quantum chemical cal-
culations has provided a consistent view of the reactivity going on
in (CH3)3Si+–acetylene adducts in gaseous environments at widely
different pressures. In this context, it is interesting to compare
the behavior of the gaseous (CH3)3Si+–acetylene complex with a
related system that has been recently investigated using matrix
isolation IR spectroscopy and quantum chemical calculations [71].
Trimethylsilylacetylenes ((CH3)3SiC CR, R = H, CH3, Si(CH3)3) have
been codeposited with SbF5 and submitted to vibrational analy-
sis after deposition and upon warming of the matrix. New bands
appearing in the 1600 cm−1 spectral region have been assigned to
silaallylcations such as 2 and 6 suggested to arise by protonation
and ensuing isomerization processes. Vibrational spectroscopy of
the gaseous sample ion S using IR multiple photon dissociation
spectroscopy [72,73] is a forthcoming effort to settle the signifi-
cant features of the potential energy surface of gaseous C5H11Si+
ions.
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Acknowledgements
Financial support from the Università degli Studi di Roma
La Sapienza and by the Italian Ministero dell’Università e della
Ricerca is gratefully acknowledged. We also gratefully acknowl-
edge support of the work at Ulm University by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
We thank Thomas Nau, Computer Center, University of Ulm, for
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