Ab Initio Study of Silyloxonium Ions

5942 Organometallics, Vol. 16, No. 26, 1997 Cypryk and Apeloig
Table 1. Calculated Geometrical Parameters a of
the SiOSi Unit in Siloxanes 4a-7a, Protonated
Siloxanes 4b-7b, and Silylated Siloxanes 4c-7c
species param
siloxane
(na) na + H + na + SiH3 +
H3SiOSiH3 (4a) Si-O1.626 1.807, 1.811 1.786
Si-H 1.474 1.456 1.458
Si-O-Si 170.0 130.3 120.0
H-Si-H 108.9 114.7 114.3
(H2SiO)3 (5a) Si-O1.640 1.819 1.797
Si-H 1.466 1.453 1.454
Si-O-Si 133.5 130.4 124.0
O-Si-O 106.5 101.4 104.8
H-Si-H 110.8 115.6 115.2
(H2SiO)4 (6a) Si-O1.626 1.806 1.785
Si-H 1.467 1.453 1.454
Si-O-Si 159.6 129.5 118.5
O-Si-O 110.4 103.9, 106.7 104.9, 106.2
(CH2SiH2)2O (7a) Si-O 1.657 1.808 1.808
Si-H 1.476 1.459 1.460
Si-O-Si 117.2 117.3 113.5
O-Si-C 99.9 94.5 97.4
H-Si-H 108.6 113.5 112.8
a Bond distances in angstroms and bond angles in degrees.
Table 2. GIAO and IGAIM 29 Si NMR Chemical
Shifts (Relative to TMS a ) of Siloxanes and
Silyloxonium Ions Calculated Using
B3LYP/6-311+G**//6-31G*
species GIAO IGAIM exptl b
4a -50.9 -45.8 -38.0 (7.0)
4b 17.1 21.7 (22.9) c
4c 11.0 16.3 (51.1) d
5a
5b
-41.0 -36.3 (-9.2)
Si1 -12.7 -7.9
Si2 5c
-29.6 -24.9
Siexo -13.0 -7.9 (50.2) d
Si1 -21.1 -15.8 (10.0)
Si2 -31.3 -26.5 (-9.6)
6a -60.2 -54.9 -47.2e (-19.4)
8c 29.0 32.6 (66.9) f
9c 20.1 24.4 (59.0) g
a δ(TMS) ) 339.5 ppm (GIAO), 343.5 ppm (IGAIM). b Values
in parentheses correspond to the permethyl-substituted analogues.
24 c Reference 3. d Reference 5. e Reference 7. f Chemical
shift in Me3SiOEt2 + . 4 g Chemical shift in (Me3Si)2OEt + . 4
The calculated 29 Si chemical shifts δ (at B3LYP/6-
311+G**//HF/6-31G*), of H3SiOSiH3 are -50.0 ppm
(GIAO) and -46.2 ppm (IGAIM). These values are
8-12 ppm higher than the reported experimental vaue
of -38.0 ppm. 24 However, calculations using the MP2/
6-31G* optimized geometry gave 29 Si chemical shifts of
-41.4 ppm (GIAO) and -37.0 ppm (IGAIM), in very
good agreement with experiment. This suggests that
for correct replacement of the magnetic properties of
siloxanes, the correct Si-O-Si angle should be used.
The measured chemical shift of Me3SiOSiMe3 is 6.8
ppm, ca. 45 ppm downfield from that of 4a. 24 In order
to test the reliability of the calculated 29 Si chemical
shifts of 4b,c and of 5a-c, we compared them with the
well-known experimental chemical shifts of their methyl-substituted
analogues, assuming that substitution of
Si-H bySi-Me is additive and is independent of the
siloxane structure. This assumption can be supported
by comparison of known chemical shifts of linear and
cyclic oligosiloxanes with those of their methylated
analogues. The SiH2 groups in linear and unstrained
cyclic siloxanes appear in the range -48 to -52 ppm,
Table 3. Calculated Charges (6-31G*) around the
Oxonium Center in Model Siloxanes According to
Mulliken and NBO Population Analysisa Mulliken NBO
a b c a b c
4 H(O) 0.55 0.58
O -0.84 -0.95 -1.02 -1.35 -1.16 -1.32
Si 0.92 0.91 0.91 1.47 1.37 1.36
H(Si) -0.17 -0.06 -0.08 -0.27 -0.19 -0.19
SiH3 b 0.42 0.7 0.67 0.67 0.78 0.78
5 H1 0.54 0.57
Siexo 0.91 1.35
H3Siexo 0.66 0.76
O1 -0.87 -0.96 -1.03 -1.35 -1.18 -1.35
Si1 1.21 1.22 1.22 1.93 1.9 1.89
H(Si1 ) -0.17 -0.09 -0.09 -0.29 -0.24 -0.24
Si1H2 b 0.87 1.04 1.03 1.35 1.42 1.41
6 H1 0.54 0.57
Siexo 0.9 1.36
H3Siexo 0.65 0.64
O1 -0.84 -0.95 -1.02 -1.37 -1.18 -1.34
Si1 1.2 1.21 1.23 1.95 1.89 1.89
H(Si1 ) -0.18 -0.1 -0.09 0.29 0.23 -0.21
Si1H2 b 0.84 1.01 1.0 1.37 1.45 1.49
7 H1 0.55 0.58
Siexo 0.91 1.37
H3Siexo 0.66 0.75
O1 -0.87 -0.95 -1.02 -1.32 -1.16 -1.33
Si2 1.02 1.01 1.02 1.66 1.61 1.61
H(Si2 ) -0.17 -0.08 -0.08 -0.27 -0.21 -0.21
Si2H2 b 0.69 0.86 0.89 1.13 1.2 1.2
a For atom numbering see Chart 1. b Charge on the entire group.
whereas SiHMe groups appear at -34 to -37 ppm and
SiMe2 groups at -19 to -24 ppm. 7,24 Thus, substitution
of one H by Me shifts the 29 Si signal by 13-15 ppm
downfield. Disiloxanes R3SiOSiR3, where R ) HorMe,
obey this correction as well. 24 Further support for this
assumption is provided by the fact that the difference
between the calculated chemical shift of 5a and that
measured for D3 is 31.8 ppm (GIAO) and 27.0 ppm
(IGAIM), as expected for the substitution of two Si-H
bonds by two Si-Me groups.
For silyloxonium cations the change in the 29 Si
chemical shift due to substitution of Si-H bySi-Me is
difficult to determine because of the lack of experimental
data. For instance, according to IGAIM calculations,
the 29 Si signal of (H3Si)3O + is shielded by 35 ppm
compared to that of (Me3Si)3O + . 6 This difference between
the 29 Si chemical shifts of the exocyclic silicon
atoms in 5c and 2 is 58 ppm (IGAIM), while the
corresponding difference between the endocyclic Si 1
atoms in these species is 26 ppm. 6 Calculations suggest
that in the silylated disiloxane 4c the Si atoms are more
shielded than those in the corresponding protonated
analogue 4b. This is in contrast to the data reported
for (Me3Si)2OH + and (Me3Si)3O + (Table 2). 4,6
Charge Distribution and Orbital Analysis. The
atomic charges in all the studied species were calculated
using both the Mulliken population analysis 11 and the
natural population analysis (NPA), which is based on
natural bond orbital (NBO) theory. 12 The results are
collected in Table 3, and the numbering system is given
in Chart 1. The two methods gave different absolute
values (as is well-known) but similar qualitative results,
and they are therefore discussed together.
The calculations show that the charge at the silicon
atoms directly bonded to the oxonium oxygen is approximately
the same as in the corresponding neutral