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