Ab Initio Study of Silyloxonium Ions

More documents

Recommendations

Info

Ab Initio Study of Silyloxonium Ions Organometallics, Vol. 16, No. 26, 1997 5941 Figure 1. Optimized geometries (6-31G*) of model siloxanes 4a-7a and of silyloxonium ions 4b-7b and 4c-9c.
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
Page 1 and 2: 5938 Organometallics 1997, 16, 5938
Page 3: 5940 Organometallics, Vol. 16, No.
Page 7 and 8: 5944 Organometallics, Vol. 16, No.

Ab Initio Study of Silyloxonium Ions

Create successful ePaper yourself

Delete template?

Save as template?