"Front Matter". In: Organosilanes in Radical Chemistry - Index of

More documents

Recommendations

Info

Radical Chemistry on Silicon Surfaces 205 actions determine the reactivity and selectivity of the reaction [46]. On the other hand, in radical chemistry consequences should be attenuated. 8.5.1 OXIDATION OF HYDROGEN-TERMINATED SILICON SURFACES The reaction of hydrogen-terminated Si(111), H w Si(111), with molecular oxygen under photochemical conditions has been investigated in some detail [48]. Figure 8.4 highlights the photoinduced reactivity of H w Si(111) in air followed by FTIR. Spectrum (a) shows that the exposure of material to air in the dark does not affect its stability, whereas after 30 min of photolysis with a mercury lamp, the area of the original peak decreases to 14 % (spectrum d). Xray photoelectron spectroscopy (XPS) of the irradiated surface shows a strong O(1s) signal and a shifted Si(2p) signal indicating the formation of SiO2. Ellipsometry studies reveal the presence of an 8 A ˚ film. When the surface is exposed to 350 nm light for the same total energy, the loss of Si w H bonds is decreased appreciably (spectrum c). With 450 nm light no detectable loss of Si w H is observed (spectrum b). It is worth mentioning that the photooxidation of porous silicon behaves differently [49]. Indeed, FTIR spectra show that there is a tremendous increase in nSi O, without a correspondingly large loss of nSi H peak intensity. The decrease of the nSi H band is offset by an increase in the nOSi H band, resulting in no net loss of hydride species on the surface during the course of the photooxidation reaction. These data apparently suggest that oxidation does not result in the removal of H atoms, implying that Si w Si bonds are attacked directly. Arb. Units dark 99�1% 0 98�2% 0 91�2% 0 (a) 450 nm (b) (c) (d) 350 nm 250 nm 14%, 8 2150 2100 (cm−1 ) 2050 Figure 8.4 ATR-FTIR spectra of H w Si(111) after exposure to air (a) in the dark, (b) with 450 nm (broadband) illumination, (c) with 350 nm (broadband) illumination, and (d) with Hg lamp illumination. Reprinted with permission from Reference [48]. Copyright 2000 American Chemical Society.
206 Silyl Radicals in Polymers and Materials The mechanism for the photooxidation of silicon surfaces is not well understood [48,49]. An early mechanistic proposal in which O2 absorbs to silicon surface atoms and dissociates before it inserts into Si w Si bonds seems to have been abandoned. More recently, it was suggested that surface silyl radicals (48), formed by UV irradiation of H w Si(111), react with oxygen to form a peroxyl radical (49) that can abstract a neighbouring hydrogen to produce a new surface dangling bond (50) (Scheme 8.10) [48]. How the migration of oxygen into the Si w Si backbone of the lattice occurs and how the regeneration of surface silyl radical or Si w H bond arises is not yet understood. If we keep the analogy with the oxidation of (Me3Si) 3Si w H and (Me3Si) 2MeSi w H and poly(hydrosilane)s described earlier in this chapter, the mechanism reported in Scheme 8.10 can be proposed. The peroxyl radical 49 rearranges to silyloxyl radical 51 by the oxygen insertion step, which is found to be ca 10 4 s 1 for the analogous reaction in (Me3Si) 3Si w H. The alternative neighbouring hydrogen abstraction to give 50 is expected to be much slower, for analogy with cumylperoxyl radical hydrogen abstraction from (Me3Si) 3Si w H occurring with a rate constant of 66 M 1 s 1 at 73 8C. In turn radical 51 is expected to undergo a fast 1,2-silyl shift to give silyl radical 52, which can add to oxygen to give peroxyl radical 53 followed by oxygen insertion to the remaining g(Si w Si) bond. The resulting silyloxyl radical 54 is ready to undergo 1,5 hydrogen shift to give another surface silyl radical (55). Radical 51 can also abstract neighbouring hydrogen via a six-membered transition state from the side that already inserted an oxygen atom to give another surface silyl radical (56). The latter could add to oxygen to give 57 and continue the chain, eventually until complete oxidation of the surface. Both 1,5 hydrogen and 1,2 silyl shifts from radical 51 are expected to be very fast. The bimolecular rate constant for hydrogen abstraction from (Me3Si) 3SiH by alkoxyl radical is 1 10 8 M 1 s 1 which suggests that the intramolecular version could be even two orders of magnitude faster. On the other hand, a rate constant > 10 8 s 1 is estimated for the 1,2 silyl shift in the oxidation of (Me3Si) 3SiH. Therefore, it is likely that the preferred path will strongly depend on entropic factors determined by the rigidity of the surface. It should also be noticed that the silyl radical 52 has two silyloxyl substituents and a 1,x H shift from an (Si) 3Si w H moiety is expected to be strongly exothermic. When favourable transition states are formed such 1,x H shifts should be quite fast to give 59 (the silyl radical moiety is not shown). Therefore, the reaction mechanism for the surface oxidation given in Scheme 8.10 could lead to Si w OH or Si w Has terminal groups depending on the type of silicon surface. The oxidation of hydrogen-terminated silicon surfaces by molecular oxygen also occurs without irradiation. Scanning tunnelling microscopy (STM) investigation shows that the exposure of H w Si(111) to O2 induces surface modification that is assigned to the insertion of oxygen atoms into the Si w Si backbone [50]. However, the exposure of hydrogen-terminated silicon surfaces either to dry molecular oxygen or to deoxygenated water gives low oxide growth rates, whereas the combination of water and oxygen results in a significantly faster
Page 1 and 2:
Organosilanes in Radical Chemistry
Page 3 and 4:
DEDICATION To my parents XrZstoB an
Page 5 and 6:
viii Contents 3.6 Hydrogen Atom: An
Page 7 and 8:
PREFACE A large number of papers de
Page 9 and 10:
ACKNOWLEDGEMENTS I thank Keith U. I
Page 11 and 12:
2 Formation and Structures of Silyl
Page 13 and 14:
Page 15 and 16:
Page 17 and 18:
Page 19 and 20:
10 Formation and Structures of Sily
Page 21 and 22:
Page 23 and 24:
Page 25 and 26:
Page 27 and 28:
2 Thermochemistry 2.1 GENERAL CONSI
Page 29 and 30:
Bond Dissociation Enthalpies 21 Tab
Page 31 and 32:
Bond Dissociation Enthalpies 23 2.2
Page 33 and 34:
Ion Thermochemistry 25 Table 2.4 Re
Page 35 and 36:
Ion Thermochemistry 27 Table 2.5 El
Page 37 and 38:
References 29 3. Goumri, A., Yuan,
Page 39 and 40:
32 Hydrogen Donor Abilities of Sili
Page 41 and 42:
Page 43 and 44:
Page 45 and 46:
Page 47 and 48:
Page 49 and 50:
Page 51 and 52:
Page 53 and 54:
Page 55 and 56:
4 Reducing Agents 4.1 GENERAL ASPEC
Page 57 and 58:
General Aspects of Radical Chain Re
Page 59 and 60:
Tris(trimethylsilyl)silane 53 Therm
Page 61 and 62:
Tris(trimethylsilyl)silane 55 4.3.1
Page 63 and 64:
Tris(trimethylsilyl)silane 57 A goo
Page 65 and 66:
Tris(trimethylsilyl)silane 59 C(O)C
Page 67 and 68:
Tris(trimethylsilyl)silane 61 radic
Page 69 and 70:
Tris(trimethylsilyl)silane 63 86% y
Page 71 and 72:
Tris(trimethylsilyl)silane 65 RO RO
Page 73 and 74:
Tris(trimethylsilyl)silane 67 O AcO
Page 75 and 76:
Tris(trimethylsilyl)silane 69 Tris(
Page 77 and 78:
Other Silicon Hydrides 71 The reduc
Page 79 and 80:
Other Silicon Hydrides 73 The decre
Page 81 and 82:
Other Silicon Hydrides 75 Ph MeS O
Page 83 and 84:
Other Silicon Hydrides 77 Table 4.6
Page 85 and 86:
Silicon Hydride / Thiol Mixture 79
Page 87 and 88:
Silylated Cyclohexadienes 81 and (4
Page 89 and 90:
References 83 34. Kawashima, E., Uc
Page 91 and 92:
References 85 104. Gimisis, T., Bal
Page 93 and 94:
88 Addition to Unsaturated Bonds te
Page 95 and 96:
90 Addition to Unsaturated Bonds ap
Page 97 and 98:
92 Addition to Unsaturated Bonds 5.
Page 99 and 100:
94 Addition to Unsaturated Bonds R
Page 101 and 102:
96 Addition to Unsaturated Bonds Ph
Page 103 and 104:
98 Addition to Unsaturated Bonds EP
Page 105 and 106:
100 Addition to Unsaturated Bonds 5
Page 107 and 108:
102 Addition to Unsaturated Bonds T
Page 109 and 110:
104 Addition to Unsaturated Bonds R
Page 111 and 112:
106 Addition to Unsaturated Bonds a
Page 113 and 114:
Page 115 and 116:
110 Addition to Unsaturated Bonds S
Page 117 and 118:
112 Addition to Unsaturated Bonds T
Page 119 and 120:
114 Addition to Unsaturated Bonds I
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
120 Unimolecular Reactions 1 Si(H)M
Page 127 and 128:
122 Unimolecular Reactions t-Bu t-B
Page 129 and 130:
124 Unimolecular Reactions MeO Si O
Page 131 and 132:
126 Unimolecular Reactions t-Bu t-B
Page 133 and 134:
128 Unimolecular Reactions R 1 R 2
Page 135 and 136:
130 Unimolecular Reactions From the
Page 137 and 138:
132 Unimolecular Reactions rearrang
Page 139 and 140:
134 Unimolecular Reactions H 3 C +
Page 141 and 142:
136 Unimolecular Reactions Br O Si(
Page 143 and 144:
138 Unimolecular Reactions R3Si C C
Page 145 and 146:
140 Unimolecular Reactions Me3Si O
Page 147 and 148:
142 Unimolecular Reactions 43. Albe
Page 149 and 150:
144 Consecutive Radical Reactions c
Page 151 and 152:
146 Consecutive Radical Reactions O
Page 153 and 154:
148 Consecutive Radical Reactions a
Page 155 and 156:
150 Consecutive Radical Reactions d
Page 157 and 158:
152 Consecutive Radical Reactions T
Page 159 and 160: 154 Consecutive Radical Reactions M
Page 161 and 162: 156 Consecutive Radical Reactions F
Page 163 and 164: 158 Consecutive Radical Reactions O
Page 167 and 168: 162 Consecutive Radical Reactions i
Page 169 and 170: 164 Consecutive Radical Reactions A
Page 173 and 174: 168 Consecutive Radical Reactions r
Page 175 and 176: 170 Consecutive Radical Reactions E
Page 179 and 180: 174 Consecutive Radical Reactions A
Page 181 and 182: 176 Consecutive Radical Reactions P
Page 183 and 184: 178 Consecutive Radical Reactions f
Page 187 and 188: 182 Consecutive Radical Reactions 1
Page 189 and 190: 184 Consecutive Radical Reactions 8
Page 191 and 192: 186 Silyl Radicals in Polymers and
Page 209: 204 Silyl Radicals in Polymers and
Page 225 and 226: 220 List of Abbreviations PED photo
Page 227 and 228: 222 Index Carbonyl sulfide 111 Carb
Page 229 and 230: 224 Index Organosilicon boranes 2,
Page 231 and 232: 226 Index Triethylsilane as mediato
show all

"Front Matter". In: Organosilanes in Radical Chemistry - Index of

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?