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

Ion Thermochemistry 25
Table 2.4 Recommended R w H bond dissociation enthalpies for some selected organic
molecules (kJ/mol) a
X w H DH(X w H) X w H DH(X w H)
H3C H 439 0:5 H2NH 452:5 1:5
w w
MeCH2 H 423 1:5 MeNH H 419
w w b
Me2CH H 412:5 1:5 PhNH H 368
w w b
Me3C H 404 1:5 HO H 499:15 0:20
w w
CH2
w CH w H 465 3:5 MeO H 436 4
w
C6H5 H 465 3:5 PhO H 371:3 2:3
w w c
CH2 w CHCH2 w H 369 9 HS w H 381:5 3
PhCH2 w H 370 6 MeS w H 365:5 2
HC(O)CH2 w H 394:5 9 PhS w H 349:4 4:5 d
N w CCH2 w H 396:5 9 H3Ge w H 349 8
HOCH2 w H 402 0:5 Bu3Ge w H 368 e
HSCH2 w H 393 8 Ph3Ge w H 356 e
MeC(O) w H 374 1:5 Bu3Sn w H 326 e
a From Reference [7], unless otherwise mentioned.
b The DH(Xw H) in MeNH þ 3 is 447 kJ/mol (i.e., 28 kJ/mol stronger) and in PhNHþ 3
(i.e., 60.5 kJ/mol weaker); see Reference [19].
c From Reference [20].
d From Reference [21].
e Data in solution [8].
is 307.5 kJ/mol
Si w F > Si w Cl > Si w Br > Si w I, or Si w O > Si w S, or Si w C > Si w Si. (ii)
For a particular row, the bond strength increases going from left to right, i.e.,
Si w C < Si w N < Si w O < Si w F, or Si w Si < Si w S < Si w Cl.
In Table 2.4, we have collected background information for discussion in the
following chapters. Recommended C w H bond dissociation enthalpies of
selected organic compounds are reported in the first two columns, followed
by a variety of heteroatom–hydrogen bond strengths including N w H, O w H,
S w H, Ge w H, and Sn w H bonds.
2.3 ION THERMOCHEMISTRY
Thermochemical information about neutral species can also be obtained from
measurements of ions. Indeed, accurate bond dissociation energies for neutral
molecules have been obtained from gas-phase ion chemistry techniques. In this
section, we will summarize both the negative-ion and hydride-affinity cycles
involving silicon hydrides (R3SiH) which are connected to electron affinity
(EA) and ionization potential (IP) of silyl radicals, respectively [22–24].
2.3.1 NEGATIVE-ION CYCLES
Thermodynamic properties related to R3SiH can be obtained from negative-ion
gas-phase studies. The following thermochemical cycle (cf. Scheme 2.1):