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

20 Thermochemistry
R 3Si +
(2.21)
R 3 SiH
(2.16)
(2.16)
R 3Si
(2.1)
R 3 SiH
(2.13)
(2.11)
R 3Si −
R 3 SiH
(2.14)
Scheme 2.1 Relationships between R3SiH and its related radicals, ions, and radical ions (The
numbers refer to reaction numbers as they appear in the text)
absence of hydroxylic or acidic bonds). This is true also in organosilanes since
good agreement between the gas-phase and the liquid-phase DHs has been
observed.
2.2 BOND DISSOCIATION ENTHALPIES
Knowledge of bond dissociation enthalpies (DH) has always been considered
fundamental for understanding kinetics and mechanisms of free radicals. DHs
offer an interesting window through which to view stability of radicals. Indeed,
based on Reaction (2.1) the bond dissociation enthalpy of silanes DH(R3Si H)
w
is related to enthalpy of formation of silyl radicals, DHf (R3Si:), by Equation
(2.2).
R3Si H !R3Si: þ H: (2:1)
w
DH(R3Si w H) ¼ DH f (R3Si:) þ DH f (H:) DH f (R3SiH) (2:2)
Several techniques have been used to determine silane bond enthalpies: radical
kinetics, gas-phase acidity cycles and photoacoustic calorimetry. However, the
following sections concentrate on the results rather than the details of experimental
techniques. DHs of silanes have changed substantially over time. For
example, the DH(Me3Si w H) has changed significantly over the last 60 years.
Indeed, values of 314, 339, 377 and 397 kJ/mol were reported in 1942, 1971, 1982,
1994, respectively [1]. Consequently, many bond dissociation enthalpies that were
derived using these reference values have changed as well. Nevertheless, it is worth
recalling that today the majority of data are consistent within the different experimental
approaches. For consistency, all the DH and DH f values are normally
extrapolated to standard state conditions (i.e., 258C in the gas phase).
2.2.1 RADICAL KINETICS
Studies of the kinetics of chemical equilibrium (Reactions 2:3= 2:3) have
provided very accurate thermodynamic information from the series