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

22 Thermochemistry
2.2.2 PHOTOACOUSTIC CALORIMETRY
Photoacoustic calorimetry is a thermodynamic method to determine a bond
strength in solution [8]. Indeed, this technique has been used to quantify the
enthalpy change occurring in a photoinduced reaction of di-tert-butyl peroxide
with a silane (Reaction 2.7). Few bond dissociation enthalpies of silanes have
been measured by this technique. In the original reports [9,10], the absolute
values of DHs were underestimated by ca 20 kJ/mol due mainly to the reaction
volume changes and the change in solvation enthalpies [1,8]. Therefore, relative
data by this technique should be reliable since solvation correction is not
necessary. Table 2.2 reports the relative bond dissociation enthalpies (DHrel)
for a few silanes. The data demonstrate that silicon–hydrogen bonds can be
dramatically weakened by successive substitution of the Me3Si group at the
Si w H functionality. A substantial decrease in bond strength is also observed by
replacing alkyl with methylthio groups. It is worth mentioning that in the
analogous experiments with other group 14 hydrides, the bond strengths decrease
by 27 and 69 kJ/mol, going from Et3Si w H to Bu3Ge w H and to
Bu3Sn w H, respectively [11,12].
t-BuOOBu-t þ 2R3SiH ! hn 2 t-BuOH þ 2R3Si: (2:7)
The relative bond enthalpies from the photoacoustic calorimetry studies can
be placed on an absolute scale by assuming that the value for DH(Et3Si w H) is
similar to DH(Me3Si w H). In Table 2.2 we have converted the DHrel values to
absolute DH values (third column). On the basis of thermodynamic data, an
approximate value of DH(Me3SiSiMe2 w H) ¼ 378 kJ=mol can be calculated
that it is identical to that in Table 2.2 [1]. A recent advancement of photoacoustic
calorimetry provides the solvent correction factor for a particular
solvent and allows the revision of bond dissociation enthalpies and conversion
to an absolute scale, by taking into consideration reaction volume effects and
heat of solvation [8]. In the last column of Table 2.2 these values are reported
and it is gratifying to see the similarities of the two sets of data.
Table 2.2 Relative and absolute bond dissociation enthalpies (kJ/mol) a
Silane (R3SiH) DHrel b DH(R3Si w H) c
Et3Si w H 0 398 402
Me3SiSiMe2 w H 20 378 381
(MeS) 3Si w H 32 366 364
(Me3Si) 3Si w H 46:5 351.5 351.5
a At 25 8C.
b From References [9,10].
c Estimated based on the DHrel (see text).
d Revised values taking into consideration solvent correction factors [8].
DH(R3Si w H) d