Thermochemistry of organosilicon compounds: II. 1-Organylsilatranes

The enthalpies of combustion and sublimation of 1-organylsilatranes with the general formula X Si(OCH 2CH 2) n [OCH(CH 3)CH 2] 3− n N ( n = 0–3) have been determined. The enthalpies of formation in the solid and liquid state and the enthalpy of atomization have been evaluated. The enthalpies of atomization have been calculated by an additive scheme ignoring the total stress energy of the silatrane skeleton and the transannular interaction between the silicon and nitrogen atoms ( E cycl.- E Si ← N). The enthalpies of formation of the silatrane group Si(OCH 2CH 2) 3N (Δ H° f,298.15 Si(OCH 2CH 2) 3N ) have been estimated as −782 ± 10 kJ mole −1. The proportion of this value due to contributions by the heterocyclic system stress and the transannular Si ← N interaction, the latter ranging from 30 to 50 kJ mole −1 depending on the nature of the substituent X at the silicon atom, was also estimated. Sequential introduction of methyl groups to the 3-, 7- and 10-positions of the silatrane skeleton increases the E cycl.- E Si ← N value up to 62.8 and 90.0 kJ mole −1, respectively, because of a concomitant increase in the Prelog strain. The enthalpy of formation of finely dispersed hydrated amorphous silicon dioxide produced in a calorimetric bomb on detonation has been measured.