The Thermochemistry of Boron and Some of Its Compounds. The Enthalpies of Formation of Orthoboric Acid, Trimethylamineborane, and Diammoniumdecaborane1,2

A rotating-bomb calorimetric technique was developed to solve the problem of determining accurate values of the enthalpies of combustion of boron and boron compounds. The enthalpies of combustion of crystalline boron, trimethylamineborane ((CH/sub 3/)/sub 3/NBH/sub 3/), and diammoniumdecaborane (B/sub 10/H/sub 18/N/sub 2/) were determined. The combustion products were carbon dioxide, water, fluoroboric acid in excess aqueous HF, and (in the case of the boron compounds) gaseous nitrogen. The enthalpy of solution of orthoboric acid was measured in a hydrofluoric acid solution chosen to yield the same fluoroboric acid solution obtained in the boron combustion experiments. The derived values of the enthalpies of formation, ..delta..H line integral/sup 0//sub 298.15/, from graphite, crystalline boron(..beta.., rhombohedral), and gaseous hydrogen and nitrogen are: orthoboric acid(c), -261.4/sub 7/ +- 0.2/sub 0/ kcal. mole/sup -1/; trimethylamineborane(c, III), -34.0/sub 4/ +- 0.5/sub 5/ kcal. mole/sup -1/; and diammoniumdecaborane(c), -85.8/sub 4/ +- 2.5/sub 0/ kcal. mole/sup -1/. The value of the enthalpy of formation of orthoboric acid was used with numerous other thermochemical data from the literature to derive values of the enthalpies of formation of the boron oxides, diborane, boron trichloride, and boron tribromide referred directly to crystalline boron. The value of the enthalpy of formation ofmore » trimethylamineborane was used with other literature data to derive another value of the enthalpy of formation of diborane.« less