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The enthalpy of combustion of crystalline hexaglycine (C12H20N6O7cr) has been determined by combustion bomb calorimetry. The molar enthalpy of combustion at 298.15 K for the following reaction: C12H20N6O7(cr) + 13.5O2(g) = 12CO2(g) + 10H2O(1) + 3N2(g) is ΔcHmo = −(5930.3 ± 11.3) kJ mol−1; the corresponding enthalpy of formation at 298.15 K is ΔfHmo = −(1650.1 ± 11.7) kJ mol−1. Because of the difficulty in establishing the moisture content of the sample, the enthalpy of combustion is based upon the mass of carbon dioxide formed in each calorimetric experiment rather than upon the mass of sample prior to combustion. A variety of NMR measurements have been made, both 1H NMR and CP-MAS 13C, to characterize hexaglycine and several other oligoglycines, and to determine the water content of the hexaglycine. The energy contribution to the enthalpy of formation of crystalline hexaglycine for the repeating unit, -(CH2-CO-NH)-, is compared to similar values derived from related peptides and amino acids in the solid phase. |
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