Thermochemistry of α-D-xylose(cr)

The thermochemistry of α-D-xylose(cr) was studied by means of oxygen bomb calorimetry and a Physical Property Measurement System (PPMS) in zero magnetic field. The sample of α-D-xylose(cr) used in this study was one well-characterized by HPLC, Karl Fischer analysis, NMR, and by carbon dioxide analysis. The standard molar enthalpy of combustion was found to be ΔcHmo = −(2342.2 ± 0.8) kJ·mol−1 at T = 298.15 K and at the standard pressure p° = 0.1 MPa. The standard molar heat capacity for α-D-xylose(cr) was measured with the PPMS over the temperature range 1.9001 ⩽ T/K ⩽ 303.66. At T = 298.15 K, Cp,mo = (178.1 ± 1.8) J·K−1·mol−1. The values of Cp,mo were fit as a function of T   by using theoretical and empirical models for appropriate temperature ranges. The results of these fits were used to calculate values of Cp,mo, the entropy increment Δ0TSmo, Δ0THmo, and Φmo=(Δ0TSmo-Δ0THmo/T) from T = 0.5 K to T = 300 K. Derived quantities for α-D-xylose(cr) are the standard molar enthalpy of formation ΔfHmo = −(1054.5 ± 1.1) kJ·mol−1, the third law standard molar entropy Smo = (175.3 ± 1.9) J·K−1·mol−1, and the standard molar Gibbs energy of formation ΔfGmo = −(750.5 ± 1.0) kJ·mol−1. A comparison of values of ΔcHmo and Smo for the five-carbon aldoses demonstrated a striking similarity in the values of these respective properties for α-D-xylose(cr), D-ribose(cr), and D-arabinose(cr). Thermochemical network calculations were performed that led to values of the standard formation properties at T = 298.15 K for a variety of biochemical substances: D-xylose(aq), D-xylose−(aq), D-xylose2−(aq), D-lyxose(cr and aq), D-lyxose−(aq), D-xylulose(aq), xylitol(aq), 1,4-β-D-xylobiose(am and aq), and 1,4-β-D-xylotriose(am and aq).

► Well-characterized material.
► Oxygen bomb calorimetry.
► Heat capacities obtained by using a Physical Property Measurement System.
► Thermochemical Network Calculations.
► Accurate thermodynamic property values of a key biochemical substance.