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Abstract: Structures, internal rotation, and ideal gas thermodynamic properties of 40 organophosphorus(V) compounds are determined by ab initio and density functional calculations. The enthalpies of formation are calculated at the G3X level of theory using the atomization energy procedure. The thermodynamic functions (entropy, heat capacity, and enthalpy) are calculated using a rigid-rotor harmonic-oscillator molecular model. The contributions to the thermodynamic functions due to the internal rotation are computed by the direct summation of the internal rotational energy levels obtained by diagonalization of the Hamiltonian matrix of hindered internal rotations. The needed structural parameters, vibrational frequencies, and potential functions for internal rotation are determined at the B3LYP/6-31G(d,p) level. The computational results are compared with available experimental values of enthalpy of formation and entropy. [Copyright &y& Elsevier]