xAb initio calculations were performed on CH3CH2OOH, CH3CHClOOH, and CH3CCl2OOH molecules using the Gaussian92 system of programs. Geometries of stable rotational conformers and transition states for internal rotation were optimized at the RHF/6-31G* and MP2/6-31G* levels of theory. Harmonic vibrational frequencies were computed at the RHF/6-31G* level of theory. Potential barriers for internal rotations were calculated at the MP2/6-31G**/HF/6-31G* level. Parameters of the Fourier expansion of the hindrance potentials have been tabulated. Standard entropies (S degrees(298)) and heat capacities (C-p(T)’s, 300 less than or equal to T/K less than or equal to 1500) were calculated using the rigid-rotor-harmonic-oscillator approximation based on the information obtained from the ab initio studies. Contributions from hindered rotors were calculated by summation over the energy levels obtained by direct diagonalization of the Hamiltonian matrix of hindered internal rotations. Enthalpies of formation for these three molecules were calculated using isodesmic reactions. Enthalpies of formation were calculated to be Delta H degrees(f298)(CH3CH2OOH) = -41.5 +/- 1.5 kcal mol(-1), Delta H degrees(f298)(CH3CHClOOH) = -50.9 +/- 3.4 kcal mol(-1), and Delta H degrees(f298)(CH3CCl2OOH) = -55.3 +/- 2.2 kcal mol(-1). Entropies (S degrees(298)) are calculated to be 76.1, 79.2 and 86.6 cal mol(-1) K-1 for CH3CH2OOH, CH3CHClOOH, and CH3CCl2OOH, respectively.