Structures, internal rotational barriers, and thermochemical properties of 2-propanol, 2-chloro-2-propanol, isopropoxy radical, 1-chloro-1-methylethoxy, 2-hydroxypropyl, and 2-chloro-2-hydroxypropyl were computed by ab initio and density functional calculations. Molecular structures and vibration frequencies were determined at the B3LYP/6-31G(d,p) density functional level, with single-point calculations for the energy at the B3LYP/ 6-311 +G(3df,2p) and CBSQ//B3LYP/6-31G(d,p) levels. The Sdegrees(298) and C-p(T)'s (0 less than or equal to T less than or equal to 5000 K) from 29 vibrational, translational, and external rotational contributions were calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. Potential barriers for the internal rotations were calculated at the B3LYP/6-3 I G(d,p) level, and hindered rotational contributions to S'. and Cp(T)'s were calculated by using direct integration over energy levels of the internal rotational 29 potentials. The DeltaHdegrees(f.298) values for the above species were calculated using several isodesmic reactions for each species, and bond energies were also determined. The C/C-2/Cl/O group value was derived for use in Benson-type group additivity. Hydrogen bond increment groups for C2CO., C2CClO., (CC)-C-.-(C)CIOH, and (CC)-C-.(C)ClOH were also developed. The recommended DeltaHdegrees(f.298) values for (CH3)(2)CHOH, (CH3)(2)CHOH, (CH3)(2)(CH3)CHOH, (CH3)(2)CClOH, (CH3)(2)CCIO., and (CH2CCl)-H-.(OH)CH3 are -69.19 +/- 2.2, -11.85 +/- 1.9, -14.95 +/- 2.8, -79.83 +/- 2.1, -25.88 +/- 2.0, and -29.00 +/- 2.8 kcaUmol, respectively. The radical (CH3)(2)CCIO. has low stability with a short atmospheric lifetime. Bond energies for the methyl hydrogens and the hydroxyl hydrogen are 102 and 105 kcaUmol in 2-propanol and 103 and 106 kcal/mol in 2-chloro-2-propanol. The recommended HBI values for oxy(chloro)alkanes are presented for use in group additivity.