Semiempirical (MNDO, AMI, and PM3) and ab initio (HF/3-21+G(d,p), BVWN/3-21+G(d,p), MP2/3-21+G(d,p), HF/6-311+G(d,p), BVWN/6-311+G(d,p), and MP2/6-311+G(d,p)) energy minimization and frequency calculations have been carried out on three hydrogen chloride polymers, (HCl)(n), n = 2-4 (the semiempirical methods were employed for n = 5 and 6 as well); on four polychlorohydrogenate(1-) anions, HnCln+1-, n = 1-4; and on two polychlorohydrogen(I) cations, Hn+1Cln+, n = 1 and 2. From the results, Delta G degrees, Delta H degrees, and Delta S degrees values have been calculated for a number of reactions involving these species. The thermodynamic data are presented for 21 reactions using calculations at the MP2/6-311+G(d,p) level, along with a comparison of Delta G degrees values obtained from all nine molecular orbital methods. H-1 NMR shielding data have also been calculated for three species, HCl, HCl2-, and H2Cl+, using the gauge-independent atomic orbital (GIAO) and the continuous set of gauge transformations (CSGT) methods and density functional techniques with large basis sets.