Low-temperature C-13 NMR spectra of cyclodecane (1) showed the presence of a minor conformation, assigned to the twist-boat-chair-chair (TBCC), in addition to the expected boat-chair-boat (BCB). If only the TBCC and BCB conformations were assumed to be appreciably populated, then a free-energy difference between the two conformations of 0.73 +/- 0.3 kcal/mol could be obtained from the five area measurements over a temperature range of -148.6 to -131.0 degrees C, with populations of 5.2 and 94.8% for the TBCC and BCB conformations at -146.1 degrees C. However, an alternative description of the conformations of 1 was suggested by the ab initio calculations, which predicted that the twist-boat-chair (TBC) and TBCC conformations have comparable free energies and populations. Equal amounts of TBCC and TBC would give populations of 5.2, 5.2, and 89.6% and relative free energies of 0.72, 0.72, and 0.00 kcal/mol for the TBCC, TBC, and BCB conformations at -146.1 degrees C, based on the experimental areas at this temperature. The experimental spectra could neither confirm nor disprove the presence of the TBC. Saunders＇ calculations of the strain energies of 1 using Allinger＇s MM3 program were reproduced to obtain a complete set of these parameters and drawings of the conformations, and free energies and populations were obtained at +25 and -171.1 degrees C. Free energies were also calculated at the HF/6-31G* and HF/6-311G* levels, and chemical shifts were obtained for three conformations at the HF/6-311G* level by the GIAO method. Chlorocyclodecane (2) was shown by C-13 and H-1 NMR spectroscopy to have three conformations at -165.5 degrees C. To aid in conformational assignments, the C-13 chemical shifts were calculated for all of the BCB and TBCC conformations of 2 using the GIAO method at the HF/6-311G* level. The free energies for each of the possible BCB, TBCC, and TBC conformations were also calculated using Allinger＇s MM3 program. From the line shape changes in the experimental C-13 NMR spectra, the free-energy barriers, a consideration of the X-ray structures of substituted cyclodecanes, and these calculated chemical shifts and free energies, the three conformations of 2 at -165.5 degrees C were suggested to be 2e BCB (31.2%), 2a BCB (14.9%), and a TBCC conformation (53.9%) (numbering as in Figure 1); the 2e and 2a BCB assignments could be reversed. Free-energy barriers for interconversion of BCB conformations of 2 at -159.8 degrees C were 5.4 +/- 0.2 and 5.5 +/- 0.2 kcal/mol, and the free-energy barriers at -120.9 degrees C for equilibration of the TBCC conformation with the rapidly interconverting BCB conformations were 7.07 +/- 0.2 and 7.08 +/- 0.2 kcal/mol. The C-13 NMR spectrum of cyclodecyl acetate (3) at -160.0 degrees C showed a similar pattern of chemical shifts and intensities for the substituted ring carbon.