Populations and free-energy differences for the E and Z conformations of S-methyl, cyclopropyl, isopropyl, and cyclopentyl thioformate were determined by low-temperature 1H NMR spectroscopy, and free-energy barriers of 10.63 and 11.84 kcal/mol were obtained for interconversion of E and Z conformations of S-methyl thioformate at −52.4 °C. Populations and free-energy differences were also determined at room temperature by using 13C NMR for a series of N-substituted formamides and N-cyclopropylacetamide in 1% solutions in CD2Cl2/CH2Cl2. In both sets of compounds, electron-withdrawing groups attached to sulfur or nitrogen appear to favor the E conformations. The electronegativities of the groups are taken to increase in the order methyl < vinyl ∼ phenyl ∼ cyclopropyl < hydrogen < ethynyl. Data from the literature are discussed in these terms, including the E−Z energy differences for formic acid and its ethynyl, vinyl, and methyl esters.

Low-temperature 13C 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 °C, with populations of 5.2 and 94.8% for the TBCC and BCB conformations at −146.1 °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 °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 °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 13C and 1H NMR spectroscopy to have three conformations at −165.5 °C. To aid in conformational assignments, the 13C 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 13C 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 °C were suggested to be 2e BCB (31.2%), 2a BCB (14.9%), and a TBCC conformation (53.9%) (numbering as in Figure ); the 2e and 2a BCB assignments could be reversed. Free-energy barriers for interconversion of BCB conformations of 2 at −159.8 °C were 5.4 ± 0.2 and 5.5 ± 0.2 kcal/mol, and the free-energy barriers at −120.9 °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 13C NMR spectrum of cyclodecyl acetate (3) at −160.0 °C showed a similar pattern of chemical shifts and intensities for the substituted ring carbon.

[reaction: see text] Low-temperature 13C NMR spectra of cis-1,4-di-tert-butylcyclohexane (1) showed signals for the twist-boat (1a) and chair (1b) conformations. 13C NMR signals were assigned to specific carbons based on the different populations, different symmetries (time-averaged C(2v) for 1a and time-averaged C(s) for 1b), and calculated chemical shifts (GIAO, HF/6-311+G*). In addition to slow ring inversion and interconversion of the chair and twist-boat conformations, slow rotation of the tert-butyl groups was found. Most of the expected 13C peaks were observed. Free-energy barriers of 6.83 and 6.35 kcal/mol were found for interconversion of 1a (major) and 1b (minor) at -148.1 degrees C. Conformational space was searched with Allinger's MM3 and MM4 programs, and free energies were obtained for several low-energy conformations 1a-c. Calculations were repeated with ab initio methods up to the HF/6-311+G* level. Molecular symmetries, relative free energies, relative enthalpies and entropies, frequencies, and NMR chemical shifts were obtained. A boat conformation (1d; C(2v) symmetry) was generated and optimized as a transition state by ab initio, MM3, and MM4 calculations.

Interconversion of carbon sites in boat-chair-boat (BCB) cyclodecane occurs by way of the twist-boat-chair (TBC) conformation, which predicts that C-1 exchanges with C-4, etc. The previously obtained low-temperature 13C spectra could be matched by assuming (1,4) or (1,3) exchange, but not (1,2) exchange. A free-energy barrier of 5.54 kcal/mol was obtained for conversion of BCB to TBC at -137.4 degrees C. The major conformation of chlorocyclodecane and cyclodecyl acetate is assigned to the IIIe BCB.