Is the molecular structure of 1,4,7-trithiacyclononane([9]aneS3) as symmetrical in the gas phase as it is in the crystal? An electron diffraction study

The molecular structure of [9]aneS3 has been studied by gas electron diffraction (GED) and by molecular-mechanics calculations (MM2). Four molecular models have been fitted to the experimental data, one having D3 symmetry, one C3 symmetry, one C2 symmetry and one C1 symmetry. The D3 model was incompatible with the GED data and was discarded. The C1 model gave the lowest R-factor in the GED analysis, and the C2 model gave only a slightly poorer fit to the data. The C3 model, which corresponds to the conformation observed in the crystalline state, could not be completely excluded, but the model gave a significantly worse fit to the GED data than the C2 and C1 models. MM2 calculations also favoured the C1 model, but with the C3 model only 0.13 kJ mol–1 higher in strain energy. The C2 and D3 models were higher in energy by 8.3 and 9.9 kJ mol–1 respectively.