data (left) with spectra simulated using the values in Table II (right) SM4: 60 Mz simulation of H2-H5 signals using values in Table II (left) compared with experimental spectrum reproduced from Ref. 19 (right) SM5. The singly occupied molecular orbital of the 1 2 A / state, which is much more compact than the ground state, with higher bow and stern angles. SM6. The transition state for thermal interconversion of CHT to NCD. SM7. The HOMO for the transition state for thermal interconversion of CHT to NCD. SM8. MP3 and MP4SDQ harmonic vibrational frequencies for CHT compared with anharmonic values at the MP2 level, including rotational constants for the vibrationally excited states, some of which are present in the microwave spectrum. SM9. The singly occupied molecular orbital of the 1 2 A / state, which is much more compact than the ground state, with higher bow and stern angles. SM10. MP4SDQ harmonic vibrational frequencies for 1 2 A / state of CHT. SM11. The X 2 A // surface as a function of the two dihedral angles chosen. See main Paper. SM12. A 2-dimensional representation of SM11, showing the regular pattern generated as the G-16 'scan' process occurs. See main text for details. SM13. The avoided crossing under C 1 symmetry of the 2 A / and 2 A // states in C S symmetry.