Molecular structures of SeSCH32and TeSCH32using gas-phase electron diffraction and ab initioand DFT geometry optimisationsElectronic supplementary information (ESI) available: Table S1: Nozzle-to-film distances, weighting functions, scale factors, correlation parameters and electron wavelengths, used in the electron diffraction studies of Se(SCH3)2and Te(SCH3)2. Table S2: Comparison of rC–C and various amplitudes of vibration for the fractional weight and bilinear methods of digital pixel interpolation for the GED scattering pattern for benzene. Tables S3–S6: Calculated [HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), MP2/LanL2DZ(d)] coordinates for Se(SCH3)2. Tables S7–S10: Calculated [HF/3-21G(d), HF/LanL2DZ(d), B3LYP/LanL2DZ(d), MP2/LanL2DZ(d)] coordinates for Te(SCH3)2. Tables S11 and S12: Least-squares correlation matrix for Se(SCH3)2and Te(SCH3)2. Fig. S1 and S2: Experimental and difference (experimental − theoretical) molecular-scattering intensities for Se(SCH3)2and Te(SCH3)2. See h

The molecular structures of SeSCH32and TeSCH32were investigated using gas-phase electron diffraction GED and ab initioand DFT geometry optimisations. While parameters involving H atoms were refined using flexible restraints according to the SARACEN method, parameters that depended only on heavy atoms could be refined without restraints. The GED-determined geometric parameters rh1 are: rSe–S 219.11, rS–C 183.21, rC–H 109.64 pm; ∠S–Se–S 102.93, ∠Se–S–C 100.62, ∠S–C–H mean 107.45, S–Se–S–C 87.920, Se–S–C–H 178.819° for SeSCH32, and rTe–S 238.12, rS–C 184.13, rC–H 110.06 pm; ∠S–Te–S 98.96, ∠Te–S–C 99.74, ∠S–C–H mean 109.29, S–Te–S–C 73.048, Te–S–C–H 180.119° for TeSCH32. Ab initioand DFT calculations were performed at the HF, MP2 and B3LYP levels, employing either full-electron basis sets 3-21Gd or 6-31Gd or an effective core potential with a valence basis set LanL2DZd. The best fit to the GED structures was achieved at the MP2 level. Differences between GED and MP2 results for rS–C and ∠S–Te–S were explained by the thermal population of excited vibrational states under the experimental conditions. All theoretical models agreed that each compound exists as two stable conformers, one in which the methyl groups are on the same side gg−conformer and one in which they are on different sides ggconformer of the S–Y–S plane Y Se, Te. The conformational composition under the experimental conditions could not be resolved from the GED data. Despite GED R-factors and ab initioand DFT energies favouring the ggconformer, it is likely that both conformers are present, for SeSCH32as well as for TeSCH32.