Microwave spectrum, molecular structure, conformational equilibrium, vibrational frequencies and quantum chemical calculations for methyl vinyl sulfide

The microwave (MW) spectrum of methyl vinyl sulfide (CH2=CHSCH3) has been investigated in the 20.0–60.5 GHz spectral region at dry ice temperature. The most stable rotamer is planar with the methyl and vinyl groups in the syn conformation. A less stable skew rotamer has been assigned for the first time. Its C=C–S–C torsional angle is approximately 154° from syn. Isotopic species of all the heavy atoms of the skeleton of the syn conformation have been assigned. This allowed the following structure parameters to be calculated as: C1–C2=133.15 (18) pm, C1–S3=174.11 (9) pm, S3–C4=179.70 (9) pm, C2–C1–S3=130.23 (11)° and C1–S3–C4=101.99 (9)°. The uncertainties are one standard deviation. Ten vibrationally excited states of the syn conformer were assigned, while the ground state and two excited states of the skew conformer were assigned. The frequencies of several of these excited states have been determined by relative intensity measurements. The syn form was found to be 5.0 (3) kJ mol-1 more stable than the skew conformer by relative intensity measurements. The uncertainty is one standard deviation. Quantum chemical calculations at HF, MP2 (full), DFT and QCISD levels of theory using the 6-311++G**, cc-pVTZ and AUG-ccpVTZ basis sets have been made to assist the experimental work.