High-resolution rotational spectrum analysis of 3-methylthiophene, an atmospheric precursor of sulfur-containing secondary organic aerosols

Sulfur-containing heterocyclic compounds emitted from natural and anthropogenic sources form organosulfur-rich secondary organic aerosols (SOAs) that impact climate. Accurately quantifying these precursors requires millimeter-wave spectroscopic characterisation and ground state molecular parameters. In this work, we focus on 3-methylthiophene, an atmospherically relevant sulfur heterocycle. Its rotational spectrum was recorded in the frequency range 70–100 GHz using the LPCA spectrometer at Dunkirk. Internal rotation splittings from the methyl group were fitted using the BELGI-Cs code, yielding a unitless standard deviation of 0.79 for 977 assigned transitions (vt = 0 and vt = 1). Eighteen spectroscopic parameters were determined, including the barrier to methyl internal rotation V3 = 268.66(62) cm−1 for which a comparison with structurally related molecules is made.Experimental spectrum (upper trace) recorded between 85.5 and 88.0 GHz compared to the theoretical spectrum calculated using the BELGI-Cs code (lower trace), illustrating different assigned a-type transitions JKa′,Kc′′←J′Ka′′,Kc′′′ for 3-methylthiophene.