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Microwave and millimeter-wave rotational spectra of 46 thiophene isotopologues are used to determine a new semi-experimental equilibrium (reSE) structure for thiophene. The large dataset of isotopologues—roughly twice as large as any dataset used for a semi-experimental equilibrium structure determination for a molecule of this size—and the broad frequency coverage enable a detailed investigation concerning previously reported discrepancies between the structural parameters of the reSE and re structures of thiophene. The observation of rotational spectra for 23 new thiophene-dx isotopologues, many with additional 34S or 13C substitution at natural abundance, is made possible by the high sensitivity of the PARIS FTMW spectrometer in the microwave frequency region. Frequency coverage for thiophene and single heavy-atom isotopologues in the millimeter-wave region has been expanded to include the range of 82–750 GHz. Bond distances and bond angles of the new highly precise and accurate reSE structure are determined with precisions of ≤0.0002 Å and ≤0.02°, respectively. This study provides strong evidence that the discrepancies can be addressed only by improvements to the theoretical re structure and that the precision and accuracy of the semi-experimental equilibrium (reSE) structural parameters are approaching the limit of structure determination. 

Abstract Sulfur is one of the most abundant elements in the interstellar medium (ISM) and a key component for life, yet little is understood about its chemistry in the ISM. While increasingly larger molecules containing sulfur and oxygen are being observed in the ISM, the largest organic molecule containing oxygen and sulfur, monothioformic acid (HC(O)SH), was only recently detected. There is still no identification of a complex organic molecule (COM, carbon-bearing molecule with six or more atoms) containing both oxygen- and sulfur-bearing functional groups. We extended the laboratory rotational spectrum of 2-mercaptoethanol (HSCH₂CH₂OH), one of the simplest saturated COMs containing both oxygen and sulfur, into millimeter/submillimeter wavelengths, providing an improved spectral catalog at frequencies required for its interstellar identification. Millimeter/submillimeter transitions were measured for 2-mercaptoethanol from ∼82 to 450 GHz. Using the resulting spectral catalog, we searched for its rotational emission toward the Galactic Center molecular cloud G+0.693-0.027, the high-mass star-forming region Sgr B2(N), the cold dark core TMC-1, the hot core in Orion, and toward the hot corino surrounding the low-mass protostar IRAS 16293-2422B. An extensive analysis of 8584 transitions of 2-mercaptoethanol with $J_{\max }^{″}$= 104, and $K_{a\max }^{″}$= 64 is provided. The resulting fit includes a full set of quartic, sextic, and octic distortion constants. We report the nondetection of 2-mercaptoethanol and provide column density upper limits toward each source. While our interstellar search for 2-mercaptoethanol did not result in a detection, the upper limits on its column density provide important constraints for chemical models on the formation of oxygen- and sulfur-bearing COMs.