Pure Rotational Spectroscopy Of Vinyl Mercaptan

Vinyl mercaptan (ethenethiol, CH$_2$$=$CHSH) exists in the gas phase in two distinct rotameric forms, \textit{syn} (planar) and \textit{anti} (quasi-planar in the ground vibrational state). The microwave spectra of these two isomers were investigated previously\footnote{M. Tanimoto et al. \textit{J. Mol. Spectrosc.} 78, 95--105 \& 106--119 (1979)} however not exceeding frequencies of about 65 GHz. In the present investigation, the pure rotational spectra of both species have been investigated at millimeter wavelengths. Vinyl mercaptan was produced in a radiofrequency discharge through a constant flow of ethanedithiol at low pressure. Both \textit{syn} and \textit{anti} rotamers were observed and new extensive sets of molecular parameters were obtained. Owing to its close structural relationship to vinyl alcohol and the astronomical abundance of complex sulfur-bearing molecules, vinyl mercaptan is a plausible candidate for future radio astronomical searches

HIGH RESOLUTION FAR-INFRARED SPECTROSCOPY OF TRANS- AND GAUCHE-BUTADIENE

Much attention has surrounded 1,3-butadiene because this conjugated diene, the simplest of all, is an ideal candidate to observe the effects of $\pi$-electron delocalization, and because the species is of central importance in a wide range of chemistry applications. Butadiene exists in two forms, the most stable planar \textit{trans} and the long-elusive \textit{gauche} lying 12 kJ/mol higher in energy. The later was only conclusively detected in the gas phase by some of us recently, in a work that allowed the determination of a partial structure from a combination of pure rotational measurements and high level quantum chemical calculations. In an attempt to detect the ro-vibrational spectrum of \textit{gauche}-butadiene, we have re-investigated the far-infrared spectrum of the species at both the SOLEIL synchrotron facility and the Canadian light source using three experimental set-ups allowing to reach rotational temperatures ranging from 30 to 350 K. We will present our results on new fundamentals of the \textit{trans} form, and on the \textit{gauche} form

Millimeter-wave spectroscopy of OSSO

Sulfur is the element with the largest number of known binary oxides\footnote{Steudel, \textit{Top. Curr. Chem.} 231, 203--230 (2003)} and as such has attracted the curiosity of chemists and physicists for decades. In particular, the simpler ones are of great interest for diverse scientific disciplines like structural and theoretical chemistry, astrochemistry, atmospheric chemistry and molecular physics. Out of those, the simplest sulfur rich oxides and dioxides S$_2$O and S$_2$O$_2$ have been studied spectroscopically to some extent in the past\footnote{Thorwirth et al., \textit{J. Mol. Struct.} 795, 1-3 (2006), and refs. therein} but still pose challenging problems to future gas-phase investigations. In the present study, the pure rotational spectrum of S$_2$O$_2$ has been investigated in the ground and $\nu_3$ states. In addition, ground state transitions of OS$^{34}$SO were observed. OSSO was produced in a radio frequency discharge through SO$_2$. Experimental measurements have been supported by high-level CCSD(T) calculations. An extensive set of molecular parameters has been derived

Resolving a long-standing ambiguity: the non-planarity of gauche-1,3-butadiene revealed by microwave spectroscopy

The preferred conformation of \textit{cis}-1,3-butadiene (CH$_2$=CH$-$CH=CH$_2$) has been of long-standing importance in organic chemistry because of its role in Diels-Alder transition states. The molecule could adopt a planar \textit{s-cis} conformation, in favor of conjugations in the carbon chain, or a non-planar \textit{gauche} conformation, as a result of steric interactions between the terminal H atoms. To resolve this ambiguity, we have now measured the pure rotational spectrum of this isomer in the microwave region, unambiguously establishing a significant inertial defect, and therefore a \textit{gauche} conformation. Experimental measurements of \textit{gauche}-1,3-butadiene and several of its isotopologues using cavity Fourier-transform microwave (FTMW) spectroscopy in a supersonic expansion and chirped-pulse FTMW spectroscopy in a 4 K buffer gas cell will be summarized, as will new quantum chemical calculations

INVESTIGATING ISOMERS OF ASTROPHYSICAL MOLECULES BY ROTATIONAL SPECTROSCOPY: THE CASE OF [C2H2O] COMPOUNDS

Detection of isomers in the interstellar medium is a valuable tool toward a better understanding of the formation and destruction mechanisms at play, especially because kinetics effects are thought to be as important as thermodynamic ones, if not preponderant. About a third of the interstellar species discovered so far are isomers but little remains known on reactive isomers of relatively large astrophysical species (5 atoms and more), in part due to the difficulty to both produce and detect these species in the laboratory. Ketene is one such species: the molecule is a known interstellar species but none of its isomers has so far been detected by mean of rotational spectroscopy, preventing any interstellar detection so far. We have undertaken an experimental and theoreticcal investigation of the rotational spectrum of the isomers of ketene, and in particular of its two close-shell isomers, hydroxyacetylene (HCCOH) and oxirene ($c$-C$_2$H$_2$O). We will report our results on these compounds and propects of using a recent experimental technique -- spectral taxonomy -- to investigate isomers of astrophysical interest in the millimeter and submillimeter domains

Automated microwave double resonance spectroscopy: a tool to identify and characterize chemical compounds

Owing to its unparalleled structural specificity, rotational spectroscopy is a powerful technique to unambiguously identify and characterize volatile, polar molecules. We present here a new experimental approach, automated microwave double resonance (AMDOR) spectroscopy, to rapidly determine the rotational constants of these compounds without any \textit{a priori} knowledge of elemental composition or molecular structure. This task is achieved by rapidly acquiring the classical (frequency vs. intensity) broadband spectrum of a molecule using chirped-pulse Fourier transform microwave (FTMW) spectroscopy, and subsequently analyzing it in near-real time using complementary cavity FTMW detection and double resonance. AMDOR measurements provide a unique ``barcode'' for each compound from which rotational constants can be extracted. To illustrate the power of this approach, AMDOR spectra of three aroma compounds --- \textit{trans}-cinnamaldehyde, $\alpha$- and $\beta$-ionone --- have been recorded and analyzed. The prospects to extend this approach to mixture characterization and purity assessment are described

LABORATORY INVESTIGATION OF ASTRONOMICAL REACTIVE SPECIES: THE VIBRATIONAL SATELLITES OF c-C3H2 RE-VISITED

Cyclopropenylidene ($c$-C$_3$H$_2$) is one of the few polyatomic hydrocarbons ubiquitous in our galaxy, despite its reactive carbene nature (see e.g. [1]). Because it is so widely distributed in space, and because its $^{13}$C, D, and D$_2$ isotopologues have also been detected (see e.g. [2]), $c$-C$_3$H$_2$ is an ideal probe of the physical conditions in various astrophysical objects. It is surprising though that its vibrational satellites have yet to be detected in the interstellar medium. To enable the interstellar detection of vibrationally excited $c$-C$_3$H$_2$, and observe for the first time the elusive $\nu_4$ and $\nu_9$ vibrational modes, we have undertaken an extensive investigation of its spectrum from the centimeter to the submillimeter wavelengths, resulting in the observation of many new vibrational satellites in a promising spectral region for astronomical observations. Our measurements are supported by anharmonic rovibrational calculations using a high-quality ab initio potential energy surface, with particular attention paid to the $\nu_4$/$\nu_9$ Coriolis interaction. \medskip \noindent [1] S. Spezzano \textit{et al.}, \textit{The Astrophysical Journal Supplement Series} \textbf{200}, 1 (2012) \noindent [2] S. Spezzano \textit{et al.}, \textit{The Astrophysical Journal Letters}, \textbf{769}, L19 (2013

THE SOLEIL VIEW ON SULFUR OXIDES: THE S2O BENDING MODE _2 AT 380 cm_1�AND ITS ANALYSIS USING AN AUTOMATED SPECTRAL ASSIGNMENT PROCEDURE (ASAP)

The fundamental vibrational bending mode $nu_2$ of disulfur monoxide, S$_2$O, and the associated hot band $2nu_2 - nu_2$ have been observed at high spectral resolution for the first time at the SOLEIL synchrotron facility using Fourier-transform far-infrared spectroscopy. This transient species has been produced using a radio-frequency discharge by flowing SO$_2$ over elemental sulfur. The spectroscopic analysis has been performed using an Automated Spectral Assignment Procedure (ASAP) which has enabled the accurate determination of more than 3500 energy levels of the $v_2=1$ and $v_2=2$ vibrational states. In addition to the high-resolution synchrotron study, pure rotational spectra of S$_2$O in the $v_2=1$ and 2 vibrational states were observed in the frequency range 250 -- 500 GHz in a long-path absorption cell

DETECTION OF HSNO, A CRUCIAL INTERMEDIATE LINKING NO AND H2S CHEMISTRIES

The simplest S-nitrosothiol, thionitrous acid (HSNO), is a reactive molecule of both biological and astronomical interest. Here we report the first detection of both textit{cis}- and textit{trans}-HSNO by means of Fourier-transform microwave spectroscopy and double resonance experiments. Surprisingly, HSNO is readily produced in a gas expansion of H$_2$S and NO, i.e. without applying any discharge. Once formed, HSNO appears quite stable, as evidenced by its high steady-state concentration. A precise empirical molecular equilibrium structure was derived from a combination of theory and experiment

THE STRUCTURE OF gauche-BUTADIENE: INSIGHTS FROM THE CENTIMETER, MILLIMETER, AND FIR-INFRARED HIGH RESOLUTION SPECTRA

\noindent \begin{minipage}{0.67\textwidth} Recent investigation of the centimeter spectrum of \textit{gauche}-butadiene has unambiguously established a non-planar conformation for this fundamental, archetypal diene for the Diels-Alder reaction [1]. We will present subsequent theoretical and experimental investigations aimed at determining a highly accurate molecular structure and the barrier height for interconversion between the two equivalent tunneling \textit{gauche} forms. \medskip \noindent [1] J. H. Baraban, M.-A. Martin-Drumel, \textit{et al.}, \textit{Angewandte Chemie}, \textbf{57}, 1821 (2018) \end{minipage} \begin{minipage}{0.3\textwidth} \includegraphics[width=\textwidth]{butadiene_pict.eps} \end{minipage