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The rotational spectra of two new silicon-bearing carbon chains, H 2 C 3 Si and HSiCCH have been detected by means of Fourier-transform microwave spectroscopy in a supersonic jet source equipped with an electrical discharge. On the basis of measurements between 10 and 42 GHz, precise spectroscopic constants have been determined for both molecules. Along with H29SiCCH and H30SiCCH, which were detected in natural abundance, several other rare isotopic species of HSiCCH were observed using samples enriched in carbon-13 and D. From this isotopic data, a precise semi-experimental equilibrium structure was derived and compared with a re-evaluated semi-experimental equilibrium structure of the parent silylene SiH 2 ; the deuterium isotopic measurements also provide important clues as to the formation pathway of HSiCCH in our discharge. Because other small Si-bearing chains have been detected in the circumstellar envelope of the evolved carbon star IRC+10216, both chains may be of astronomical interest. [Display omitted] • Laboratory detection of two new silicon-bearing carbon chains. • The HSiC angle of HSiCCH is close to that of SiH2. • Isotopic studies point to formation pathway of SiiCCH. [ABSTRACT FROM AUTHOR]

Recent advances in circumstellar metal chemistry and laser-coolable molecules have spurred interest in the spectroscopy and electronic properties of alkaline earth metal-bearing polyatomic molecules. We report the microwave rotational spectra of two members of this important chemical family, the linear magnesium-carbon chains MgC 4 H and MgC 3 N, detected with cavity Fourier transform microwave spectroscopy of a laser ablation-electric discharge expansion. The rotation, fine, and hyperfine parameters have been derived from the precise laboratory rest frequencies. These experimental results, combined with a theoretical quantum chemical analysis, confirm the recent identification of MgC 4 H and MgC 3 N in the circumstellar envelope of the evolved carbon-rich star IRC+10216. The spectroscopic data also provide insight into the structural and electronic properties that influence the metal-based optical cycling center in this unique class of laser-coolable polyatomics. [Display omitted] • MgC 4 H and MgC 3 N were detected in the laboratory with microwave spectroscopy. • Confirmation of the astronomical identification in carbon-rich circumstellar envelopes. • Fine and hyperfine structure reveals a highly metal-localized unpaired electron. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rotational spectrum (130–360 GHz) measured for chlorobenzene. • Constants determined for ground vibrational states of [35Cl]-, [37Cl]-chlorobenzene. • Constants determined for 7 fundamental states, 8 overtone/combination states of [35Cl]-chlorobenzene. • Constants determined for 4 fundamental states of [37Cl]-chlorobenzene. • Data enable detailed comparisons of spectroscopic constants with structurally related compounds. The rotational spectra of [35Cl]- and [37Cl]-chlorobenzene (C 6 H 5 Cl) have been studied from 2 to 18 GHz and 130–360 GHz, resulting in the measurement, assignment, and least-squares fitting of almost 40,000 transitions of twenty-one vibrational states. Previously measured [35Cl]- and [37Cl]-chlorobenzene ground-state transitions were combined with newly measured transitions and fit to A-reduced, partial sextic Hamiltonian models with low-error (σ total fit <0.05 MHz). Analysis of the 2–18 GHz spectrum allowed for refinement of the nuclear hyperfine coupling constants for the ground-state spectra of both isotopologues, while measurement of the 130–360 GHz spectrum provided sufficient information to determine the sextic centrifugal distortion constants of the ground states for the first time. From these millimeter-wave data collected at room temperature, the spectroscopic constants for the lowest-energy fundamentals of [35Cl]-chlorobenzene (ν 20 , ν 30 , ν 11 , ν 14 , ν 19 , ν 29 and ν 18) and [37Cl]-chlorobenzene (ν 20 , ν 30 , ν 11) were determined. As with previous studies of chloroarenes, the computed (B3LYP/6–311+G(2d,p)) spectroscopic constants show quite close agreement with the experimentally determined values. [ABSTRACT FROM AUTHOR]

Infrared action spectra of the protonated forms of two fundamental organic nitriles, vinyl cyanide, C2H3CN , and ethyl cyanide, C2H5CN , have been observed using the FELion 22-pole ion trap apparatus and the Free Electron Laser for Infrared eXperiments (FELIX) at Radboud University, Nijmegen (The Netherlands). Vibrational bands of both ions were observed in the wavenumber regime between 300 and 2500 cm−1 and spectroscopically assigned based on results from anharmonic force field calculations performed here at the coupled cluster level of theory. As the action spectroscopy scheme used in the present study probes the Ne-tagged weakly bound complexes, C 2 H 3 CNH + − Ne and C 2 H 5 CNH + − Ne, corresponding quantum-chemical calculations of these systems were also performed. Significant blueshifts observed for the strong -CNH + bending modes indicate that the neon atom is attached to the proton bound to the nitrile group in both molecular ions. [Display omitted] • First infrared spectroscopic study of protonated vinyl- and ethyl cyanide. • Free Electron Laser for Infrared eXperiments, FELIX, used as radiation source. • Detection via infrared photodissociation of the Ne-ion clusters. • Experimental study complemented by high-level CCSD(T) calculations. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rotational transitions from 8 to 38 and 130 to 360 GHz used to study anti- and gauche- 4-cyano-1-butyne (an isomer of pyridine). • Spectroscopic constants for the ground vibrational states determined for the first time. • Hyperfine resolved transitions included to determine quadrupole coupling constants (χ aa , χ bb , and χ cc). • Experimental energy difference determined between the conformers (Δ E anti – gauche). The analysis of the rotational spectrum of 4-cyano-1-butyne, an open-chain isomer of pyridine, is presented herein for the first time. Transitions for the ground vibrational states of both conformational isomers, anti and gauche , were observed in the 8–38 GHz and 130–360 GHz frequency ranges and fit to effective sextic distorted-rotor Hamiltonians with low uncertainty (σ fit < 50 kHz), allowing for their potential detection by radioastronomy or in harsh reaction environments. The microwave region includes hyperfine-split transitions for both conformers, allowing for the experimental determination of the χ aa , χ bb , and χ cc quadrupole coupling constants. In the millimeter-wave region, not every observable ground-state transition could be satisfactorily addressed by a single-state Hamiltonian due to Coriolis coupling with the low-energy vibrationally excited states. Thus, effective single-state fits of the ground states were obtained by including only K a -series transitions without obviously perturbed energy levels. Importantly, the effective fit does include the most intense transitions throughout the observed frequency ranges. The predicted values (MP2/cc-pVTZ) are in close agreement with the experimental spectroscopic constants. These data provide a sufficient foundation for the identification of anti- and gauche- 4-cyano-1-butyne in the interstellar medium by radioastronomy. The gauche conformer is estimated to lie 1.00 (5) kcal/mol higher in energy than the anti conformer, as determined using experimental transition intensities. [ABSTRACT FROM AUTHOR]