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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]

Rotational spectroscopy is an invaluable tool to unambiguously determine the molecular structure of a species, and sometimes even to establish its very existence. This article illustrates how experimental and theoretical state‐of‐the‐art tools can be used in tandem to investigate the rotational structure of molecules, with particular emphasis on those that have long remained elusive. The examples of three emblematic species—gauche‐butadiene, disilicon carbide, and germanium dicarbide—highlight the close, mutually beneficial interaction between high‐level theoretical calculations and sensitive microwave measurements. Prospects to detect other elusive molecules of chemical and astronomical interest are discussed. [ABSTRACT FROM AUTHOR]

We report the gas-phase detection and spectroscopic characterisation of ethynethiol (), a metastable isomer of thioketene () using a combination of Fourier-transform microwave and submillimetre-wave spectroscopies. Several a-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, b-type lines using double resonance provided accurate predictions in the submillimetre region. With these, searches using a millimetre-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280–660 GHz, ultimately yielding nearly 100 transitions up to and. From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of HCCSH and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction , which is calculated to be highly exothermic (−477 kJ/mol) using the HEAT345(Q) thermochemical scheme. [ABSTRACT FROM AUTHOR]

Abstract: The planarity of the second stable conformer of 1,3‐butadiene, the archetypal diene for the Diels–Alder reaction in which a planar conjugated diene and a dienophile combine to form a ring, is not established. The most recent high level calculations predicted the species to adopt a twisted, gauche structure owing to steric interactions between the inner terminal hydrogens rather than a planar, cis structure favored by the conjugation of the double bonds. The structure cis‐1,3‐butadiene is unambiguously confirmed experimentally to indeed be gauche with a substantial dihedral angle of 34°, in excellent agreement with theory. Observation of two tunneling components indicates that the molecule undergoes facile interconversion between two equivalent enantiomeric forms. Comparison of experimentally determined structures for gauche‐ and trans‐butadiene provides an opportunity to examine the effects of conjugation and steric interactions. [ABSTRACT FROM AUTHOR]

The phenyl radical (C6H5.) is the prototypical σ-type aryl radical and one of the most common aromatic building blocks for larger ring molecules. Using a combination of rotational spectroscopy of singly substituted isotopic species and vibrational corrections calculated theoretically, an extremely accurate molecular structure has been determined. Relative to benzene, the phenyl radical has a substantially larger C-Cipso-C bond angle [125.8(3)° vs. 120°], and a shorter distance [2.713(3) Å vs. 2.783(2) Å] between the ipso and para carbon atoms. [ABSTRACT FROM AUTHOR]

Abstract: Transitions between the spin–rotational levels of the 12CH radical in the v =1 level of the X 2Π state have been studied by the technique of laser magnetic resonance at far-infrared wavelengths. The data have been combined with a measurement of lambda-doubling transition frequencies at 7GHz to determine an improved set of molecular parameters for CH in the v =1 level. The parameters provide information on the effects of vibrational excitation on the structural properties of CH. Accurate predictions of the transition frequencies between the low-lying levels of the radical in the absence of a magnetic field have also been made. Small inconsistencies in the least-squares fit of the laser magnetic resonance data prompted re-measurement of three far-infrared laser frequencies, the 122.5μm line of CH2F2 pumped by 9R(22), the 122.5μm line of CH2F2 pumped by 9P(8) and the 554.4μm line of CH2CF2 pumped by 10P(14). The new measurements differ by as much as 3.8MHz from those made previously and are more accurate; they also remove the inconsistencies in the fit. The re-measured frequencies of the two 122.5μm lines are identical within experimental error which suggests that the far-infrared lasing transition is the same, namely the rR23(32) transition in the v 9=1 level of CH2F2. [Copyright &y& Elsevier]

[Display omitted] • New high-speed method for fitting and predicting rotational spectra. • Factors of hundreds faster than existing methods. • Flexible, lightweight, and parallelizable code. The advent of chirped-pulse Fourier transform microwave spectroscopy has dramatically increased the acquisition rate of high-resolution data. However, this data is only useful when it can be translated into molecule-specific information, and ultimately chemical knowledge. To adapt to this high data rate regime, new data analysis tools, which do not result in commensurate increases in time, are needed. Several methods, both computational and experimental, have been developed to simplify and automate the assignment of microwave spectra, and such tools can also make microwave spectroscopy far more approachable and usable for non-experts. Computationally efficient spectral assignment is a particularly promising avenue to enable rapid and nearly exhaustive spectral identification. Towards this end, we have recently developed an efficient, high-speed program for the prediction and fitting of rotational spectra. This program is built on a simple framework that is applicable to a wide variety of molecules. We present this program, discuss its performance, and demonstrate its use in fitting broadband spectra at centimeter wavelengths. [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]

[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]