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Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new Ka band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire Ka band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert-butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13C and 18O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements. [ABSTRACT FROM AUTHOR]

Photodissociation is one of the main destruction pathways for dicarbon (C2) in astronomical environments, such as diffuse interstellar clouds, yet the accuracy of modern astrochemical models is limited by a lack of accurate photodissociation cross sections in the vacuum ultraviolet range. C2 features a strong predissociative F 1 Π u − X 1 Σ g + electronic transition near 130 nm originally measured in 1969; however, no experimental studies of this transition have been carried out since, and theoretical studies of the F1Πu state are limited. In this work, potential energy curves of excited electronic states of C2 are calculated with the aim of describing the predissociative nature of the F1Πu state and providing new ab initio photodissociation cross sections for astrochemical applications. Accurate electronic calculations of 56 singlet, triplet, and quintet states are carried out at the DW-SA-CASSCF/MRCI+Q level of theory with a CAS(8,12) active space and the aug-cc-pV5Z basis set augmented with additional diffuse functions. Photodissociation cross sections arising from the vibronic ground state to the F1Πu state are calculated by a coupled-channel model. The total integrated cross section through the F1Πuv = 0 and v = 1 bands is 1.198 × 10−13 cm2 cm−1, giving rise to a photodissociation rate of 5.02 × 10−10 s−1 under the standard interstellar radiation field, much larger than the rate in the Leiden photodissociation database. In addition, we report a new 2 1 Σ u + state that should be detectable via a strong 2 1 Σ u + − X 1 Σ g + band around 116 nm. [ABSTRACT FROM AUTHOR]

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 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 - trans-cinnamaldehyde, α-, and β-ionone - have been recorded and analyzed. The prospects to extend this approach to mixture characterization and purity assessment are described. [ABSTRACT FROM AUTHOR]

Because of its structural specificity, rotational spectroscopy has great potential as an analytical tool for characterizing the chemical composition of complex gas mixtures. However, disentangling the individual molecular constituents of a rotational spectrum, especially if many of the lines are entirely new or unknown, remains challenging. In this paper, we describe an empirical approach that combines the complementary strengths of two techniques, broadband chirped-pulse Fourier transform microwave spectroscopy and narrowband cavity Fourier transform microwave spectroscopy, to characterize and assign lines. This procedure, called microwave spectral taxonomy, involves acquiring a broadband rotational spectrum of a rich mixture, categorizing individual lines based on their relative intensities under series of assays, and finally, linking rotational transitions of individual chemical compounds within each category using double resonance techniques. The power of this procedure is demonstrated for two test cases: a stable molecule with a rich spectrum, 3,4-difluorobenzaldehyde, and products formed in an electrical discharge through a dilute mixture of C2H2 and CS2, in which spectral taxonomy has enabled the identification of propynethial, HC(S)CCH. [ABSTRACT FROM AUTHOR]

HOCO is an important intermediate in combustion and atmospheric processes because the OH + CO → H + CO2 reaction represents the final step for the production of CO2 in hydrocarbon oxidation, and theoretical studies predict that this reaction proceeds via various intermediates, the most important being this radical. Isotopic investigations of trans- and cis-HOCO have been undertaken using Fourier transform microwave spectroscopy and millimeter-wave double resonance techniques in combination with a supersonic molecular beam discharge source to better understand the formation, chemical bonding, and molecular structures of this radical pair. We find that trans- HOCO can be produced almost equally well from either OH + CO or H + CO2 in our discharge source, but cis-HOCO appears to be roughly two times more abundant when starting from H + CO2. Using isotopically labelled precursors, the OH + C18O reaction predominately yields HOC18O for both isomers, but H18OCO is observed as well, typically at the level of 10%-20% that of HOC18O; the opposite propensity is found for the 18OH + CO reaction. DO + C18O yields similar ratios between DOC18O and D18OCO as those found for OH + C18O, suggesting that some fraction of HOCO (or DOCO) may be formed from the back-reaction H + CO2, which, at the high pressure of our gas expansion, can readily occur. The large 13C Fermi-contact term (aF) for trans- and cis-HO13CO implicates significant unpaired electronic density in a σ-type orbital at the carbon atom, in good agreement with theoretical predictions. By correcting the experimental rotational constants for zero-point vibration motion calculated theoretically using second-order vibrational perturbation theory, precise geometrical structures have been derived for both isomers. [ABSTRACT FROM AUTHOR]

The chemical reaction H3+ + H2 → H2 + H3+ is the simplest bimolecular reaction involving a polyatomic, yet is complex enough that exact quantum mechanical calculations to adequately model its dynamics are still unfeasible. In particular, the branching fractions for the 'identity,' 'proton hop,' and 'hydrogen exchange' reaction pathways are unknown, and to date, experimental measurements of this process have been limited. In this work, the nuclear-spin-dependent steady-state kinetics of the H3+ + H2 reaction is examined in detail, and employed to generate models of the ortho:para ratio of H3+ formed in plasmas of varying ortho:para H2 ratios. One model is based entirely on nuclear spin statistics, and is appropriate for temperatures high enough to populate a large number of H3+ rotational states. Efforts are made to include the influence of three-body collisions in this model by deriving nuclear spin product branching fractions for the H5+ + H2 reaction. Another model, based on rate coefficients calculated using a microcanonical statistical approach, is appropriate for lower-temperature plasmas in which energetic considerations begin to compete with the nuclear spin branching fractions. These models serve as a theoretical framework for interpreting the results of laboratory studies on the reaction of H3+ with H2. [ABSTRACT FROM AUTHOR]

The nuclear spin dependence of the chemical reaction H3++ H2 → H2 + H3+ has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho- and para-H3+ (o-H3+ and p-H3+) in hydrogenic plasmas of varying para-H2 (p-H2) enrichment. The ratio of the rates of the proton hop (kH) and hydrogen exchange (kE) reactions α ≡ kH/kE is inferred from the observed p-H3+ fraction as a function of p-H2 fraction using steady-state chemical models. Measurements have been performed both in uncooled (Tkin ∼ 350 K) and in liquid-nitrogen-cooled (Tkin ∼ 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of α has been found to decrease from 1.6 ± 0.1 at 350 K to 0.5 ± 0.1 at 135 K. [ABSTRACT FROM AUTHOR]

In an effort to develop a source of H3+ that is almost entirely in a single quantum state (J=K=1), we have successfully generated a plasma that is enriched to ∼83% in para-H3+ at a rotational temperature of 80 K. This enrichment is a result of the nuclear spin selection rules at work in hydrogenic plasmas, which dictate that only para-H3+ will form from para-H2, and that para-H3+ can be converted to ortho-H3+ by subsequent reaction with H2. This is the first experimental study in which the H2 and H3+ nuclear spin selection rules have been observed at cold temperatures. The ions were produced from a pulsed solenoid valve source, cooled by supersonic expansion, and interrogated via continuous-wave cavity ringdown spectroscopy. [ABSTRACT FROM AUTHOR]

The determination of the dissociative recombination rate coefficient of H3+ has had a turbulent history, but both experiment and theory have recently converged to a common value. Despite this convergence, it has not been clear if there should be a difference between the rate coefficients for ortho-H3+ and para-H3+. A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of H3+ in the diffuse interstellar medium, where H3+ has been widely observed. We present the results of an experiment at the CRYRING ion storage ring in which we investigated the dissociative recombination of highly enriched (∼83.6%) para-H3+ using a supersonic expansion source that produced ions with Trot∼60–100 K. We observed an increase in the low energy recombination rate coefficient of the enriched para-H3+ by a factor of ∼1.25 in comparison to H3+ produced from normal H2 (ortho:para=3:1). The ratio of the rate coefficients of pure para-H3+ to that of pure ortho-H3+ is inferred to be ∼2 at low collision energies; the corresponding ratio of the thermal rate coefficients is ∼1.5 at electron temperatures from 60 to 1000 K. We conclude that this difference is unlikely to have an impact on the interstellar ortho:para ratio of H3+. [ABSTRACT FROM AUTHOR]

Diffuse interstellar bands (DIBs) are absorption features commonly observed in optical/nearinfrared spectra of stars and thought to be associated with polyatomic molecules that comprise a significant reservoir of organic material in the Universe. However, the central wavelengths of almost all DIBs do not correspond with electronic transitions of known atomic or molecular species and the specific physical nature of their carriers remains inconclusive despite decades of observational, theoretical and experimental research. It is well established that DIB carriers are located in the interstellar medium, but the recent discovery of time-varying DIBs in the spectra of the extragalactic supernova SN 2012ap suggests that some may be created in massive star environments. Here, we report evidence of short time-scale (∼10–60 d) changes in DIB absorption line substructure towards 3 of 17 massive stars observed as part of a pathfinder survey of variable DIBs conducted with the 1.5-m Tillinghast telescope and Tillinghast Reflector Echelle Spectrograph (TRES) at Fred L. Whipple Observatory. The detections are made in high-resolution optical spectra (R ∼ 44 000) having signal-to-noise ratios of 5–15 around the 5797 and 6614 Å features, and are considered significant but requiring further investigation. We find that these changes are potentially consistent with interactions between stellar winds and DIB carriers in close proximity. Our findings motivate a larger survey to further characterize these variations and may establish a powerful new method for probing the poorly understood physical characteristics of DIB carriers. [ABSTRACT FROM AUTHOR]

Silaisocyanoacetylene, HCCNSi, silaisocyanodiacetylene, HC4NSi, and silaisocyanogen, NCNSi, have been identified spectroscopically for the first time. All three transient species were observed at high spectral resolution at centimeter wavelengths (5–40 GHz) by microwave spectroscopy. From detection of less abundant isotopic species and high-level quantum-chemical calculations, accurate empirical equilibrium structures have been derived for HCCNSi and NCNSi. All three molecules are promising candidates for future radio astronomical detection owing in part to large calculated dipole moments. [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]

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]

Carbenes are reactive molecules of the form R1C̈R2 that play a role in topics ranging from organic synthesis to gas-phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HOC̈OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high-level coupled-cluster calculations. The semi-experimental equilibrium structure derived from five isotopic variants of HOC̈OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HOC̈OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH2O2 chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH2OO. [ABSTRACT FROM AUTHOR]

Carbenes are reactive molecules of the form R1C̈R2 that play a role in topics ranging from organic synthesis to gas-phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HOC̈OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high-level coupled-cluster calculations. The semi-experimental equilibrium structure derived from five isotopic variants of HOC̈OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HOC̈OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH2O2 chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH2OO. [ABSTRACT FROM AUTHOR]

A direct current discharge has been coupled with a continuous supersonic expansion to provide a source of rotationally cold molecular ions for gas phase spectroscopy. Constructed primarily of machinable ceramic and stainless steel, this source design is modular, customizable, and robust. Its performance has been assessed by recording transitions within the ν2 fundamental band of H3+ using cavity ringdown spectroscopy to determine the rotational temperature of ions produced in the free-jet expansion. Temperature and column density were recorded as a function of discharge current as the source was operated over a period of 200 h. Observed temperatures ranged between 50–110 K, and the ion column densities between 8×1010 and 2×1012 cm-2. [ABSTRACT FROM AUTHOR]