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Recent detections of aromatic species in dark molecular clouds suggest that formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC-1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain code NAUTILUS and an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by 2 orders of magnitude compared to its observed value. Predicted abundances for the chemically related C4H3N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in polycyclic aromatic hydrocarbon formation. [ABSTRACT FROM AUTHOR]

The unique optical cycling efficiency of alkaline earth metal–ligand molecules has enabled significant advances in polyatomic laser cooling and trapping. Rotational spectroscopy is an ideal tool for probing the molecular properties that underpin optical cycling, thereby elucidating the design principles for expanding the chemical diversity and scope of these platforms for quantum science. We present a comprehensive study of the structure and electronic properties in alkaline earth metal acetylides with high-resolution microwave spectra of 17 isotopologues of MgCCH, CaCCH, and SrCCH in their ²Σ+ ground electronic states. The precise semiexperimental equilibrium geometry of each species has been derived by correcting the measured rotational constants for electronic and zero-point vibrational contributions calculated with high-level quantum chemistry methods. The well-resolved hyperfine structure associated with the 1,2H, 13C, and metal nuclear spins provides further information on the distribution and hybridization of the metal-centered, optically active unpaired electron. Together, these measurements allow us to correlate trends in chemical bonding and structure with the electronic properties that promote efficient optical cycling essential to next-generation experiments in precision measurement and quantum control of complex polyatomic molecules. [ABSTRACT FROM AUTHOR]

The rotational spectra of five isotopic species of the Ar–SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si–O stretch, in which lines up to v = 12 (∼14 500 cm−1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar–Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar–CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar–SiO is predicted to be T-shaped with a well depth of 152 cm−1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm−1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar–SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar–SiO, and the putative Ka = ±1 levels for Ar–28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential. [ABSTRACT FROM AUTHOR]

The structure and bonding of H2NNO, the simplest N-nitrosamine, and a key intermediate in deNOx processes, have been precisely characterized using a combination of rotational spectroscopy of its more abundant isotopic species and high-level quantum chemical calculations. Isotopic spectroscopy provides compelling evidence that this species is formed promptly in our discharge expansion via the NH2 + NO reaction and is collisionally cooled prior to subsequent unimolecular rearrangement. H2NNO is found to possess an essentially planar geometry, an NNO angle of 113.67(5)°, and a N—N bond length of 1.342(3) Å; in combination with the derived nitrogen quadrupole coupling constants, its bonding is best described as an admixture of uncharged dipolar (H2N—N=O, single bond) and zwitterion (H2N+=N—O-, double bond) structures. At the CCSD(T) level, and extrapolating to the complete basis set limit, the planar geometry appears to represent the minimum of the potential surface, although the torsional potential of this molecule is extremely flat. [ABSTRACT FROM AUTHOR]

A systematic search for carbon-chain cumulenones beyond H2C4O has been undertaken using microwave spectral taxonomy in combination with a pulsed jet discharge source. No evidence was found for the C2V isomer of H2C5O or its longer derivatives, but HC(O)C4H, the longer variant of propynal, HC(O)CCH, was identified instead. Its rotational and leading centrifugal distortion constants have been derived to high accuracy from detection of both a- and b-type lines; those below 40 GHz were measured using a Fabry-Perot cavity, while lines between 40 and 72 GHz were recorded by double resonance techniques. Overwhelming evidence for the identification is provided by detection of HC(O)C4D, DC(O)C4H, and HC(18O)C4H at the expected frequencies using isotopically enriched samples. Because HC(18O)C4H is produced with comparable abundance when using either 18O2 or C18O as the source of oxygen, and because H13C(O)C4H is not preferentially formed when starting from 13CO, atomic oxygen appears to be a key reactant in formation, plausibly viaOinsertion, e.g., H2CC2n+2H + O → HC(O)C2n+2H + H. Under the same experimental conditions, HC(O)CCH is more than 10 times more abundant than H2C3O, regardless of the source of oxygen, and no evidence is found for cyclopropenone, c-C3H2O. Taken together, these observations indicate that propynal and longer chains with an odd number of carbon atoms are either energetically more stable than cumulenones of the same size, are kinetically favored products, or both. On the basis of the HC(O)C4H discovery, searches for the isovalent sulfur species, HC(S)C4H, and HC(O)C6H have been conducted. Guided by new quantum chemical calculations, the rotational spectra of both were observed in the centimeter-wave band with the same spectrometer. [ABSTRACT FROM AUTHOR]

The rotational spectra of the singly substituted carbon-13 isotopic species of six sulphur-terminated carbon chains have been detected between 5 and 40 GHz using a supersonic discharge jet source in combination with a cavity Fourier transform microwave spectrometer. These molecules are H C 3 H S , a closed-shell isomer of H 2 C 3 S , and five radicals: H C C S , H C 3 S , H C 4 S , H C 5 S and C 4 S . Experiments were carried out with precursors enriched in 13 C, either high purity H 13 C 2 H , a statistical sample of 13 C/ 12 C acetylene, or 13 C S 2 . From the 13 C hyperfine coupling constants, the unpaired electronic density along the chain can be quantified for the radical species, while precise experimental structures ( r 0 ) can be derived for each chain except H C 5 S by a least-squares fit to the rotational constants. The use of 13 C S 2 is particularly revealing in that it provides important clues as to the major formation pathway for each chain in our discharge nozzle. Somewhat surprisingly, 13 C from this precursor appears to be substituted in one of three distinct ways: statistical, a single C site, or not at all. This propensity is molecule specific, implying that multiple formation pathways are operative. [ABSTRACT FROM AUTHOR]

The thermal decomposition of syn-ethanal-oxide (syn-CH3CHOO) through vinyl hydrogen peroxide (VHP) leading to hydroxyl radical is characterized using a modification of the HEAT thermochemical protocol. The isomerization step of syn-CH3CHOO to VHP via a 1,4 H-shift, which involves a moderate barrier of 72 kJ/mol, is found to be rate determining. A two-dimensional master equation approach, in combination with semi-classical transition state theory, is employed to calculate the time evolution of various species as well as to obtain phenomenological rate coefficients. This work suggests that, under boundary layer conditions in the atmosphere, thermal unimolecular decomposition is the most important sink of syn-CH3CHOO. Thus, the title reaction should be included into atmospheric modeling. The fate of cold VHP, the intermediate stabilized by collisions with a third body, has also been investigated. [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]

We report the gas-phase optical detection of Si2C near 390 nm and the first experimental investigation of the rovibrational structure of its ¹A1 ground electronic state using mass-resolved and fluorescence spectroscopy and variational calculations performed on a high-level ab initio potential. From this joint study, it is possible to assign all observed Ka = 1 vibrational levels up to 3800 cm−1 with confidence, as well as a number of levels in the Ka = 0, 2, and 3 manifolds. Dixon-dip plots for the bending coordinate (ν2) allow an experimental determination of a barrier to linearity of 783(48) cm−1 (2σ), in good agreement with theory (802(9) cm−1). The calculated (Ka, ν2) eigenvalue lattice shows an archetypal example of quantum monodromy (absence of a globally valid set of quantum numbers) that is reflected by the experimentally observed rovibrational levels. The present study provides a solid foundation for infrared and optical surveys of Si2C in astronomical objects, particularly in the photosphere of N- and J-type carbon stars where the isovalent SiC 2 molecule is known to be abundant. [ABSTRACT FROM AUTHOR]

Air pollutant concentrations are often higher near major roadways than in the surrounding environments owing to emissions from on-road mobile sources. In this study, we quantified the gradient in black carbon (BC) and air toxics concentrations from the I-70 freeway in the Elyria-Swansea environmental justice neighborhood in Denver, Colorado, during three measurement campaigns in 2017–2018. The average hourly upwind-downwind gradient of BC concentrations from the roadway was 500–800 ng/m3, equal to an increment of approximately 30-80% above local background levels within 180 m of the freeway. When integrated over all wind directions, the gradients were smaller, approximately 150–300 ng/m3 (~11-18%) over the course of nearly four months of measurements. No statistically significant gradient in air toxics (e.g., benzene, formaldehyde, etc.) was found, likely because the uncertainties in the mean concentrations were larger than the magnitude of the gradient (<25%). This finding is in contrast to some earlier studies in which small gradients of benzene and other VOCs were found. We estimate that sample sizes of at least 100 individual measurements would have been required to estimate mean concentrations with sufficient certainty to quantify gradients on the order of ±10% uncertainty. These gradient estimates are smaller than those found in previous studies over the past two decades; more stringent emissions standards, the local fleet age distribution, and/or the steady turnover of the vehicle fleet may be reducing the overall impact of roadway emissions on near-road communities. Implications: Gradients of near-road pollution may be declining in the near-road environment as tailpipe emissions from the vehicle fleet continue to decrease. Near-road concentration gradients of mobile source air toxics, including benzene, 1,3-butadiene, and ethylbenzene, will require higher sample sizes to quantify as emissions continue to decline. [ABSTRACT FROM AUTHOR]

The carbon chain ions HC 3 O + and HC 3 S + – longer variants of the famous 'X-ogen' line carrier HCO + – have been observed for the first time using two cryogenic 22-pole ion trap apparatus (FELion, Coltrap) and two different light sources: the Free Electron Laser for Infrared eXperiments (FELIX), which was operated between 460 and 2500 cm − 1 , and an optical parametric oscillator operating near 3200 cm − 1 ; signals from both experiments were detected by infrared predissociation action spectroscopy. The majority of vibrational fundamentals were observed for both ions and their wavenumbers compare very favourably with results from high-level anharmonic force field calculations performed here at the coupled-cluster singles and doubles level augmented by a perturbative treatment of triple excitations, CCSD(T). As the action scheme employed here probes the Ne-tagged weakly bound variants, Ne–HC 3 O + and Ne–HC 3 S + , corresponding calculations of these systems were also performed. Differences in the structures and molecular force fields between the bare ions and their Ne-tagged complexes are found to be very small. [ABSTRACT FROM AUTHOR]

The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H2O and O2 heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H2 and O2 as precursor gases. Using equal mixtures of normal and 18O2, it has been possible to detect three new isotopic species, H18OOO, HO18O18O, and H18O18O18O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O2 → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r0) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O-O bond (1.684 Å), in agreement with earlier work, a markedly shorter O-H bond distance (0.913 Å), and a more acute ∠HOO angle (92.4°) when compared to equilibrium (re) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (reemp) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen-oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines. [ABSTRACT FROM AUTHOR]

The rotational spectra of two small silicon sulfides, silanethione H2SiS and the disilicon sulfide ring Si2S, have been detected in the centimeter band by Fourier transform microwave spectroscopy of a molecular beam; lines of H2SiS were also observed in the millimeter band up to 377 GHz in a glow discharge. Precise rotational and centrifugal distortionconstants have been determined for the normal and a number of the more abundant rare isotopic species of both closed-shell molecules. Theoretical equilibrium (re) structures of H2SiS and Si2S were derived from coupled-cluster calculations that included triple and quadruple excitations, core correlation, and extrapolation to the basis-set limit. The re structures agree to within 5×10-4 Å and 0.1°with empirical equilibrium (reemp) structures derived from the experimental rotational constants, combined with theoretical vibrational and electronic corrections. Both H2SiS and Si2S are good candidates for radioastronomical detection in the circumstellar shells of evolved carbon-rich stars such as IRC+10216, because they are fairly polar and are similar in composition to the abundant astronomical molecule SiS. [ABSTRACT FROM AUTHOR]

By means of Fabry-Pérot Fourier transform microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a cis- and a trans-geometry, is reported. The search for both isomers was based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects. At a similarly high level of theory, the cis-isomer is calculated to be the global minimum on the potential energy surface, but the trans-isomer is predicted to lie only a few kcal/mol higher in energy. A total of seven lines, including a- and b-type transitions, has been observed for both isomers, and precise rotational constants have been derived. Because sulfur dioxide, SO2, is a widespread and very abundant astronomical species, and because it possesses a large proton affinity, HOSO+ is an excellent candidate for radioastronomical detection. [ABSTRACT FROM AUTHOR]

Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space. [ABSTRACT FROM AUTHOR]

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2–OCS, ortho-H2–OCS, HD–OCS, para-D2–OCS, and ortho-D2–OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2–OCS relative to ortho-H2–OCS increases exponentially with decreasing normal H2 component in H2/He gas mixtures, making the observation of para-H2–OCS in the presence of the more strongly bound ortho-H2–OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2–OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2–OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD–OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2–OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2–OCS. [ABSTRACT FROM AUTHOR]

The rotational spectra of five weakly bonded hydrogen-OCS complexes (paraH2, orthoH2, HD, orthoD2, and paraD2) are measured. Hyperfine structure is resolved and analyzed in all except the complex with paraH2, where I=0. For the two j=1 species, orthoH2-OCS and paraD2-OCS, nuclear hyperfine coupling constants are found to be da=21.2(2) and 8.4(2) kHz, respectively, indicative of nearly free uniaxial rotation of the hydrogen around the b-inertial axis. Similar analyses for HD-OCS and orthoD2-OCS yield the quadrupole coupling constants eqQa=16(2) and 30(2) kHz, respectively, showing that the internal rotational motions of HD and orthoD2 in the complex are slightly hindered producing a small nonspherical distribution. For orthoD2-OCS, the observed hyperfine structure indicates that the nuclear spin states I=0 and 2 are strongly coupled in the rotation of the complex. [ABSTRACT FROM AUTHOR]

Vibrational excitation and relaxation of five linear polyatomic molecules, OCS, OC3S, HC3N, HC5N, and SiC2S, have been studied by Fourier transform microwave spectroscopy in a supersonic expansion after the application of a low-current dc electric discharge. For each chain, the populations in bending and stretching modes have been characterized as a function of the applied discharge current; for stable OCS and HC3N, vibrational populations were studied as well in the absence of a discharge. With no discharge present the derived vibrational temperatures are slightly below T, the temperature of the gas before the supersonic expansion (i.e., 300 K). In the presence of the discharge, vibrational excitation occurs via inelastic collisions with the electrons and the vibrational temperatures rise as the applied current increases. Global vibrational relaxation is governed by rapid vibration–vibration (VV) energy transfer and slow vibration–translation (VT) energy transfer. The latter process is rate-determining and depends primarily on the wave number of the vibration. Vibrational modes with wave numbers near and below kT/hc (where T=300 K and kT/hc∼210 cm-1) are efficiently cooled by VT transfer because a sufficient number of collisions occur in the initial stages of the supersonic expansion. Vibrational modes with wave numbers around 450 cm-1 appear to be inefficiently cooled in the molecular beam; at these energies VV and VT rates are probably comparable. For high-frequency vibrations, VV energy transfer dominates. For the longer chains OC3S and HC5N, higher-lying modes are generally not detectable and vibrational temperatures of most lower-lying modes were found to be lower than those of OCS and HC3N, suggesting that as the size of the molecules increases, intermode VV transfer becomes more efficient, plausibly due to the higher density of vibrational levels. New high resolution spectroscopic data have been obtained for several vibrationally excited states of OC3S, HC3N, and HC5N. Rotational lines of the 13C and 15N isotopic species of HC5N have been measured, yielding improved rotational and centrifugal distortion constants; 14N nitrogen quadrupole coupling constants for the isotopic species of HC5N with 13C have been determined for the first time. [ABSTRACT FROM AUTHOR]

Fourier transform microwave spectroscopy has been used to investigate vibrational excitation and relaxation of diatomic molecules produced by an electric discharge in the throat of a supersonic nozzle. Rotational transitions of SO, SiO, and SiS, in vibrational states up to v=33 for [sup 32]S[sup 16]O, v=45 for [sup 28]Si[sup 16]O, and v=51 for [sup 28]Si[sup 32]S in their ground electronic states have been detected. The isotopic species [sup 33]S[sup 16]O, [sup 34]S[sup 16]O, [sup 29]Si[sup 16]O, [sup 28]Si[sup 18]O, [sup 29]Si[sup 32]S, and [sup 28]Si[sup 34]S have also been observed in highly excited vibrational states. Microwave transitions include up to v=22 for the second lowest excited electronic state b [sup 1]Σ[sup +] of SO (∼10 510 cm-1 above ground) have also been detected. Effective vibrational temperatures have been derived for each species, and a general model is proposed to qualitatively explain the observations. Vibrational excitation is caused by inelastic collisions with the hot electrons produced in the discharge. The subsequent vibrational populations are largely determined by vibration–vibration energy transfer via molecule–molecule binary collisions. Two regions can be inferred from the data: one characterized by a temperature of around 1000 K and a second region with a temperature of several thousand degrees Kelvin. Improved Dunham coefficients and correction terms for the breakdown of the Born–Oppenheimer approximation have been determined for b [sup 1]Σ[sup +] SO, X [sup 1]Σ[sup +] SiO, and X [sup 1]Σ[sup +] SiS. Nuclear spin-rotation hyperfine structure for the [sup 29]Si isotopic species of SiO and SiS has been observed in all highly excited vibrational states. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

On the basis of extensive coupled cluster calculations, the rotational spectra of the linear silicon- and sulfur-containing carbon chains SiC[sub 2n] S (n = 1 -3) in their singlet electronic ground state and SiC[sub 3]S in its triplet electronic ground state have been detected and characterized by means of molecular beam Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined to high accuracy as well as the spin-spin coupling constant for triplet SiC[sub 3] S. In addition, the [sup 29]Si, [sup 34]S, and both [sup 13]C isotopic species of SiC[sub 2]S have been detected, allowing the determination of both an effective r[sub 0] structure, as well as a mixed experimental-theoretical structure resulting from the combination of the measured rotational constants with the vibrationrotation coupling constants calculated ab initio. Several rotational satellite lines have also been observed for SiC[sub 2]S and, on the basis of their predicted vibration-rotation and l-type doubling constants, were assigned to the two highest-frequency stretching modes υ[sub 1] and υ[sub 2], and to a progression in the second lowest-frequency bending mode υ 4 up to 3 υ 4. Equilibrium structures and various spectroscopic properties are predicted for all SiC[sub n]S species with n = 1 - 8. [ABSTRACT FROM AUTHOR]

Following the recent discovery of T-shaped GeC2, rotational spectra of three larger Ge carbides, linear GeC4, GeC5, and GeC6 have been observed using chirped pulse and cavity Fourier transform microwave spectroscopy and a laser ablation molecule source, guided by new high-level quantum chemical calculations of their molecular structure. Like their isovalent Si-bearing counterparts, Ge carbides with an even number of carbon atoms beyond GeC2 are predicted to possess 1Σ ground electronic states, while odd-numbered carbon chains are generally 3Σ; all are predicted to be highly polar. For the three new molecules detected in this work, rotational lines of four of the five naturally occurring Ge isotopic variants have been observed between 6 and 22 GHz. Combining these measurements with ab initio force fields, the Ge–C bond lengths have been determined to high precision: the derived values of 1.776 Å for GeC4, 1.818 Å for GeC5, and 1.782 Å for GeC6 indicate a double bond between these two atoms. Somewhat surprisingly, the spectrum of GeC5 very closely resembles that of a 1Σ molecule, implying a spin–spin coupling constant λ in excess of 770 GHz for this radical, a likely consequence of the large spin–orbit constant of atomic Ge (∼1000 cm−1). A systematic comparison between the production of SiCn and GeCn chains by laser ablation has also been undertaken. The present work suggests that other large metal-bearing molecules may be amenable to detection by similar means. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols are large clusters of molecules and particulate matter that profoundly affect the Earth's radiation budget and climate. Gas-phase oxidation of volatile organic compounds is thought to play a key role in nucleation and aerosol growth, but remains poorly understood. One reaction proposed to trigger formation of condensable, low volatility organic compounds is that between Criegee intermediates and carboxylic acids to yield hydroperoxide esters. Here we isolate in high yield the simplest hydroperoxide ester, hydroperoxymethyl formate (HOOCH2OCHO), as a secondary product in the ozonolysis of ethylene, and establish by rotational spectroscopy that this ester adopts a nearly-rigid cyclic structure owing to a strong hydrogen bond between the peroxy hydrogen and carbonyl oxygen. Subsequent detection of this ester in the ozonolysis of propylene and isoprene suggests that terminal alkenes readily undergo specific types of second-order oxidation reactions that have been implicated in the formation of atmospheric aerosols. [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]

We describe an instrument which can be used to analyze complex chemical mixtures at high resolution and high sensitivity. Molecules are collisionally cooled with helium gas at cryogenic temperatures (∼4–7 K) and subsequently detected using chirped pulse microwave spectroscopy. Here, we demonstrate three significant improvements to the apparatus relative to an earlier version: (1) extension of its operating range by more than a factor of two, from 12–18 GHz to 12–26 GHz, which allows a much wider range of species to be characterized; (2) improved detection sensitivity owing to the use of cryogenically cooled low-noise amplifiers and protection switches; and (3) a versatile method of sample input that enables analysis of solids, liquids, gases, and solutions, without the need for chemical separation (as demonstrated with a 12–16 GHz spectrum of lemon oil). This instrument can record broadband microwave spectra at comparable sensitivity to high Q cavity spectrometers which use pulsed supersonic jets, but up to 3000 times faster with a modest increase in the sample consumption rate. [ABSTRACT FROM AUTHOR]

The recent astronomical detection of benzonitrile (C6H5CN) in the cold, starless cloud TMC-1 demonstrates that aromatic chemistry is efficient even in the primordial stages of star and planet formation. C6H5CN may serve as a convenient observational proxy for benzene, which is otherwise challenging to detect in space, provided the chemistry linking these two molecules is tightly constrained. Here we present a high-resolution microwave spectroscopic study in combination with an accurate thermochemical treatment of the formation chemistry of C6H5CN and benzene. We demonstrate that C6H5CN is a highly useful tracer for benzene in the presence of CN radical, either in space or in the laboratory, and by inference, that the reaction C2H + CH2(CH)2CH2 yields benzene, along with its high-energy polar isomer fulvene. In addition, we find that the higher energy isomer, C6H5NC, is formed at ＜0.1% abundance relative to C6H5CN. By exploiting –CN tagging, formation pathways to produce benzene using a variety of acyclic hydrocarbon precursors are then explored. A robust, self-consistent, and chemically accurate theoretical treatment has also been undertaken for several key reactions. The results are discussed both in the context of aromatic molecule synthesis and astrochemistry. [ABSTRACT FROM AUTHOR]

Context. Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotal for the determination of important physical parameters for the evolution of the interstellar medium (e.g. gas ionisation fraction) or as tracers of non-polar species which are not directly observable. The identification of these molecular species through radioastronomical observations is directly linked to precise laboratory spectral characterisation. Aims. The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronic state of protonated carbonyl sulfide (HSCO+) and its deuterium substituted isotopomer (DSCO+). At the same time, we show how implementing different laboratory techniques allows for the determination of different spectroscopical properties of asymmetric-top protonated species. Methods. Three different high-resolution experiments were used in conjunction to detect for the first time the b-type rotational spectrum of HSCO+, and to extend, well into the sub-millimetre region, the a-type spectrum of the same molecular species and DSCO+. The electronic ground-state of both ions was investigated in the 273–405 GHz frequency range, allowing for the detection of 60 and 50 new rotational transitions for HSCO+ and DSCO+, respectively. Results. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the most astronomically relevant transitions to be predicted to better than 100 kHz for both HSCO+ and DSCO+ up to 500 GHz, equivalent to better than 60 m s−1 in terms of equivalent radial velocity. Conclusions. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency. Each instrument addressed a complementary part of the same spectroscopic challenge, demonstrating the potential of such an approach for future studies of similar reactive species. [ABSTRACT FROM AUTHOR]

Uses the sideband optical-optical double resonance Zeeman effect spectroscopy to study the electronic spectrum of the gas-phase platinum-hydrogen compound. Analysis of the sub-Doppler Zeeman spectrum of each feature using a systematic quasi-two-dimensional approach; Radio frequency magnetic resonances.

Applies the sideband optical-optical double resonance Zeeman effect spectroscopy to study transition metal hydrides. Resolution of the extremely small ground state photon hyperfine splittings; Record of the sub-dopper spectra; Deviation of the g values in each state from the nominal Hund's coupling case.

Several variants of the one-laser, two-color technique of sideband optical–optical double resonance Zeeman (SOODRZ) spectroscopy, which enables Zeeman and hyperfine splittings to be measured at sub-Doppler resolution, are described; a detailed theory of the line shape and signal-to-noise ratio for each variant is also presented. In the single-frequency variant of SOODRZ spectroscopy, radio frequency sidebands are imposed onto a single-frequency continuous wave laser beam to determine energy splittings between pairs of closely spaced levels. Each pair of closely spaced levels is coupled to a third isolated level by optical transitions and is tuned into resonance by the Zeeman effect. A theoretical treatment of the SOODRZ effect is presented here. It is shown that both the real and imaginary contributions to the complex third-order optical susceptibility tensor χ(3) can be observed and that the SOODRZ signal originates from a cross term between the amplitude of the unperturbed frequency-modulated beam of the laser and the nonlinear signal amplitude Re{EL*·ENL}. The SOODRZ signal is directly proportional to ΔNμ4MI20 (where M is the modulation index), rather than (ΔN)2, μ8, and I30 as in degenerate four-wave mixing and other nonlinear spectroscopies, which means the detection sensitivity of SOODRZ spectroscopy is considerably higher for low laser intensities and at lower number densities.Moreover, the linewidth of the sub-Doppler feature is independent of the both the Doppler width of the optical transition and the natural width of the common linked level and, hence, is immune from the effects such as upper state predissociation. Two different types of modulators, acousto-optic and electro-optic, can be used in the single-frequency variant of SOODRZ spectroscopy. The unique features of SOODRZ schemes incorporating both types of modulators are described. SOODRZ spectroscopy can also be performed with a multimode broadband... [ABSTRACT FROM AUTHOR]

The technique of frequency-modulation enhanced magnetic rotation spectroscopy (FM-MRS) is applied here to the study of the electronic spectra of gas-phase transition metal hydrides and deuterides. Experimental FM-MR results for several bands of PdH, PdD, NiH, and CuH are presented. In the case of PdH, an Ω=0.5–XΩ=0.5 band at 4371 Å has been observed and assigned. Prior to this work, although several bands of PdD had been reported previously, no electronic transitions of PdH had been assigned. All of the PdH, PdD, and NiH bands studied in the 4200–4600 Å wavelength region are strongly predissociated, and in the case of PdH and PdD, strongly and multiply perturbed as well. [ABSTRACT FROM AUTHOR]

The technique of frequency-modulation enhanced magnetic rotation spectroscopy (FM-MRS) is described. An experimental comparison between FM-MRS, frequency-modulation, and magnetic rotation spectroscopies clearly demonstrates that the double null absorption-based FM-MR technique is more sensitive than either FM or MRS separately. The theoretical basis for this sensitivity enhancement can be attributed to a 1/θ (where θ is the small uncrossing angle between two nearly crossed polarizers) improvement in the sensitivity of the combined FM-MR technique relative to that of standard FM spectroscopy. The sensitivity improvement of FM-MRS relative to MRS is even better than that for FM spectroscopy. In practical terms, this enhancement can improve the experimentally achievable sensitivity of FM and magnetic rotation spectroscopies by factors of O(101) to O(102) and O(102) to O(104), respectively, levels that approach the theoretical quantum noise limit. In addition, because FM-MRS is based on the Zeeman effect, Zeeman information is encoded into FM-MR line profiles in the form of relative phase labels and line shapes. The relative phases and line shapes of rotational lines can be used to group features by rotational branch and to differentiate between magnetically active (Λ≠0, Σ≠0) and inactive (Λ=0, Σ=0) features. The theory of FM-MRS, including a discussion of line shapes and a sensitivity analysis, is reviewed here; applications of the FM-MR technique are presented separately in the following paper. [ABSTRACT FROM AUTHOR]

The equilibrium structure for the ground state of the HNCN radical is calculated at the levels of the self-consistent field theory (SCF), the second-order Mo\ller–Plesset perturbation approximation (MP2), and the full single and double excitation coupled cluster theory including all connected triples in a noniterative manner [CCSD(T)], using various extended basis sets starting from 6–311 G(d,p). At the CCSD(T) level, the outer C–N bond is more than 0.1 Å shorter than the inner one and the N–C–N group departs from linearity by 6°. The total N–C–N length is in good agreement with the experimental value [Herzberg and Warsop, Can. J. Phys. 41, 286 (1963)], however, the H–N–C angle is about 6° smaller. The N–H bond is very close to a normal N–H bond but is about 0.2 Å smaller than the experimental estimate. Except for the smaller H–N–C angle, the geometrical parameters for HNCN closely parallel those for the triplet HCCN molecule. The dipole moment, harmonic frequencies, electric quadrupole, and Fermi contact coupling constants of HNCN are also calculated. The calculated harmonic frequencies confirm the preliminary assignments of Wu, Hall, and Sears [J. Chem. Soc. Faraday Trans. 89, 615 (1993)]. The quadrupole coupling constants for the inner and outer N atoms are comparable, implying a complex pattern of hyperfine split components in the lowest rotational transitions. The present calculation may thus serve as a useful guide for the interpretation of the rotational spectrum. [ABSTRACT FROM AUTHOR]

The application of magnetic rotation spectroscopy (MRS) to the electronic spectra of gas phase NiH and CeF is examined. Experimental results for both MRS and laser excitation spectroscopy are presented for the NiH B 2Δ5/2–X 2Δ5/2 (1,0) and 2[uppercase_phi_synonym]7/2–X 2Δ5/2 (0,0) bands. A comparison between these two techniques clearly illustrates the ability of MRS to simplify and presort spectra. The power of MRS originates from its specificity for low-J, ΔΩ=ΔJ transitions and its ability, based on the sign and magnitude of the MRS effect, to partially presort rotational branches. A simple theoretical explanation of the MRS results, based on the structure of the underlying Zeeman pattern, is presented. Also, the extension of MRS to the more complicated and congested spectrum of CeF is presented. Using MRS, a previously unassigned CeF band, near 568 nm, has been definitively assigned as Ω’=4.5←Ω‘=3.5. [ABSTRACT FROM AUTHOR]

We have developed a method to quantify reaction product ratios using high resolution microwave spectroscopy in a cryogenic buffer gas cell. We demonstrate the power of this method with the study of the ozonolysis of isoprene, CH2=C(CH3)–CH=CH2, the most abundant, non-methane hydrocarbon emitted into the atmosphere by vegetation. Isoprene is an asymmetric diene, and reacts with O3 at the 1,2 position to produce methyl vinyl ketone (MVK), formaldehyde, and a pair of carbonyl oxides: [CH3CO–CH=CH2 + CH2=OO] + [CH2=O + CH3COO–CH=CH2]. Alternatively, O3 could attack at the 3,4 position to produce methacrolein (MACR), formaldehyde, and two carbonyl oxides [CH2=C(CH3)–CHO + CH2=OO] + [CH2=O + CH2=C(CH3)–CHOO]. Purified O3 and isoprene were mixed for approximately 10 seconds under dilute (1.5–4% in argon) continuous flow conditions in an alumina tube held at 298 K and 5 Torr. Products exiting the tube were rapidly slowed and cooled within the buffer gas cell by collisions with cryogenic (4–7 K) He. High resolution chirped pulse microwave detection between 12 and 26 GHz was used to achieve highly sensitive (ppb scale), isomer-specific product quantification. We observed a ratio of MACR to MVK of 2.1 ± 0.4 under 1 : 1 ozone to isoprene conditions and 2.1 ± 0.2 under 2 : 1 ozone to isoprene conditions, a finding which is consistent with previous experimental results. Additionally, we discuss relative quantities of formic acid (HCOOH), an isomer of CH2=OO, and formaldehyde (CH2=O) under varying experimental conditions, and characterize the spectroscopic parameters of the singly-substituted 13C trans-isoprene and 13C anti-periplanar-methacrolein species. This work has the potential to be extended towards a complete branching ratio analysis, as well towards the ability to isolate, identify, and quantify new reactive intermediates in the ozonolysis of alkenes. [ABSTRACT FROM AUTHOR]

Sulfur monoxide (SO) is a highly reactive molecule and thus, eludes bulk isolation. We report here on synthesis and reactivity of a molecular precursor for SO generation, namely 7-sulfinylamino- 7-azadibenzonorbornadiene (1). This compound has been shown to fragment readily driven by dinitrogen expulsion and anthracene formation on heating in the solid state and in solution, releasing SO at mild temperatures (<100 °C). The generated SO was detected in the gas phase by MS and rotational spectroscopy. In solution, 1 allows for SO transfer to organic molecules as well as transition metal complexes. [ABSTRACT FROM AUTHOR]

We present a microwave spectral taxonomy study of several hydrocarbon/CS2 discharge mixtures in which more than 60 distinct chemical species, their more abundant isotopic species, and/or their vibrationally excited states were detected using chirped-pulse and cavity Fourier-transform microwave spectroscopies. Taken together, in excess of 85 unique variants were detected, including several new isotopic species and more than 25 new vibrationally excited states of C2S, C3S, and C4S, which have been assigned on the basis of published vibration–rotation interaction constants for C3S, or newly calculated ones for C2S and C4S. On the basis of these precise, low-frequency measurements, several vibrationally exited states of C2S and C3S were subsequently identified in archival millimeter-wave data in the 253–280 GHz frequency range, ultimately providing highly accurate catalogs for astronomical searches. As part of this work, formation pathways of the two smaller carbon–sulfur chains were investigated using 13C isotopic spectroscopy, as was their vibrational excitation. The present study illustrates the utility of microwave spectral taxonomy as a tool for complex mixture analysis, and as a powerful and convenient ‘stepping stone’ to higher frequency measurements in the millimeter and submillimeter bands. [ABSTRACT FROM AUTHOR]