Rotational Spectrum of the Phenoxy Radical.

We report the hyperfine-resolved rotational spectrum of the gas-phase phenoxy radical in the 8-25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates that are yet undetected at radio wavelengths are discussed.

Direct Probes of π-Delocalization in Prototypical Resonance-Stabilized Radicals: Hyperfine-Resolved Microwave Spectroscopy of Isotopic Propargyl and Cyanomethyl.

Delocalization of the unpaired electron in π-conjugated radicals has profound implications for their chemistry, but direct and quantitative characterization of this electronic structure in isolated molecules remains challenging. We apply hyperfine-resolved microwave rotational spectroscopy to rigorously probe π-delocalization in propargyl, CH2CCH, a prototypical resonance-stabilized radical and key reactive intermediate. Using the spectroscopic constants derived from the high-resolution cavity Fourier transform microwave measurements of an exhaustive set of 13C- and 2H-substituted isotopologues, together with high-level ab initio calculations of zero-point vibrational effects, we derive its precise semiexperimental equilibrium geometry and quantitatively characterize the spatial distribution of its unpaired electron. Our results highlight the importance of considering both spin-polarization and orbital-following contributions when interpreting the isotropic hyperfine coupling constants of π radicals. These physical insights are strengthened by a parallel analysis of the isoelectronic species cyanomethyl, CH2CN, using new 13C measurements also reported in this work. A detailed comparison of the structure and electronic properties of propargyl, cyanomethyl, and other closely related species allows us to correlate trends in their chemical bonding and electronic structure with critical changes in their reactivity and thermochemistry.

Structural and electronic trends of optical cycling centers in polyatomic molecules revealed by microwave spectroscopy of MgCCH, CaCCH, and SrCCH.

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.

Hyperfine-Resolved Rotational Spectroscopy of Phenyl Radical.

We present an analysis of the hyperfine-resolved rotational spectrum of gas-phase phenyl radical, c-C6H5, between 9 and 35 GHz. The isotropic and anisotropic hyperfine parameters of all five protons and the electronic spin-rotation fine structure parameters are accurately determined from this study, which allow detailed insight into the distribution and interactions of the unpaired electron in this prototypical σ-radical. The implications for laboratory and astronomical investigations of phenyl that are reliant on a precise centimeter-wave catalog are discussed, as are the prospects for detecting and assigning the hyperfine-resolved rotational spectra of other large, weakly polar hydrocarbon chain and ring radicals.

Low-Temperature Gas-Phase Kinetics of Ethanol-Methanol Heterodimer Formation.

The structures of gas-phase noncovalently bound clusters have long been studied in supersonic expansions. This method of study, while providing a wealth of information about the nature of noncovalent bonds, precludes observation of the formation of the cluster, as the clusters form just after the orifice of the pulsed valve. Here, we directly observe formation of ethanol-methanol dimers via microwave spectroscopy in a controlled cryogenic environment. Time profiles of the concentration of reagents in the cell yielded gas-phase reaction rate constants of kMe-g = (2.8 ± 1.4) × 10-13 cm3 molecule-1 s-1 and kMe-t = (1.6 ± 0.8) × 10-13 cm3 molecule-1 s-1 for the pseudo-second-order ethanol-methanol dimerization reaction at 8 K. The relaxation cross section between the gauche and trans conformers of ethanol was also measured using the same technique. In addition, thermodynamic relaxation between conformers of ethanol over time allowed for selection of conformer stoichiometry in the ethanol-methanol dimerization reaction, but no change in the ratio of dimer conformers was observed with changing ethanol monomer stoichiometry.

Cortical microtubules contribute to division plane positioning during telophase in maize.

Cell divisions are accurately positioned to generate cells of the correct size and shape. In plant cells, the new cell wall is built in the middle of the cell by vesicles trafficked along an antiparallel microtubule and a microfilament array called the phragmoplast. The phragmoplast expands toward a specific location at the cell cortex called the division site, but how it accurately reaches the division site is unclear. We observed microtubule arrays that accumulate at the cell cortex during the telophase transition in maize (Zea mays) leaf epidermal cells. Before the phragmoplast reaches the cell cortex, these cortical-telophase microtubules transiently interact with the division site. Increased microtubule plus end capture and pausing occur when microtubules contact the division site-localized protein TANGLED1 or other closely associated proteins. Microtubule capture and pausing align the cortical microtubules perpendicular to the division site during telophase. Once the phragmoplast reaches the cell cortex, cortical-telophase microtubules are incorporated into the phragmoplast primarily by parallel bundling. The addition of microtubules into the phragmoplast promotes fine-tuning of the positioning at the division site. Our hypothesis is that division site-localized proteins such as TANGLED1 organize cortical microtubules during telophase to mediate phragmoplast positioning at the final division plane.

Grain-Surface Hydrogen-Addition Reactions as a Chemical Link Between Cold Cores and Hot Corinos: The Case of H2CCS and CH3CH2SH.

Recently, searches were made for H2CCS and HCCSH in a variety of interstellar environments─all of them resulted in nondetections of these two species. Recent findings have indicated the importance of destruction pathways, e.g., with atomic hydrogen, in explaining the consistent nondetection of other species, such as the H2C3O family of isomers. We have thus performed ab initio calculations looking at reactions of H2CCS, HCCSH, and related species with atomic hydrogen. Our results show that H2CCS and HCCSH are both destroyed barrierlessly by atomic hydrogen, thus providing a plausible explanation for the nondetections. We further find that subsequent reactions with atomic hydrogen can barrierlessly lead to CH3CH2SH, which has been detected. Astrochemical simulations including these reactions result not only in reproducing the observed abundance of H2CCS in TMC-1 but also show that CH3CH2SH, produced via our H-addition pathways and subsequently trapped on grains, can desorb in warmer sources up to abundances that match previous observations of CH3CH2SH in Orion KL. These results, taken together, point to the importance of grain-surface H-atom addition reactions and highlight the chemical links between cold prestellar cores and their subsequent, warmer evolutionary stages.

First Laboratory Detection of N13CO- and Semiexperimental Equilibrium Structure of the NCO- Anion.

The cyanate anion (NCO-) is a species of considerable astrophysical relevance. It is widely believed to be embedded in interstellar ices present in young stellar objects but has not yet been detected in the dense gas of the interstellar medium. Here we report highly accurate laboratory measurements of the rotational spectrum of the N13CO- isotopologue at submillimeter wavelengths along with the detection of three additional lines of the parent isotopologue up to 437.4 GHz. With this new data, the rotational spectrum of both isotopologues can be predicted to better 0.25 km s-1 in equivalent radial velocity up to 1 THz, more than adequate for an astronomical search in any source. Moreover, a semiexperimental equilibrium structure of the anion is derived by combining the experimental ground-state rotational constants of the two isotopologues with theoretical vibrational corrections, obtained by using the coupled-cluster method with inclusion of single and double excitations and perturbative inclusion of triple excitations (CCSD(T)). The estimated accuracy of the two bond distances is on the order of 5 × 10-4 Å: a comparison to the values obtained by geometry optimization with the CCSD(T) method and the use of a composite scheme, including additivity and basis-set extrapolation techniques, reveals that this theoretical procedure is very accurate.

Aromatics and Cyclic Molecules in Molecular Clouds: A New Dimension of Interstellar Organic Chemistry.

Astrochemistry lies at the nexus of astronomy, chemistry, and molecular physics. On the basis of precise laboratory data, a rich collection of more than 200 familiar and exotic molecules have been identified in the interstellar medium, the vast majority by their unique rotational fingerprint. Despite this large body of work, there is scant evidence in the radio band for the basic building blocks of chemistry on earth-five- and six-membered rings-despite long-standing and sustained efforts during the past 50 years. In contrast, a peculiar structural motif, highly unsaturated carbon in a chainlike arrangement, is instead quite common in space. The recent astronomical detection of cyanobenzene, the simplest aromatic nitrile, in the dark molecular cloud TMC-1, and soon afterward in additional prestellar and possibly protostellar sources, establishes that aromatic chemistry is likely widespread in the earliest stages of star formation. The subsequent discovery of cyanocyclopentadienes and even cyanonaphthalenes in TMC-1 provides further evidence that organic molecules of considerable complexity are readily synthesized in regions with high visual extinction but where the low temperature and pressure are remarkably low. This review focuses on laboratory efforts now underway to understand the rich transition region between linear and planar carbon structures using microwave spectroscopy. We present key features, advantages, and disadvantages of current detection methods, a discussion of the types of molecules found in space and in the laboratory, and approaches under development to identify entirely new species in complex mixtures. Studies focusing on the cyanation of hydrocarbons and the formation of benzene from acyclic precursors are highlighted, as is the role that isotopic studies might play in elucidating the chemical pathways to ring formation.

Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering.

Unidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon (PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group-functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules.

Assessment of mobile source air toxics in an Environmental Justice Denver community adjacent to a freeway.

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.

Automated Construction of Potential Energy Surfaces Suitable to Describe van der Waals Complexes with Highly Excited Nascent Molecules: The Rotational Spectra of Ar-CS(v) and Ar-SiS(v).

Some reactions produce extremely hot nascent products which nevertheless can form sufficiently long-lived van der Waals (vdW) complexes-with atoms or molecules from a bath gas-as to be observed via microwave spectroscopy. Theoretical calculations of such unbound resonance states can be much more challenging than ordinary bound-state calculations depending on the approach employed. One encounters not just the floppy, and perhaps multiwelled potential energy surface (PES) characteristic of vdWs complexes, but in addition, one must contend with excitation of the intramolecular modes and its corresponding influence on the PES. Straightforward computation of the (resonance) rovibrational levels of interest, involves the added complication of the unbound nature of the wave function, often treated with techniques such as introducing a complex absorbing potential. Here, we have demonstrated that a simplified approach of making a series of vibrationally effective PESs for the intermolecular coordinates-one for each reaction product vibrational quantum number of interest-can produce vdW levels for the complex with spectroscopic accuracy. This requires constructing a series of appropriately weighted lower-dimensional PESs for which we use our freely available PES-fitting code AUTOSURF. The applications of this study are the Ar-CS and Ar-SiS complexes, which are isovalent to Ar-CO and Ar-SiO, the latter of which we considered in a previously reported study. Using a series of vibrationally effective PESs, rovibrational levels and predicted microwave transition frequencies for both complexes were computed variationally. A series of shifting rotational transition frequencies were also computed as a function of the diatom vibrational quantum number. The predicted transitions were used to guide and inform an experimental effort to make complementary observations. Comparisons are given for the transitions that are within the range of the spectrometer and were successfully recorded. Calculations of the rovibrational level pattern agree to within 0.2% with experimental measurements.

Exhaustive Product Analysis of Three Benzene Discharges by Microwave Spectroscopy.

Using chirped and cavity microwave spectroscopies, automated double resonance, new high-speed fitting and deep learning algorithms, and large databases of computed structures, the discharge products of benzene alone, or in combination with molecular oxygen or nitrogen, have been exhaustively characterized between 6.5 and 26 GHz. In total, more than 3300 spectral features were observed; 89% of these, accounting for 97% of the total intensity, have now been assigned to 152 distinct chemical species and 60 of their variants (i.e., isotopic species and vibrationally excited states). Roughly 50 of the products are entirely new or poorly characterized at high resolution, including many heavier by mass than the precursor benzene. These findings provide direct evidence for a rich architecture of two- and three-dimensional carbon and indicate that benzene growth, particularly the formation of ring-chain molecules, occurs facilely under our experimental conditions. The present analysis also illustrates the utility of microwave spectroscopy as a precision tool for complex mixture analysis, irrespective of whether the rotational spectrum of a product species is known a priori or not. From this large quantity of data, for example, it is possible to determine with confidence the relative abundances of different product masses, but more importantly the relative abundances of different isomers with the same mass. The complementary nature of this type of analysis to traditional mass spectrometry is discussed.

Synchrotron-Based High Resolution Far-Infrared Spectroscopy of trans-Butadiene.

The high resolution far-infrared spectrum of trans-butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm-1, ν10 and ν24, have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν24 analysis was rather straightforward, ν10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν9 + ν12 state. Effective rotational constants have been derived for both the v10 = 1 and v24 = 1 states. Since only one weak, infrared active fundamental band (ν23) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

Detecting Laser-Volatilized Salts with a Miniature 100-GHz Spectrometer.

Rotational transitions are unique identifiers of molecular species, including isotopologues. This article describes the rotational detections of two laser-volatilized salts, NaCl and KCl, made with a miniature Fourier transform millimeter-wave (FTmmW) cavity spectrometer that could one day be used to measure solid composition in the field or in space. The two salts are relevant targets for icy moons in the outer solar system, and in principle, other molecular solids could be analyzed with the FTmmW instrument. By coupling the spectrometer to a collisionally cooling laser ablation source, (a) we demonstrate that the FTmmW instrument is sensitive enough to detect ablation products, and (b) we use the small size of the FTmmW cavity to measure ablation product signal along the carrier gas beam. We find that for 532 nm nanosecond pulses, ablated molecules are widely dispersed in the carrier-gas jet. In addition to the miniature spectrometer results, we present several complementary measurements intended to characterize the laser ablation process. For pulse energies between 10 and 30 mJ, the ablation product yield increases linearly, reaching approximately 1012 salt molecules per 30 mJ pulse. Using mass spectrometry, we observe Li+, Na+, and K+ in the plumes of ablated NaCl, KCl, and LiCl, which implies dissociation of the volatilized material. We do not observe salt ions (e.g., NaCl+). However, with 800 nm femtosecond laser pulses, the triatomic ion clusters Li2Cl+, Na2Cl+, and K2Cl+ are produced. Finally, we observe incomplete volatilization with the nanosecond pulses: some of the ejecta are liquid droplets. The insights about ablation plume physics gleaned from these experiments should guide future implementations of the laser-volatilization technique.

Assessment of Ambient Air Toxics and Wood Smoke Pollution among Communities in Sacramento County.

Ambient air monitoring and phone survey data were collected in three environmental justice (EJ) and three non-EJ communities in Sacramento County during winter 2016-2017 to understand the differences in air toxics and in wood smoke pollution among communities. Concentrations of six hazardous air pollutants (HAPs) and black carbon (BC) from fossil fuel (BCff) were significantly higher at EJ communities versus non-EJ communities. BC from wood burning (BCwb) was significantly higher at non-EJ communities. Correlation analysis indicated that the six HAPs were predominantly from fossil fuel combustion sources, not from wood burning. The HAPs were moderately variable across sites (coefficient of divergence (COD) range of 0.07 for carbon tetrachloride to 0.28 for m- and p-xylenes), while BCff and BCwb were highly variable (COD values of 0.46 and 0.50). The BCwb was well correlated with levoglucosan (R2 of 0.68 to 0.95), indicating that BCwb was a robust indicator for wood burning. At the two permanent monitoring sites, wood burning comprised 29-39% of the fine particulate matter (PM2.5) on nights when PM2.5 concentrations were forecasted to be high. Phone survey data were consistent with study measurements; the only significant difference in the survey results among communities were that non-EJ residents burn with indoor devices more often than EJ residents.

Measuring Spatial and Temporal PM2.5 Variations in Sacramento, California, Communities Using a Network of Low-Cost Sensors.

Low-cost sensors can provide insight on the spatio-temporal variability of air pollution, provided that sufficient efforts are made to ensure data quality. Here, 19 AirBeam particulate matter (PM) sensors were deployed from December 2016 to January 2017 to determine the spatial variability of PM2.5 in Sacramento, California. Prior to, and after, the study, the 19 sensors were deployed and collocated at a regulatory air monitoring site. The sensors demonstrated a high degree of precision during all collocated measurement periods (Pearson R2 = 0.98 - 0.99 across all sensors), with little drift. A sensor-specific correction factor was developed such that each sensor reported a comparable value. Sensors had a moderate degree of correlation with regulatory monitors during the study (R2 = 0.60 - 0.68 at two sites). In a multi-linear regression model, the deviation between sensor and reference measurements of PM2.5 had the highest correlation with dew point and relative humidity. Sensor measurements were used to estimate the PM2.5 spatial variability, finding an average pairwise coefficient of divergence of 0.22 and a range of 0.14 to 0.33, indicating mostly homogeneous distributions. No significant difference in the average sensor PM concentrations between environmental justice (EJ) and non-EJ communities (p value = 0.24) was observed.

Generation and structural characterization of Ge carbides GeCn (n = 4, 5, 6) by laser ablation, broadband rotational spectroscopy, and quantum chemistry.

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.

Characterization of the simplest hydroperoxide ester, hydroperoxymethyl formate, a precursor of atmospheric aerosols.

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.

High sensitivity microwave spectroscopy in a cryogenic buffer gas cell.

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.