This work was motivated by difficulties encountered while trying to calibrate laser excitation spectra, taken in short (1 cm⁻¹) scans around 438 nm, by matching optogalvanic transitions from a Uranium-Argon hollow cathode lamp to peaks listed in a widely circulated `informal report' on the Uranium spectrum (11000 -25900 cm⁻¹) from Los Alamos, published in 1980An atlas of uranium emission intensities in a hollow cathode discharge; Palmer, Keller & Engleman, Los Alamos report LA 8251-MS, (1980) Short pieces of excitation spectra often fell between secure calibration lines, because many of the weaker features had been excluded from the printed linelist. To remedy this, we have re-recorded emission from a commercial Uranium hollow-cathode lamp 19800 - 27400 cm⁻¹ on a Fourier transform spectrometer, at an instrumental resolution of at 0.04 cm⁻¹. The wavenumber scale was fine-tuned to match earlier reference data^aComparing the emission spectra of U and Th hollow cathode lamps, and a new U line list; Sarmiento et  al., A & A, 618, A118, (2018)Uranium and iodine standards measured by means of Fourier-transform spectroscopy; Gerstenkorn, et  al., A & A, 58, 255-66, (1977) to within 0.003 cm⁻¹. This spectrum (together with its peak list) is proposed in ascii formatA uranium atlas, from 365 to 505 nm; Ross et  al. J Mol Spectrosc (accepted) 2020s a possible aid to calibration of laser excitation spectra in the blue, violet and near UV. It extends the spectrum reported by Sarmiento and co-workers^b that focused on calibration of astronomical spectrographs in the near IR and visible. An atlas of uranium emission intensities in a hollow cathode discharge; Palmer, Keller & Engleman, Los Alamos report LA 8251-MS, (1980). Comparing the emission spectra of U and Th hollow cathode lamps, and a new U line list; Sarmiento et  al., A & A, 618, A118, (2018)

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A uranium atlas, from 365 to 505 nm; Ross et  al. J Mol Spectrosc (accepted) 2020a

The development of portable millimeter-wave gas sensors that operate using pulsed Fourier transform detection schemes and meet the stringent demands for space deployment depend on the advancement of several key technologies. A prototype systemNemchick, et al., A 90-102 GHz CMOS based pulsed Fourier transform spectrometer: New approaches for in situ chemical detection and millimeter-wave cavity-based molecular spectroscopy Rev. Sci. Inst., vol. 89, pp. 073109:1-12, 2018hat operates at 90-100 GHz has already been demonstrated with efforts now focusing on a system to target the 3_1,3 ← 2_2,0 (J^′′_{K^′′a,K^′′c} ← J^′_{K^′a,K^′c} ) H₂O transition at 183.310 GHz. The performance properties of a 180-190 GHz CMOS-based pulsed transmitter and heterodyne receiver set has already proven viable for system incorporation.Nemchick, et al., Ä 180 GHz Pulsed Transmitter and Heterodyne Receiver 28 NM CMOS Chipset for Molecular Sensing", International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, 2019nother key component is the hybrid coupling plate that interfaces with the integrated circuit transmitter and receiver chips and serves as the cavity end mirror of the resonant sample cell. The performance of the waveguide fed version of this cavity system will be discussed including demonstrative examples of cavity quality factor measurements (Q > 10000) and molecular detections deploying both CMOS and traditional mm-wave sources/detectors. System performance will be discussed in the context of realizing a dual band system capable of near simultaneous detection of both H₂O (at 183.310 GHz) and D₂O (at 80.359 GHz).

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Nemchick, et al., A 90-102 GHz CMOS based pulsed Fourier transform spectrometer: New approaches for in situ chemical detection and millimeter-wave cavity-based molecular spectroscopy Rev. Sci. Inst., vol. 89, pp. 073109:1-12, 2018t Nemchick, et al., Ä 180 GHz Pulsed Transmitter and Heterodyne Receiver 28 NM CMOS Chipset for Molecular Sensing", International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, 2019A

Although many organo-sulfur species have been detected in molecular clouds and star-forming regions, they are insufficient to account for the expected sulfur content. In an attempt to identify new potential sulfur reservoirs, we have studied the discharge of benzene with both H₂S and CS₂. Thiobenzaldehyde is the first species we have characterized at high resolution using chirped pulse and cavity enhanced microwave spectroscopy in the 2-40 GHz region. Production of thiobenzaldehyde was particularly prominent in the discharge of benzene with carbon disulfide. Isotopic substitution with ¹³CS₂ indicates a relatively simple formation pathway for thiobenzaldehyde, initiated by attack of the benzene ring by ¹³CS followed by an H-atom shift from the benzene ring. Although much weaker, observation of thiobenzaldehyde in the discharge of benzene with H₂S suggests that other pathways may be relevant.

Current spectroscopic methods Sérgio R. Domingos et al. ,Sensing Chirality with Rotational Spectroscopy Annual Review of Physical Chemistry 69:1, 499-519 (2018)., Nafie, L. A.; Dukor, R. K.; Freedman, T. B. Handbook of Vibrational Spectroscopy: Vibrational Circular Dichroism; Wiley Online Library, 2006.an only detect and quantify chirality if the molecular composition of the sample has been determined. Therefore, before extracting enantiomeric information from a spectrum of an unknown sample, one needs to compare it to previously measured spectra, or to simulated spectra generated with previously determined rotational constants. We describe new developments towards an updated version of the microwave three-wave mixing technique (M3WM)Patterson, D. Et al , Enantiomer-specific detection of chiral molecules via microwave spectroscopy. Nature 497, 475–477 (2013).n a buffer gas cell, that can unambiguously identify the presence of chiral species in an unknown sample with no need for prior spectral assignment or knowledge of the sample composition. This work combines the established sensitivity of 3WM in a buffer gas cell in determining enantiomeric information with broadband microwave fields, and an optimized sample input, and it opens up new directions in chirality detection towards real-life molecular samples and complicated mixtures.

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Sérgio R. Domingos et al. ,Sensing Chirality with Rotational Spectroscopy Annual Review of Physical Chemistry 69:1, 499-519 (2018). Nafie, L. A.; Dukor, R. K.; Freedman, T. B. Handbook of Vibrational Spectroscopy: Vibrational Circular Dichroism; Wiley Online Library, 2006.c Patterson, D. Et al , Enantiomer-specific detection of chiral molecules via microwave spectroscopy. Nature 497, 475–477 (2013).i

The ongoing development of rotational spectroscopy through the growth of broadband capabilities and automated acquisition schemes regularly generates a wealth of data to be analyzed. However, assigning these data is often a bottleneck to obtaining useful chemical information. This is particularly true for unknown carriers for which no initial guess or constraint is available. Development of automated spectral analysis tools is therefore critical to fully utilize rotational spectroscopy data. We have previously reported the development of a high speed algorithm for the calculation of asymmetric rotor spectra. The initial report demonstrated the efficacy of the program to calculate spectra hundreds of times faster than conventional methods. Building on the underlying engine, we have constructed a set of high speed spectral fitting and analysis tools. These tools include a general spectral line fitter and brute force searching algorithms. We have combined these into a pipeline for automated spectral assignment. An initial version of the pipeline has been successfully used for automated assignment of double resonance data, and has been expanded to analyze data using only rotational spectra as an input. A preliminary version is capable of producing a list of probable matches that can be readily evaluated by hand. We will present this work, and discuss the efficacy and limitations of our approach as well as potential future development.

Our chirped pulse millimeter wave spectrometer for complex molecules of astrophysical interest is operational between 75 and 110 GHz, which is coincident with the Atacama Large Millimeter/Submillimeter Array (ALMA) Band 3. High sensitivity and stability is a focus of our research to be able to measure isotopic species of molecules in natural abundance on the one hand and to observe fragments of molecules, which are produced with a high voltage DC discharge in combination with a supersonic jet on the other hand. For the latter application, first tests were performed with methyl cyanide (CH₃CN). We observed HCN as well as HNC discharge products. As the detector side of our instrument coincides in many aspects with our emission spectrometers [1,2] a comparison of chirped pulse measurements and emission spectroscopy will be discussed briefly. Additionally, we show and discuss current improvements and developments of our chirped pulse instrument. Other candidate molecules are ions or radicals created by the discharge and other means with the aim to record their fingerprint-like rotational spectra.
References: [1] N. Wehres, B. Heyne, F. Lewen, M. Hermanns, B. Schmidt, C. Endres, U. U. Graf, D. R. Higgins, and S. Schlemmer, Proceedings of the International Astronomical Union, 13(S332), 332-345. DOI:10.1017/S1743921317007803 [2] N. Wehres, J. Maßen, K. Borisov, B. Schmidt, F. Lewen, U. U. Graf, C. E. Honingh, D. R. Higgins, and S. Schlemmer, Phys. Chem. Chem. Phys. 20, 5530–5544 (2018), DOI:10.1039/C7CP06394F

As made clear by E. Bright Wilson’s prescient thesis published in Science in 1968,Wilson, E. B., Science 1968, 162, 3849.pplicability to analytical chemistry has long been a goal for microwave spectroscopy. This comes as no surprise, as the high-resolution nature of microwave spectroscopy leads to unparalleled sensitivity to the structures of molecular analytes. However, in the absence of isotopologues, determination of an unknown molecule's identity can be difficult by the experimental rotational constants alone. Although a structure maps trivially into its rotational constants, the reverse is not necessarily true as some structural information is lost. Yet, one can imagine a trained machine that outputs likely molecular structures by intelligent interpretation of the observed rotational spectroscopy. This is the inverse problem for rotational spectroscopy – instead of predicting constants from structures, the goal is to predict structures from constants. Recent developments in machine learning have led to success in decoding multidimensional chemical parameter spaces from massive, enumerated libraries of molecular structures.[1] Zhou, Z.; Kearnes, S.; Li, L.; Zare, R. N.; Riley, P., Sci. Rep. 2019, 9, 10752. [2] Gómez-Bombarelli, R.; Wei, J. N.; Duvenaud, D.; et al., ACS Cent. Sci. 2018, 4, 268.n order to apply such methods to the inverse problem, there are critical questions that are still unanswered. For instance, is the mapping of spectrum to molecule single-valued within the uncertainty of calculated rotational constants? If not, what is the average number of reasonable molecules consistent for a random but valid set of rotational constants? Or, is there an efficient and convergent “similarity” metric that compares a target spectrum to a chemical search space, key for developing a molecular “search engine”? With these questions in mind, this talk will present an exploration of, and a proposal for, a potential solution to the inverse problem.

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Wilson, E. B., Science 1968, 162, 3849.a [1] Zhou, Z.; Kearnes, S.; Li, L.; Zare, R. N.; Riley, P., Sci. Rep. 2019, 9, 10752. [2] Gómez-Bombarelli, R.; Wei, J. N.; Duvenaud, D.; et al., ACS Cent. Sci. 2018, 4, 268.I

Gas chromatography is a gold-standard technique for analysis of mixtures of volatile compounds, and can be coupled to a range of detectors including many allowing for molecular identification (e.g., mass spectrometry). Recently, we have explored the use of molecular rotational resonance spectroscopy, with its extremely high sensitivity to subtle changes in molecular structure, as a rapid detector for gas chromatography effluents. Results from an initial studyD.W. Armstrong et al., Angew. Chem. Int. Ed. 2020, 59, 192-196ill be presented, where GC-MRR with a 75-110 GHz chirped-pulse millimeter-wave spectrometer was used to identify and quantitate mixtures, including co-eluting isotopologues and isotopomers which could not be resolved by other methods. Challenges for ongoing development, including data collection and interpretation, sensitivity optimization, and the column-spectrometer interface, will also be discussed.

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D.W. Armstrong et al., Angew. Chem. Int. Ed. 2020, 59, 192-196w

Molecular rotational resonance (MRR) spectroscopy is used to determine the distribution of deuterium isotopomers of varenicline produced in catalytic hydrogen isotope exchange of the active pharmaceutical ingredient (API) using Ni(I)-X complexes with bulky α-diimine ligands.C. Zarate, et al., J. Am. Chem. Soc. 2019, 141(12), 5034-5044.n this reaction, four hydrogen atoms on varenicline can be exchanged for deuterium, leading to 10 distinct isotopic forms of the API. The sensitivity of MRR spectra to mass distribution, via the principle moments-of-inertia, enables the spectral signatures of all 10 isotopomers to be resolved in the reaction product. The MRR spectrum of the deuterated API is measured from the crude reaction product (including the metal complex) using a 2-8 GHz chirped-pulse Fourier transform microwave spectrometer.C. Pérez, et al., Chem. Phys. Lett. 2013, 571, 1-15.wo samples were analyzed that differed in reaction time: 8 hours (70 mg crude product) and 24 hours (40 mg crude product). The API is volatilized by direct heating to 175^°C and the vapor is entrained in neon. Isotopomers are identified by comparing the experimental spectrum with MRR spectrum predictions of each isotopic species. Theoretical rotational constants of each isotopic species are calculated from a single quantum chemistry reference geometry (B3LYP D3BJ 6-311++G(d,p)) and then scaled using a comparison of the theoretical and experimental rotational constants of the normal isotopic species. For both samples, the average number of deuterium substitutions per molecule determined by MRR spectroscopy is in agreement with the mass spectrometry characterization. The relative abundances of the 10 isotopomers at 8-hour and 24-hour reaction times are modeled using a first-order kinetics model. This model indicates deuterium incorporation at the two chemically distinct reaction sites occurs at the same rate with a rate constant of 0.125/hr. The ability to rapidly monitor the isotopomer distribution is demonstrated using the BrightSpec IsoMRR instrument based on the cavity-enhanced Balle-Flygare instrument.T. J. Balle, W. H. Flygare, Rev. Sci. Instrum. 1981, 52(1), 33-45.html:<hr /><h3>Footnotes: C. Zarate, et al., J. Am. Chem. Soc. 2019, 141(12), 5034-5044.I C. Pérez, et al., Chem. Phys. Lett. 2013, 571, 1-15.T T. J. Balle, W. H. Flygare, Rev. Sci. Instrum. 1981, 52(1), 33-45.

As a part of the efforts to extend the applications of rotational spectroscopy to downstream petroleum processing, a Benchtop K-Band molecular rotational resonance spectroscopy has been employed to record rotationally-resolved spectra of polar toxins in a gasoline and gumout mixtures. The analysis of the observed rotational spectra by matching them to those of available references reveals the presence of multiple polar toxins, including oxygen-nitrogen containing compounds, due to K-band MRR’s sensitivity to only polar compounds. The complex hydrocarbon matrix, which in many analytical instruments obscures the signals from low concentration impurities, is eliminated. The capability for K-band MRR to extract small polar toxins in different petroleum products is being evaluated and the results will be given in this talk.