l0pt Figure Raman Spectroscopy is a great way to distinguish different kinds of API polymorphs.Shijie Zhang, et al. Dynamic Sparse Sampling for Confocal Raman Microscopy, Analytical Chemistry 2018owever,adversarial attacks on spectral classifiers were shown to enable identification of potential vulnerabilities in common dimension reduction analyses of Raman spectra. We tackled this susceptibility with the Generative Adversarial Linear Discriminant Analysis (GALDA) approach. GALDA is analogous to Generative Adversarial Nets (GAN) in the machine learning context. Conceptually in a typical GAN, two models were simultaneously trained, a generative model G that attempts to estimate the sample distribution and a discriminative model D that classifies the output of G.These two models seek to achieve Nash equilibrium during the iterative adversarial training process.Goodfellow, Ian, et al. Generative adversarial nets, Advances in neural information processing systems. 2014e herein incorporated this concept into spectral classification. Spectral classification in the spectroscopic context seems to lag albeit the mass development in the computer science area. As such, the analyzation methods' robustness and susceptibility to malicious attack is considered even less. Therefore, GALDA aims to test against the robustness of LDA classifiers and provide a new framework for considerations for classification strategies less susceptible to spurious misclassification.

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Shijie Zhang, et al. Dynamic Sparse Sampling for Confocal Raman Microscopy, Analytical Chemistry 2018H Goodfellow, Ian, et al. Generative adversarial nets, Advances in neural information processing systems. 2014W

A major bottleneck in the analysis of broadband chirped-pulse microwave spectra is the identification of unknown molecules. Often, a set of molecular frequencies are fit to an effective Hamiltonian, whereby a set of spectroscopic parameters are used to infer the molecular carrier. These constants are reproduced with electronic structure calculations through a trial and error process, accompanied by chemical intuition involving the precursors used, and the reaction conditions (e.g. electrical discharges). As the size of the molecules increase, the combinatorics of many hundreds to thousands of possible isomers becomes an intractable problem for chemical intuition alone. Since spectroscopic parameters are only weakly informative, we turn to statistical inference to complement conventional spectroscopic analysis. In this talk, I will discuss a new framework for identifying unknown molecules by performing inference on spectroscopic parameters with probabilistic deep learning. Using a series of decoder architectures, we are able to infer the approximate molecular composition/formula and what functional groups are present using only the rotational constants and derived quantities κ (the asymmetry parameter) and ∆ (the inertial defect), and approximate magnitudes and projections of the dipole moments.

By means of 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 three benzene mixtures - alone or with oxygen and nitrogen - have been exhaustively characterized between 6.5 and 26 GHz. In total, more than 3300 spectral features were observed; 88% of these, accounting for roughly 97% of the total intensity, have now been assigned to 160 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 than the precursor benzene. These findings provide evidence for a rich architecture of two- and three-dimensional carbon, and indicate that benzene growth, particularly formation of ring-chain molecules, occurs facilely under our experimental conditions. The present analysis vividly demonstrates 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 derive a mass spectrum for each discharge that is analogous to a traditional mass spectrum, but with exquisite isomeric resolution.

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

Adipic acid (C₆H₁₀O₄), a linear dicarboxylic acid, is of great importance in industry as a precursor for the production of nylon and as food additive (E355) to regulate the acidity of the product. Adipic acid is a stable compound at standard conditions; under heating it decarboxylates giving cyclopentanone as the main product and adipic anhydride as a by-product. This behavior is contrary to other aliphatic dicarboxylics acids, which tend to produce the anhydride analogue rather than the ketone.J. W. Hill, J. Am. Chem. Soc. 1930, 52, 4110–4114.e investigated the self-reaction of adipic acid under heating employing high-resolution rotational spectroscopy, which allows identifying each component in the gas phase, together with structure elucidation and quantification of molecules. The microwave spectrum was measured using a segmented chirped-pulse Fourier transform microwave (FTMW) spectrometer working in the 18-26 frequency range. The most intense lines were assigned to cyclopentanone, but we were able to identify also two conformations of the adipic anhydride, one conformation of adipic acid, and even a complex between cyclopentanone and one water molecule. We have analyzed the spectrum in time segments of one hour each to investigate the reaction pathways of adipic acid and the temporal evolution of the components of the spectrum.

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J. W. Hill, J. Am. Chem. Soc. 1930, 52, 4110–4114.W

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

Laser desorption time-of-flight mass spectrometry (LD-ToF-MS) experiments on pressed-pellet samples of polycyclic aromatic hydrocarbons (PAHs) exhibit the formation of covalently-bonded dimers at masses (m/z) of 2M-2 and 2M-4 (where M is the parent mass). Through replication of these LD-ToF-MS conditions at higher throughput, PAH dimers have been produced and collected in milligram quantities. The formation of the covalently-bonded dimers was confirmed under similar LD-ToF-MS conditions and through HPLC separation with a UV-Vis detector. For collected samples of pyrene, differential sublimation has removed the residual monomer to leave a mixture of dimerized pyrene. Decrease of the monomer mass peak was confirmed using LD-ToF-MS, and the samples are analyzed using several spectroscopic methods, including UV-Vis, IR, and Raman spectroscopy. Theoretical studies of possible dimer structures have been calculated using density functional theory with the CAM-B3LYP method at the def2TZV level of theory using the Gaussian 09 program. Theory calculations were calibrated and checked by comparison with the monomer samples, and used to determine the lowest energy dimer structures. The simulated spectra were compared with collected spectra of isolated dimers to determine the contributing structures of the dimerized PAHs.

Graph representations of spectroscopic information have been increasing in popularity due to their efficiency and scalability in encoding large volumes of data; simply, energy levels are represented as nodes, and transitions as edges between each level. Thus, all quantum mechanical information pertaining to a molecule can be readily manipulated and transformed using a single data format, allowing computations and visualizations to be performed with ease. One application of spectroscopic graphs is to assist in the analysis of high resolution spectra of complex mixtures, comprising many observed transitions from an unknown number of molecules. Analysis of such mixtures comprises two coupled tasks: assignment of features to their respective signal carriers, and to decompose the full spectrum into known molecules. Viewing this problem from the perspective of spectroscopic graphs, our task is to fully reconstruct the subgraphs (i.e. molecules) from sparse information obtained with techniques such as AMDOR and MST. Here, we apply the use of deep graph learning techniques for spectroscopic graph reconstruction. Using convolutional autoencoder architectures, we experiment with the possibility of parameterizing graph neural networks to reproduce complex graphs when given only a limited number of "observed" energy levels and/or transitions. As part of this work, we perform a comprehensive investigation into the successes and challenges of our approach, including the interpretability of learned representations, how they can be manipulated and analyzed, and its applicability in complex mixture analysis.

Coherent anti-Stokes Raman scattering (CARS), a four-wave mixing parametric process, is used for flow diagnostics due to its excellent spatial and temporal resolution. In this work we present a dual-pump combined CARS (DPCC) system for simultaneously obtaining the pure-rotational and vibrational spectra of O₂. O₂ is a an important molecule in combustion as well as in many high-speed non-reacting aerodynamic flows. Excellent temperature accuracy, in DPCC, is obtained via sensitivity in Boltzmann distribution of the pure-rotational CARS spectra. r0pt Figure Pressure information, in DPCC, is obtained via relative sensitivity of the rotational and vibrational CARS spectrum to collision dynamics. As pressure increases, at a fixed temperature, the frequency of molecular collisions also increases. Only the vibrational spectra of O₂ undergoes collisional narrowing due to mixing between the closely-spaced Q-branch transitions. We employed the DPCC system in an underexpanded jet outside the exit of a converging nozzle. Figure shows a single spectrum, obtained 250 μm downstream of the nozzle exit, along the jet centerline. We compared different Raman lineshape models for extracting temperature and pressure from the DPCC spectra. The rotational diffusion model for collisional narrowing predicts a higher pressure compared to the Voigt model.

Here, a two-color O₂ vibrational temperature diagnostic was developed by utilizing spectroscopic models to inform optimal wavelength candidates for both a continuous-wave (CW), ultraviolet (UV) laser and a picosecond pulsed, UV laser. Cross-sections of shock-heated O₂ were measured using a CW UV laser, and results over a range of wavelengths and temperatures are compared against a Stanford model, developed to simulate oxygen absorption cross-sections in the Schumann-Runge system under vibrational non-equilibrium conditions, and Specair, a spectroscopic model for high-temperature air species developed by Laux et al. All measurements were completed behind reflected shocks in 2% and 5% O₂ in argon (Ar) mixtures. Vibrational temperatures for cross-section measurements were calculated for plateaus and peaks in experimental absorbances using a Bethe-Teller relaxation model up to 6,000 K and a steady-state approach above 6,000 K. Temperature sweep measurements were fixed around 223.237 nm, while wavelength sweep measurements were taken around 4550 K and ranged between 223.23 nm to 223.27 nm. Temperature sweep cross-sections agree to within 15% of Specair modeled cross-sections, with most measurements falling within 10% of Specair predictions. Wavelength sweep cross-sections agree at shorter wavelengths with Specair cross-sections, but longer wavelength features are offset from both the Stanford model and Specair predictions. Using the spectroscopic model developed here to inform appropriate wavelength selection, the UV laser systems in this work become tools for directly tracking both vibrational temperature and populations in specific vibrational states of O₂ as it undergoes vibrational relaxation and dissociation behind strong shock waves. These temperature and population time histories provide important experimental data needed to evaluate current computational models that seek to capture the molecular energy transfer present in high-enthalpy airflows.

A spectroscopic technique based on the principle of Vernier frequency comb spectroscopy is developed, which combines the broadband spectral coverage and high spectral resolution with the signal enhancement due to a resonant optical cavity. The resonant cavity serves the dual role of optical path enhancement along with Vernier filtering of the comb modes, allowing individual comb modes to be resolved with a grating. The method uses only one frequency comb laser and by using multiple exposures and taking a sequence of images of enhanced comb modes (i.e. taking video-recording) a constraint imposed on the bandwidth can removed. The bandwidth can be expanded even more by simultaneously recording with the IR camera frequency comb modes from different spectral intervals. The methane absorption spectrum in ambient air in the mid-IR with a frequency comb obtained as a difference frequency generation was measured. Processing the sequence of images recorded with the mid-IR camera as the resonant cavity length is scanned allows the retrieval of the absorption spectrum. The experimental results and the discussion of the advantages and limitations of this techniques are presented. This work was supported by Robert A. Welch Foundation(grant No A1546) and a T3 grant from Texas A and M University.

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