Continuous Monitoring of Sulfur Dioxide Removal Using K-Band Molecular Rotational Resonance Spectroscopy

Sulfur dioxide (SO2), an air pollutant, poses significant threats to both public health and the environment. It is one of the six air pollutants regulated by the U.S. Environmental Protection Agency under the Clean Air Act. In efforts to determine the application of molecular rotational resonance (MRR) spectroscopy for monitoring SO2 and its removal from point sources, a K-band MRR technique was evaluated. This method was applied to measure the products of heated mixtures of SO2 and oxygen (O2) in the presence of ammonium metavanadate (NH4VO3) as a catalyst. The observed MRR spectrum revealed the presence of SO2, water vapor (H2O), and ammonia (NH3) due to the sensitivity of MRR to only polar species. SO2 removal was further confirmed by the disappearance of SO2 as NH3 formed. The work presented here analyzed the measurements of SO2 and validated K-band MRR for monitoring SO2 removal. It was observed that the K-band MRR maintains its linearity and other polar species in the mixture did not interfere with MRR signature of SO2. The limit of detection, better than 1%, was determined by evaluating targeted K-band MRR signal response of SO2 removal obtained at varying partial pressures of SO2 in the mixture and using the MRR signal of pure SO2 at 3 mTorr as a reference (100%). Additionally, the results showed that the accuracy and precision of K-band MRR for measuring SO2 partial pressure were satisfactory.

Conformational isomerism in trans-3-methoxycinnamic acid: From solid to gas phase

The rotational spectrum of laser ablated trans-3-methoxycinnamic acid has been observed in the 2-8 GHz range using chirped-pulse Fourier transform microwave spectroscopy coupled to a supersonic jet and adapted to support a laser ablation vaporization system (LA-CP-FTMW). Eight stable conformers were theoretically predicted to exist at B3LYP-D3BJ/6-311++(2d,p) level, all of which were experimentally detected. The experimental rotational parameters data evidence the essentially planar structures for all the conformers. The relative population distribution of conformers in the supersonic jet was investigated from relative intensity measurements. Cooling in the jet brings rotational temperatures close to 1 K for all the conformers. The theoretical predictions for the rotational constants and electric dipole moments show good agreement with the experimental constants and selection rules observed. The population distribution of conformers in the supersonic jet was found to be close to the equilibrium distribution calculated at temperatures lower than the stagnation temperature. Finally, the correlation of the observed conformers structures with those found in condensed phases was investigated.

Perfluorination of Aromatic Compounds Reinforce Their van der Waals Interactions with Rare Gases: The Rotational Spectrum of Pentafluoropyridine-Ne

The rotational spectrum of the pentafluoropyridine-Ne complex, generated in a supersonic jet, has been investigated using chirped-pulse microwave Fourier transform spectroscopy in the 2–8 GHz range. The spectra of the ²⁰Ne and ²²Ne species have been observed, and the rotational constants have been used to determine the structure of the complex. This structure, and those of the previously experimentally studied complexes benzene-Ne and pyridine-Ne, are an excellent benchmark for the theoretical calculations on these adducts. These complexes and hexafluorobenzene-Ne have been investigated at the CCSD/6-311++G(2d,p) level. The calculations reproduce the experimental structures well and show how the van der Waals complexes are stronger for the perfluorinated compound.

Decoding the Structure of Non-Proteinogenic Amino Acids: The Rotational Spectrum of Jet-Cooled Laser-Ablated Thioproline

The broadband rotational spectrum of jet-cooled laser-ablated thioproline was recorded. Two conformers of this system were observed and identified with the help of DFT and ab initio computations by comparison of the observed and calculated rotational constants and ¹⁴N quadrupole coupling constants as well as the predicted energies compared to the observed relative populations. These conformers showed a mixed bent/twisted arrangement of the five-membered ring similar to that of the related compound thiazolidine with the N–H bond in axial configuration. The most stable form had the COOH group in an equatorial position on the same side of the ring as N-H. The arrangement of the C=O group close to the N-H bond led to a weak interaction between them (classified as type I) characterized by a noncovalent interaction analysis. The second form had a trans-COOH arrangement showing a type II O–H···N hydrogen bond. In thioproline, the stability of conformers of type I and type II was reversed with respect to proline. We show how the conformation of the ring depends on the function associated with the endocyclic N atom when comparing the structures of isolated thioproline with its zwitterion observed in condensed phases and with peptide forms.

Chiral analysis of pantolactone with molecular rotational resonance spectroscopy

A molecular rotational resonance spectroscopy method for measuring the enantiomeric excess of pantolactone, an intermediate in the synthesis of panthenol and pantothenic acid, is presented. The enantiomers are distinguished via complexation with a small chiral tag molecule, which produces diastereomeric complexes in the pulsed jet expansion used to inject the sample into the spectrometer. These complexes have distinct moments of inertia, so their spectra are resolved by MRR spectroscopy. Quantitative enantiomeric excess (EE) measurements are made by taking the ratio of normalized complex signal levels when a chiral tag sample of high, known EE is used, while the absolute configuration of the sample can be determined from electronic structure calculations of the complex geometries. These measurements can be performed without the need for reference samples with known enantiopurity. Two instruments were used in the analysis. A broadband, chirped-pulse spectrometer is used to perform structural characterization of the complexes. The broadband spectrometer is also used to determine the EE; however, this approach requires relatively long measurement times. A targeted MRR spectrometer is also used to demonstrate EE analysis with approximately 15-min sample-to-sample cycle time. The quantitative accuracy of the method is demonstrated by comparison with chiral gas chromatography and through the measurement of a series of reference samples prepared from mixtures of (R)-pantolactone and (S)-pantolactone samples of known EE.

How Aromatic Fluorination Exchanges the Interaction Role of Pyridine with Carbonyl Compounds: The Formaldehyde Adduct

The rotational spectrum of the weakly bound complex pentafluoropyridine⋅⋅⋅formaldehyde has been investigated using Fourier transform microwave spectroscopy. From the analysis of the rotational parameters of the parent species and of the 13 C and 15 N isotopologues, the structural arrangement of the adduct has been unambiguously established. The full ring fluorination of pyridine has a dramatic effect on its binding properties: It alters the electron density distribution at the π-cloud of pyridine creating a π-hole and changing its electron donor-acceptor capabilities. In the complex, formaldehyde lies above the aromatic ring with one of the oxygen lone pairs, as conventionally envisaged, pointing toward its centre. This lone pair⋅⋅⋅π-hole interaction, reinforced by a weak C-H⋅⋅⋅N interaction, indicates an exchange of the electron-acceptor roles of both molecules when compared to the pyridine⋅⋅⋅formaldehyde adduct. Tunnelling doublets due to the internal rotation of formaldehyde have also been observed and analysed leading to a discussion on the competition between lone pair⋅⋅⋅π-hole and π⋅⋅⋅π stacking interactions.

Direct regioisomer analysis of crude reaction mixtures via molecular rotational resonance (MRR) spectroscopy

Direct analyses of crude reaction mixtures have been carried out using molecular rotational resonance (MRR) spectroscopy. Two examples are presented, a demonstration application in photocatalytic CH-arylation as well as generation of an intermediate in a natural product synthesis. In both cases, the reaction can proceed at more than one site, leading to a mixture of regioisomers that can be challenging to distinguish. MRR structural parameters were calculated for the low lying conformers for the desired compounds, and then compared to the experimental spectra of the crude mixtures to confirm the presence of these species. Next, quantitation was performed by comparing experimentally measured line intensities with simulations based on computed values for the magnitude and direction of the molecular dipole moment of each species. This identification and quantification was performed without sample purification and without isolated standards of the compounds of interest. The values obtained for MRR quantitation were in good agreement with the chromatographic values. Finally, previously unknown impurities were discovered within the photocatalytic CH-arylation work. This paper demonstrates the utility of MRR as a reaction characterization tool to simplify analytical workflows.

Direct Measurements of Small Polar Impurities in Gasoline Mixtures Using Molecular Rotational Resonance Spectroscopy

This paper reports our efforts to determine whether rotational spectroscopy is a useful tool for petroleum analysis. These efforts include the use of a BrightSpec molecular rotational resonance (MRR) spectrometer, which operates in the 260-290 GHz frequency range, to record rotational spectra of small polar contaminants in commercial gasoline. The observed rotational spectra showed rich, but assignable, patterns due to the sensitivity of the MRR to only small polar compounds. Any interference from a complex hydrocarbon matrix, which in conventional chromatographic methods obscures signals from small polar contaminants, is nearly eliminated. In addition to the evident rotational spectrum of ethanol, the spectra of toluene, ethyl cyanide, and acetaldehyde have also been detected. A quantitative method for ethanol has been developed and demonstrated in this paper, whereas the specific analyses of the other polar impurities will be reported in the future. The validity of MRR to be used as an analytical instrument has been examined by constructing a standard linear curve using dilutions of ethanol in water. The linearity and percentage recovery parameters are satisfactory.