r0pt Figure 1,2-BN Cyclohexene was produced from 1,2-BN Cyclohexane through the loss of H₂ and characterized and identified using a pulsed-beam Fourier-transform microwave spectrometer. The first microwave spectra for 1,2-¹⁰BN Cyclohexene 1,2-¹¹BN Cyclohexene have been measured in the frequency range of 5.5-12.5 GHz, providing accurate rotational constants and nitrogen and boron quadrupole coupling strengths for two isotopologues. High-level ab initio calculations provided rotational constants and quadrupole coupling strengths for the precursor 1,2-BN Cyclohexane (C₄H₁₂BN) and 1,2-BN Cyclohexene(C₄H₁₀BN). Calculated molecular properties for 1,2-BN Cyclohexene are in very good agreement with measured parameters. Calculated parameters for the starting material, 1,2-BN Cyclohexane do not agree with the experimental data. Rotational constants for 1,2-¹¹BN Cyclohexene are A = 4702.058(2) MHz, B = 4360.334(1) MHz and C = 2494.407(1) MHz. The inertial defect is ∆₀ = -20.78 amu-Ų clearly indicating a nonplanar structure. These microwave experiments show that heating the initial compound, 1,2-BN Cyclohexane, to 60 C in a 1 atm neon stream results in the loss of H₂ and conversion to 1,2-BN Cyclohexene. This appears to be the first characterization of the 1,2-BN Cyclohexene monomer.

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 any 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.

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H₂S biochemistries. Its existence and stability in vivo, however, remain controversial. By means of Fourier-transform microwave spectroscopy, we establish that HSNO is spontaneously formed in high concentration when NO and H₂S gases are simply mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed with high efficiency by the reaction H₂S and N₂O₃ to produce HSNO and HNO₂, where N₂O₃ is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H₂S may form HNO and HSSH. The length of the S-N bond has been derived to high precision from isotopic studies, and is found to be unusually long, 1.84 Å - the longest S-N bond reported to date for an SNO compound. The present structural and reactivity investigations of this elusive molecule provide a firm fundation to better understand its physiological chemistry and propensity to undergo S-N bond homolysis in vivo.

Dihydroartemisinic acid (DHAA, C₁₅H₂₄O₂, five chiral centers) is a precursor in proposed low-cost synthetic routes to the antimalarial drug artemisinin. In one reaction process being considered in pharmaceutical production, DHAA is formed from an enantiopure sample of artemisinic acid through hydrogenation of the alkene. This reaction needs to properly set the stereochemistry of the asymmetric carbon for the synthesis to produce artemisinin. A recrystallization process can purify the diastereomer mixture of the hydrogenation reaction if the unwanted epimer is produced in less than 10% abundance. There is a need in the process analytical chemistry to rapidly (less than 1 min) measure the diastereomer excess and current solutions, such a HPLC, lack the needed measurement speed. The rotational spectrum of DHAA has been measured at 300:1 signal-to-noise ratio in a chirped-pulsed Fourier transform microwave spectrometer operating from 2-8 GHz using simple heating of the compound. The ¹³C isotope analysis provides a carbon atom structure that confirms the diastereomer. This structure is in excellent agreement with quantum chemistry calculations at the B2PLYPD3/ 6-311++G** level of theory. The DHAA spectrum is expected to be fully resolved from the unwanted diastereomer raising the potential for fast diastereomer excess measurement by rotational spectroscopy in the pharmaceutical production process.

Because HSNO is formed abundantly and selectively from H₂S and N₂O₃ in the presence of metallic surfaces, it may be feasible to synthesize larger RSNOs in analogous reactions using RSH precursors. To critically explore this possibility, products of the CH₃SH + N₂O₃ reaction have been studied using a combination of chirped-pulse microwave spectroscopy and automated double resonance techniques. As with HSNO, we find that anti-CH₃SNO is formed in high abundance under similar experimental conditions, suggesting that this production method might be extended to study still larger S-nitrothiols in the gas-phase. This talk will provide a status report of our analysis, high-level quantum chemical calculations of minima on the CH₃SNO potential energy surface, and searches for secondary products.

Recent advances in Fourier transform millimeter-wave spectroscopy techniques have renewed the application reach of molecular rotational spectroscopy for analytical chemistry. We present a sampling method for sub ppm analysis of low volatility impurities by thermal evolution from solid powders using a millimeter-wave Fourier transform molecular rotational resonance (FT-MRR) spectrometer for detection. This application of FT-MRR is relevant to the manufacturing of safe oral pharmaceuticals. Low volatility impurities can be challenging to detect at 1 ppm levels with chromatographic techniques. One such example of a potentially mutagenic impurity is acetamide (v.p. 1 Torr at 40 C, m.p. 80 C). We measured the pure reference spectrum of acetamide by flowing the sublimated vapor pressure of acetamide crystals through the FT-MRR spectrometer. The spectrometer lower detection level (LDL) for a broadband (> 20 GHz, 10 min.) spectrum is 300 nTorr, 30 pmol, or 2 ng. For a 50 mg powder, perfect sample transfer efficiency can yield a w/w % detection limit of 35 ppb. We extended the sampling method for the acetamide reference measurement to an acetaminophen sample spiked with 5000 ppm acetamide in order to test the sample transfer efficiency when liberated from an pharmaceutical powder. A spectral reference matching algorithm detected the presence of several impurities including acetaldehyde, acetic acid, and acetonitrile that evolved at the melting point of acetaminophen, demonstrating the capability of FT-MRR for identification without a routine chemical standard. The method detection limit (MDL) without further development is less than 10 ppm w/w %. Resolved FT-MRR mixture spectra will be presented with a description of sampling methods.

Nitric oxide has been of strong biological interest for nearly 40 years due to its role in cardiovascular and nervous signaling. It has been shown that S-nitrosocompounds are the main carrier molecule for nitric oxide in biological systems. These compounds are also of interest due to their relationship to several diseases including muscular dystrophy, stroke, myocardial infarction, Alzheimer's disease, Parkinson's disease, cystic fibrosis, asthma, and pulmonary arterial hypertension. Understanding the role of these S-nitrosocompounds in these diseases requires concentration studies in healthy and diseased tissues as well as metabolic studies using isotopically labeled S-nitroso precursors such at ¹⁵N-arginine. The current widely used techniques for these studies include chemiluminescence, which is blind to isotopic substitution, and mass spectrometry, which is known to artificially create and break S-NO bonds in the sample preparation stages. To this end we have designed and constructed a mid-IR cavity ring-down spectrometer for the detection of nitric oxide released from the target S-nitrosocompounds. Progress toward measuring S-NO groups in biological samples using the CRDS instrument will be presented.

The A-band of oxygen has been measured at low resolution at temperatures between 90 K and 373 K using the phase shift cavity ring down (PS-CRD) technique. For temperatures between 90 K and 295 K, the PS-CRD technique presented here involves an optical cavity attached to a cryostat. The static cell and mirrors of the optical cavity are all inside a vacuum chamber at the same temperature of the cryostat. The temperature of the cell can be changed between 77 K and 295 K. For temperatures above 295 K, a hollow glass cylindrical tube without windows has been inserted inside an optical cavity to measure the temperature of air flowing through the tube. The cavity consists of two highly reflective mirrors which are mounted parallel to each other and separated by a distance of 93 cm. In this experiment, air is passed through a heated tube. The temperature of the air flowing through the tube is determined by measuring the intensity of the oxygen absorption as a function of the wavenumber. The A-band of oxygen is measured between 298 K and 373 K, with several air flow rates. Accuracy of the temperature measurement is determined by comparing the calculated temperature from the spectra with the temperature obtained from a calibrated thermocouple inserted at the center of the tube.

Specificity and sensitivity in atmospheric pressure remote sensing have always been big challenges. This is especially true for approaches that involve the submillimeter/terahertz (smm/THz) spectral region because atmospheric pressure broadening precludes taking advantage of the small Doppler broadening in the region. The Infrared-submillimeter (IR-smm) double resonance spectroscopic technique allows us to obtain a more specific two-dimensional signature as well as a means of modulating the molecular signal to enhance its separation from background and system variation. Applying this technique at atmospheric pressure presents a unique bandwidth requirement on the IR pump laser, and the smm/THz receiver. We will discuss the pump system comprising of a CO2 TEA laser, plasma switch and a free induction decay hot cell designed to produce fast IR pulses on the time scale of atmospheric pressure relaxation and a high bandwidth fast pulse smm/THz receiver. System diagnostics will also be discussed. Results as a function of pressure and pump pulse width will be presented.

The effect of alkyl chain structure on pyrolysis mechanisms has been investigated in a series of aldehydes. Isovaleraldehyde, CH₃CH(CH₃)CH₂CHO, and pivaldehyde, (CH₃)₃CCHO, were subject to thermal decomposition in a resistively heated SiC tubular reactor at 800−1200 ^°C. Matrix-isolation FTIR spectroscopy was used to identify pyrolysis products. Carbon monoxide and isobutene were major products from each of the aldehydes, which is consistent with what is known from previous studies of unbranched alkyl-chain aldehydes. Other products observed include vinyl alcohol, propene, acetylene, and ethylene, revealing complexities to be considered in the pyrolysis of large, branched-chain aldehydes.

A quantified characterization of a spacecraft’s thruster plume is obtainable through measurements of fluorescence from the plume. Fluorescence in a plume is due to electronic excitation of a plume’s molecular species, such as water and ammonia, from solar photons. For instance, electronic excitation of water with Lyman-alpha (121.6 nm) causes photodissociation to OH radical by following one of several possible pathways. One pathway leads to an electronically excited OH radical which fluoresces at  310 nm. Here, the emission spectra of H₂O excited at wavelengths ranging from  128-121 nm are presented and the role of temperature in fluorescence is discussed.

Largest wetland systems are situated on the territory of the Tomsk region. They are characterized by the high content of organic matter (OM), which undergoes transformation as a result of physical, chemical and biological processes. The composition of peat OM is determined by the nature of initial peat-forming plants, their transformation products and bacteria. An experiment in stimulated microbial impact was carried out for estimating the influence of biodegradation on the composition of peat lipids. The composition of the functional groups in the bacterial biomass, initial peat and peat after biodegradation was determined by IR-spectroscopy using the spectrometer NICOLET 5700. The IR spectra of peat and bacteria organic matter are characterized by the presence of absorption bands in ranges: 3400-3200 cm⁻¹, which refers to the stretching vibrations of OH-group of carboxylic acids and various types of hydrogen bonds; 1738-1671 cm⁻¹ – characteristic stretching vibrations of the C = O group of carboxylic acids and ketones; 1262 cm⁻¹ – stretching vibrations of C-O of carboxylic acids. Group and individual composition of organic compounds in studied samples was determined by gas chromatography-mass-spectrometry.

The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of benzene. The pump and probe thermal lens technique has been found to be very sensitive for detecting samples of low concentration in transparent solvents. The C-H fifth vibrational (∆υ = 6) overtone spectrum of benzene is detected at room temperature for compositions per volume in the range (1 to 1×10⁻⁴) using n-C₆H₁₄ as the solvent. By detecting the absorption band in a 100 ppm solution, the peak absorption of the signal is approximately (2.2 ± 0.3)×10⁻⁷ cm⁻¹. The parameters that determine the magnitude of the thermal lens signal such as the pump laser power and the thermodynamic properties of the solvent and solute are discussed. A plot of normalized integrated intensity as a function of composition of benzene in solution reveals a non-linear behavior. The non-linearity cannot be explained assuming solvent enhancement at low concentrations. A two color absorption model that includes the simultaneous absorption of the pump and probe lasers could explain the enhanced magnitude and the non-linear behavior of the thermal lens signal for solutions of composition below 0.01.

We observe that the pressure-broadening coefficients depend on the ortho-para levels. The spectrum is taken with a dual-comb spectrometer which has the resolution of 48 MHz and the frequency accuracy of 8 digit when the signal-to-noise ratio is more than 20S. Okubo et al., Applied Physics Express 8, 082402 (2015) r0pt Figure In this study, about 4.4-Tz wide spectra of the P(31) to R(31) transitions in the ν₁+ν₃ vibration band of ¹²C₂H₂ are observed at the pressure of 25, 60, 396, 1047, 1962 and 2654 Pa. Each rotation-vibration absorption line is fitted to Voight function and we determined pressure-broadening coefficients for each rotation-vibration transition. The Figure shows pressure broadening coefficient as a function of m. Here m is J”+1 for R and –J” for P-branch. The graph shows obvious dependence on ortho and para. We fit it to Pade function considering the population ratio of three-to-one for the ortho and para levels. This would lead to detailed understanding of the pressure boarding mechanism.

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Footnotes:

S. Okubo et al., Applied Physics Express 8, 082402 (2015).