743 research outputs found
Metrics for the performance of chirped-pulse fourier transform microwave spectrometers
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Previous issue date: 26We have been pursuing several applications of molecular rotational resonance (MRR) spectroscopy in quantitative analysis relevant to the pharmaceutical industry. These applications include chiral analysis (absolute configuration and enantiomeric excess), diastereomer identification, regioisomer identification, and isotopologue/isotopomer analysis. Because this work requires quantitative determination of species abundances, we have examined issues related to signal-to-noise determination and quantitation in the low signal regime. Models for the noise distribution in broadband, chirped-pulse Fourier transform microwave (CP-FTMW) spectrometers where the spectra are reported as the magnitude Fourier transform will be presented. The noise distribution is also found to be the same in cavity FTMW spectrometers. We have also modeled the accuracy of signal levels observed in the magnitude Fourier transform as a function of signal-to-noise ratio. This modeling shows that measured transition intensities are accurate down to the root-mean-squared noise level of the instrument without the need to correct for noise power in the signal channel. Finally, using the proper definitions for the noise in magnitude FTMW spectra, we have determined the performance metric introduced by Porterfield et al.\footnote{J.P. Porterfield, L. Satterthwaite, S. Eibenberger, D. Patterson, M.C. McCarthy, \textit{Rev. Sci. Instrum. } \textbf{2019}, \textit{90}, 053104.} for the UVA 6-18 GHz broadband spectrometer. The CP-FTMW instrument speed metric is S = 36,000,000 MHz/min using OCS as the test sample. The reported value for the cryogenic buffer gas cell instrument described in Ref. 1 is S = 4,400,000 MHz/min. These results are in strong disagreement with the CP-FTMW speed metric reported in Ref. 1, S = 60,360 MHz/min, that was the basis for claiming that buffer gas cooling spectrometers have almost a factor of 100 better performance than CP-FTMW instruments
The analysis of complex chemical mixtures by broadband rotational spectroscopy
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Previous issue date: 6Broadband rotational spectroscopy has several experimental advantages as a technique for the analysis of complex chemical mixtures without the need for chromatography to separate the distinct chemical species prior to analysis. The technique has high spectral resolution so that mixtures with a large number of components can be analyzed without spectral overlap, the frequency accuracy is excellent so that library spectra can be transferred between instruments, and the measurement has high dynamic range so that low level impurities can be detected in the presence of dominant species like the solvent in a direct-from-flask reaction mixture. It also has the special feature of a spectroscopic signature that is dependent on the molecular mass distribution so that isomers can be resolved – a problem that can be a challenge for the high-sensitivity analytical chemistry methods based on mass spectrometry. The problem of decomposing a measured spectrum into the individual rotational spectra of each different sample molecule is common to many applications of broadband rotational spectroscopy including reaction product screening in laboratory astrochemistry, the identification of different molecular clusters in the study of weakly bound complexes, and the analysis of chemical samples from a variety of chemistry fields including pharmaceutical science. In this talk we will discuss strategies that have been developed by the spectroscopy community to solve the problem of decomposing a measurement into its constituent spectra. These approaches include traditional analytical chemistry approaches like the creation of large chemical libraries. Instrumental methods that exploit broadband detection to implement efficient double-resonance methods will also be summarized. Two approaches that may deserve additional consideration in the future will also be discussed. One approach measures the spectrum as a function of a continuously variable external parameter and then uses computer algorithms to group transitions with similar parametric dependence as a way to separate the measurement into molecule-specific spectra. The second approach uses the fact that the Hamiltonian for rotational spectroscopy is known and that only certain patterns of transitions are consistent with it. An early example of this idea is the AUTOFIT routine. The possibility of extending this approach into a fully automated computer analysis of the spectrum will be considered
Molecular size limits for rotational spectroscopy and the high-J limit of the rigid rotor
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Previous issue date: 2019-06-19Made available in DSpace on 2020-01-25T19:30:40Z (GMT). No. of bitstreams: 5
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Previous issue date: 2019-06-19Molecular rotational spectroscopy is well suited for the analysis of regioisomers, which has applications in pharmaceutical synthesis. Rotational spectroscopy offers important advantages over other chemical analysis techniques resulting from extreme sensitivity to molecular mass distribution, high accuracy molecular structure (and, therefore, rotational constant) predictions from quantum calculations, and the combination of high-spectral resolution and dynamic range. The sensitivity and range of the technique allow low abundance regioisomer impurities to be quantitatively measured without chemical separation. The challenge for rotational spectroscopy is that actual measurement challenges in pharmaceutical chemistry involve molecules in the 200-500 Da mass range. Unique features of the rotational kinetic energy levels lead to decreasing peak transition strength as the molecular size increases as well as a decrease in the frequency where the strongest transitions occur. However, special features of the high-J limit (semiclassical limit) of the rigid rotor Hamiltonian potentially mitigate these experimental difficulties and suggest that rotational spectroscopy can be applied to much larger molecules than previously expected. The rotational spectra of a series of high vapor pressure large molecules in both the limiting prolate and oblate limits are analyzed. For the prospect of analyzing larger molecules, the prolate limit appears to offer major advantages. However, experiments using pulsed-jet sources show rapid relaxation of these high-energy rotational levels and, at present, appear to reduce the major advantages suggested by the rotational spectroscopy in the high-J limit
C–H···π Interactions in the CHBrF<sub>2</sub>···HCCH Weakly Bound Dimer
No full textThe microwave spectra of four isotopologues of the CHBrF(2)···HCCH weakly bound dimer have been measured in the 6-18 GHz region using chirped-pulse and Balle-Flygare Fourier-transform microwave spectroscopy. Spectra of (13)CH(79)BrF(2) and (13)CH(81)BrF(2) monomers have also been measured, and spectroscopic constants are reported. Measurement of spectra for the (79)Br and (81)Br isotopologues of CHBrF(2) complexed with both (12)C(2)H(2) and (13)C(2)H(2) have allowed the determination of a structure with C(s) symmetry for this complex. CHBrF(2) interacts with the triple bond of acetylene via a C-H···π contact (R(H···π) = 2.670(8) Å) with the Br atom lying in the ab plane, located 3.293(40) Å from a hydrogen atom of the HCCH molecule. The structure of CHBrF(2)···HCCH has been compared with recently studied related acetylene complexes, including a comparison with (and further structural analysis of) the CHClF(2)···HCCH complex
Online Stereochemical Process Monitoring by Molecular Rotational Resonance Spectroscopy
No full textA molecular
rotational resonance (MRR) spectrometer designed to
monitor the product composition of an asymmetric continuous flow reaction
online is presented. The MRR technique is highly sensitive to small
changes in molecular structure and, as such, is capable of rapidly
quantifying isomers as well as other impurities in a complex mixture,
without chromatographic separation or chemometrics. The spectrometer
in this study operates by automatically drawing a portion of the reaction
solution into a reservoir, volatizing it by heating, and measuring
the highly resolved MRR spectra of each of the components of interest
in order to determine their relative quantity in the mixture. The
reaction under study was the hydrogenation of artemisinic acid, an
intermediate step in the semisynthesis of the antimalarial drug artemisinin.
Four analytes were characterized in each measurement: the starting
material, the product, a diastereomer of the product, and an overreduction
byproduct that was not directly quantifiable by either HPLC or NMR
methods. The MRR instrument has a measurement cycle time of approximately
17 min for this analysis and can run for several hours without any
user interaction
DIRECT MEASUREMENT OF CATALYTIC OXIDATION OF SO₂ BY A K-BAND MOLECULAR ROTATIONAL RESONANCE SPECTROSCOPY
No full textA Molecular rotational resonance (MRR) spectrometer, which operates in the 18-26GHz, has been evaluated for monitoring the oxidation process of SO₂ and O₂ in the presence of NH₄VO₃. This work is performed as a part of effort to determine the utility of rotational spectroscopy as a tool for monitoring the conversion of SO₂ to H₂SO₄. The initial MRR measurements revealed the reduction of SO₂and the presence of small polar impurities (i.e., water vapor and ammonia). The current data have been further employed to validate K-Band MRR for SO₂ removal. The MRR maintains its linearity confirming its strength to monitor the removal of SO₂ in presence of other polar impurities. Work to improve this analytical procedure is underway and will be reported in this talk
Dynamics of the semiclassical limit quartet rotational transitions of large molecules
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Previous issue date: 25Large molecules, molecules with about 15 or more heavy atoms, have high angular momentum quantum states populated even at the low temperatures produced by seeded pulsed jet expansion. The rotational spectroscopy of these molecules is often more easily described using semiclassical energy expressions and selection rules. One prominent feature of the rotational spectra of molecules in the semiclassical limit is a set of strong, equally spaced transitions that correlate to the collapse of the well-known quartets in the rotational spectra of asymmetric top molecules. These quartets occur for classically stable rotation about the principal axis with both the smallest and largest moments-of-inertia. Over several measurements of the broadband, chirped-pulse Fourier transform microwave spectra of large molecules with semiclassical limit spectra, we have noticed that the quartets for motion around the a-principal axis (prolate quartets) frequently show very fast decay of the free-induction decay (FID) relative to the oblate quartet transitions and the “normal” asymmetric top rotational transitions. To further explore this effect, we have performed Hahn echo experiments that show the rapid decay is caused by dephasing instead of population relaxation. It has also been observed that these transitions do not show significantly faster FID dynamics in a coaxial nozzle cavity FTMW instrument. A summary of results over a series of molecules and possible physical origins of the short dephasing times of the prolate quartets will be presented
THE LOW FREQUENCY BROADBAND FOURIER TRANSFORM MICROWAVE SPECTROSCOPY OF HEXAFLUOROPROPYLENE OXIDE, CFCFOCF
No full textthe John B. Stephenson Fellowship Of Aca Is Gratefully; Shipman, Steven T.; Neill, Justin L.; Lesarri, Alberto; Pate, Brooks H.Author Institution: Department of Natural Sciences, Union College, Barbourville, KY 40906; Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904; Departmento de Quimica Fisica, Universidad de Valladolid, Facultad de Ciencias, Valladolid, 47005 SPAIN; Department of Chemistry, University of Virginia, McCormick Rd., P.O. Box 400319, Charlottesville, VA 22904The pure rotational spectra of hexafluoropropylene oxide (HFPO), CFCFOCF, as well as its C (1.07\%) and O (0.205\%) isotopomers were recorded in natural abundance from 2.0 to 26 GHz. Low frequency transitions (2 - 8 GHz) were measured by a recently designed chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer at the University of Virginia. The observed spectra lines of C isotopomers (in natural abundance) demonstrate the capability and sensitivity of the CP-FTMW spectrometer operating in 2 - 8 GHz region. The spectra in 8.0 - 26 GHz region were recorded by a Fabry-Perot cavity Fourier transform microwave (FP-FTMW) spectrometer. All the five isotopologues of HFPO were found, and their spectroscopic constants were fit from assigned spectral lines using JB95 and Picketts SPFIT suite of programs. For the dominate HFPO isotopomer: = 2217.04887(11) MHz, = 1101.48958(5) MHz, = 936.60131(5) MHz, = 55.0(2) Hz, = 107.5(9) Hz, = -20(2) Hz, = 8.49(6) Hz, and = -266(2) Hz. The experimentally determined molecular structure and rotational constants are in a good agreement with our density functional theory calculation using B3LYP/6-31g(d) method
Sampling requirements for mixture analysis using molecular rotational resonance spectroscopy
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Previous issue date: 2019-06-18Made available in DSpace on 2020-01-25T19:29:42Z (GMT). No. of bitstreams: 4
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Previous issue date: 2019-06-18"Over the past few years, a number of studies have been performed that show the capability for gas-phase molecular rotational spectroscopy to perform quantitative mixture analysis. In particular, the ability of this technique to identify new compounds in a mixture on the basis of comparison to electronic structure theory is extremely powerful. For a number of reasons, however, the sample introduction and volatilization methods employed warrant new development. First, mixtures often contain components with different vapor pressures, and so care is required in extrapolating concentration information from observed signals.\footnote{C. West \textit{et al.}, ""Analysis of pear ester flavoring samples using broadband rotational spectroscopy,"" 2018 International Symposium on Molecular Spectroscopy, talk RH06.} Additionally, operator-to-operator variability, measurement cycle time, and ease of use are factors that should be considered. We will discuss our efforts to develop sampling interfaces to enable routine quantitative mixture analysis using molecular rotational spectroscopy, as well as challenges that the field still faces.
SPECTROSCOPIC CHARACTERIZATION OF SMALL POLAR IMPURITIES IN GASOLINE
No full textSmall polar compounds in gasoline have been identified using a BrightSpec Fourier Transform Microwave Rotational Resonance (FT-MRR) spectrometer in the 260-290 GHz band with Headspace Sampling Module. The design of this spectrometer is based on segmented Chirped Pulse Fourier Transform millimeter wave (CP-FTmmW) spectroscopy, which exploits recent advances in digital electronics to allow the measurement of broadband rotational spectra in a few minutes. As part of efforts to determine applications for rotational spectroscopy to petrochemical problems, FT-MRR has been employed to record rotationally resolved spectra of small polar compounds in gasoline. Preliminary analysis of the observed features using the BrightSpec spectral database reveals a rich, but interpretable, pattern, due to the sensitivity of FT-MRR to only polar compounds. The complex hydrocarbon matrix, which in many analytical instruments obscures the signals from low concentration impurities, is nearly invisible in FT-MRR. Spectroscopic and quantitative analyses of detected polar compounds are underway and will be given in this talk
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