224 research outputs found
Using concentration dependence of microwave spectra of 2-component mixtures to identify single component clusters ‒ Application to (fluoroethylene)n and (1,1-difluoroethylene)n
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Previous issue date: 2019-06-18Made available in DSpace on 2020-01-25T19:29:54Z (GMT). No. of bitstreams: 4
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Previous issue date: 2019-06-18Over the past two years, we have implemented automated analysis of chirped-pulse microwave spectra of two-component mixtures of fluoroethylene (FE) or 1,1-difluoroethylene (DFE) and \chem{CO_2} to facilitate identification of the numerous cluster spectra present in a single scan.\footnote{Rebecca A. Peebles, Prashansa B. Kannangara, Sean A. Peebles, Brooks H. Pate, 73rd International Symposium on Molecular Spectroscopy, Talk TH02, June 19, 2018.} This approach has led to assignment of ten \chem{(FE)_n(CO_2)_m} clusters and one \chem{(DFE)_n(CO_2)_m} cluster, so far. These scans also include spectra of multiple single-component clusters (for instance, \chem{(FE)_n}), which were not apparent in earlier analyses, since “monomer only” peaks were filtered from data sets during analysis. Present efforts utilize intensity variation amongst two-component scans as a way to identify these single-component clusters. Previously unobserved spectra for five clusters involving only FE or only DFE (and in some cases including neon) have now been assigned.
Identifying groups of related transitions and assigning their spectra has proven relatively straightforward, but determining compositions and structures of the carriers of these spectra is challenging. Several approaches, including analysis of the concentration dependence of transition intensity and implementation of rapid force-field based structure optimizations, have allowed some progress on determining details of the observed species
Influence of halogen variation on structure and interactions in vinyl halide (H2C=CHX)…CO2 (X = F, Cl, Br) complexes
No full textChirped-pulse and resonant cavity Fourier-transform microwave spectroscopy have been used to investigate dimers of CO with vinyl fluoride (VF), vinyl chloride (VCl) and vinyl bromide (VBr). For all three complexes, CO is aligned adjacent to the X--C--H end (X = F, Cl, Br) of the ethylene subunit, with C--XC and C--HO contacts. For VFCO, a second isomer is also observed, with CO roughly parallel to the H--C=C--F side of VFhowever, there is no spectroscopic indication that similar structures are present for VClCO or VBrCO.
For vinyl fluorideCO, a full structural analysis has previously been published,\footnote {C. L. Christenholz, R. E. Dorris, R. A. Peebles, S. A. Peebles, {\it J. Phys. Chem. A}, {\bf 118}, (2014), 8765-8772.} while for the Cl- and Br-containing species, insufficient data are presently available for complete structure determinations. However, structural information from ab initio calculations, Cl/Cl and Br/Br isotopic substitution, and analysis of chlorine and bromine nuclear quadrupole coupling constants will be presented. In addition, for this series of dimers containing C--HO contacts, further insight into the nature of the weak interactions may be obtained from Quantum Theory of Atoms in Molecules (QTAIM) and other {\it ab initio} analyses that are presently in progress.Made available in DSpace on 2017-01-26T21:39:41Z (GMT). No. of bitstreams: 3
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Previous issue date: 2016-06-2
Rotational spectroscopic studies of C-H · · · F interactions in the vinyl fluoride · · · difluoromethane complex
No full textRotational spectra of the normal isotopic species and three (13)C isotopologues of the 1:1 complex between vinyl fluoride (CH2 ═ CHF) and difluoromethane (CH2F2) have been measured using 480 MHz bandwidth chirped-pulse Fourier-transform microwave spectroscopy in the 6.5-20 GHz region. A structure for this dimer has been determined by fitting the moments of inertia of all isotopologues and confirmed by calculation of Kraitchman single isotopic substitution coordinates. The structure is consistent with that determined by ab initio geometry optimization at the MP2/6-311++G(2d,2p) level and has the difluoromethane subunit located on the CHF side of the vinyl fluoride subunit with three C-H · · · F contacts and with the hydrogen atoms of the CH2F2 straddling the vinyl fluoride symmetry plane
Reduced bandwidth chirped-pulse microwave spectroscopy for analysis of weakly bound dimers: Rotational spectrum and structural analysis of CH2ClF⋯FHCCH2
No full textWeak interactions and CO2 microsolvation in the cis-1,2-difluoroethylene...CO2 complex
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Previous issue date: 6Made available in DSpace on 2018-01-29T23:05:21Z (GMT). No. of bitstreams: 4
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Previous issue date: 6The need for a deep understanding of chem{CO_2} interactions is significant given the importance of supercritical chem{CO_2} (sc-chem{CO_2}) as a green solvent. Fluorinated compounds often have higher solubility in sc-chem{CO_2} than their hydrocarbon analogs, and the reasons for this are not well understood. Investigations of dimers of one chem{CO_2} molecule with a simple fluorinated hydrocarbon provide an initial step towards understanding the complex balance of forces that is likely to be present as a larger solvation shell of sc-chem{CO_2} is built._x000d_
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The weakly bound dimer {it cis}-1,2-difluoroethylene...chem{CO_2} is the latest in a series of complexes of chem{CO_2} with fluorinated ethylenes that has recently been studied using chirped-pulse (CP) Fourier-transform microwave spectroscopy. Unlike all previous members of the series, the observed structure of {it cis}-1,2-difluoroethylene...chem{CO_2} is nonplanar, with chem{CO_2} sitting above the ethylene plane and crossed relative to the C=C bond. This nonplanar arrangement is consistent with predictions made using symmetry adapted perturbation theory (SAPT), where the dispersion energy of the nonplanar structure is significantly more favorable than for a structure where chem{CO_2} lies in the same plane as the ethylene moiety. Observed transitions are doubled as a result of chem{CO_2} tunneling between equivalent positions above and below the ethylene plane, leading to inversion of the dipole moment component. Observed transitions for the most abundant isotopologue have been fitted to a two state Hamiltonian to give an energy difference between tunneling states of MHz, and analysis using Meyer's one dimensional model to determine the barrier to inversion is presently in progress
The molecular structure of difluoroisocyanato silane: A combined microwave spectral and theoretical study
No full textModeling Co2 Microsolvation: Microwave Spectroscopic Studies Of Difluoroethylene (dfe)/co2 Clusters, (dfe)1(co2)x, For A Trimer, Tetramer, And Pentamer
No full textMicrowave spectroscopy allows for analysis of weakly-bound clusters in a mixture of difluoroethylene (DFE) and \chem{CO_2}. The present study probes variations in interactions and orientations of DFE and \chem{CO_2} within weakly-bound clusters as cluster size increases. Four chirped-pulse FTMW spectra of DFE/\chem{CO_2} mixtures were obtained from 2-8 GHz, where the concentration of \chem{CO_2} was varied from 1\% to 4\%, with a constant DFE concentration of 1\%. This experimental design allowed variation in intensity to be observed based on the variation of \chem{CO_2} concentration, where the pattern of intensity variation was used to identify transitions belonging to a particular cluster. In addition, patterns of intensity variation provided information about the size and DFE:\chem{CO_2} ratio of the cluster. Using these methods based on intensity variation analysis,\footnote{H. Fino, R.A. Peebles, S.A. Peebles, C. West, B. Pate, International Symposium on Molecular Spectroscopy (Virtual), Talk FH12, June 25, 2021; R.A. Peebles, S.A. Peebles, P. Kannangara, H. Fino, International Symposium on Molecular Spectroscopy (Virtual), Talk FH13, June 25, 2021} three separate sets of transitions, each with unique intensity variation patterns, were extracted from the original raw spectra. Cluster composition was hypothesized based on further evidence from the intensity variation analyses, leading to compositions of \chem{(DFE)_1(CO_2)_2}, \chem{(DFE)_1(CO_2)_3}, and \chem{(DFE)_1(CO_2)_4}. Fitted rotational constants for the spectra were compared to the results of ab initio calculations, which further supported hypothesized cluster compositions for the trimer, tetramer and pentamer. These results indicate that instead of forming a solvation shell around DFE, \chem{CO_2} molecules appear preferably to interact with other \chem{CO_2} molecules to form arrangements more closely resembling pure \chem{CO_2} clusters, with DFE on the outside of the cluster
New approaches to decoding rotational spectra: Applications to fluoroethylene microsolvation by CO2
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Previous issue date: 6Made available in DSpace on 2018-12-12T22:37:31Z (GMT). No. of bitstreams: 4
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Previous issue date: 6Chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometers such as the instrument at the University of Virginia can acquire spectra with high sensitivity in a short amount of time. This necessitates new approaches to spectroscopic analysis to ensure identification of all species in each recorded spectrum. With an intensity range covering four orders of magnitude after averaging 1 million free induction decays, a recent spectrum of a fluoroethylene (FE)/CO2 mixture in the 2 – 8 GHz range has over 11,000 lines with signal-to-noise ratio above ∼2.5. These transitions may arise from a combination of monomer, dimer and larger cluster species, including low abundance isotopes and complexes with carrier gas, water or other contaminants.
Our current focus is identifying spectra of FE(CO2)n clusters, containing progressively larger numbers of CO2 molecules. Several methods have been used to facilitate identification of lines for these spectra, which are expected to lose 1-2 orders of magnitude of intensity for each increase in cluster size. These approaches include subtraction of transitions that are observed in the FE-only spectrum from the spectrum of the FE/CO2 mixture, visual identification of patterns characteristic of asymmetric molecules, and application of extended cross correlation (XCC) techniques.a In the XCC approach, several spectra with systematically varied conditions (such as pressure or concentration) are compared, and a combination of graphs and computerized algorithms is used to identify transitions that behave similarly under the changing conditions. In addition to applications for fundamental spectroscopic studies, this approach has potential application to identification of the components of complex mixtures
Engaging Undergraduate Students In Spectroscopy Research Via Development And Incorporation Of Advanced Data Analysis Techniques
No full textThe rapidity with which large amounts of spectroscopic data can now be collected is presently driving interest in developing techniques to improve the speed with which spectra can be analyzed. While desirable in a research setting to avoid bottlenecks in the lab, these techniques will also be essential to the commercialization of high resolution spectroscopic methods for analysis of complex mixtures. At the same time, many undergraduate students are intrigued by the concept of data analytics and attracted by the growing job market related to this field. We will present our incorporation of analysis techniques appropriate for large data sets into undergraduate spectroscopy research experiences. Through analysis of high sensitivity microwave spectra of complex mixtures of weakly bound complexes, undergraduate students from a wide range of majors gain skill sets that put them ahead of their peers in areas such as problem solving, basic coding, and computer skills (Excel, DOS, Linux, Python, Mathcad). The majority of spectroscopy undergraduate research students at Eastern Illinois University do not go on to chemistry careers, and these additional skills that they learn provide excellent preparation for a wide range of career choices.Made available in DSpace on 2021-09-24T21:08:48Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-06-25Made available in DSpace on 2022-01-21T16:10:07Z (GMT). No. of bitstreams: 4
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Previous issue date: 2021-06-2
Engaging undergraduate students in spectroscopy research via development and incorporation of advanced data analysis techniques
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Previous issue date: 23The rapidity with which large amounts of spectroscopic data can now be collected is presently driving interest in developing techniques to improve the speed with which spectra can be analyzed. While desirable in a research setting to avoid bottlenecks in the lab, these techniques will also be essential to the commercialization of high resolution spectroscopic methods for analysis of complex mixtures. At the same time, many undergraduate students are intrigued by the concept of data analytics and attracted by the growing job market related to this field. We will present our incorporation of analysis techniques appropriate for large data sets into undergraduate spectroscopy research experiences. Through analysis of high sensitivity microwave spectra of complex mixtures of weakly bound complexes, undergraduate students from a wide range of majors gain skill sets that put them ahead of their peers in areas such as problem solving, basic coding, and computer skills (Excel, DOS, Linux, Python, Mathcad). The majority of spectroscopy undergraduate research students at Eastern Illinois University do not go on to chemistry careers, and these additional skills that they learn provide excellent preparation for a wide range of career choices
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