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Melanie Schnell - One of the best experts on this subject based on the ideXlab platform.
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Dynamic chiral self-recognition in aromatic dimers of styrene oxide revealed by Rotational Spectroscopy
Communications Chemistry, 2021Co-Authors: Sérgio R. Domingos, Cristobal Perez, Nora M. Kreienborg, Christian Merten, Melanie SchnellAbstract:Chiral molecular recognition is a pivotal phenomenon in biomolecular science, governed by subtle balances of intermolecular forces that are difficult to quantify. Non-covalent interactions involving aromatic moieties are particularly important in this realm, as recurring motifs in biomolecular aggregation. In this work, we use high-resolution broadband Rotational Spectroscopy to probe the dynamic conformational landscape enclosing the self-pairing topologies of styrene oxide, a chiral aromatic system. We reach a definite assignment of four homochiral and two heterochiral dimers using auxiliary quantum chemistry calculations as well as structure-solving methods based on experimental isotopic information. A complete picture of the dimer conformational space is obtained, and plausible routes for conformational relaxation are derived. Molecular structures are discussed in terms of conformational flexibility, the concerted effort of weak intermolecular interactions, and their role in the expression of the molecular fit. Chiral molecular recognition is critical in many natural processes, but probing the conformational landscape of chiral aromatic systems can be challenging. Here, high-resolution broadband Rotational Spectroscopy data is combined with quantum chemical computations to reveal the dynamic self-pairing topologies of the chiral molecule styrene oxide.
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Assessing the performance of Rotational Spectroscopy in chiral analysis
Chemical Science, 2020Co-Authors: Sérgio R. Domingos, Cristobal Perez, Mark D. Marshall, Helen O. Leung, Melanie SchnellAbstract:The capabilities of Rotational Spectroscopy-based methods as tools to deliver accurate and precise chirality-sensitive information are still breaking ground, but their applicability in the challenging field of analytical chemistry is already clear. In this mini review, we explore the current abilities and challenges of two emergent techniques for chiral analysis based on Rotational Spectroscopy. For that, we will showcase the two methods (microwave 3-wave mixing and chiral tag Rotational Spectroscopy) while testing their performance to solve the absolute configuration and the enantiomeric excess of a blind sample containing a mixture of enantiomers of styrene oxide.
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Analysis of thyme essential oils using gas-phase broadband Rotational Spectroscopy.
Physical chemistry chemical physics : PCCP, 2019Co-Authors: María Mar Quesada-moreno, Anna Krin, Melanie SchnellAbstract:A semi-quantitative analysis of the components of two natural essential oils has been carried out using broadband Rotational Spectroscopy, which is inherently molecule specific. The samples under study were two thyme essential oils from Spain with different compositions: (a) with thymol as the most abundant species (thyme I) and (b) with linalool and 4-carvomenthenol being the most abundant ones (thyme II). Relative intensity measurements of selected Rotational transitions were carried out to estimate the abundances of the different species present in these complex mixtures, taking into account the square of the respective dipole moment components. One strength of Rotational Spectroscopy is its structure sensitivity. Here, we also re-investigated the microwave spectrum of linalool and determined the accurate experimental gas-phase structures of thymol and linalool through the assignment of all 13C isotopologues of their lowest energy conformers. A characteristic splitting pattern of the Rotational transitions due to internal rotation of two non-equivalent methyl groups of linalool was observed in the thyme II spectrum. Their internal rotation barriers were experimentally determined to 4.7703(96) kJ mol-1 and 9.2581(74) kJ mol-1, respectively.
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Sensing Chirality with Rotational Spectroscopy.
Annual review of physical chemistry, 2018Co-Authors: Sérgio R. Domingos, Cristobal Perez, Melanie SchnellAbstract:Chiroptical Spectroscopy techniques for the differentiation of enantiomers in the condensed phase are based on an established paradigm that relies on symmetry breaking using circularly polarized light. We review a novel approach for the study of chiral molecules in the gas phase using broadband Rotational Spectroscopy, namely microwave three-wave mixing, which is a coherent, nonlinear, and resonant process. This technique can be used to generate a coherent molecular Rotational signal that can be detected in a manner similar to that in conventional Fourier transform microwave Spectroscopy. The structure (and thermal distribution of conformations), handedness, and enantiomeric excess of gas-phase samples can be determined unambiguously by employing tailored microwave fields. We discuss the theoretical and experimental aspects of the method, the significance of the first demonstrations of the technique for enantiomer differentiation, and the method’s rapid advance into a robust choice to study molecular chir...
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Quantitative Chiral Analysis by Molecular Rotational Spectroscopy
Chiral Analysis, 2018Co-Authors: Brooks H Pate, Cristobal Perez, Javix Thomas, Luca Evangelisti, Walther Caminati, David Patterson, Melanie SchnellAbstract:Abstract Recent advances in the application of molecular Rotational Spectroscopy to chiral analysis are described. These include the use of broadband Rotational Spectroscopy for the analysis of diastereomers of molecules with multiple chiral centers. Two new approaches to enantiomer-specific Rotational Spectroscopy are presented. One method adapts strategies from nuclear magnetic resonance Spectroscopy by using noncovalent attachment of a chiral tag molecule to convert enantiomers into diastereomers. Applications of this method to the high-confidence determination of absolute stereochemistry and the measurement of the enantiomeric excess without reference samples of the analyte are illustrated with recent measurements. The second approach is a novel three-wave mixing method that generates a phase-dependent chiral emission signal. This method offers the possibility for direct chiral analysis of molecules in complex chemical mixtures without the need for chemical separation protocol development.
Cristobal Perez - One of the best experts on this subject based on the ideXlab platform.
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Dynamic chiral self-recognition in aromatic dimers of styrene oxide revealed by Rotational Spectroscopy
Communications Chemistry, 2021Co-Authors: Sérgio R. Domingos, Cristobal Perez, Nora M. Kreienborg, Christian Merten, Melanie SchnellAbstract:Chiral molecular recognition is a pivotal phenomenon in biomolecular science, governed by subtle balances of intermolecular forces that are difficult to quantify. Non-covalent interactions involving aromatic moieties are particularly important in this realm, as recurring motifs in biomolecular aggregation. In this work, we use high-resolution broadband Rotational Spectroscopy to probe the dynamic conformational landscape enclosing the self-pairing topologies of styrene oxide, a chiral aromatic system. We reach a definite assignment of four homochiral and two heterochiral dimers using auxiliary quantum chemistry calculations as well as structure-solving methods based on experimental isotopic information. A complete picture of the dimer conformational space is obtained, and plausible routes for conformational relaxation are derived. Molecular structures are discussed in terms of conformational flexibility, the concerted effort of weak intermolecular interactions, and their role in the expression of the molecular fit. Chiral molecular recognition is critical in many natural processes, but probing the conformational landscape of chiral aromatic systems can be challenging. Here, high-resolution broadband Rotational Spectroscopy data is combined with quantum chemical computations to reveal the dynamic self-pairing topologies of the chiral molecule styrene oxide.
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Assessing the performance of Rotational Spectroscopy in chiral analysis
Chemical Science, 2020Co-Authors: Sérgio R. Domingos, Cristobal Perez, Mark D. Marshall, Helen O. Leung, Melanie SchnellAbstract:The capabilities of Rotational Spectroscopy-based methods as tools to deliver accurate and precise chirality-sensitive information are still breaking ground, but their applicability in the challenging field of analytical chemistry is already clear. In this mini review, we explore the current abilities and challenges of two emergent techniques for chiral analysis based on Rotational Spectroscopy. For that, we will showcase the two methods (microwave 3-wave mixing and chiral tag Rotational Spectroscopy) while testing their performance to solve the absolute configuration and the enantiomeric excess of a blind sample containing a mixture of enantiomers of styrene oxide.
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Sensing Chirality with Rotational Spectroscopy.
Annual review of physical chemistry, 2018Co-Authors: Sérgio R. Domingos, Cristobal Perez, Melanie SchnellAbstract:Chiroptical Spectroscopy techniques for the differentiation of enantiomers in the condensed phase are based on an established paradigm that relies on symmetry breaking using circularly polarized light. We review a novel approach for the study of chiral molecules in the gas phase using broadband Rotational Spectroscopy, namely microwave three-wave mixing, which is a coherent, nonlinear, and resonant process. This technique can be used to generate a coherent molecular Rotational signal that can be detected in a manner similar to that in conventional Fourier transform microwave Spectroscopy. The structure (and thermal distribution of conformations), handedness, and enantiomeric excess of gas-phase samples can be determined unambiguously by employing tailored microwave fields. We discuss the theoretical and experimental aspects of the method, the significance of the first demonstrations of the technique for enantiomer differentiation, and the method’s rapid advance into a robust choice to study molecular chir...
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Quantitative Chiral Analysis by Molecular Rotational Spectroscopy
Chiral Analysis, 2018Co-Authors: Brooks H Pate, Cristobal Perez, Javix Thomas, Luca Evangelisti, Walther Caminati, David Patterson, Melanie SchnellAbstract:Abstract Recent advances in the application of molecular Rotational Spectroscopy to chiral analysis are described. These include the use of broadband Rotational Spectroscopy for the analysis of diastereomers of molecules with multiple chiral centers. Two new approaches to enantiomer-specific Rotational Spectroscopy are presented. One method adapts strategies from nuclear magnetic resonance Spectroscopy by using noncovalent attachment of a chiral tag molecule to convert enantiomers into diastereomers. Applications of this method to the high-confidence determination of absolute stereochemistry and the measurement of the enantiomeric excess without reference samples of the analyte are illustrated with recent measurements. The second approach is a novel three-wave mixing method that generates a phase-dependent chiral emission signal. This method offers the possibility for direct chiral analysis of molecules in complex chemical mixtures without the need for chemical separation protocol development.
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communication structural locking mediated by a water wire a high resolution Rotational Spectroscopy study on hydrated forms of a chiral biphenyl derivative
Journal of Chemical Physics, 2016Co-Authors: Melanie Schnell, Cristobal Perez, Sérgio R. DomingosAbstract:We report the observation of structural changes in an axially chiral molecule, biphenyl-2-carboxaldehyde, due to aggregation with water. Using high-resolution broadband Rotational Spectroscopy we find that two water molecules link opposite sides of the molecule, resembling a water wire. We show that this effect can be explained by a cooperative rearrangement of both molecule and a water dimer. Hydrogen bonding interactions are shown to change the original structure upon aggregation of water. This phenomenon is insightful on the role of microsolvation in assisting structural morphing of stereo-selective chiral molecular systems.
Sérgio R. Domingos - One of the best experts on this subject based on the ideXlab platform.
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Dynamic chiral self-recognition in aromatic dimers of styrene oxide revealed by Rotational Spectroscopy
Communications Chemistry, 2021Co-Authors: Sérgio R. Domingos, Cristobal Perez, Nora M. Kreienborg, Christian Merten, Melanie SchnellAbstract:Chiral molecular recognition is a pivotal phenomenon in biomolecular science, governed by subtle balances of intermolecular forces that are difficult to quantify. Non-covalent interactions involving aromatic moieties are particularly important in this realm, as recurring motifs in biomolecular aggregation. In this work, we use high-resolution broadband Rotational Spectroscopy to probe the dynamic conformational landscape enclosing the self-pairing topologies of styrene oxide, a chiral aromatic system. We reach a definite assignment of four homochiral and two heterochiral dimers using auxiliary quantum chemistry calculations as well as structure-solving methods based on experimental isotopic information. A complete picture of the dimer conformational space is obtained, and plausible routes for conformational relaxation are derived. Molecular structures are discussed in terms of conformational flexibility, the concerted effort of weak intermolecular interactions, and their role in the expression of the molecular fit. Chiral molecular recognition is critical in many natural processes, but probing the conformational landscape of chiral aromatic systems can be challenging. Here, high-resolution broadband Rotational Spectroscopy data is combined with quantum chemical computations to reveal the dynamic self-pairing topologies of the chiral molecule styrene oxide.
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Assessing the performance of Rotational Spectroscopy in chiral analysis
Chemical Science, 2020Co-Authors: Sérgio R. Domingos, Cristobal Perez, Mark D. Marshall, Helen O. Leung, Melanie SchnellAbstract:The capabilities of Rotational Spectroscopy-based methods as tools to deliver accurate and precise chirality-sensitive information are still breaking ground, but their applicability in the challenging field of analytical chemistry is already clear. In this mini review, we explore the current abilities and challenges of two emergent techniques for chiral analysis based on Rotational Spectroscopy. For that, we will showcase the two methods (microwave 3-wave mixing and chiral tag Rotational Spectroscopy) while testing their performance to solve the absolute configuration and the enantiomeric excess of a blind sample containing a mixture of enantiomers of styrene oxide.
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Sensing Chirality with Rotational Spectroscopy.
Annual review of physical chemistry, 2018Co-Authors: Sérgio R. Domingos, Cristobal Perez, Melanie SchnellAbstract:Chiroptical Spectroscopy techniques for the differentiation of enantiomers in the condensed phase are based on an established paradigm that relies on symmetry breaking using circularly polarized light. We review a novel approach for the study of chiral molecules in the gas phase using broadband Rotational Spectroscopy, namely microwave three-wave mixing, which is a coherent, nonlinear, and resonant process. This technique can be used to generate a coherent molecular Rotational signal that can be detected in a manner similar to that in conventional Fourier transform microwave Spectroscopy. The structure (and thermal distribution of conformations), handedness, and enantiomeric excess of gas-phase samples can be determined unambiguously by employing tailored microwave fields. We discuss the theoretical and experimental aspects of the method, the significance of the first demonstrations of the technique for enantiomer differentiation, and the method’s rapid advance into a robust choice to study molecular chir...
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communication structural locking mediated by a water wire a high resolution Rotational Spectroscopy study on hydrated forms of a chiral biphenyl derivative
Journal of Chemical Physics, 2016Co-Authors: Melanie Schnell, Cristobal Perez, Sérgio R. DomingosAbstract:We report the observation of structural changes in an axially chiral molecule, biphenyl-2-carboxaldehyde, due to aggregation with water. Using high-resolution broadband Rotational Spectroscopy we find that two water molecules link opposite sides of the molecule, resembling a water wire. We show that this effect can be explained by a cooperative rearrangement of both molecule and a water dimer. Hydrogen bonding interactions are shown to change the original structure upon aggregation of water. This phenomenon is insightful on the role of microsolvation in assisting structural morphing of stereo-selective chiral molecular systems.
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flexibility unleashed in acyclic monoterpenes conformational space of citronellal revealed by broadband Rotational Spectroscopy
Physical Chemistry Chemical Physics, 2016Co-Authors: Melanie Schnell, Cristobal Perez, Sérgio R. Domingos, Chris Medcraft, Pablo PinachoAbstract:Conformational flexibility is intrinsically related to the functionality of biomolecules. Elucidation of the potential energy surface is thus a necessary step towards understanding the mechanisms for molecular recognition such as docking of small organic molecules to larger macromolecular systems. In this work, we use broadband Rotational Spectroscopy in a molecular jet experiment to unravel the complex conformational space of citronellal. We observe fifteen conformations in the experimental conditions of the molecular jet, the highest number of conformers reported to date for a chiral molecule of this size using microwave Spectroscopy. Studies of relative stability using different carrier gases in the supersonic expansion reveal conformational relaxation pathways that strongly favour ground-state structures with globular conformations. This study provides a blueprint of the complex conformational space of an important biosynthetic precursor and gives insights on the relation between its structure and biological functionality.
Wolfgang Jäger - One of the best experts on this subject based on the ideXlab platform.
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conformational landscape chirality recognition and chiral analyses Rotational Spectroscopy of tetrahydro 2 furoic acid propylene oxide conformers
ChemPhysChem, 2021Co-Authors: Fan Xie, Nathan A Seifert, Arsh Hazrah, Wolfgang JägerAbstract:A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using Rotational Spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol-1 . A rich broadband Rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag Rotational Spectroscopy, an exciting new analytical application of Rotational Spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.
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Conformational Landscape, Chirality Recognition and Chiral Analyses: Rotational Spectroscopy of Tetrahydro‐2‐Furoic Acid⋅⋅⋅Propylene Oxide Conformers
Chemphyschem : a European journal of chemical physics and physical chemistry, 2021Co-Authors: Fan Xie, Nathan A Seifert, Arsh Hazrah, Wolfgang JägerAbstract:A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using Rotational Spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol-1 . A rich broadband Rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag Rotational Spectroscopy, an exciting new analytical application of Rotational Spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.
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Rotational Spectroscopy of chiral tetrahydro-2-furoic acid: Conformational landscape, conversion, and abundances.
The Journal of chemical physics, 2018Co-Authors: Fan Xie, Nathan A Seifert, Javix Thomas, Wolfgang JägerAbstract:The conformational landscape of tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid which is often used as a precursor in syntheses of pharmaceuticals, was investigated using Rotational Spectroscopy and theoretical modeling. Extensive manual searches were carried out to identify possible conformers related to the relative orientations of the carbonyl and hydroxyl groups in the COOH functional group, the COOH rotation, and the ring puckering motions in the system. A large number of initial conformational geometries were generated in parallel using a joint semiempirical-molecular dynamics simulation program. The final geometry optimizations were carried out at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6-311++G(2d,p), and MP2/6-311G++(2d,p) levels of theory. Eight conformers within a relative energy span of 10 kJ mol−1 after zero-point energy corrections were identified. Rotational spectra of three conformers were detected experimentally and assigned, as were the spectra of all the 13C isotopologues of the most stable conformer. Based on the achieved experimental sensitivity and the predicted relative abundances at the sample source, some conformers are unexpectedly missing or experience significant depletion, whereas others show noticeable enrichment. Detailed analyses of the conformational conversion barriers were carried out to satisfactorily explain the observed phenomena. The combined experimental Rotational spectroscopic and theoretical investigation provides significant insights into the complex conformational landscape of THFA.The conformational landscape of tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid which is often used as a precursor in syntheses of pharmaceuticals, was investigated using Rotational Spectroscopy and theoretical modeling. Extensive manual searches were carried out to identify possible conformers related to the relative orientations of the carbonyl and hydroxyl groups in the COOH functional group, the COOH rotation, and the ring puckering motions in the system. A large number of initial conformational geometries were generated in parallel using a joint semiempirical-molecular dynamics simulation program. The final geometry optimizations were carried out at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6-311++G(2d,p), and MP2/6-311G++(2d,p) levels of theory. Eight conformers within a relative energy span of 10 kJ mol−1 after zero-point energy corrections were identified. Rotational spectra of three conformers were detected experimentally and assigned, as were the spectra of all the 13C isotopologues of ...
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Rotational Spectroscopy OF THE METHYL GLYCIDATE-WATER COMPLEX
Proceedings of the 72nd International Symposium on Molecular Spectroscopy, 2017Co-Authors: Jason T. A. Gall, Javix Thomas, Zhibo Wang, Wolfgang JägerAbstract:Many biologically important molecules are chiral and perform their biological functions in an aqueous medium. In this study, we investigate the intermolecular interactions of methyl glycidate, a chiral epoxy ester, with water using Rotational Spectroscopy. We examine the competition among the three hydrogen-bond acceptor sites at methyl glycidate: the epoxy oxygen, the carbonyl oxygen, and the ester oxygen when interacting with water. We also probe how interaction with water modifies the methyl internal rotation barriers and conformational distribution of methyl glycidate. The possible large amplitude and tunnelling motions associated with water are investigated and analyzed
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Encyclopedia of Inorganic and Bioinorganic Chemistry - Microwave Rotational Spectroscopy
Encyclopedia of Inorganic Chemistry, 2008Co-Authors: Wolfgang JägerAbstract:Since its inception in 1934, microwave Rotational Spectroscopy has provided some of the most accurate data for numerous small to medium sized molecules, including structural parameters, force field parameters, and electronic charge distributions. We give a very brief review of the history of microwave Spectroscopy, leading up to the most recent developments, that is, instruments that use Fourier transform techniques. Since these Fourier transform microwave spectrometers are not commercially available, a brief discussion about their general design is given. Analyses of the recorded Rotational spectra can yield a number of spectroscopic parameters, depending on the molecular symmetry and the various angular momenta present, such as nuclear spins and electronic momenta. The physical significance of the more common spectroscopic parameters, such as Rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants, etc. is given. We also indicate how these parameters can be used to characterize the molecule in terms of, for example, structural parameters, dynamical behavior, and bond character. The remainder of the article is devoted to case studies. These include the discovery of covalent bonds involving the noble gas atom argon, structural characterization of organometallic compounds, such as ferrocenes, bonding in diatomic transition metal-containing compounds, a description of laboratory studies of molecules which are of atmospheric and astrophysical interest, and studies of weakly bound complexes with relevance to inorganic chemistry. Keywords: microwave Rotational Spectroscopy; Fourier transform Spectroscopy; molecular expansion; laser ablation; electric discharge; Rotational constants; centrifugal distortion constants; nuclear quadrupole coupling constants; electric dipole moment; molecular geometry and structure; force field; electron distribution
Nathan A Seifert - One of the best experts on this subject based on the ideXlab platform.
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conformational landscape chirality recognition and chiral analyses Rotational Spectroscopy of tetrahydro 2 furoic acid propylene oxide conformers
ChemPhysChem, 2021Co-Authors: Fan Xie, Nathan A Seifert, Arsh Hazrah, Wolfgang JägerAbstract:A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using Rotational Spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol-1 . A rich broadband Rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag Rotational Spectroscopy, an exciting new analytical application of Rotational Spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.
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Conformational Landscape, Chirality Recognition and Chiral Analyses: Rotational Spectroscopy of Tetrahydro‐2‐Furoic Acid⋅⋅⋅Propylene Oxide Conformers
Chemphyschem : a European journal of chemical physics and physical chemistry, 2021Co-Authors: Fan Xie, Nathan A Seifert, Arsh Hazrah, Wolfgang JägerAbstract:A chiral adduct formed between a chiral carboxylic acid, tetrahydro-2-furoic acid (THFA), and a chiral ester, propylene oxide (PO), was investigated using Rotational Spectroscopy and DFT calculations. Isolated THFA exists dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the trans-COOH configuration and III having the cis-COOH configuration. We utilized CREST, a conformational ensemble space exploration tool, to identify the possible conformations of the binary adduct, THFA⋅⋅⋅PO. Subsequent DFT geometry optimizations predicted about two hundred homochiral and heterochiral binary structures with 28 low energy structures within an energy window of 15 kJ mol-1 . A rich broadband Rotational spectrum was obtained with a mixture of trace amounts of THFA+PO in neon in a supersonic jet expansion. Six THFA⋅⋅⋅PO conformers were identified experimentally. Kinetically favored binary products which contain trans-COOH I dominate among the observed conformers, while thermodynamically more stable adducts were also detected. Detailed analyses of the structures of the observed conformers show interesting chirality-controlled structural preferences. Such non-covalently bound chiral contact pairs are the foundation of chiral-tag Rotational Spectroscopy, an exciting new analytical application of Rotational Spectroscopy for determination of enantiomeric excess. Enantiomeric excess analyses were performed and the results are discussed.
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Rotational Spectroscopy of chiral tetrahydro-2-furoic acid: Conformational landscape, conversion, and abundances.
The Journal of chemical physics, 2018Co-Authors: Fan Xie, Nathan A Seifert, Javix Thomas, Wolfgang JägerAbstract:The conformational landscape of tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid which is often used as a precursor in syntheses of pharmaceuticals, was investigated using Rotational Spectroscopy and theoretical modeling. Extensive manual searches were carried out to identify possible conformers related to the relative orientations of the carbonyl and hydroxyl groups in the COOH functional group, the COOH rotation, and the ring puckering motions in the system. A large number of initial conformational geometries were generated in parallel using a joint semiempirical-molecular dynamics simulation program. The final geometry optimizations were carried out at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6-311++G(2d,p), and MP2/6-311G++(2d,p) levels of theory. Eight conformers within a relative energy span of 10 kJ mol−1 after zero-point energy corrections were identified. Rotational spectra of three conformers were detected experimentally and assigned, as were the spectra of all the 13C isotopologues of the most stable conformer. Based on the achieved experimental sensitivity and the predicted relative abundances at the sample source, some conformers are unexpectedly missing or experience significant depletion, whereas others show noticeable enrichment. Detailed analyses of the conformational conversion barriers were carried out to satisfactorily explain the observed phenomena. The combined experimental Rotational spectroscopic and theoretical investigation provides significant insights into the complex conformational landscape of THFA.The conformational landscape of tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid which is often used as a precursor in syntheses of pharmaceuticals, was investigated using Rotational Spectroscopy and theoretical modeling. Extensive manual searches were carried out to identify possible conformers related to the relative orientations of the carbonyl and hydroxyl groups in the COOH functional group, the COOH rotation, and the ring puckering motions in the system. A large number of initial conformational geometries were generated in parallel using a joint semiempirical-molecular dynamics simulation program. The final geometry optimizations were carried out at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6-311++G(2d,p), and MP2/6-311G++(2d,p) levels of theory. Eight conformers within a relative energy span of 10 kJ mol−1 after zero-point energy corrections were identified. Rotational spectra of three conformers were detected experimentally and assigned, as were the spectra of all the 13C isotopologues of ...
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Chiral analysis of isopulegol by fourier transform molecular Rotational Spectroscopy
Proceedings of the 71st International Symposium on Molecular Spectroscopy, 2016Co-Authors: Luca Evangelisti, Brooks H Pate, Lorenzo Spada, Nathan A SeifertAbstract:Chiral analysis on molecules with multiple chiral centers can be performed using pulsed-jet Fourier transform Rotational Spectroscopy. This analysis includes quantitative measurement of diastereomer products and, with the three wave mixing methods developed by Patterson, Schnell, and Doyle (Nature 497, 475-477 (2013)), quantitative determination of the enantiomeric excess of each diastereomer. The high resolution features enable to perform the analysis directly on complex samples without the need for chromatographic separation. Isopulegol has been chosen to show the capabilities of Fourier transform Rotational Spectroscopy for chiral analysis. Broadband Rotational Spectroscopy produces spectra with signal-to-noise ratio exceeding 1000:1. The ability to identify low-abundance (0.1-1\%) diastereomers in the sample will be described. Methods to rapidly identify Rotational spectra from isotopologues at natural abundance will be shown and the molecular structures obtained from this analysis will be compared to theory. The role that quantum chemistry calculations play in identifying structural minima and estimating their spectroscopic properties to aid spectral analysis will be described. Finally, the implementation of three wave mixing techniques to measure the enantiomeric excess of each diastereomer and determine the absolute configuration of the enantiomer in excess will be described
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Broadband Fourier transform Rotational Spectroscopy for structure determination: The water heptamer
Chemical Physics Letters, 2013Co-Authors: Cristobal Perez, Daniel P Zaleski, Nathan A Seifert, Berhane Temelso, George C Shields, Zbigniew Kisiel, Simon Lobsiger, Brooks H PateAbstract:Over the recent years chirped-pulse, Fourier-transform microwave (CP-FTMW) spectrometers have changed the scope of Rotational Spectroscopy. The broad frequency and large dynamic range make possible structural determinations in molecular systems of increasingly larger size from measurements of heavy atom ( 13 C, 15 N, 18 O) isotopes recorded in natural abundance in the same spectrum as that of the parent isotopic species. The design of a broadband spectrometer operating in the 2‐8 GHz frequency range with further improvements in sensitivity is presented. The current CP-FTMW spectrometer performance is benchmarked in the analyses of the Rotational spectrum of the water heptamer, (H 2 O) 7 , in both 2‐ 8 GHz and 6‐18 GHz frequency ranges. Two isomers of the water heptamer have been observed in a pulsed supersonic molecular expansion. High level ab initio structural searches were performed to provide plausible low-energy candidates which were directly compared with accurate structures provided from broadband Rotational spectra. The full substitution structure of the most stable species has been obtained through the analysis of all possible singly-substituted isotopologues (H 2 18 O and HDO), and a
