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Tunneling and conformational preferences in racemic- and meso-2,4-pentanediol: A rotational spectroscopic and theoretical investigation
No full textPublished under an exclusive license by AIP PublishingThe rotational spectra of a mixture of 2,4-pentanediol (PDL) isomers, comprising both the meso isomers [(2R, 4S) and (2S, 4R)] and the racemic isomers [(2R, 4R) and (2S, 4S)], were recorded using a chirped-pulse Fourier transform microwave spectrometer coupled to a supersonic jet expansion. The conformational landscapes of meso- and racemic-PDL were examined using the Conformer-Rotamer Ensemble Sampling Tool and high-level quantum chemical calculations, generating 26 and 25 conformers, respectively. Five sets of rotational transitions were observed and assigned, with two attributed to meso-PDL and the remaining three attributed to racemic-PDL. Furthermore, tunneling splittings were observed for both meso-PDL conformers, and the nudged elastic band method was utilized to map the corresponding tunneling pathways and obtain tunneling barriers. To rationalize relative abundances of the observed conformers and the non-observation of other low-energy conformers, possible conversion pathways among meso-PDL conformers and racemic-PDL conformers were also explored. Finally, ten carbon-13 isotopologues, five from each of the lowest-energy meso- and racemic-PDL conformers, were detected in natural abundance and used to establish the corresponding geometries. These results offer valuable insights into the stereoisomeric behaviors and conformational preferences within flexible diols.This work was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Alberta. M.D., J.M., A.N.M., and A.I. acknowledge their respective financial support from an NSERC undergraduate summer research award, a Marshall Syska Chemistry Graduate Scholarship, an NSERC Canada Graduate Scholarship (Master’s Program), and a Basque Government postdoctoral fellowship, respectively.Peer reviewe
2,2,3,3,3-Pentafluoro-1-propanol and its dimer: structural diversity, conformational conversion, and tunnelling motion
No full textRotational spectra of 2,2,3,3,3-pentafluoro-1-propanol (PFP) were measured using cavity and chirped pulse Fourier transform microwave spectrometers. Of the nine possible PFP configurations which include four mirror-imaged pairs and an achiral conformer, the two most stable monomeric PFP imaged pairs, i.e., PFPG+g+/G−g− and PFPTg+/Tg− were observed and assigned, along with the 13C, 18O and deuterated isotopologues of PFPG+g+/G−g−. The rotational transitions of PFPTg+/Tg− exhibit large tunnelling splittings and were analyzed in detail. CREST, a recently developed conformational search tool that was used for systematic conformational searches of possible binary PFP conformers and the subsequent DFT calculations at the B3LYP-D3(BJ)/def2-QZVP level produced nearly 80 stable, binary PFP geometries, where ten of them are within a narrow energy window of ∼1 kJ mol−1, highlighting the structural diversity of the system. Rotational spectra of five (PFP)2 conformers were assigned and were identified as the five most stable binary conformers predicted. A closer examination reveals that the assigned binary conformers are made exclusively of the two most stable PFP monomeric subunits observed experimentally. A combined kinetic and thermodynamic model was proposed to explain the observation or non-observation of low energy conformers, and the analysis was further verified by the ‘argon test’. The non-covalent intermolecular interactions of PFP and its binary conformers are also discussed with the aid of quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, as well as the effects of fluorination by comparing with 1-propanol and its dimers.This work was funded by the Natural Sciences and Engineering Research Council of Canada and the University of Alberta. A. I. acknowledge the support of a H. E. Gunning Research Fellowship, and a research award through the Mobility Program: Egonlabur 2020–2021, the University of the Basque Country UPV/EHU and a predoctoral fellowship from the Basque Government, Spain, respectively. S. O. thanks the Fonds der Chemischen Industrie for financial support.Peer reviewe
FIRST MONOSACCHARIDE-WATER COMPLEX CAUGHT BY MICROWAVE SPECTROSCOPY
Get PDFCarbohydrates are the main source of energy and make up the main backbone of DNA and RNA, which make them, among many biological systems, of particular interest. Furthermore, they take part in the recognition process, where has been demonstrated that water is actively involved. The role that water molecules play in glycobiology is crucial and is not yet well understood, in part, for the lack of experimental information about how water comes into play in the monosaccharide conformations. Here we present xylopyranose-water complex, first microhydrated monosaccharide studied in gas phase using the combination of laser ablation and broadband microwave spectroscopy. The revealed way in which water binds to the xylopyranose molecule, among the many possibilities due to the high capacity to form hydrogen bonds, can be considered the first step in understanding the mutarotation processes that monosaccharides undergo in biological environments
A BROADBAND ROTATIONAL SPECTROSCOPIC STUDY OF TETRAHYDRO-2H-PYRAN-2-OL
Get PDFTetrahydro-2H-pyran-2-ol (THP) is an alcohol containing pyranose ring. The lack of substituents in THP minimizes the conformational flexibility and the non-covalent interactions within the structure, drastically simplifying the system in relation to physiologically relevant carbohydrates, such as glucose. This makes THP a prototypical system to study various phenomena and dynamics of carbohydrate chemistry. Such phenomena include the anomeric effect, which describes the preference for the axial conformation over the less hindered and more energetically favoured equatorial conformation. When in solution, THP is in equilibrium with its linear counterpart, 5-hydroxypentanal, and readily interconverts between its two chiral forms. A high resolution gas phase study of THP may not only confirm phenomena such as the anomeric effect, but also allow for the study of chiral interconversion in the gas phase. An in-depth study of THP will also lay a foundation for future gas phase carbohydrate systems with more physiological relevance. However, before we can study the various phenomena and dynamics, it is first beneficial to study the structure of THP in great detail. To elucidate the THP structure we analyzed rotational spectra measured with a chirped-pulse Fourier transform microwave spectrometer, operating in the 2-6 GHz range. In addition to the monomers, some dimer conformations were assigned to the rotational spectrum. The intermolecular interactions within the dimers were then visualized using a non-covalent interactions (NCI) analysis
Espektroskopia errotazionala tresna gisa azukreak eta interakzio molkularrak aztertzeko
No full textTesis presentada para la obtención del título de Doctor por la Universidad del País Vasco.La espectroscopía rotacional es la técnica de mayor resolución que proporciona datos estructurales de las moléculas y pequeños agregados en fase gas, así como información sobre sus dinámicas moleculares. Esta tesis está mayoritariamente focalizada al estudio de azucares utilizando la combinación de la espectroscopia rotacional y los cálculos químico cuánticos. En la tesis se presentan varios estudios, partiendo del estudio de la eritrulosa en la que combinamos los datos obtenidos en el laboratorio con los datos de los radio-telescopios para su intento de detección en el medio interestelar y continuamos con varios estudios de azucares derivados de glucosa, galactosa y manosa. Nuestros resultados muestran como las pequeñas mutaciones en los azucares (de oxidaciones, lactonizaciones, oxidaciones) producen grandes cambios estructurales de la estructura intrínseca de los monosacáridos en fase gas.Por último, se presentan varios estudios de procesos químicos de relevante interés en la biología. Entre ellos la transferencia de protones en el dimero de ácido furoico y la micro-solvatación del metil benzoato.En general demostramos como la espectroscopia de rotación permite el estudio de las estructuras y de varios procesos de importancia en la biología
URONIC ACIDS IN GAS PHASE
Get PDFSugars are versatile molecules that play a variety of roles in the organism. For example, they are important in energy storage processes, cellular recognition or as structural scaffolds. Here, we focus on uronic acids derivatives of the most abundant monosaccharides in the nature, concretely in glucuronic acid (GlcA) and galacturonic acid (GalA). By the combination of high resolution spectroscopy, supersonic expansions and laser ablation as vaporization technique with quantum chemistry calculations we are able to obtain valuable information in gas phase about the bare structures of both anomers of GlcA and GalA (with rotational spectroscopy) and water clusters of the 1-O-phenylated derivative of GlcA (using ion-dip spectroscopy (IR-UV)). Previous studies in the gas phase had been performed on α/β- Glucose1,2 and β-Galactose2,3 where the importance of –CH2OH backbone in H-bond orientation and hydration was reported. In this work, we focus on how the characteristic –COOH group of uronic acids and their anomeric hydroxyl in the H-bond network cooperativity/orientation and microsolvation process
The amino alcohol prolinol: a rotational study
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The amino alcohol prolinol is a reduction product of the chiral amino acid proline, which is widely used as a chiral auxiliary in noted reactions. Up to date, only IR data for prolinol is available.\footnote{J.J. Lee, S. Hesse, M.A. Suhm. \textit{Journal of Molecular Structure}, \textbf{2010}, \underline{976}, 397–404} Here, we report the first high-resolution microwave study of this system. Two endo-like and exo-like conformations stabilized by strong intramolecular hydrogen bonds O-H···N have been detected in the supersonic expansion of our CP-FTMW technique.\footnote{I. Uriarte, C. P\'{e}rez, E. Caballero-Mancebo, F. J. Basterretxea, A. Lesarri, J. A. Fern\'{a}ndez and E. J. Cocinero, \textit{Chem. - A Eur. J.}, \textbf{2017}, \underline{23}, 7156}\footnote{C. Calabrese, W. Li, G. Prampolini, L. Evangelisti, I. Uriarte, I. Cacelli, S. Melandri,and E. J. Cocinero, \textit{Angew.Chem.}, \textbf{2019}, \underline{131},8525–8530} These structures have been successfully identified by comparison of the experimental rotational and N nuclear quadrupole coupling constants with those predicted theoretically. Given the high sensitivity reached in our experiment, the seven monosubstituted species of C, N and O have also been observed in their natural abundance for both conformers, and the corresponding r substitution structure has been derived. The monohydrate of the most abundant endo form has also been detected. The water interacts simultaneously, acting as a proton donor to the imine nitrogen atom and accepting a proton from the hydroxyl group. This configuration benefits from two intermolecular hydrogen bonds, OH···O and O-H···N, closing a six-membered ring configuration. These first results pave the way for future studies on the chiral recognition of prolinol
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
ROTATIONAL SPECTROSCOPIC STUDIES OF PARA-NITROBENZOIC ACID, PARA-AMINOBENZOIC ACID, PARA-CHLOROBENZOIC ACID, AND PARA-HYDROXYBENZOIC ACID
Get PDFpara-substituted benzoic acids, such as para-nitrobenzoic acid, para-aminobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid, play important roles in atmospheric chemistry and are used as precursors for a wide range industrial products. Surprisingly, there are no high-resolution spectroscopic studies of these compounds, and we describe here their rotational spectroscopic investigations combined with electronic structure calculations. The rotational spectra were recorded using a 2 to 6 GHz chirped-pulse Fourier transform microwave spectrometer, which is based on the design by Pate et al.¹ Experimentally, all four substituted benzoic acids were found to exist in the cis-configuration, the global minimum energy configuration, of the carboxylic acid group. In all instances, except para-aminobenzoic acid, the global minimum structure is planar. Nudged elastic band calculations² for the wagging motion of the amino group in para-aminobenzoic acid suggest that it is a barrierless large amplitude motion and indeed, no tunnelling splittings were observed in the spectra. For para-chlorobenzoic acid, the nuclear quadrupole hyperfine structures of the ³⁵Cl and ³⁷Cl isotopologues were measured and analyzed. In the case of para-hydroxybenzoic acid two conformers were experimentally and theoretically identified. We noticed in the theoretical structures of the trans-conformers that the carboxylic acid group is pushed slightly out of plane, depending on the nature of the para-substituent. We were able to correlate the out-of-plane angle with the corresponding Hammett constant³ via the molecular electrostatic potentia
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