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12 pages, 9104 KiB

Open AccessFeature PaperArticle

A Scent of Peppermint—A Microwave Spectroscopy Analysis on the Composition of Peppermint Oil

by Anna Krin, María Mar Quesada Moreno, Cristóbal Pérez and Melanie Schnell

Symmetry 2022, 14(6), 1262; https://doi.org/10.3390/sym14061262 - 18 Jun 2022

Cited by 3 | Viewed by 2364

Essential oils have a vast number of applications in different areas of our daily life. Detailed chiral analysis and structural characterization of their constituents remains an important subject in analytical chemistry. Here, we report on a broadband rotational spectroscopy study of peppermint oil [...] Read more.

Essential oils have a vast number of applications in different areas of our daily life. Detailed chiral analysis and structural characterization of their constituents remains an important subject in analytical chemistry. Here, we report on a broadband rotational spectroscopy study of peppermint oil in the frequency range 2–8 GHz. We focus on an unambiguous determination of the excess enantiomers of the oil constituents menthone and isomenthone in the oil by applying chirality-sensitive rotational spectroscopy, the so-called microwave three-wave mixing (M3WM) technique. Additionally, a new menthol conformer, not previously characterized, was experimentally observed, and the gas-phase structures of the two conformers of menthol and menthone were determined experimentally based on the assignment of their

^{13} C

-isotopologues in natural abundance. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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12 pages, 2662 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

MCD and Induced CD of a Tetraphenoxyperylene-Based Dye in Chiral Solvents: An Experimental and Computational Study

by Simone Ghidinelli, Marco Fusè, Giuseppe Mazzeo, Sergio Abbate and Giovanna Longhi

Symmetry 2022, 14(6), 1108; https://doi.org/10.3390/sym14061108 - 28 May 2022

Cited by 2 | Viewed by 2234

Abstract

The magnetic circular dichroism (MCD) spectrum of N,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxydiimide, also known as Lumogen Red 300 or ROT-300, has been recorded both in achiral and chiral solvents. The induced CD spectra in chiral solvents have, similarly, been recorded. A discussion of the spectroscopic response, both [...] Read more.

The magnetic circular dichroism (MCD) spectrum of N,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxydiimide, also known as Lumogen Red 300 or ROT-300, has been recorded both in achiral and chiral solvents. The induced CD spectra in chiral solvents have, similarly, been recorded. A discussion of the spectroscopic response, both in CD and in MCD experiments, is presented in this paper. Both types of spectra have been predicted most satisfactorily by DFT calculations; the CD spectra were obtained by assuming the prevalence of one “enantiomeric” conformer and the same set of conformers could also be used for MCD, since “enantiomeric” structures present identically in MCD spectra. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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14 pages, 2150 KiB

Open AccessFeature PaperArticle

Accurate Determination of Enantiomeric Excess Using Raman Optical Activity

by Pavel Michal, Jana Hudecová, Radek Čelechovský, Milan Vůjtek, Michal Dudka and Josef Kapitán

Symmetry 2022, 14(5), 990; https://doi.org/10.3390/sym14050990 - 12 May 2022

Cited by 5 | Viewed by 2136

Abstract

The optical purity of a chiral sample is of particular importance to the analytical chemistry and pharmaceutical industries. In recent years, the vibrational optical activity (VOA) has become established as a sensitive and nondestructive technique for the analysis of chiral molecules in solution. [...] Read more.

The optical purity of a chiral sample is of particular importance to the analytical chemistry and pharmaceutical industries. In recent years, the vibrational optical activity (VOA) has become established as a sensitive and nondestructive technique for the analysis of chiral molecules in solution. However, the relatively limited accuracy in the range of about 1–2% reported in published papers and the relatively small spread of experimental facilities to date have meant that vibrational spectroscopy has not been considered a common method for determining enantiomeric excess. In this paper, we attempt to describe, in detail, a methodology for the determination of enantiomeric excess using Raman optical activity (ROA). This method achieved an accuracy of 0.05% for neat α-pinene and 0.22% for alanine aqueous solution, after less than 6 h of signal accumulation for each enantiomeric mixture, which we believe is the best result achieved to date using vibrational optical activity techniques. An algorithm for the elimination of systematic errors (polarization artifacts) is proposed, and the importance of normalizing ROA spectra to correct for fluctuations in excitation power is established. Results comparable to those obtained with routinely used chemometric analysis by the partial least squares (PLS) method were obtained. These findings show the great potential of ROA spectroscopy for the quantitative analysis of enantiomeric mixtures. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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10 pages, 1955 KiB

Open AccessFeature PaperArticle

Chiral Analysis of Linalool, an Important Natural Fragrance and Flavor Compound, by Molecular Rotational Resonance Spectroscopy

by Reilly E. Sonstrom, Donald M. Cannon and Justin L. Neill

Symmetry 2022, 14(5), 917; https://doi.org/10.3390/sym14050917 - 30 Apr 2022

Cited by 7 | Viewed by 2748

Abstract

The chiral analysis of terpenes in complex mixtures of essential oils, necessary for authentication, has been further developed using chiral tagging molecular rotational resonance (MRR) spectroscopy. One analyte that is of particular interest is linalool (3,7-dimethyl-1,6-octadien-3-ol), a common natural chiral terpene found in [...] Read more.

The chiral analysis of terpenes in complex mixtures of essential oils, necessary for authentication, has been further developed using chiral tagging molecular rotational resonance (MRR) spectroscopy. One analyte that is of particular interest is linalool (3,7-dimethyl-1,6-octadien-3-ol), a common natural chiral terpene found in botanicals with its enantiomers having unique flavor, fragrance, and aromatherapy characteristics. In this MRR demonstration, resolution of the enantiomers is achieved through the addition of a chiral tag, which creates non-covalent diastereomeric complexes with distinct spectral signatures. The relative stereochemistry of the complexes is identified by the comparison of calculated spectroscopic parameters with experimentally determined parameters of the chiral complexes with high accuracy. The diastereomeric complex intensities are analyzed to determine the absolute configuration (AC) and enantiomeric excess (EE) in each sample. Here, we demonstrate the use of chiral tagging MRR spectroscopy to perform a quantitative routine enantiomer analysis of linalool in complex essential oil mixtures, without the need for reference samples or chromatographic separation. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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17 pages, 4881 KiB

Open AccessFeature PaperArticle

Rational Pulse Design for Enantiomer-Selective Microwave Three-Wave Mixing

by Monika Leibscher, Jonas Kalveram and Christiane P. Koch

Symmetry 2022, 14(5), 871; https://doi.org/10.3390/sym14050871 - 24 Apr 2022

Cited by 4 | Viewed by 2327

Abstract

Microwave three-wave mixing allows for enantiomer-selective excitation of randomly oriented chiral molecules into rotational states with different energy. The random orientation of molecules is reflected in the degeneracy of the rotational spectrum with respect to the orientational quantum number M and reduces, if [...] Read more.

Microwave three-wave mixing allows for enantiomer-selective excitation of randomly oriented chiral molecules into rotational states with different energy. The random orientation of molecules is reflected in the degeneracy of the rotational spectrum with respect to the orientational quantum number M and reduces, if not accounted for, enantiomer-selectivity. Here, we show how to design pulse sequences with maximal enantiomer-selectivity from an analysis of the M-dependence of the Rabi frequencies associated with rotational transitions induced by resonant microwave drives. We compare different excitations schemes for rotational transitions and show that maximal enantiomer-selectivity at a given rotational temperature is achieved for synchronized three-wave mixing with circularly polarized fields. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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13 pages, 3721 KiB

Open AccessArticle

Construction and Demonstration of a 6–18 GHz Microwave Three-Wave Mixing Experiment Using Multiple Synchronized Arbitrary Waveform Generators

by Nicole T. Moon, Klaus Woelk and Garry S. Grubbs II

Symmetry 2022, 14(5), 848; https://doi.org/10.3390/sym14050848 - 19 Apr 2022

Cited by 2 | Viewed by 3361

Abstract

This manuscript details the construction and demonstration of the first known microwave three-wave mixing (M3WM) experiment utilizing multiple arbitrary waveform generators (AWGs) completely operable in the 6–18 GHz frequency range for use in chirality determination and quantification. Many M3WM techniques, which involve two [...] Read more.

This manuscript details the construction and demonstration of the first known microwave three-wave mixing (M3WM) experiment utilizing multiple arbitrary waveform generators (AWGs) completely operable in the 6–18 GHz frequency range for use in chirality determination and quantification. Many M3WM techniques, which involve two orthogonal, subsequent Rabi π/2 and π microwave pulses, suffer from flexibility in pulse types and timings as well as frequency due to most instruments only using one, one-channel AWG and the M3WM probability decreasing with an increasing quantum number, J. In this work, we presented an M3WM instrument that allows that flexibility by introducing multiple, synchronized AWGs and adheres to the high probability transition loop pathways in carvone. The functionality and reliability of the instrument were demonstrated using a series of experiments and mixtures of the R and S enantiomers and determined to be of similar accuracy to other reported M3WM setups with the additional benefit of flexibility in pulsing schemes. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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19 pages, 1220 KiB

Open AccessEditor’s ChoiceArticle

Attaching Onto or Inserting Into an Intramolecular Hydrogen Bond: Exploring and Controlling a Chirality-Dependent Dilemma for Alcohols

by Manuel Lange, Elisabeth Sennert and Martin A. Suhm

Symmetry 2022, 14(2), 357; https://doi.org/10.3390/sym14020357 - 11 Feb 2022

Cited by 3 | Viewed by 2176

Abstract

Prereactive complexes in noncovalent organocatalysis are sensitive to the relative chirality of the binding partners and to hydrogen bond isomerism. Both effects are present when a transiently chiral alcohol docks on a chiral

α

-hydroxy ester, turning such 1:1 complexes into elementary, non-reactive [...] Read more.

Prereactive complexes in noncovalent organocatalysis are sensitive to the relative chirality of the binding partners and to hydrogen bond isomerism. Both effects are present when a transiently chiral alcohol docks on a chiral

α

-hydroxy ester, turning such 1:1 complexes into elementary, non-reactive model systems for chirality induction in the gas phase. With the help of linear infrared and Raman spectroscopy in supersonic jet expansions, conformational preferences are investigated for benzyl alcohol in combination with methyl lactate, also exploring p-chlorination of the alcohol and the achiral homolog methyl glycolate to identify potential London dispersion and chirality effects on the energy sequence. Three of the four combinations prefer barrierless complexation via the hydroxy group of the ester (association). In contrast, the lightest complex predominantly shows insertion into the intramolecular hydrogen bond, such as the analogous lactate and glycolate complexes of methanol. The experimental findings are rationalized with computations, and a uniform helicality induction in the alcohol by the lactate is predicted, independent of insertion into or association with the internal lactate hydrogen bond. p-chlorination of benzyl alcohol has a stabilizing effect on association because the insertion motif prevents a close contact between the chlorine and the hydroxy ester. After simple anharmonicity and substitution corrections, the B3LYP-D3 approach offers a fairly systematic description of the known spectroscopic data on alcohol complexes with

α

-hydroxy esters. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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10 pages, 1600 KiB

Open AccessArticle

Sub-Hz Differential Rotational Spectroscopy of Enantiomers

by Lincoln Satterthwaite, Greta Koumarianou, Daniel Sorensen and David Patterson

Symmetry 2022, 14(1), 28; https://doi.org/10.3390/sym14010028 - 27 Dec 2021

Cited by 5 | Viewed by 2698

Abstract

We demonstrate for the first time high-precision differential microwave spectroscopy, achieving sub-Hz precision by coupling a cryogenic buffer gas cell with a tunable microwave Fabry–Perot cavity. We report statistically limited sub-Hz precision of (0.08 ± 0.72) Hz, observed between enantiopure samples of (R)-1,2-propanediol [...] Read more.

We demonstrate for the first time high-precision differential microwave spectroscopy, achieving sub-Hz precision by coupling a cryogenic buffer gas cell with a tunable microwave Fabry–Perot cavity. We report statistically limited sub-Hz precision of (0.08 ± 0.72) Hz, observed between enantiopure samples of (R)-1,2-propanediol and (S)-1,2-propanediol at frequencies near 15 GHz. We confirm highly repeatable spectroscopic measurements compared to traditional pulsed-jet methods, opening up new capabilities in probing subtle molecular structural effects at the 10⁻¹⁰ level and providing a platform for exploring sources of systematic error in parity-violation searches. We discuss dominant systematic effects at this level and propose possible extensions of the technique for higher precision. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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13 pages, 1680 KiB

Open AccessArticle

Molecular Recognition, Transient Chirality and Sulfur Hydrogen Bonding in the Benzyl Mercaptan Dimer

by Rizalina Tama Saragi, Marcos Juanes, Ruth Pinacho, José Emiliano Rubio, José A. Fernández and Alberto Lesarri

Symmetry 2021, 13(11), 2022; https://doi.org/10.3390/sym13112022 - 26 Oct 2021

Cited by 11 | Viewed by 2160

Abstract

The homodimers of transiently chiral molecules offer physical insight into the process of molecular recognition, the preference for homo or heterochiral aggregation and the nature of the non-covalent interactions stabilizing the adducts. We report the observation of the benzyl mercaptan dimer in the [...] Read more.

The homodimers of transiently chiral molecules offer physical insight into the process of molecular recognition, the preference for homo or heterochiral aggregation and the nature of the non-covalent interactions stabilizing the adducts. We report the observation of the benzyl mercaptan dimer in the isolation conditions of a supersonic jet expansion, using broadband (chirped-pulse) microwave spectroscopy. A single homochiral isomer was observed for the dimer, stabilized by a cooperative sequence of S-H···S and S-H···π hydrogen bonds. The structural data, stabilization energies and energy decomposition describe these non-covalent interactions as weak and dispersion-controlled. A comparison is also provided with the benzyl alcohol dimer. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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Review

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13 pages, 3012 KiB

Open AccessFeature PaperEditor’s ChoiceReview

How Symmetry Influences the Dissociation of Protonated Cyclic Peptides

by Ariel F. Pérez-Mellor, Riccardo Spezia and Anne Zehnacker

Symmetry 2022, 14(4), 679; https://doi.org/10.3390/sym14040679 - 25 Mar 2022

Cited by 4 | Viewed by 2145

Abstract

Protonated cyclic dipeptides undergo collision-induced dissociation, and this reaction mechanism strongly depends on the symmetry and the nature of the residues. We review the main dissociation mechanism for a series of cyclic dipeptides, obtained through chemical dynamics simulations. The systems range from the [...] Read more.

Protonated cyclic dipeptides undergo collision-induced dissociation, and this reaction mechanism strongly depends on the symmetry and the nature of the residues. We review the main dissociation mechanism for a series of cyclic dipeptides, obtained through chemical dynamics simulations. The systems range from the symmetrical cyclo-(glycyl-glycyl), with two possible symmetrical protonation sites located on the peptide ring, to cyclo-(tyrosyl-prolyl), where the symmetry of protonation sites on the peptide ring is broken by the dissimilar nature of the different residues. Finally, cyclo-(phenylalanyl-histidyl) shows a completely asymmetric situation, with the proton located on one of the dipeptide side chains, which explains the peculiar fragmentation mechanism induced by shuttling the proton, whose efficiency is strongly dependent on the relative chirality of the residues. Full article

(This article belongs to the Special Issue Asymmetric Molecules and Chirality Recognition)

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