Online Stereochemical Process Monitoring by Molecular Rotational Resonance Spectroscopy

A Rotational Study of 2-tert-Butylphenol and Its 1 : 1 Argon Complex

The chirped-pulse Fourier Transform microwave spectrum of 2-tert-butylphenol, an industrial intermediate for the production of antioxidants, has been investigated in the 2-8 GHz frequency range. The spectral analysis has allowed obtaining precise structural information on the most stable conformer and its complex with argon. The conformation of the monomer reveals that the hydroxyl group is coplanar with the ring but points in the opposite direction to the tert-butyl group, reducing steric interactions. In the tert-butyl group one methyl group is coplanar and the other two are symmetrically staggered respect to the ring. The complex shows the rare gas sitting above the aromatic ring. Interestingly, neither the monomer nor the complex exhibit large-amplitude hydroxyl torsion motions, previously observed in 2,6-disubstituted phenols such as 2,6-di-tert-butylphenol or propofol. The experimental results are supported by computational calculations, validating the molecular structure. Additionally, symmetry-adapted perturbation theory has allowed determining the van der Waals intermolecular interaction energy of the complex.

Absolute configuration assignment of highly fluorinated carboxylic acids via VCD and MRR spectroscopy

Chiral analysis has become a crucial step in studying the stereospecific synthesis of Active Pharmaceutical Ingredients (APIs). Both Vibrational Circular Dichroism (VCD) and Molecular Rotational Resonance (MRR) spectroscopy are capable of determining absolute configurations (ACs) via comparison of experimental and calculated data. In this regard, each technique has its own caveats. In VCD analysis, accurate prediction of the normal modes as well as rigorous conformational searches of both the analyte and potential (self-)aggregation products are required to optimally match experimental spectra. In MRR analysis, chiral species are resolved through complexation with a chiral tag to prepare spectrally distinct diastereomeric complexes. Although individual complex isomers can be distinguished, spectral assignments need to be matched to unique isomer geometries for unambiguous AC assignment. In this work, the ACs of two highly fluorinated carboxylic acids were successfully assigned using VCD and MRR spectroscopy. In the VCD analysis, the M06-2X functional was demonstrated to be superior to B3LYP and B3LYP-GD3 in accurately predicting the C-F normal modes and both monomeric and dimeric spectral contributions were observed. In a similar analysis with broadband MRR, most experimentally identified geometries had more than one possible computational match. Nevertheless, careful consideration of the chiral tag, as well as additional isomer assignments, resulted in successful assignment of the AC. This comparative study demonstrates the power of contemporary VCD analysis and the unique contributions of MRR to the analytical toolbox.

Rapid Enantiomeric Excess Measurements of Enantioisotopomers by Molecular Rotational Resonance Spectroscopy

Recent work in drug discovery has shown that selectively deuterated small molecules can improve the safety and efficacy for active pharmaceutical ingredients. The advantages derive from changes in metabolism resulting from the kinetic isotope effect when deuterium is substituted for a hydrogen atom at a structural position where rate limiting C-H bond breaking occurs. This application has pushed the development of precision deuteration strategies in synthetic chemistry that can install deuterium atoms with high regioselectivity and with stereocontrol. Copper-catalyzed alkene transfer hydrodeuteration chemistry has recently been shown to have high stereoselectivity for deuteration at the metabolically important benzyl C-H position. In this case, stereocontrol results in the creation of enantioisotopomers-molecules that are chiral solely by virtue of the deuterium substitution-and chiral analysis techniques are needed to assess the reaction selectivity. It was recently shown that chiral tag molecular rotational resonance (MRR) spectroscopy provides a routine way to measure the enantiomeric excess and establish the absolute configuration of enantioisotopomers. High-throughput implementations of chiral tag MRR spectroscopy are needed to support optimization of the chemical synthesis. A measurement methodology for high-throughput chiral analysis is demonstrated in this work. The high-throughput ee measurements are performed using cavity-enhanced MRR spectroscopy, which reduces measurement times and sample consumption by more than an order-of-magnitude compared to the previous enantioisotopomer analysis using a broadband MRR spectrometer. It is also shown that transitions for monitoring the enantiomers can be selected from a broadband rotational spectrum without the need for spectroscopic analysis. The general applicability of chiral tag MRR spectroscopy is illustrated by performing chiral analysis on six enantioisotopomer reaction products using a single molecule as the tag for chiral discrimination.

Essential Oil Composition and Enantioselective Profile of Agastache urticifolia (Lamiaceae) and Monardella odoratissima (Lamiaceae) from Utah

Two species within the Lamiaceae (mint) family, Agastache urticifolia and Monardella odoratissima, are aromatic plants that are native to the Intermountain Region (USA). Essential oil produced through steam distillation was examined to establish the essential oil yield and both the achiral and chiral aromatic profiles of both plant species. The resulting essential oils were analyzed by GC/MS, GC/FID, and MRR (molecular rotational resonance). For A. urticifolia and M. odoratissima, achiral essential oil profiles were largely composed of limonene (71.0%, 27.7%), trans-β-ocimene (3.6%, 6.9%), and pulegone (15.9%, 4.3%), respectively. Between the two species, eight chiral pairs were analyzed and, interestingly, the dominant enantiomer (calculated as ee%) of limonene and pulegone switched between the two species. Where enantiopure standards were not commercially available, MRR was used as a reliable analytical technique for chiral analysis. This study verifies the achiral profile for A. urticifolia and, for the first time to the authors' knowledge, establishes the achiral profile for M. odoratissima and chiral profile for both species. Additionally, this study confirms the utility and practicality of using MRR for determining chiral profiles in essential oils.

Direct Construction of Peaks from Free Induction Decay Curves for Gas Chromatography-Molecular Rotational Resonance Spectroscopy without Fourier Transforms

The concept of coupling gas chromatography with molecular rotational resonance spectroscopy (GC-MRR) was introduced in 2020, combining the separation capabilities of GC with the unparalleled specificity of MRR. In this study, we address the challenge of the high data throughput of MRR spectrometers, as GC-MRR spectrometers can generate thousands to millions of data points per second. In the previous GC-MRR studies, a free induction decay (FID) measurement was Fourier transformed to generate each point on the chromatogram. Such extensive calculations limit the performance, sensitivity, and speed of GC-MRR. A direct approach is proposed here to extract peak intensity from FID using the Gram-Schmidt vector orthogonalization method. First, analyte-free FIDs are used to construct a basis set representing the instrument's background noise, and then the remaining FIDs are orthogonalized to this fixed basis set. Each FID yields a single intensity value after Gram-Schmidt orthogonalization. The magnitude of the orthogonalized analyte FID is the signal intensity plotted in the chromatogram. This approach is computationally much faster (up to 10 times) than the conventional Fourier transform algorithm, is at least as sensitive as the FT algorithm, and maintains or improves the chromatographic peak shape. We compare the sensitivity, linearity, and chromatographic peak shapes for the Fourier transform and Gram-Schmidt approaches using both synthetically generated FIDs and instrumental data. This approach would allow the summed peak intensity to be displayed essentially in real-time, following which identified peaks can be further investigated to identify and quantify the species associated with each.

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

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.

Assignment-free chirality detection in unknown samples via microwave three-wave mixing

Straightforward identification of chiral molecules in multi-component mixtures of unknown composition is extremely challenging. Current spectrometric and chromatographic methods cannot unambiguously identify components while the state of the art spectroscopic methods are limited by the difficult and time-consuming task of spectral assignment. Here, we introduce a highly sensitive generalized version of microwave three-wave mixing that uses broad-spectrum fields to detect chiral molecules in enantiomeric excess without any prior chemical knowledge of the sample. This method does not require spectral assignment as a necessary step to extract information out of a spectrum. We demonstrate our method by recording three-wave mixing spectra of multi-component samples that provide direct evidence of enantiomeric excess. Our method opens up new capabilities in ultrasensitive phase-coherent spectroscopic detection that can be applied for chiral detection in real-life mixtures, raw products of chemical reactions and difficult to assign novel exotic species.

Design, Development, and Analysis of an Automated Sampling Loop for Online Monitoring of Chiral Crystallization

Enantiomeric purity is of prime importance for several industries, specifically in the production of pharmaceuticals. Crystallization processes can be used to obtain pure enantiomers in a suitable solid form. However, some process variants inherently rely on kinetic enhancement (preferential crystallization) of the desired enantiomer or on complex interactions of several phenomena (e.g., attrition-enhanced deracemization and Viedma ripening). Thus, a process analytical technology able to measure the enantiomeric composition of both the solid phase and the liquid phase would be valuable to track and eventually control such processes. This study presents the design and development of a novel automated analytical monitoring system that achieves this. The designed setup tracks the enantiomeric excess (ee) using a continuous closed-loop sampling loop that is coupled to a polarimeter and an attenuated total reflection Fourier transform infrared spectroscopy spectrometer. By heating the loop and alternately sampling either the liquid or the suspension, the combination of these measurements allows tracking of the ee of both the liquid and the solid. This work demonstrates a proof of concept of both the experimental and theoretical aspects of the new system.

Enhancing Sensitivity for High-Selectivity Gas Chromatography-Molecular Rotational Resonance Spectroscopy

A next-generation gas chromatograph-molecular rotational resonance (MRR) spectrometer (GC-MRR) with instrumental improvements and higher sensitivity is described. MRR serves as a structural information-rich detector for GC with extremely narrow linewidths and capabilities surpassing ¹H nuclear magnetic resonance/Fourier transform infrared spectroscopy/mass spectrometry (MS) while offering unparalleled specificity in regard to a molecule's three-dimensional structure. With a Fabry-Pérot cavity and a supersonic jet incorporated into a GC-MRR, dramatic improvements in sensitivity for molecules up to 244 Da were achieved in the microwave region compared to the only prior work, which demonstrated the GC-MRR idea for the first time with millimeter waves. The supersonic jet cools the analytes to ∼2 K, resulting in a limited number of molecular rotational and vibrational levels and enabling us to obtain stronger GC-MRR signals. This has allowed the limits of detection of the GC-MRR to be comparable to a GC thermal conductivity detector with an optimized choice of gases. The performance of this GC-MRR system is reported for a range of molecules with permanent dipole moments, including alcohols, nitrogen heterocyclics, halogenated compounds, dioxins, and nitro compounds in the molecular mass range of 46-244 Da. The lowest amount of any substance yet detected by MRR in terms of mass is reported in this work. A theoretically unexpected finding is reported for the first time about the effect of the GC carrier gas (He, Ne, and N₂) on the sensitivity of the analysis in the presence of the gas driving the supersonic jet (He, Ne, and N₂) in the GC-MRR. Finally, the idea of total molecule monitoring in the GC-MRR analogous to selected ion monitoring in GC-MS is illustrated. Structural isomers and isotopologues of bromobutanes and bromonitrobenzenes are used to demonstrate this concept.

Chiral analysis of pantolactone with molecular rotational resonance spectroscopy

A molecular rotational resonance spectroscopy method for measuring the enantiomeric excess of pantolactone, an intermediate in the synthesis of panthenol and pantothenic acid, is presented. The enantiomers are distinguished via complexation with a small chiral tag molecule, which produces diastereomeric complexes in the pulsed jet expansion used to inject the sample into the spectrometer. These complexes have distinct moments of inertia, so their spectra are resolved by MRR spectroscopy. Quantitative enantiomeric excess (EE) measurements are made by taking the ratio of normalized complex signal levels when a chiral tag sample of high, known EE is used, while the absolute configuration of the sample can be determined from electronic structure calculations of the complex geometries. These measurements can be performed without the need for reference samples with known enantiopurity. Two instruments were used in the analysis. A broadband, chirped-pulse spectrometer is used to perform structural characterization of the complexes. The broadband spectrometer is also used to determine the EE; however, this approach requires relatively long measurement times. A targeted MRR spectrometer is also used to demonstrate EE analysis with approximately 15-min sample-to-sample cycle time. The quantitative accuracy of the method is demonstrated by comparison with chiral gas chromatography and through the measurement of a series of reference samples prepared from mixtures of (R)-pantolactone and (S)-pantolactone samples of known EE.

Testing the Scalability of the HS-AUTOFIT Tool in a High-Performance Computing Environment

In the last years, the development of broadband chirped-pulse Fourier transform microwave spectrometers has revolutionized the field of rotational spectroscopy. Currently, it is possible to experimentally obtain a large quantity of spectra that would be difficult to analyze manually due to two main reasons. First, recent instruments allow obtaining a considerable amount of data in very short times, and second, it is possible to analyze complex mixtures of molecules that all contribute to the density of the spectra. AUTOFIT is a spectral assignment software application that was developed in 2013 to support and facilitate the analysis. Notwithstanding the benefits AUTOFIT brings in terms of automation of the analysis of the accumulated data, it still does not guarantee a good performance in terms of execution time because it leverages the computing power of a single computing machine. To cater to this requirement, we developed a parallel version of AUTOFIT, called HS-AUTOFIT, capable of running on high-performance computing (HPC) clusters to shorten the time to explore and analyze spectral big data. In this paper, we report some tests conducted on a real HPC cluster aimed at providing a quantitative assessment of HS-AUTOFIT’s scaling capabilities in a multi-node computing context. The collected results demonstrate the benefits of the proposed approach in terms of a significant reduction in computing time.

Conformational Landscape, Chirality Recognition and Chiral Analyses: Rotational Spectroscopy of Tetrahydro-2-Furoic Acid⋅⋅⋅Propylene Oxide Conformers

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.

Direct regioisomer analysis of crude reaction mixtures via molecular rotational resonance (MRR) spectroscopy

Direct analyses of crude reaction mixtures have been carried out using molecular rotational resonance (MRR) spectroscopy. Two examples are presented, a demonstration application in photocatalytic CH-arylation as well as generation of an intermediate in a natural product synthesis. In both cases, the reaction can proceed at more than one site, leading to a mixture of regioisomers that can be challenging to distinguish. MRR structural parameters were calculated for the low lying conformers for the desired compounds, and then compared to the experimental spectra of the crude mixtures to confirm the presence of these species. Next, quantitation was performed by comparing experimentally measured line intensities with simulations based on computed values for the magnitude and direction of the molecular dipole moment of each species. This identification and quantification was performed without sample purification and without isolated standards of the compounds of interest. The values obtained for MRR quantitation were in good agreement with the chromatographic values. Finally, previously unknown impurities were discovered within the photocatalytic CH-arylation work. This paper demonstrates the utility of MRR as a reaction characterization tool to simplify analytical workflows.

Terpenoids: shape and non-covalent interactions. The rotational spectrum of cis-verbenol and its 1 : 1 water complex

In isolated and mono-hydrated verbenol, as in simpler allyl alcohols, the conformational leading force is the OH⋯π interaction.