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.

Influence of microsolvation on vibrational circular dichroism spectra in dimethyl sulfoxide solvent: A Bottom-Up approach using Quantum cluster growth

Chiroptical spectroscopic measurements serve as routine methods to assign the absolute configuration of chiral compounds and interpret their conformational behavior in solution. One common challenge is the use of strongly hydrogen-bonding solvents, which can significantly bias the conformational ensemble and affect the vibrational circular dichroism (VCD) active bands in solution. One such solvent is dimethyl sulfoxide (DMSO)-an excellent solvent for stubborn compounds-that must be explicitly considered in VCD analysis. Explicit consideration of solvent remains a critical challenge in chiroptical spectroscopy due to the need to explore solute-solvent conformational space and the computational expense in modeling these clusters. Interested in the recent development of the Quantum Cluster Growth (QCG) program by the Grimme lab, we set out to model and interpret previously reported VCD spectra for several molecules using their efficient program. Our purposes are two-fold: (1) to investigate the applicability of the QCG program to the problem of reproducing VCD spectra in DMSO solvent and (2) to identify limitations in using this approach. We find that we can conveniently model and analyze the VCD spectra of investigated molecules in DMSO. However, the final set of conformers used for VCD calculations are functional dependent and different sets of conformers can provide satisfactory quantitative agreement between experimental and predicted VCD spectra. We hope that this study provides guidance for future chiroptical studies in the challenging DMSO solvent.

Matrix effects in MI-VCD spectra of two chiral oxiranes and their potential microscopic origin

Combining vibrational circular dichroism (VCD) spectroscopy with the matrix isolation (MI) technique opens up interesting possibilities to study chiral molecules. MI involves the isolation of guest species in inert solid matrices at cryogenic temperatures. Hence, MI-VCD measures are solid-state VCD measurements, and as such, can suffer from mostly birefringance-related artefacts in the same way as common solid-state VCD measurements. In this contribution, we demonstrate that the sample preparation condition have tremendous impact on the quality and reliability of the recorded MI-VCD spectra. While MI-IR spectra are basically blind to these artefacts, the variation of deposition temperatures and host flow rates seem to control whether high quality MI-VCD spectra are obtained or if depolarization effects lead to completely obscured spectra. For two selected examples, styrene oxide (SO) and 1-phenyl propylene oxide (PPO), we discuss how the various experimental conditions may lead to the aforementioned effects and give a microscopic description of their origin.

Synthesis and VCD Spectroscopic Characterization of a Series of Azacryptands from a Chiral Valine-Based Derivative of Tris(2-aminoethyl)amine (TREN)

Utilizing experimental and computational vibrational circular dichroism (VCD) spectroscopy, we explored the conformational preferences of a series of chiral C3 -symmetric octaazacryptands with tris(2-aminoethyl)-amine head groups derived from valine. While the spectra of the smallest azacryptand with p-phenyl linkers and its elongated derivative with p-biphenyls linker were found to match well with the computed spectra, the computed conformational preferences of the m-biphenyl-based azacryptand did not seem to reflect the conformations dominating in chloroform solution. A detailed analysis revealed that structural changes resulting in a collapsed cage structure gave a notably better match with the experiment. It could subsequently be concluded from the VCD analysis, that the octaazacryptands prefer a collapsed structure, which is not predicted by density functional theory (DFT) calculations as the global minimum structures. These findings are expected to have consequences also for future studies on inclusion complexes of such azacryptands.

Modelling solute-solvent interactions in VCD spectra analysis with the micro-solvation approach

Vibrational circular dichroism (VCD) spectroscopy has become an important part of the (stereo-)chemists' toolbox as a reliable method for the determination of absolute configurations. Being the chiroptical version of infrared spectroscopy, it has also been recognized as being very sensitive to conformational changes and intermolecular interactions. This sensitivity originates from the fact that the VCD spectra of individual conformers are often more different than their IR spectra, so that changes in conformational distributions or band positions and intensities become more pronounced. What is an advantage for studies focussing on intermolecular interactions can, however, quickly turn into a major obstacle during AC determinations: solute-solvent interactions can have a strong influence on spectral signatures and they must be accurately treated when simulating VCD and IR spectra. In this perspective, we showcase selected examples which exhibit particularly pronounced solvent effects. It is demonstrated that it is typically sufficient to model solute-solvent interactions by placing single solvent molecules near hydrogen bonding sites of the solute and subsequently use the optimized structures for spectra simulations. This micro-solvation approach works reasonably well for medium-sized, not too conformationally flexible molecules. We thus also discuss its limitations and outline the next steps that method development needs to take in order to further improve the workflows for VCD spectra predictions.

7-Azaindole breaks carboxylic acid dimers and simplifies VCD spectra analyses of natural products

The determination of absolute configurations of carboxylic acids by vibrational circular dichroism (VCD) spectroscopy is often complicated by self-aggregation and the subsequent need to compute the spectra of the aggregates. We show that 7-azaindole effectively breaks up these aggregates by stronger complemental hydrogen bonding to the COOH moiety, enabling drastic simplification and acceleration of VCD spectra calculations.

Dynamic Stereochemistry of a Biphenyl‐Bisprolineamide Model Catalyst and its Imidazolidinone Intermediates

In this study, we characterize the dynamic stereochemistry of a biphenyl‐2,2’‐bis(proline amide) catalyst in chloroform and DMSO as representative weakly and strongly hydrogen bonding solvents. Using vibrational circular dichroism (VCD) spectroscopy and density functional theory (DFT) based spectra calculations, we show that the preferred axial stereochemistry of the catalyst is determined by solute‐solvent interactions. Explicitly considering solvation with DMSO molecules is found to be essential to correctly predict the conformational preferences of the catalyst. Furthermore, we investigate the stereochemistry of the corresponding enamines and imidazolidinones that are formed upon reaction with isovaleraldehyde. The enamines are found to rapidly convert to endo‐imidazolidinones and the thermodynamically favored exo‐imidazolidinones are formed only slowly. The present study demonstrates that the stereochemistry of these imidazolidinones can be deduced directly from the VCD spectra analysis without any further detailed analysis of NMR spectra. Hence, we herein exemplify the use of VCD spectroscopy for an in situ characterization of intermediates relevant in asymmetric catalysts.

A Computational Protocol for Vibrational Circular Dichroism Spectra of Cyclic Oligopeptides

Cyclic peptides are a promising class of compounds for next-generation antibiotics as they may provide new ways of limiting antibiotic resistance development. Although their cyclic structure will introduce some rigidity, their conformational space is large and they usually have multiple chiral centers that give rise to a wide range of possible stereoisomers. Chiroptical spectroscopies such as vibrational circular dichroism (VCD) are used to assign stereochemistry and discriminate enantiomers of chiral molecules, often in combination with electronic structure methods. The reliable determination of the absolute configuration of cyclic peptides will require robust computational methods than can identify all significant conformers and their relative population and reliably assign their stereochemistry from their chiroptical spectra by comparison with ab initio calculated spectra. We here present a computational protocol for the accurate calculation of the VCD spectra of a series of flexible cyclic oligopeptides. The protocol builds on the Conformer-Rotamer Ensemble Sampling Tool (CREST) developed by Grimme and co-workers ( Phys. Chem. Chem. Phys. 2020, 22, 7169-7192 and J. Chem. Theory. Comput. 2019, 15, 2847-2862) in combination with postoptimizations using B3LYP and moderately sized basis sets. Our recommended computational protocol for the computation of VCD spectra of cyclic oligopeptides consists of three steps: (1) conformational sampling with CREST and tight-binding density functional theory (xTB); (2) energy ranking based on single-point energy calculations as well as geometry optimization and VCD calculations of conformers that are within 2.5 kcal/mol of the most stable conformer using B3LYP/6-31+G*/CPCM; and (3) VCD spectra generation based on Boltzmann weighting with Gibbs free energies. Our protocol provides a feasible basis for generating VCD spectra also for larger cyclic peptides of biological/pharmaceutical interest and can thus be used to investigate promising compounds for next-generation antibiotics.

Automated Molecular Cluster Growing for Explicit Solvation by Efficient Force Field and Tight Binding Methods

An automated and broadly applicable workflow for the description of solvation effects in an explicit manner is introduced. This method, termed quantum cluster growth (QCG), is based on the semiempirical GFN2-xTB/GFN-FF methods, enabling efficient geometry optimizations and MD simulations. Fast structure generation is provided using the intermolecular force field xTB-IFF. Additionally, the approach uses an efficient implicit solvation model for the electrostatic embedding of the growing clusters. The novel QCG procedure presents a robust cluster generation tool for subsequent application of higher-level (e.g., DFT) methods to study solvation effects on molecular geometries explicitly or to average spectroscopic properties over cluster ensembles. Furthermore, the computation of the solvation free energy with a supermolecular approach can be carried out with QCG. The underlying growing process is physically motivated by computing the leading-order solute-solvent interactions first and can account for conformational and chemical changes due to solvation for low-energy barrier processes. The conformational space is explored with the NCI-MTD algorithm as implemented in the CREST program, using a combination of metadynamics and MD simulations. QCG with GFN2-xTB yields realistic solution geometries and reasonable solvation free energies for various systems without introducing many empirical parameters. Computed IR spectra of some solutes with QCG show a better match to the experimental data compared to well-established implicit solvation models.

VCD spectroscopy reveals conformational changes of chiral crown ethers upon complexation of potassium and ammonium cations

Two chiral derivatives of 18-crown-6, namely the host molecules 2,3-diphenyl- and 2-phenyl-18c6, serve as model systems to investigate whether VCD spectroscopy can be used to monitor conformational changes occurring upon complexation of guests. Host-guest complexes of both crown ethers were prepared by addition of KNO₃. The more bulky 2,3-diphenyl-18c6 is found to undergo major conformational changes upon encapsulation of K⁺, which are revealed as characteristic changes of the VCD spectral signatures. In contrast, while 2-phenyl-18c6 also incorporates K⁺ into the macrocycle, strong conformational changes are not occurring and thus spectral changes are negligible. With an octyl ammonium cation as guest molecule, 2,3-diphenyl-18c6 shows the same conformational and spectral changes that were observed for K⁺-complexes. In addition, the asymmetric NH₃-deformation modes are found to gain VCD intensity through an induced VCD process. An analysis of the vibrational spectra enables a differentiation of VCD active and inactive guest modes: There appears to be a correlation between the symmetry of the vibrational mode and the induced VCD intensity. While this finding makes the host-guest complexes interesting systems for future theoretical studies on the origin of induced VCD signatures, the observations described in this study demonstrate that VCD spectroscopy is indeed a suitable technique for the characterization of supramolecular host-guest complexes.

Insights in the vibrational optical activity spectra of the antibiotic vancomycin in DMSO

Vibrational circular dichroism (VCD) and Raman optical activity (ROA) are two spectroscopic techniques that are sensitive towards the conformational behaviour of molecules, and are often complementary herein. In this work we pursue the determination of the conformational ensemble of the antibiotic glycopeptide vancomycin in DMSO through comparison of experimental and computational spectra, both for VCD and ROA. ROA is found to be highly suitable for the task, identifying an ensemble that strongly resembles the NMR conformation. In the case of VCD, however, a too high sensitivity of the intensities with respect to minor conformational changes hampers a reliable conformational analysis. Whence attempting to improve the match between the VCD experiment and calculations by any means - e.g., by inducing minor conformational changes or including solvent effects in the calculations - we show that there is the risk of going down the rabbit hole. In conclusion, this work contributes to the broader understanding of where, when and how VCD and ROA can be deployed as techniques for conformational analysis.

Solvation of the Boc-Val-Phe-nPr peptide characterized by VCD spectroscopy and DFT calculations

The conformational preferences of peptides are strongly determined by hydrogen bonding interactions. Intermolecular solute-solvent interactions compete with intramolecular interactions, which typically stabilize the secondary structure of the peptide. The analysis of vibrational circular dichroism (VCD) spectra can give insights into solvation-induced changes in the conformational distribution of small peptides. Here we describe the VCD spectroscopic characterization of the model peptide Boc-Val-Phe-nPr in chloroform as representative for a weakly interacting solvent and dimethyl sulfoxide (DMSO-d₆) as a strongly hydrogen bonding solvent. We show that the conformational preferences of the peptide in chloroform are well-described by the computationally predicted distribution of the isolated molecule assuming only implicit solvation effects through a continuum solvation model. In order to simulate the spectra recorded in DMSO-d₆, solvation was accounted for explicitly by computed microsolvated structures containing one to three solvent molecules. A good match of the computed spectra with the experimental data is obtained by this method. Comparing the conformational distributions in deuterated chloroform-d₁ and DMSO-d₆, structures with intramolecular hydrogen bonds such as the (δ,δ)-conformer family contribute to the conformational distribution only when there is no strong interaction with the solvent. This is in contrast to the results for the related Boc-Pro-Phe-nPr studied before, for which the intramolecular interaction was found to persist in DMSO-d₆. Furthermore, we discuss the influence of hydrogen bonding to different numbers of solvent molecules on the spectral signatures and show that the structure of the peptide in DMSO-d₆ is best described as a mixture of twofold-solvated (δ,β)- and threefold-solvated (β,β)-conformers.

The many forms of alpha-methoxy phenylacetic acid in the gas phase: flexibility, internal dynamics, and their intramolecular interactions

Five conformers of the flexible molecule alpha-methoxy phenylacetic acid were identified using rotational spectroscopy. The conformational landscape, internal dynamics, and intramolecular interactions were investigated.

Anion-binding of a chiral tris(2-aminoethyl)amine-based tripodal thiourea: A spectroscopic and computational study

Thioureas are well-known structural motifs in supramolecular anion recognition. Their conformational preferences are typically characterized by detailed NMR spectroscopy and crystallography, which are often complemented with computational results from geometry...

VCD spectroscopy distinguishes the enamine and iminium ion of a 1,1’-binaphthyl azepine

We present a VCD spectroscopic characterization of a chiral 1,1’-binaphthyl azepine catalyst and show that the VCD spectra of an in-situ generated enamine and an ex-situ prepared iminium ion are...

Vibrational CD study on the solution phase structures of the MacMillan catalyst and its corresponding iminium ion

We demonstrate that VCD spectroscopy can reveal insights into the conformational preferences of the iminium ion obtained from MacMillan's imidazolidinone catalyst. For both the isolated and in situ generated iminium ion, the comparison of experimental and computed VCD spectra directly confirms that conformer 2b ("Houk-conformer") is the dominant structure in solution. This conclusion is reached without any in-depth interpretation of the spectroscopic data, just by visual comparison of the spectral signatures. For the parent catalyst 1 and its salts 1·HCl and 1·HClO₄, we report a comprehensive analysis of the conformational preferences in two solvents. VCD spectroscopy is subsequently shown to be able to reveal small conformational changes induced by solute-solvent and solute-anion interactions.

Double Hydrogen Bonding Dimerization Propensity of Aqueous Hydroxy Acids Investigated Using Vibrational Optical Activity

Lactic and malic acids are key substances in a number of biochemical processes in living cells and are also utilized in industry. Vibrational spectroscopy represents an efficient and sensitive way to study their structure and interactions. Since water is the natural environment, proper understanding of their molecular dynamics in aqueous solutions is of critical importance. To this end, we employed Raman spectroscopy and Raman optical activity (ROA) to study the conformation of l-lactic and l-malic acids in water (while varying pH, temperature, and concentration), with special emphasis on their double hydrogen bonding dimerization propensity. Raman and ROA experimental data were supported by extensive theoretical calculations of the vibrational properties and by additional experiments (IR absorption, vibrational circular dichroism, and NMR). Conformational behavior of the acids in water was described by molecular dynamics simulations. Reliability of the results was verified by calculating the vibrational properties of populated conformers and by comparing thus obtained spectral features with the experimental data. Calculations estimated the incidence of H-bonded dimers in water to be low in lactic acid and comparable to monomers in malic acid. The "hybrid" approach presented here reveals limitations of relying on the experimental spectra alone to study dimer formation.

Competition between inter and intramolecular hydrogen bond evidenced by vibrational circular dichroism spectroscopy: The case of (1S,2R)-(-)-cis-1-amino-2-indanol

The infrared (IR) absorption and vibrational circular dichroism (VCD) spectra of an intramolecularly hydrogen-bonded chiral amino-alcohol, (1S,2R)-(-)-cis-1-amino-2-indanol, are studied in DMSO-d₆ . The spectra are simulated at the density functional theory (DFT) level within the frame of the cluster-in-the-liquid model. Both IR and VCD spectra show a clear signature of the formation of intermolecular hydrogen bonds at the detriment of the intramolecular OH … N interaction present in the isolated molecule. Two solvent molecules are necessary to reproduce the experimental spectra. Whereas the first DMSO molecule captures the main spectral modifications due to hydrogen bond formation between the solute and the solvent, the second DMSO molecule is necessary for a good description of the Boltzmann contribution of the different complexes, based on their Gibbs free energy.

Induced VCD and conformational chirality in host-guest complexes of a chiral ammonium salt with crown ethers

The hydrogen bonded complexes of the chiral ammonium salt α-methylbenzyl ammonium chloride (MBA-H⁺Cl^-) and the achiral crown ethers 18c6 and 15c5 serve as model systems to investigate the effect of host-guest complex formation on the conformational preferences of the macrocycles. We demonstrate that the intermolecular interactions result in new VCD signatures, that can be assigned to vibrational modes of the crown ethers. Based on a detailed conformational analysis, we investigate the origin of these signatures and discuss induced VCD (iVCD) and conformational chirality as possible sources of VCD intensity. The macrocycle in the MBA-H⁺/18c6 complex prefers either an achiral D_3d-symmetric conformation, which gives rise to iVCD, or chiral conformations, that feature individual contributions to the VCD spectrum. For the MBA-H⁺/15c5 complex, the contributions of the macrocycle to the VCD signatures are less pronounced and found to arise solely from conformational chirality. Therefore, analysis of the VCD signatures confirms that the small chiral guest molecule is able to affect the conformational preferences of a macrocyclic host. The study thus demonstrates the suitability of VCD spectroscopy for the characterization of analogous supramolecular host-guest complexes.

The important role of non-covalent interactions for the vibrational circular dichroism of lactic acid in aqueous solution

We present a computational study of vibrational circular dichroism (VCD) in solutions of (S)-lactic acid, relying on ab initio molecular dynamics (AIMD) and full solvation with bulk water. We discuss the effect of the hydrogen bond network on the aggregation behaviour of the acid: while aggregates of the solute represent conditions encountered in a weakly interacting solvent, the presence of water drastically interferes with the clusters - more strongly than originally anticipated. For both scenarios we computed the VCD spectra by means of nuclear velocity perturbation theory (NVPT). The comparison with experimental data allows us to establish a VCD-structure relationship that includes the solvent network around the chiral solute. We suggest that fundamental modes with strong polarisation such as the carbonyl stretching vibration can borrow VCD from the chirally restructured solvent cage, which extends the common explanatory models of VCD generation in aqueous solution.

VCD spectroscopy reveals that a water molecule determines the conformation of azithromycin in solution

We report the IR and VCD spectra of azithromycin, a macrolide antibiotic with a total of 18 stereogenic centers. The computational analysis of the spectra reveals that a single water molecule has to be considered in the conformational search. Its key role is the stabilization of an extended hydrogen bonding network and an otherwise unstable conformation that determines the VCD spectral signatures.

How important are the intermolecular hydrogen bonding interactions in methanol solvent for interpreting the chiroptical properties?

Two crispine A analogs and tetrahydrofuro[2,3-b]furan-3,3a(6aH)-diol, endowed with hydroxyl groups that can participate in intramolecular hydrogen bonding, have been synthesized and experimental vibrational circular dichroism (VCD) spectra and optical rotatory dispersion (ORD) data have been measured in CD₃OD/CH₃OH solvents. The absolute configurations (ACs) of these compounds have been determined using their synthetic schemes, supplemented wherever possible with X-ray diffraction data. The ACs are also analyzed with quantum chemical (QC) calculations of VCD and ORD utilizing implicit solvation as well as explicit solvation models, with the later employing classical molecular dynamics (MD) simulations. It is found that VCD calculations with implicit solvation model are adequate for determining the ACs, despite propensity of studied compounds for intermolecular hydrogen bonding between solute and solvent molecules. This observation is important because time-consuming MD simulations may not be necessary in the type of situations studied here. Additionally, it is found that the QC predicted VCD spectra provided enough diastereomer discrimination for determining the correct AC of studied compounds independently. The same observation did not apply to ORD.

Solvation and the secondary structure of a proline-containing dipeptide: insights from VCD spectroscopy

In this study we investigate the IR and VCD spectra of the diastereomeric dipeptide Boc-Pro-Phe-(n-propyl) 1 in chloroform-d₁ (CDCl₃) and the strongly hydrogen bonding solvent dimethylsulfoxide-d₆ (DMSO-d₆). From comparison of the experimental spectra, the amide II spectral region is identified as marker signature for the stereochemistry of the dipeptide: the homochiral LL-1 features a (+/-)-pattern in the amide II region of the VCD spectrum, while the amide II signature of the diastereomer LD-1 is inverted. Computational analysis of the IR and VCD spectra of LL-1 reveals that the experimentally observed amide II signature is characteristic for a β_I-turn structure of the peptide. Likewise, the inverted pattern found for LD-1 arises from a β_II-turn structure of the dipeptide. Following a micro-solvation approach, the experimental spectra recorded in DMSO-d₆ are computationally well reproduced by considering only a single solvent molecule in a hydrogen bond with N-H groups. Considering a second solvent molecule, which would lead to a cleavage of intramolecular hydrogen bonds in 1, is found to give a significantly worse match with the experiment. Hence, the detailed computational analysis of the spectra of LL- and LD-1 recorded in DMSO-d₆ confirms that the intramolecular hydrogen bonding pattern, that stabilizes the β-turns and other conformations of LL- and LD-1 in apolar solvents, remains intact. Our findings also show that it is essential to consider solvation explicitly in the analysis of the IR and VCD spectra of dipeptides in strongly hydrogen bonding solvents. As the solute-solvent interactions affect both conformational preferences and spectral signatures, it is also demonstrated that this inclusion of solvent molecules cannot be circumvented by applying fitting procedures to non-solvated structures.

Increased Nematic-Isotropic Transition Temperature on Doping a Liquid Crystal with Molecularly Rigid Carboxylic Acids

A thermotropic nematic liquid crystal (LC) becomes an isotropic liquid at the nematic-isotropic transition temperature (T_NI), which depends on the molecular order of the mesophase. By means of a polarized optical microscope and a differential scanning calorimeter, it was found that doping the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) with molecularly rigid carboxylic acids (benzoic, 1-naphthoic, 2-naphthoic, and biphenyl-4-carboxylic acids) increases T_NI without modification of the nematic-isotropic transition enthalpy. This increment in T_NI is due to the increased order caused by the formation of molecularly rigid and elongated dimers of carboxylic acids in the nematic LC, as confirmed with infrared spectra. Furthermore, T_NI increased with the length of the molecularly rigid dimers at the same concentration level. Conversely, doping the LC with molecularly flexible acids caused lowering of T_NI. A quantitative correlation was established between the T_NI increase of the rigid carboxylic acids and the length of the dimers of these acids; a predictive model for these ΔT_NI values as a function of acid molar fraction was developed. It was also demonstrated that the doping of 5CB with rigid carboxylic acids increases the rotational viscosity of the liquid crystal.

More complex, less complicated? Explicit solvation of hydroxyl groups for the analysis of VCD spectra

With increasing size of the molecules, hydrogen bonding induced solvent effects on the IR and VCD spectra become more negligible.

A combined Raman optical activity and vibrational circular dichroism study on artemisinin-type products

Artemisinin and two of its derivatives, dihydroartemisinin and artesunate, front line drugs against malaria, were studied using Raman optical activity (ROA), vibrational circular dichroism (VCD) experiments and density functional theory (DFT) calculations.

How many solvent molecules are required to solvate chiral 1,2-diols with hydrogen bonding solvents? A VCD spectroscopic study

A VCD spectroscopic analysis of selected model systems for solute–solvent interactions of chiral diols with hydrogen bonding solvents DMSO and ACN.

Assessing cluster models of solvation for the description of vibrational circular dichroism spectra: synergy between static and dynamic approaches

Solvation effects are essential for defining the shape of vibrational circular dichroism (VCD) spectra.

Absolute Configurations of Synthetic Molecular Scaffolds from Vibrational CD Spectroscopy

Vibrational circular dichroism (VCD) spectroscopy is one of the most powerful techniques for the determination of absolute configurations (AC), as it does not require any specific UV/vis chromophores, no chemical derivatization, and no growth of suitable crystals. In the past decade, it has become increasingly recognized by chemists from various fields of synthetic chemistry such as total synthesis and drug discovery as well as from developers of asymmetric catalysts. This perspective article gives an overview about the most important experimental aspects of a VCD-based AC determination and explains the theoretical analysis. The comparison of experimental and computational spectra that leads to the final conclusion about the AC of the target molecules is described. In addition, the review summarizes unique VCD studies carried out in the period 2008-2018 that focus on the determination of unknown ACs of new compounds, which were obtained in its enantiopure form either through direct asymmetric synthesis or chiral chromatography.

On the Absolute Stereochemistry of Tolterodine: A Circular Dichroism Study

Tolterodine (1) is a potent muscarinic receptor antagonist used in the treatment of overactive urinary bladder (OAB) syndrome. Tolterodine is chiral and it was patented, and is currently marketed, as the l-tartrate salt of the (R)-enantiomer. However, the existing literature does not offer an ultimate proof of a stereoselective mode of action of 1. A second open stereochemical issue concerns the absolute configuration (AC) of 1. Neither the original patents nor subsequent studies have established the AC of 1 in an unambiguous way, although the AC of the l-tartrate salt of 1 was assigned by X-ray diffractometry. Finally, neither electronic nor vibrational circular dichroism (ECD and VCD) spectra of 1 are reported so far. We performed a thorough ECD/VCD study of 1 in different solvents and at variable temperatures. Solvent and temperature dependence highlighted the existence of moderate flexibility which was confirmed by molecular modelling. ECD calculations with time-dependent density functional theory (TDDFT) accurately reproduced the experimental spectra and allowed us to confirm the AC of 1 in an independent way.

Solvation and self-aggregation of chiral alcohols: how hydrogen bonding affects their VCD spectral signatures

IR and VCD spectra of chiral alcohols in different solvents are analyzed with DFT spectra calculations. We show that for ACN or DMSO explicit solvation is needed to reproduce experimental spectra.

Solvation of 2-(hydroxymethyl)-2,5,7,8-tetramethyl-chroman-6-ol revealed by circular dichroism: a case of chromane helicity rule breaking

The primary goal of this work is to clarify why 2-(hydroxymethyl)-2,5,7,8-tetramethyl-chroman-6-ol {(S)-TMChM} deviates from the chromane helicity rule under solvent change. The rule, applicable to determining the absolute configuration of molecules containing the chromane chromophore, binds the sign of the 1Lb Cotton effect (CE) with the helicity of the dihydropyran ring. In case of TMChM, however, this CE exhibits extreme solvent dependence: it is negative in non-coordinating solvents and positive in coordinating ones, irrespective of the helicity of the heterocyclic ring. TD-DFT calculations using PCM and hybrid solvation models were performed to explain origin of this phenomenon. It turned out that the 1Lb CE sign directly depends on the position of the phenolic OH group at carbon atom C6 (OHC6). In the absence of interactions with solvents (as in CCl4 or nC6H14) or when a solvent plays proton donor role (as in CHCl3), the OHC6 lies in the phenyl plane and the 1Lb CE sign follows the P/M helicity rule. In contrast, in proton acceptor solvents, like DMSO, CH3OH or CH3CN, the OHC6 group is deflected from the phenyl plane, and the 1Lb CE sign of individual (S)-TMChM conformers depends on the sector in which the OHC6 is located. Thus, in solution, the 1Lb CE sign is an average over different orientations of the OHC6 group and can be positive (as in DMSO and CH3OH) or negative (as in CH3CN) which means that it does not follow the chromane helicity rule. The impact of OHC6 on the 1Lb CE sign and thus the conclusions for the stereochemistry of chromans are demonstrated here for the first time. Additionally, a comparison of experimental and simulated ECD spectra, supported by VCD data, allowed to determine the geometry of intermolecular clusters formed in different solvents.

Effect of puckering motion and hydrogen bond formation on the vibrational circular dichroism spectrum of a flexible molecule: the case of (S)-1-indanol

Vibrational circular dichroism spectra of (S)-1-indanol in DMSO and CCl₄ are described by cluster-in-the-bulk static calculations and first principles molecular dynamics.