Predicting 13C NMR Spectra by DFT Calculations

Date fruit melanin is primarily based on (-)-epicatechin proanthocyanidin oligomers

Plant-based melanin seems to be abundant, but it did not receive scientific attention despite its importance in plant biology and medicinal applications, e.g. photoprotection, radical scavenging, antimicrobial properties, etc. Date fruit melanin (DM) has complex, graphene-like, polymeric structure that needs characterization to understand its molecular properties and potential applications. This study provides the first investigation of the possible molecular composition of DM. High performance size-exclusion chromatography (HPSEC) suggested that DM contains oligomeric structures (569-3236 Da) and transmission electron microscopy (TEM) showed agglomeration of these structures in granules of low total porosity (10-1000 Å). Nuclear magnetic resonance (NMR) spectroscopy provided evidence for the presence of oligomeric proanthocyanidins and electron paramagnetic resonance (EPR) spectroscopy revealed a g-factor in the range 2.0034-2.005. Density functional theory (DFT) calculations suggested that the EPR signals can be associated with oligomeric proanthocyanidin structures having 4 and above molecular units of (-)-epicatechin. The discovery of edible melanin in date fruits and its characterization are expected to open a new area of research on its significance to nutritional and sensory characteristics of plant-based foods.

Machine learning-based correction for spin-orbit coupling effects in NMR chemical shift calculations

As one of the most powerful analytical methods for molecular and solid-state structure elucidation, NMR spectroscopy is an integral part of chemical laboratories associated with a great research interest in its computational simulation. Particularly when heavy atoms are present, a relativistic treatment is essential in the calculations as these influence also the nearby light atoms. In this work, we present a Δ-machine learning method that approximates the contribution to 13C and 1H NMR chemical shifts that stems from spin-orbit (SO) coupling effects. It is built on computed reference data at the spin-orbit zeroth-order regular approximation (ZORA) DFT level for a set of 6388 structures with 38 740 13C and 64 436 1H NMR chemical shifts. The scope of the methods covers the 17 most important heavy p-block elements that exhibit heavy atom on the light atom (HALA) effects to covalently bound carbon or hydrogen atoms. Evaluated on the test data set, the approach is able to recover roughly 85% of the SO contribution for 13C and 70% for 1H from a scalar-relativistic PBE0/ZORA-def2-TZVP calculation at virtually no extra computational costs. Moreover, the method is transferable to other baseline DFT methods even without retraining the model and performs well for realistic organotin and -lead compounds. Finally, we show that using a combination of the new approach with our previous Δ-ML method for correlation contributions to NMR chemical shifts, the mean absolute NMR shift deviations from non-relativistic DFT calculations to experimental values can be halved.

Synthesis of Ion-Exchange Catalysts by Introduction of Fluorinated Ponytails into Novel Mesoporous Polymers

A novel synthetic procedure for the functionalisation of styrenic cross-linked polymers with perfluorinated acyl chains has been reported. The effective significant grafting of the fluorinated moieties is supported by {¹H}-¹³C and {¹⁹F}-¹³C NMR characterisations. This kind of polymer appears promising as catalytic support for a variety of reactions requiring a highly lipophilic catalyst. Indeed, the improved lipophilicity of the materials resulted in enhanced catalytic properties of the corresponding sulfonic materials in the reaction of esterification of a solution in a vegetable oil of stearic acid with methanol.

Truncatenolide, a Bioactive Disubstituted Nonenolide Produced by Colletotrichum truncatum, the Causal Agent of Anthracnose of Soybean in Argentina: Fungal Antagonism and SAR Studies

A bioactive disubstituted nonenolide, named truncatenolide, was produced by Colletotrichum truncatum, which was collected from infected tissues of soybean showing anthracnose symptoms in Argentina. This is a devastating disease that drastically reduces the yield of soybean production in the world. The fungus also produced a new trisubstituted oct-2-en-4-one, named truncatenone, and the well-known tyrosol and N-acetyltyramine. Truncatenolide and truncatenone were characterized by spectroscopic (essentially one-dimensional (1D) and two-dimensional (2D) ¹H and ¹³C NMR and HR ESIMS) and chemical methods as (5E,7R,10R)-7-hydroxy-10-methyl-3,4,7,8,9,10-hexahydro-2H-oxecin-2-one and (Z)-6-hydroxy-3,5-dimethyloct-2-en-4-one, respectively. The geometry of the double bond of truncatenolide was assigned by the value of olefinic proton coupling constant and that of truncatenone by the correlation observed in the corresponding NOESY spectrum. The relative configuration of each stereogenic center was assigned with the help of ¹³C chemical shift and ¹H-¹H scalar coupling DFT calculations, while the absolute configuration assignment of truncatenolide was performed by electronic circular dichroism (ECD). When tested on soybean seeds, truncatenolide showed the strongest phytotoxic activity. Tyrosol and N-acetyltyramine also showed phytotoxicity to a lesser extent, while truncatenone weakly stimulated the growth of the seed root in comparison to the control. When assayed against Macrophomina phaseolina and Cercospora nicotianae, other severe pathogens of soybean, truncatenolide showed significant activity against M. phaseolina and total inhibition of C. nicotianae. Thus, some other fungal nonenolides and their derivatives were assayed for their antifungal activity against both fungi in comparison with truncatenolide. Pinolidoxin showed to a less extent antifungal activity against both fungi, while modiolide A selectively and totally inhibited only the growth of C. nicotianae. The SAR results and the potential of truncatenolide, modiolide A, and pinolidoxin as biofungicides were also discussed.

Computational 1 H and 13 C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹ H and ¹³ C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin-spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹ H and ¹³ C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.

Experimental and Theoretical DFT Study of Cu(I)/N,N-Disubstituted-N'-acylthioureato Anticancer Complexes: Actin Cytoskeleton and Induction of Death by Apoptosis in Triple-Negative Breast Tumor Cells

Six complexes with the general formula [Cu(acylthioureato)(PPh3)2] were synthesized and characterized using spectroscopic techniques (IR, UV/visible, and 1D and 2D NMR), mass spectrometry, elemental analysis, and X-ray diffraction. Interpretation of the in vitro cytotoxicity data of Cu(I) complexes took into account their stability in cell culture medium. DFT calculations showed that NMR properties, such as the shielding of carbon atoms, are affected by relativistic effects, supported by the ZORA Hamiltonian in the theoretical calculations. Additionally, the calculation of the energies of the frontier molecular orbitals predicted that the structural changes of the acylthiourea ligands did not cause marked changes in the reactivity descriptors. All complexes were cytotoxic to the evaluated tumor cell lines [MDA-MB-231 (triple-negative breast cancer, TNBC), MCF-7 (breast cancer), and A549 (lung cancer)]. In the MDA-MB-231 cell line, complex 1 significantly altered the cytoskeleton of the cells, reducing the density and promoting the condensation of F-actin filaments. In addition, the compound caused an increase in the percentage of cells in the fragmented DNA region (sub-G0) and induced cell death via the apoptotic pathway starting at the IC50 concentration. Taken together, the results show that complex 1 has cytotoxic and apoptotic effects on TNBC cells, which is a cell line originating from an aggressive, difficult-to-treat breast cancer.

Revving up 13C NMR shielding predictions across chemical space: benchmarks for atoms-in-molecules kernel machine learning with new data for 134 kilo molecules

The requirement for accelerated and quantitatively accurate screening of nuclear magnetic resonance spectra across the small molecules chemical compound space is two-fold: (1) a robust ‘local’ machine learning (ML) strategy capturing the effect of the neighborhood on an atom’s ‘near-sighted’ property—chemical shielding; (2) an accurate reference dataset generated with a state-of-the-art first-principles method for training. Herein we report the QM9-NMR dataset comprising isotropic shielding of over 0.8 million C atoms in 134k molecules of the QM9 dataset in gas and five common solvent phases. Using these data for training, we present benchmark results for the prediction transferability of kernel-ridge regression models with popular local descriptors. Our best model, trained on 100k samples, accurately predicts isotropic shielding of 50k ‘hold-out’ atoms with a mean error of less than 1.9 ppm. For the rapid prediction of new query molecules, the models were trained on geometries from an inexpensive theory. Furthermore, by using a Δ-ML strategy, we quench the error below 1.4 ppm. Finally, we test the transferability on non-trivial benchmark sets that include benchmark molecules comprising 10–17 heavy atoms and drugs.

High-field and benchtop NMR spectroscopy for the characterization of new psychoactive substances

New psychoactive substances (NPS) have become a serious threat to public health in Europe due to their ability to be sold in the street or on the darknet. Regulating NPS is an urgent priority but comes with a number of analytical challenges since they are structurally similar to legal products. A number of analytical techniques can be used for identifying NPS, among which NMR spectroscopy is a gold standard. High field NMR is typically used for structural elucidation in combination with others techniques like GC-MS, Infrared spectroscopy, together with databases. In addition to their strong ability to elucidate molecular structures, high field NMR techniques are the gold standard for quantification without any physical isolation procedure and with a single internal standard. However, high field NMR remains expensive and emerging "benchtop" NMR apparatus which are cheaper and transportable can be considered as valuable alternatives to high field NMR. Indeed, benchtop NMR, which emerged about ten years ago, makes it possible to carry out structural elucidation and quantification of NPS despite the gap in resolution and sensitivity as compared to high field NMR. This review describes recent advances in the field of NMR applied to the characterization of NPS. High-field NMR methods are first described in view of their complementarity with other analytical methods, focusing on both structural and quantitative aspects. The second part of the review highlights how emerging benchtop NMR approaches could act as a game changer in the field of forensics.

DFT/NMR Approach for the Configuration Assignment of Groups of Stereoisomers by the Combination and Comparison of Experimental and Predicted Sets of Data

Quantum mechanical/nuclear magnetic resonance (NMR) approaches are widely used for the configuration assignment of organic compounds generally comparing one cluster of experimentally determined data (e.g., 13C NMR chemical shifts) with those predicted for all possible theoretical stereoisomers. More than one set of experimental data, each related to a specific stereoisomer, may occur in some cases, and the accurate stereoassignments can be obtained by combining the experimental and computed data. We introduce here a straightforward methodology based on the simultaneous analysis, combination, and comparison of all sets of experimental/calculated 13C chemical shifts for aiding the correct configuration assignment of groups of stereoisomers. The comparison of the differences between the calculated/experimental chemical shifts instead of the shifts themselves led to the advantage of avoiding errors arising from calibration procedures, reducing systematic errors, and highlighting the most diagnostic differences between calculated and experimental data. This methodology was applied on a tetrad of synthesized cladosporin stereoisomers (cladologs) and further corroborated on a tetrad of pochonicine stereoisomers, obtaining the correct correspondences between experimental and calculated sets of data. The new MAEΔΔδ parameter, useful for indicating the best fit between sets of experimental and calculated data, is here introduced for facilitating the stereochemical assignment of groups of stereoisomers.

Computational 1 H NMR: Part 3. Biochemical studies

This is the third and the last part of three closely interrelated reviews dealing with computation of ¹ H nuclear magnetic resonance chemical shifts and ¹ H-¹ H spin-spin coupling constants. Present review deals with the computation of these parameters in biologically active natural products, carbohydrates, and other molecules of biological origin focusing on stereochemical applications of computational ¹ H nuclear magnetic resonance to these objects.

Quantitative prediction of electronic absorption spectra of copper(II)-bioligand systems: Validation and applications

The visible region of the electronic absorption spectra of Cu(II) complexes was studied by time-dependent density functional theory (TD-DFT). The performance of twelve functionals in the prediction of absorption maxima (λ_max) was tested on eleven compounds with different geometry, donors and charge. The ranking of the functionals for λ_max was determined in terms of mean absolute percent deviation (MAPD) and standard deviation (SD) and it is as follows: BHandHLYP > M06 ≫ CAM-B3LYP ≫ MPW1PW91 ~ B1LYP ~ BLYP > HSE06 ~ B3LYP > B3P86 ~ ω-B97x-D ≫ TPSSh ≫ M06-2X (MAPD) and BHandHLYP > M06 ~ HSE06 > ω-B97x-D ~ CAM-B3LYP ~ MPW1PW91 > B1LYP ~ B3LYP > B3P86 > BLYP ≫ TPSSh ≫ M06-2X (SD). With BHandHLYP functional the MAPD is 3.1% and SD is 2.3%, while with M06 the MAPD is 3.7% and SD is 3.7%. The protocol validated in the first step of the study was applied to: i) calculate the number of transitions in the spectra and relate them to the geometry of Cu(II) species; ii) determine the coordination of axial water(s); iii) predict the electronic spectra of the systems where Cu(II) is bound to human serum albumin (HSA) and to the regions 94-97 and 108-112 of prion protein (PrP). The results indicate that the proposed computational protocol allows a successful prediction of the electronic spectra of Cu(II) species and to relate an experimental spectrum to a specific structure.

A Simple Coulombic Model for 31P NMR Spectra of Cluster-Encapsulated Phosphorus Atoms

While sophisticated computational methods can predict ³¹P NMR spectra of phosphorus atoms encapsulated within Keggin-derived heteropoly tungstate and molybdate cluster anions, calculated and experimental chemical shift values typically deviate considerably from one another. Motivated by the observation that experimentally determined ³¹P chemical shift values within a series of water-soluble plenary and metal-cation substituted lacunary Keggin anions, [PM ⁿW₁₁O₃₉]^{(7- n)-} (M ⁿ = Ag⁺, Zn²⁺, Nb⁵⁺, W⁶⁺) and [(PW₁₁O₃₉)₂M ⁿ]^{(14- n)-} (M ⁿ = Y³⁺, Zr⁴⁺), varied as a linear function of the oxidation states, n, of the complexed M ⁿ cations, a linear correlation was sought between observed chemical shift values and the net Coulombic forces experienced by the encapsulated phosphorus atoms. The Coulombic model based on Shannon radii, published electronegativity values, and bond angles from X-ray crystallographic data remarkably accounted for the relative ³¹P chemical shift values of phosphorus atoms in over 50 metal-oxide cluster anions, including large structures comprised of up to four Keggin-derived fragments with an overall R² value of 0.974. With the model being applied here to three cluster anions whose ³¹P chemical shift values are reported here for the first time, predicted and experimental values differed by only ±0.4 ppm.

Toward the comprehensive calculations on the relationship between 1 H, 13 C, 31 P chemical shifts, 2 JPH , and the bonding structure of different phosphoryl benzamides

A comprehensive investigation was performed on ¹ H, ¹³ C, and ³¹ P nuclear magnetic resonance (NMR) chemical shifts (CSs) of phosphoryl benzamide derivatives (C₆ H₅ C(O)NHP(O)R₁ R₂ ), (R₁ , R₂  = aziridine [L₁ ], azetidine [L₂ ], pyrrolidine [L₃ ], piperidine [L₄ ], azepane [L₅ ], 4-methylpiperidine [L₆ ], propane-2-amine [L₇ ], and 2-methylpropane-2-amine [L₈ ]) by the gauge-independent atomic orbital method (GIAO) to find the most accordant level of theory with the experimental values. To achieve this goal, all the structures were optimized using the B3LYP, BP86, PBE1PBE, M06-2X, MPWB1K, and MP2 methods with 6-31+G* basis set. Computed structural parameters demonstrate that BP86 has the best agreement to the experimental values between the other methods. The def2-TZVP and aug-cc-pVDZ basis sets were also employed to inspect the effect of different types of basis sets with higher polarization and diffuse functions. The correlation between the empirical and computational values attests that 6-31+G* basis set is the optimum case regarding minimization of the costs and results. The comparison between calculated and experimental CSs at all mentioned combinations illustrated that in accordance with structural results, the best level of theory in CSs is also BP86/6-31+G*. Besides, ² J_PH values were computed with an acceptable agreement to experimental data at the optimum level of theory. The dependency between ² J_PH and the bonding structure of studied ligands was also scrutinized by the Natural Bond Orbital (NBO) analysis that interprets the relationship between the electronic properties and ² J_PH values.

Computation and structural elucidation of compounds formed via epoxide alcoholysis

Isobenzofuranones are known for their wide range of biological activities such as fungicide, insecticide, and anticancer. The search for novel bioactive compounds was performed by reaction of epoxide 2 with methanol, ethanol, propan-1-ol, propan-2-ol, and butan-1-ol. The mechanism for the stereoselective and stereospecific epoxide opening with methanol was reasoned by calculating the transition states for the two putative structures (rac)-3a and (rac)-3b. The compound (rac)-3a is the kinetic product as inferred from the lower energies of its transition state (TS1). The ¹ H and ¹³ C nuclear magnetic resonance (NMR) chemical shifts for these two candidate structures were calculated and compared with the experimental data using mean absolute error (MAE) and DP4 analyses. Therefore, the relative stereochemistry of (rac)-3a was established by the mechanism, MAE, and DP4 approaches. The hydroxyl group was acetylated to surpass the problem of signal overlapping of H5 and H6 in the ¹ H NMR. The relative stereochemistry of the corresponding ester determined by NMR interpretation was in agreement with the structure of (rac)-3a.

"Through-Space" Relativistic Effects on NMR Chemical Shifts of Pyridinium Halide Ionic Liquids

We have investigated, using two-component relativistic density functional theory (DFT) at ZORA-SO-BP86 and ZORA-SO-PBE0 level, the occurrence of relativistic effects on the ¹ H, ¹³ C, and ¹⁵ N NMR chemical shifts of 1-methylpyridinium halides [MP][X] and 1-butyl-3-methylpyridinium trihalides [BMP][X₃ ] ionic liquids (ILs) (X=Cl, Br, I) as a result of a non-covalent interaction with the heavy anions. Our results indicate a sizeable deshielding effect in ion pairs when the anion is I^- and I₃ ^- . A smaller, though nonzero, effect is observed also with bromine while chlorine based anions do not produce an appreciable relativistic shift. The chemical shift of the carbon atoms of the aromatic ring shows an inverse halogen dependence that has been rationalized based on the little C-2s orbital contribution to the σ-type interaction between the cation and anion. This is the first detailed account and systematic theoretical investigation of a relativistic heavy atom effect on the NMR chemical shifts of light atoms in the absence of covalent bonds. Our work paves the way and suggests the direction for an experimental investigation of such elusive signatures of ion pairing in ILs.

An automated framework for NMR chemical shift calculations of small organic molecules

When using nuclear magnetic resonance (NMR) to assist in chemical identification in complex samples, researchers commonly rely on databases for chemical shift spectra. However, authentic standards are typically depended upon to build libraries experimentally. Considering complex biological samples, such as blood and soil, the entirety of NMR spectra required for all possible compounds would be infeasible to ascertain due to limitations of available standards and experimental processing time. As an alternative, we introduce the in silico Chemical Library Engine (ISiCLE) NMR chemical shift module to accurately and automatically calculate NMR chemical shifts of small organic molecules through use of quantum chemical calculations. ISiCLE performs density functional theory (DFT)-based calculations for predicting chemical properties—specifically NMR chemical shifts in this manuscript—via the open source, high-performance computational chemistry software, NWChem. ISiCLE calculates the NMR chemical shifts of sets of molecules using any available combination of DFT method, solvent, and NMR-active nuclei, using both user-selected reference compounds and/or linear regression methods. Calculated NMR chemical shifts are provided to the user for each molecule, along with comparisons with respect to a number of metrics commonly used in the literature. Here, we demonstrate ISiCLE using a set of 312 molecules, ranging in size up to 90 carbon atoms. For each, calculation of NMR chemical shifts have been performed with 8 different levels of DFT theory, and with solvation effects using the implicit solvent Conductor-like Screening Model. The DFT method dependence of the calculated chemical shifts have been systematically investigated through benchmarking and subsequently compared to experimental data available in the literature. Furthermore, ISiCLE has been applied to a set of 80 methylcyclohexane conformers, combined via Boltzmann weighting and compared to experimental values. We demonstrate that our protocol shows promise in the automation of chemical shift calculations and, ultimately, the expansion of chemical shift libraries.

Columnar self-assembly of N,N′,N′′-trihexylbenzene-1,3,5-tricarboxamides investigated by means of NMR spectroscopy and computational methods in solution and the solid state

The columnar self-assembly resulting from units of N,N′,N′′-trihexylbenzene-1,3,5-tricarboxamide is investigated in solution and the solid state by means of NMR spectroscopy and DFT methods.

NMR spectroscopy and DFT calculations of a self-assembled arene ruthenium rectangle obtained from a combination of coordination and hydrogen bonds

Hydrogen-bonded ruthenium metalla-rectangle investigated by means of solution-phase NMR spectroscopy and DFT calculations.

Beyond DP4: an Improved Probability for the Stereochemical Assignment of Isomeric Compounds using Quantum Chemical Calculations of NMR Shifts

The DP4 probability is one of the most sophisticated and popular approaches for the stereochemical assignment of organic molecules using GIAO NMR chemical shift calculations when only one set of experimental data is available. In order to improve the performance of the method, we have developed a modified probability (DP4+), whose main differences from the original DP4 are the inclusion of unscaled data and the use of higher levels of theory for the NMR calculation procedure. With these modifications, a significant improvement in the overall performance was achieved, providing accurate and confident results in establishing the stereochemistry of 48 challenging isomeric compounds.

Conformational and NMR study of some furan derivatives by DFT methods

4'-substituted neutral/protonated furfurylidenanilines and trans-styrylfurans are able to exist in two different conformations related to the rotation around the furan ring-bridge double bond. In this work, the equilibrium geometry and the corresponding rotational barrier of the benzene ring for each furan derivative conformation were calculated by DFT methods. The trend and shape of the rotational barrier are rationalized within natural bond orbitals as well as atoms-in-molecules approach. For the corresponding equilibrium geometries, (1)H and (13)C substituent induced shifts (SIS) were calculated and compared with experimental values. Calculated shielding constants are shown to be sensitive to the substituent effect through a linear fit with substituent's Hammett constants. An alternative approach was followed for assessing the effect of substituents over SIS through comparing the differences in isotropic shielding constants with NBO charges as well as with (1)H and (13)C experimental chemical shifts.

Comparative molecular field analysis and comparative molecular similarity index analysis studies on 1H NMR chemical shift of NH group of diaryl triazene derivatives

Comparative molecular field analysis (CoMFA), comparative molecular field analysis region focusing (CoMFA-RF) for optimizing the region for the final partial least square analysis, and comparative molecular similarity indices analysis (CoMSIA) methods were employed to develop three-dimensional quantitative structure-activity relationship (3D-QSAR) models of (1)H NMR chemical shift of NH proton of diaryl triazene derivatives. The best orientation was searched by all-orientation search (AOS) strategy to minimize the effect of the initial orientation of the structures. The predictive abilities of CoMFA-RF and CoMSIA models were determined using a test set of ten compounds affording predictive correlation coefficients of 0.721 and 0.754, respectively, indicating good predictive power. For further model validation, cross validation (leave one out), progressive scrambling, and bootstrapping were also applied. The accuracy and speed of obtained 3D-QSAR models for the prediction of (1)H NMR chemical shifts of NH group of diaryl triazene derivatives were greater compared to some computational well-known procedures.

Aetheramides A and B, potent HIV-inhibitory depsipeptides from a myxobacterium of the new genus "Aetherobacter"

Aetheramides are structurally distinctive cyclic peptides isolated from a novel myxobacterial genus proposed to be termed "Aetherobacter". The structures were solved by a combination of NMR analyses, quantum mechanical calculations, and chemical derivatizations. Aetheramides which contain a unique polyketide moiety and two amino acid residues potently inhibited HIV-1 infection with IC(50) values of ~0.015 μM. Furthermore aetheramides showed cytostatic activity against human colon carcinoma (HCT-116) cells with IC(50) values of 0.11 μM.

The structure of glibenclamide in the solid state

The structure of glibenclamide, 5-chloro-N-(2-{4-[(cyclohexylamino)carbonyl] aminosulfonyl}phenyl) ethyl)-2-methoxybenzamide, an important antidiabetic drug, has been studied both in solution and in the solid state by a combination of NMR spectroscopy and theoretical calculations. The possibility that glibenclamide suffers a tautomerization under melting to afford a desmotrope was rejected.

Density functional theory study of (13)C NMR chemical shift of chlorinated compounds

The use of the standard density functional theory (DFT) leads to an overestimation of the paramagnetic contribution and underestimation of the shielding constants, especially for chlorinated carbon nuclei. For that reason, the predictions of chlorinated compounds often yield too high chemical shift values. In this study, the WC04 functional is shown to be capable of reducing the overestimation of the chemical shift of Cl-bonded carbons in standard DFT functionals and to show a good performance in the prediction of (13)C NMR chemical shifts of chlorinated organic compounds. The capability is attributed to the minimization of the contributions that intensively increase the chemical shift in the WC04. Extensive computations and analyses were performed to search for the optimal procedure for WC04. The B3LYP and mPW1PW91 standard functionals were also used to evaluate the performance. Through detailed comparisons between the basis set effects and the solvent effects on the results, the gas-phase GIAO/WC04/6-311+G(2d,p)//B3LYP/6-31+G(d,p) was found to be specifically suitable for the prediction of (13)C NMR chemical shifts of chlorides in both chlorinated and non-chlorinated carbons. Further tests with eight molecules in the probe set sufficiently confirmed that WC04 was undoubtedly effective for accurately predicting (13) C NMR chemical shifts of chlorinated organic compounds.

Application of multivariate image analysis in modeling (13) C-NMR chemical shifts of mono substituted pyridines

Multivariate image analysis (MIA) descriptors have been applied to the quantitative structure-property relationship (QSPR) study of (13) C-NMR chemical shifts of 2-mono substituted pyridines. In this method, descriptors are generated from pixels of images and are analyzed with different multivariate methods. Correlation ranking-principal component regression and correlation ranking-principal component-artificial neural networks were applied in constructing predictor models. In this article, the role of weight update function in artificial neural networks was investigated too. Obtained results using the correlation ranking-principal component-artificial neural network method showed high performance for predicting of (13) C-NMR chemical shifts of pyridine derivatives. Also, these results indicated that MIA descriptors may be useful to predict (13) C-NMR chemical shifts. Finally, The MIA-QSPR approach coupled to artificial neural networks revealed that the predictive ability of MIA descriptors is comparable or even superior for the pyridine derivatives when compared with the ChemDraw program or gauge included atomic orbital procedure for (13) C chemical shifts calculations.

Hydration and temperature dependence of 13C and 1H NMR spectra of the DMPC phospholipid membrane and complete resonance assignment of its crystalline state

Inhomogeneous line broadening due to conformational distributions of molecules is one of the troublesome problems in solid-state NMR spectroscopy. The best possible way to avoid it is to crystallize the sample. Here, we present a highly resolved (13)C cross-polarization (CP) magic angle spinning (MAS) NMR spectrum of the highly ordered crystalline 1,2-dimyrystoyl-sn-glycero-3-phosphocholine (DMPC) and completely assigned it using two-dimensional (2D) solid-state NMR spectra, dipolar heteronuclear correlation (HETCOR) spectra, scalar heteronuclear J coupling based chemical shift correlation (MAS-J-HMQC) spectra, and Dipolar Assisted Rotational Resonance (DARR) spectra. A comparison between assigned chemical shift values by solid-state NMR in this study and the calculated chemical shift values for X-ray crystal DMPC structures shows good agreement, indicating that the two isomers in the crystalline DMPC take the same conformation as the X-ray crystal structure. The phase diagram of the low hydration level of DMPC (3 ≤ n(W) ≤ 12) determined by (1)H and (13)C NMR spectra indicates that DMPC takes a crystalline state only in a very narrow region around n(W) = 4 and T < 313 K. These findings provide us with conformational information on crystalline DMPC and the physical properties of DMPC at a low hydration level and can possibly help us obtain a highly resolved solid-state NMR spectrum of microcrystalline membrane-associated protein samples.