DFT Calculations of 1H NMR Chemical Shifts of Geometric Isomers of Conjugated Linolenic Acids,Hexadecatrienyl Pheromones,and Model Triene-Containing Compounds: Structures in Solution and Revision of NMR Assignments

A DFT study of the ¹H NMR chemical shifts, δ(¹H), of geometric isomers of 18:3 conjugated linolenic acids (CLnAs), hexadecatrienyl pheromones, and model triene-containing compounds is presented, using standard functionals (B3LYP and PBE0) as well as corrections for dispersion interactions (B3LYP-D3, APFD, M06–2X and ωB97XD). The results are compared with literature experimental δ(¹H) data in solution. The closely spaced “inside” olefinic protons are significantly more deshielded due to short-range through-space H^…H steric interactions and appear close to or even beyond δ-values of aromatic systems. Several regularities of the computational δ(¹H) of the olefinic protons of the conjugated double bonds are reproduced very accurately for the lowest-energy DFT-optimized single conformer for all functionals used and are in very good agreement with experimental δ(¹H) in solution. Examples are provided of literature studies in which experimental resonance assignments deviate significantly from DFT predictions and, thus, should be revised. We conclude that DFT calculations of ¹H chemical shifts of trienyl compounds are powerful tools (i) for the accurate prediction of δ(¹H) even with less demanding functionals and basis sets; (ii) for the unequivocal identification of geometric isomerism of conjugated trienyl systems that occur in nature; (iii) for tackling complex problems of experimental resonance assignments due to extensive signal overlap; and (iv) for structure elucidation in solution.     

Recent advances in computational 31P NMR: Part 1. Chemical shifts

This is the first part of two closely related reviews dealing with the computation of phosphorus-31 nuclear magnetic resonance chemical shifts in a wide series of organophosphorus compounds including complexes, clusters, and bioorganic phosphorus compounds. In particular, the analysis of the accuracy factors, such as substitution effects, solvent effects, vibrational corrections, and relativistic effects, is presented. This review is dedicated to the Full Member of the Russian Academy of Sciences Professor Boris A. Trofimov in view of his invaluable contribution to the field of synthesis, nuclear magnetic resonance, and computation studies of organophosphorus compounds.     

Towards the Accurate Calculation of 183W NMR Chemical Shifts in Polyoxometalates: The Relevance of the Structure

DFT calculations were carried out to study ¹⁸³W NMR chemical shifts in the family of the Keggin anions with formula α‐[XW₁₂O₄₀]^q? (X=B, Al, Si, P, Ga, Ge, As, Zn), in the β‐ and γ‐[SiW₁₂O₄₀]^4? geometric isomers, in the derivative Dawson anion [P₂W₁₈O₆₂]^6?, and in the most symmetrical Lindqvist [W₆O₁₉]^2? anion and its derivative [W₁₀O₃₂]^4?. In this article, we show that the geometry employed in the calculation of NMR chemical shifts in polyoxotungstates is extremely important if we want to be quantitative. Using very large basis sets of QZ4P quality and taking into account the conductor‐like screening model (COSMO) to account for solvent effects (aqueous and organic solutions), good geometries were found for the polyoxoanions. From these optimal geometries the ¹⁸³W NMR chemical shifts were computed with the more standard basis sets of TZP quality and including spin–orbit corrections inside the zero‐order regular approximation (ZORA) to describe the relativistic effects of the internal electrons. With this strategy the mean absolute error between experimental and theoretical values was found to be less than 10 ppm, which is similar to the experimental error. We also discuss how the geometry of the polyoxoanion influences on the shielding.     

The use of variable temperature 13C solid-state MAS NMR and GIPAW DFT calculations to explore the dynamics of diethylcarbamazine citrate

Experimental ¹³C solid-state magic-angle spinning (MAS) Nuclear Magnetic Resonance (NMR) as well as Density-Functional Theory (DFT) gauge-including projector augmented wave (GIPAW) calculations were used to probe disorder and local mobility in diethylcarbamazine citrate, (DEC)⁺(citrate)⁻. This compound has been used as the first option drug for the treatment of filariasis, a disease endemic in tropical countries and caused by adult worms of Wuchereria bancrofti, which is transmitted by mosquitoes. We firstly present 2D ¹³C─¹H dipolar-coupling-mediated heteronuclear correlation spectra recorded at moderate spinning frequency, to explore the intermolecular interaction between DEC and citrate molecules. Secondly, we investigate the dynamic behavior of (DEC)⁺(citrate)⁻ by varying the temperature and correlating the experimental MAS NMR results with DFT GIPAW calculations that consider two (DEC)⁺ conformers (in a 70:30 ratio) for crystal structures determined at 293 and 235 K. Solid-state NMR provides insights on slow exchange dynamics revealing conformational changes involving particularly the DEC ethyl groups.     

GIAO Calculations of Chemical Shifts in Heterocyclic Compounds

In this review, the GIAO calculations of absolute shieldings and their relationship with experimental chemical shifts for aromatic heterocycles will be summarized. Automatic assignment, conformational analysis, E/Z isomerism, and, in particular, tautomerism, will be discussed in detail. Solid-state and solvent effects will be examined, as well as the problem of heteroaromaticity. The review ends with the discussion of some methodological problems with special emphasis on the calculation of references, such as TMS and nitromethane.     

Prediction of the 13C NMR Chemical Shifts of Fluorenone Analogues by the GIAO Method

After the geometry optimization at B3LYP/6-31+G(d,p) level,the NMR calcula-tions of a series of fluorenone analogues have been carried out by GIAO method at HF/6-31+G(d) level and B3LYP/6-311G+(2d,p) level,respectively.The 13C NMR chemical shifts calculated at HF/6-31+G(d) level show better agreement with the observed values.By a series of linear correction equations (δpred=a + bδcalc),accurate prediction of 13C chemical shifts was achieved for the new fluorenone compound.The linear correlation of δpred with δexptl is excellent,and the square of correlation coefficient,r2,is up to 0.994.The maximum absolute difference between δpred and δexptl,Δδ,is 4.6 ppm,and the root-mean-square error between δpred and δexptl is only 2.6 ppm.     

Substituent Effects on 13C NMR and 1H NMR Chemical Shifts of CH=N in Multi-substituted Benzylideneanilines

Fifty-three samples of multi-substituted benzylideneanilines XArCH=NArYs (abbreviated XBAYs) were synthesized and their NMR spectra were determined. An extensional study of substituent effects on the ¹H NMR chemical shifts (^δH(CH=N)) and ¹³C NMR chemical shifts (^δC(CH=N)) of the CH=N bridging group from di-substituted to multi-substituted XBAYs was made based on a total of 182 samples of XBAYs, together with the NMR data of other 129 samples of di-substituted XBAYs quoted from literatures. The results show that the substituent specific cross-interaction effect parameter \begin{document} $\left(\Delta\left(\displaystyle\sum\sigma\right)^2\right)$ \end{document} plays an important role in quantifying the ^δC(CH=N) values of XBAYs, but it is negligible for quantifying the ^δH(CH=N) values; the other substituent parameters also present different influences on the ^δC(CH=N) and ^δH(CH=N). On the whole, the contributions of X and Y to the ^δC(CH=N) of XBAYs are balanced, but the ^δH(CH=N) values of XBAYs mainly rely on the contributions of X.     

Computational modeling of polyoxotungstates by relativistic DFT calculations of (183)W NMR chemical shifts

The (183)W nuclear shielding in a variety of tungsten polyoxometalates (POM) (Lindqvist, Anderson, decatungstates, Keggin) of different shapes and charges has been modeled by DFT calculations that take into account relativistic effects, by means of the zero-order regular approximation (ZORA), and solvent effects, by the conductor-like screening model (COSMO) continuum method. The charge/surface area ratio (q/A) is proposed as an indicator of the charge density to which the solvation energies of all POMs are correlated in a satisfactory way. Among the various theoretical levels tested (ZORA scalar or spin-orbit, frozen-core or all-electron basis set, geometry optimization in the gas phase or in the continuum solvent, etc.), the best results are obtained when both geometry optimization in solvent and spin-orbit shielding are included (mean absolute error of delta=35 ppm). The quality of the computed chemical shifts depends systematically on the charge density as expressed by q/A; thus, POMs with low q/A ratios display the best agreement with experimental data. The performance of the method is such that computed values can aid the assignment of the (183)W NMR spectra of polyoxotungstates, as shown by the case of alpha-[PW(11)TiO(40)](5-), whose six signals are ranked computationally so as to almost reproduce the experimental ordering even though the signals are spaced by as little as 5 ppm.     

Insights into the Intramolecular Donor Stabilisation of Organostannylene Palladium and Platinum Complexes: Syntheses,Structures and DFT Calculations

The syntheses of the transition metal complexes cis‐[(4‐tBu‐2,6‐{P(O)(OiPr)₂}₂C₆H₂SnCl)₂MX₂] ( 1 , M=Pd, X=Cl; 2 , M=Pd, X=Br; 3 , M=Pd, X=I; 4 , M=Pt, X=Cl), cis‐[{2,6‐(Me₂NCH₂)₂C₆H₃SnCl}₂MX₂] ( 5 , M=Pd, X=I; 6 , M=Pt, X=Cl), trans‐[{2,6‐(Me₂NCH₂)₂C₆H₃SnI}₂PtI₂] ( 7 ) and trans‐[(4‐tBu‐2,6‐{P(O)(OiPr)₂}₂ C₆H₂SnCl)PdI₂]₂ ( 8 ) are reported. Also reported is the serendipitous formation of the unprecedented complexes trans‐[(4‐tBu‐2,6‐{P(O)(OiPr)₂}₂C₆H₂SnCl)₂ Pt(SnCl₃)₂] ( 10 ) and [(4‐tBu‐2,6‐{P(O) (OiPr)₂}₂C₆H₂SnCl)₃Pt(SnCl₃)₂] ( 11 ). The compounds were characterised by elemental analyses, ¹H, ¹³C, ³¹P, ¹¹⁹Sn and ¹⁹⁵Pt NMR spectroscopy, single‐crystal X‐ray diffraction analysis, UV/Vis spectroscopy and, in the cases of compounds 1 , 3 and 4 , also by Mössbauer spectroscopy. All the compounds show the tin atoms in a distorted trigonal‐bipyramidal environment. The Mössbauer spectra suggest the tin atoms to be present in the oxidation state III. The kinetic lability of the complexes was studied by redistribution reactions between compounds 1 and 3 as well as between 1 and cis‐[{2,6‐(Me₂NCH₂)₂C₆H₃SnCl}₂PdCl₂]. DFT calculations provided insights into both the bonding situation of the compounds and the energy difference between the cis and trans isomers. The latter is influenced by the donor strength of the pincer‐type ligands.     

GIAO/DFT evaluation of 13C NMR chemical shifts of selected acetals based on DFT optimized geometries

DFT/B3LYP calculations of the ground-state conformation of eight cyclic and acyclic acetals are presented and compared with experimental data. Results of single-point GIAO/DFT calculations at five different levels of theory show that isotropic shieldings need to be empirically scaled to achieve agreement with experimental chemical shifts. Statistical evaluation of data indicates that the most accurate prediction of 13C chemical shifts is achieved at the MPW1PW91/6-311G** level of theory. An empirical equation describing the relationship between delta values and shielding constants is postulated. This equation has been applied to the non-chair ground-state conformation of the six-membered acetonide and to the conformationally flexible benzodioxonine derivative. The agreement observed between the experimental and predicted chemical shifts shows that calculations at the MPW1PW91/6-311G** level of theory are adequate for addressing questions of conformation.     

An Investigation of 1:1 Adducts of Gallium Trihalides with Triarylphosphines by Solid‐State 69/71Ga and 31P NMR Spectroscopy

Several 1:1 adducts of gallium trihalides with triarylphosphines, X₃Ga(PR₃) (X=Cl, Br, and I; PR₃=triarylphosphine ligand), were investigated by using solid‐state ^69/71Ga and ³¹P NMR spectroscopy at different magnetic‐field strengths. The ^69/71Ga nuclear quadrupolar coupling parameters, as well as the gallium and phosphorus magnetic shielding tensors, were determined. The magnitude of the ⁷¹Ga quadrupolar coupling constants (C_Q(⁷¹Ga)) range from approximately 0.9 to 11.0 MHz . The spans of the gallium magnetic shielding tensors for these complexes, δ₁₁?δ₃₃, range from approximately 30 to 380 ppm; those determined for phosphorus range from 10 to 40 ppm. For any given phosphine ligand, the gallium nuclei are most shielded for X=I and least shielded for X=Cl, a trend previously observed for In^III–phosphine complexes. This experimental trend, attributed to spin‐orbit effects of the halogen ligands, is reproduced by DFT calculations. The signs of C_Q(^69/71Ga) for some of the adducts were determined from the analysis of the ³¹P NMR spectra acquired with magic angle spinning (MAS). The ¹J(^69/71Ga,³¹P) and ΔJ(^69/71Ga, ³¹P) values, as well as their signs, were also determined; values of ¹J(⁷¹Ga,³¹P) range from approximately 380 to 1590 Hz. Values of ¹J(^69/71Ga,³¹P) and ΔJ(^69/71Ga,³¹P) calculated by using DFT have comparable magnitudes and generally reproduce experimental trends. Both the Fermi‐contact and spin‐dipolar Fermi‐contact mechanisms make important contributions to the ¹J(^69/71Ga,³¹P) tensors. The ³¹P NMR spectra of several adducts in solution, obtained as a function of temperature, are contrasted with those obtained in the solid state. Finally, to complement the analysis of NMR spectra for these adducts, single‐crystal X‐ray diffraction data for Br₃Ga[P(p‐Anis)₃] and I₃Ga[P(p‐Anis)₃] were obtained.     

Structure of Modified Polytetrafluoroethylene According to DFT Calculations and 19F NMR Spectroscopy

The molecular structure of n-C₇F₁₆ and the ¹⁹F nuclear magnetic screening tensors are calculated by density functional theory (DFT) methods. The results of calculations are compared with ¹⁹F NMR data, and it is shown that fine polytetrafluoroethylene (PTFE) contains the terminal CF₃ groups in its structure.     

Predicting 195Pt NMR Chemical Shifts in Water-Soluble Inorganic/Organometallic Complexes with a Fast and Simple Protocol Combining Semiempirical Modeling and Machine Learning

Water-soluble Pt complexes are the key components in medicinal chemistry and catalysis. The well-known cisplatin family of anticancer drugs and industrial hydrosylilation catalysts are two leading examples. On the molecular level, the activity mechanisms of such complexes mostly involve changes in the Pt coordination sphere. Using ¹⁹⁵Pt NMR spectroscopy for operando monitoring would be a valuable tool for uncovering the activity mechanisms; however, reliable approaches for the rapid correlation of Pt complex structure with ¹⁹⁵Pt chemical shifts are very challenging and not available for everyday research practice. While NMR shielding is a response property, molecular 3D structure determines NMR spectra, as widely known, which allows us to build up 3D structure to ¹⁹⁵Pt chemical shift correlations. Accordingly, we present a new workflow for the determination of lowest-energy configurational/conformational isomers based on the GFN2-xTB semiempirical method and prediction of corresponding chemical shifts with a Machine Learning (ML) model tuned for Pt complexes. The workflow was designed for the prediction of ¹⁹⁵Pt chemical shifts of water-soluble Pt(II) and Pt(IV) anionic, neutral, and cationic complexes with halide, NO₂⁻, (di)amino, and (di)carboxylate ligands with chemical shift values ranging from −6293 to 7090 ppm. The model offered an accuracy (normalized root-mean-square deviation/RMSD) of 1.08 %/145.02 ppm on the held-out test set.     

NMR spectra of salicylohydroxamic acid in DMSO-d6 solution: a DFT study

The ¹H, ¹³C and ¹H, ¹³C COSY NMR spectra of salicylohydroxamic acid (sha) were measured in DMSO-d₆ solution. The B3LYP GIAO method with the 6-311++G(d,p) basis set was chosen to reproduce the experimental spectra. All possible zusammen and entgegen conformers of monomeric sha were computed. After geometry optimisation (B3LYP/6-311++G(d,p)) only nine independent models of the molecule were shown to be stable. Additionally, the NMR chemical shifts of the Onsager model of the most stable monomer were calculated. The computed chemical shifts for the labile protons for all aforementioned geometries meaningfully underestimated experimental results suggesting the existence of the H-bonded structure of sha in DMSO solution. The most probable two dimeric structures along with two solvent-bounded aggregates were subsequently calculated at the same level of theory. The best agreement was obtained for sha H-bonded with two DMSO molecules (confirmed by the absence of concentration effect). The relative error not exceeding 10 and 4% for chemical shifts in ¹H and ¹³C NMR spectra of sha–(DMSO)₂, respectively, showed that the applied method with the B3LYP/6-311++G(d,p) basis set was efficient to predict the NMR shifts of a compound with strong H-bonds. Thus, this allows to assign properly NMR resonances to specific structure formed in DMSO solution.     

Atomic Contributions from Spin‐Orbit Coupling to 29Si NMR Chemical Shifts in Metallasilatrane Complexes

New members of a novel class of metallasilatrane complexes [X‐Si‐(μ‐mt)₄‐M‐Y], with M=Ni, Pd, Pt, X=F, Cl, Y=Cl, Br, I, and mt=2‐mercapto‐1‐methylimidazolide, have been synthesized and characterized structurally by X‐ray diffraction and by ²⁹Si solid‐state NMR. Spin‐orbit (SO) effects on the ²⁹Si chemical shifts induced by the metal, by the sulfur atoms in the ligand, and by heavy halide ligands Y=Cl, Br, I were investigated with the help of relativistic density functional calculations. Operators used in the calculations were constructed such that SO coupling can selectively be switched off for certain atoms. The unexpectedly large SO effects on the ²⁹Si shielding in the Ni complex with X=Y=Cl reported recently originate directly from the Ni atom, not from other moderately heavy atoms in the complex. With respect to Pd, SO effects are amplified for Ni owing to its smaller ligand‐field splitting, despite the smaller nuclear charge. In the X=Cl, Y=Cl, Br, I series of complexes the Y ligand strongly modulates the ²⁹Si shift by amplifying or suppressing the metal SO effects. The pronounced delocalization of the partially covalent M←Y bond plays an important role in modulating the ²⁹Si shielding. We also demonstrate an influence from the X ligand on the ²⁹Si SO shielding contributions originating at Y. The NMR spectra for [X‐Si‐(μ‐mt)₄‐M‐Y] must be interpreted mainly based on electronic and relativistic effects, rather than structural differences between the complexes. The results highlight the sometimes unintuitive role of SO coupling in NMR spectra of complexes containing heavy atoms.