Experimental NMR Chemical Shifts Research Articles

103Rh NMR chemical shifts in organometallic complexes: a combined experimental and density functional study.

Experimental 103Rh NMR chemical shifts of mono- and binuclear rhodium(I) complexes containing s- or as-hydroindacenide and indacenediide bridging ligands with different ancillary ligands (1,5-cyclooctadiene, ethylene, carbonyl) are presented. A protocol, based on density functional theory calculations, was established to determine 103Rh NMR shielding constants in order to rationalise the effects of electronic and structural variations on the spectroscopic signal, and to gain insight into the efficiency of this computational method when applied to organometallic systems. Scalar and spin-orbit relativistic effects based on the ZORA (zeroth order regular approximation) level have been taken into account and discussed. A good agreement was found for model compounds over a wide range of chemical shifts of rhodium (approximately 10,000 ppm). This allowed us to discuss the experimental and calculated delta(103Rh) in larger complexes and to relate it to their electronic structure.

17O NMR investigation of rigid polycyclic systems: experimental and calculated chemical shifts

In this work we describe a systematic and comparative study of the 17O NMR and related parameters of 17 polycyclic compounds: bicyclic, tricyclic, tetracyclic ( endo–endo and endo–exo fusion), pentacyclic compounds (ketone; endo and exo-alcohols), hexacyclic alcohol. Concentration, temperature and solvent effects on the 17O NMR chemical shifts and the relaxation rate are also described. We compare all experimental 17O NMR chemical shifts with chemical shifts calculated from gauge-independent atomic orbitals within density functional theory (GIAO-DFT). Correlations between calculated and experimental 17O NMR chemical shifts for respective functional groups were carried out and linear equations for chemical shift predictions were generated.

A Dozen Carborane Structures Refined by Comparison of Quantum Chemical Calculated and Experimental NMR Chemical Shift Values

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ab initio calculation of the anisotropic/ring current effects of amino acid residues to locate the position of substrates in the binding site of enzymes

The ring current effects of aromatic moieties and the anisotropic effects of the CO and C–X (X=C, N, S) bonds and of the NHC(NH 2)–NH– moiety in the side chains of amino acid residues of proteins were ab initio calculated based on nuclear independent chemical shieldings as employed by P.v.R. Schleyer. Hereby, quantitative information about the spatial extension, sign and scope of the corresponding ring current/anisotropic effects was obtained and they were visualized as iso-chemical-shielding-surfaces. Examining this quantitative information compared with experimental NMR chemical shifts, the role of the corresponding amino acid residues in binding substrates in the binding site of enzymes was studied.

Application of quantum chemical calculations of 13C NMR chemical shifts to quinoxaline structure determination

Comparison of experimental and theoretical (GIAO DFT) 13C NMR chemical shifts allows the reliable assignment of isomeric structures of heteroaromatic compounds. This methodology was applied to establish the structures of isomeric quinoxalines. A modern 1D NOE technique permitted independent proof of the proposed structures.

Theoretical and experimental NMR chemical shifts of norsanguinarine and norchelerythrine

Norchelerythrine and norsanguinarine, tertiary benzo[ c]phenanthridine alkaloids, were examined by gradient-selected 2D NMR spectroscopy and the later also by extensive theoretical calculations. 1H, 13C and 15N chemical shifts assignments of the title isoquinoline alkaloids based on NOE and multiple-bond chemical-shift correlation experiments (GSQMBC) are reported. Various methods were used for the NMR chemical shifts calculations. Molecular mechanics (MM3 forcefield), AM1 method and Ab initio methods were used for optimizing the geometry. Chemical shielding constants were computed by density functional theory, GIAO and IGLO approaches were used. Chemical shifts calculated by all methods display good qualitative agreement with experimentally determined values. The best overall agreement was achieved when geometry was optimized by RHF/6-31G** method and chemical shielding constants were calculated by B3LYP/6-311G** method, GIAO approach.

Hydrogen-bonded rotamers of 2′,4′,6′-trihydroxy-3′-formyldihydrochalcone, an intermediate in the synthesis of a dihydrochalcone from Leptospermum recurvum

Synthesis of 2′,4′,6′-trihydroxy-3′-methyldihydrochalcone, isolated as a natural product for the first time ( ex Leptospermum recurvum), proceeds through 2′,4′,6′-trihydroxy-3′-formyldihydrochalcone. Two stable rotamers of this formyl derivative have been identified and the problems associated with NMR assignments of this type of compound have been attributed to conformational exchange. Rotamer ratios from molecular modelling and ab initio calculations agree well with those obtained from low temperature NMR studies. There is also excellent correlation between experimental NMR chemical shifts for the hydrogen-bonded hydroxyl protons with those derived from ab initio calculations. This formyl dihydrochalcone showed promising bioactivity in antiviral and antimicrobial assays.

Comparison of calculated DFT/B3LYP and experimental 13C and 17O NMR chemical shifts, ab initio HF/6-31G* optimised structures, and single crystal X-ray structures of some substituted methyl 5β-cholan-24-oates

Single crystal X-ray structures (monoclinic space group P2 1 ) for methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate have been solved and compared with HF/6-31G* optimised structures. In the crystalline packings the side chains are connected with weak OC(sp 3) H⋯ O-type of interactions between C25– H and C24– O–C25 and the keto ends with weak C(sp 3) H⋯ OC-type of interactions between C4– H and OC3. The orientations of the side chains, which steric configurations are of great importance to the biological activity of the molecules, are compared with the experimental structure of methyl 3α-hydroxy-5β-cholan-24-oate. Probable reasons for the observed differences are discussed. In addition, 13C and 17O NMR chemical shifts of methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate as well as the epimeric methyl 3α-hydroxy-5β-cholan-24-oate and methyl 3β-hydroxy-5β-cholan-24-oate have been calculated (DFT/B3LYP/6-311G*) and compared with the experimental values by linear regression analyses. In general, the correspondence between the theoretical and experimental parameters is good or excellent.

Three-dimensional structure determination of N-(p-Tolyl)-dodecylsulfonamide from powder diffraction data and validation of structure using solid-state NMR spectroscopy.

The three-dimensional structure, conformation, and packing of molecules in the solid state are crucial components used in the optimization of many technologically useful materials properties. Single-crystal X-ray diffraction is the traditional and most effective method of determining 3-D structures in the solid state. Obtaining single crystals that are sufficiently large and free of imperfections is often laborious, time-consuming, and, occasionally, impossible. The feasibility of an integrated approach to the determination and verification of a complete three-dimensional structure for a medium-sized organic molecule without using single crystals is demonstrated for the case of an organic stabilizer compound N-(p-tolyl)-dodecylsulfonamide. The approach uses a combination of powder XRD data, several computational packages involving Monte Carlo simulations and ab initio quantum mechanical calculations, and experimental solid-state NMR chemical shifts. Structure elucidation of N-(p-tolyl)-dodecylsulfonamide revealed that the Bravais lattice is monoclinic, with cell dimensions of a = 38.773 A, b = 5.507 A, c = 9.509 A, and beta = 86.35 degrees, and a space group of P21/c.

Phosphacarborane Chemistry: The Synthesis of the Parent Phosphadicarbaboranesnido-7,8,9-PC2B8H11 and [nido-7,8,9-PC2B8H10]−, and Their 10-Cl Derivatives − Analogs of the Cyclopentadienide Anion

The reaction of the carborane nido-5,6-C2B8H12 (1) with PCl3 in dichloromethane in the presence of a “proton sponge” [PS = 1,8-bis(dimethylamino)naphthalene], followed by hydrolysis of the reaction mixture, resulted in the isolation of the eleven-vertex nido-phosphadicarbaboranes 7,8,9-PC2B8H11 (2) and 10-Cl-7,8,9-PC2B8H10 (10-Cl-2), depending on the ratio of the reactants. Both of these compounds can be deprotonated by PS to give the nido anions [7,8,9-PC2B8H10]− (2−) and [10-Cl-7,8,9-PC2B8H9]− (10-Cl-2−). The molecular geometries of all compounds were optimized by ab initio methods at a correlated level of theory [RMP2(fc)] using the 6-31G* basis set and their correctness was assessed by a comparison of the experimental 11B NMR chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G* method. Moreover, the structure of 10-Cl-2− was determined by an X-ray diffraction analysis. The anionic compounds 2− and 10-Cl-2− are analogs of the Cp (Cp = η5-C5H5−) anion. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Configurations and conformations of sanguinarine and chelerythrine free bases stereoisomers

The configurations and conformations of bimolecular aminoacetals of benzo[ c]phenanthridine alkaloids sanguinarine and chelerythrine were investigated by NMR spectroscopy, quantum chemical calculations, and X-ray analysis. The results of the complete computational conformational analysis, the calculation of the chemical shielding of the considerably populated conformers, the determination of averaged chemical shifts and comparison with experimental NMR chemical shifts observed in solution are reported. Based on these results, the relative configurations at the stereogenic centers of two diastereomers of bis(dihydrosanguinarinyl) ether were determined. The structure of the major diastereomer of bis(dihydrosanguinarinyl) ether was confirmed by X-ray analysis and specified for the solid state.

Multinuclear 1H, 13C and 15N NMR study of some substituted 2-amino-4-nitropyridines and their N-oxides

1H, 13C and 15N NMR chemical shift assignments based on pulsed field gradient selected PFG 1H,X (X= 13C and 15N) HMQC and HMBC experiments are reported for three 4-nitropyridine N-oxides and four 4-nitropyridines. It was found that an ortho effect of a methyl group inhibits the deshielding effect of the 4-nitro group and that this effect and the so-called back donation is influenced by electronegativity and position of substituents in the multisubstituted pyridine N-oxides. The shielding effect of N-oxide group is most pronounced in the 15N NMR chemical shifts of the studied compounds. This effect is further modified by methylamino, methylnitramino, 5- or 3-methyl and 4-nitro groups. Among them the 4-nitro group exerts the highest influence on the shielding effect of the N-oxide functionality. Experimental 1H, 13C and 15N NMR chemical shifts and GIAO/DFT theoretical calculations are consistent with each other and supported by the reactivity on nucleophilic substitution, the UV spectral and the dipole moment data.

ChemInform Abstract: Calculated and Experimental NMR Chemical Shifts of p‐Menthane‐3,9‐diols. A Combination of Molecular Dynamics and Quantum Mechanics to Determine the Structure and the Solvent Effects.

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GIAO/DFT calculated chemical shifts of tautomeric species. 2‐Phenacylpyridines and (Z)‐2‐(2‐hydroxy‐2‐phenylvinyl)pyridines

Abstract1H, 13C and 15N NMR chemical shifts for 28 substituted 2‐phenacylpyridines (ketimine forms) and their enolimine tautomers, (Z)‐2‐(2‐hydroxy‐2‐phenylvinyl)pyridines, were calculated via the GIAO/DFT approach. Among four tested methods at the B3LYP level of theory, the 6–311G, 6–311++G and 6–311G** basis sets gave acceptable result for 13C NMR chemical shifts whereas the 6–311++G** basis set was the minimum needed for reproduction of 15N NMR chemical shifts. Satisfactory reproduction of 13C and 15N NMR chemical shifts for different tautomers revealed that intramolecular hydrogen bonding could be modeled reliably by these calculations when the geometry optimizations were done with the HF/3–21G method. Agreements between theoretical and experimental 13C and 15N NMR chemical shifts and also HF/3–21G and HF/6–31G** optimized structural parameters with those obtained by x‐ray crystallographic measurements suggests that it is not necessary to select too sophisticated and CPU time‐intensive methods for geometry optimizations. Copyright © 2001 John Wiley & Sons, Ltd.

Theoretical Study of [2 + 1] cycloaddition of CO and CS to acetylenes forming cyclopropenones and cyclopropenethiones.

The [2 + 1] cycloadditions of carbon monoxide and carbon monothioxide CX (X = O, S) to acetylenes (R1C triple bond CR2 with R1 = H, OH and R2 = CH3, OH, NH2, C6H5) have been studied at the B3LYP/6-311G(d,p) level. It has been shown that the reaction proceeds in two steps forming first an intermediate having the properties of both a carbene and a zwitterion followed by a ring closure leading to cyclopropenones or cyclopropenethiones. The solvent effect does not play an important role in the course of the cycloaddition. The estimation of the first vertical excitation energies by CIS and TD-B3LYP methods shows that the reactions likely take place in the ground state rather than in an excited state. All the studied cyclopropenones and cyclopropenethiones are aromatic as shown by their NICS values and confirmed by calculated and experimental NMR chemical shifts. Different reactivity criteria including HOMO coefficient, local softness, hardness, polarizability, and NICS are used to predict the site selectivity in all studied cases, and the NICS criterion seems to yield the best results among them.

Calculated and experimental NMR chemical shifts of p-menthane-3,9-diols. A combination of molecular dynamics and quantum mechanics to determine the structure and the solvent effects.

NMR chemical shifts have been experimentally measured and theoretically estimated for all the carbon atoms of (1R,3S,4S,8S)-p-menthane-3,9-diol in chloroform solution. Theoretical estimations were performed using a combination of molecular dynamics simulations and quantum mechanical calculations. Molecular dynamics simulations were used to obtain the most populated conformations of the (1R,3S:4S,8S)-p-menthane-3,9-diol as well as the distribution of the solvent molecules around it. Quantum mechanical calculations of NMR chemical shifts were performed on the most relevant conformations employing the GIAO-DFT formalism. A special emphasis was put in evaluating the effects of the surrounding solvent molecules. For this purpose, supermolecule calculations were performed on complexes constituted by the solute and n chloroform molecules, where n ranges from 3 to 16. An excellent agreement with experimental data has been obtained following this computational strategy.

The structure and dynamics of cationic zirconocene complexes with phenyl coordination †

Density functional computations (BP86/ECP1 and B3LYP/II level) have revealed that the phenyl group in [Zr(η-C5H5)(η-C5H4CR2C6H5)Me]+ (R = H) is coordinated to the Zr atom via one of the phenyl carbon atoms, rather than via an agostic Zr–H contact, as suggested previously. A stationary point with such an agostic interaction is the transition structure for phenyl rotation with a barrier higher than 50 kJ mol−1 (B3LYP/II level), consistent with results from dynamic NMR spectroscopy for a closely related compound (R = Me). The structural assignments are supported by the good accord between the computed (GIAO-B3LYP/II level) and experimental NMR chemical shifts. A second dynamic process with a very similar activation barrier is indicated to involve dissociation of the coordinated phenyl moiety from the Zr atom and inversion at the latter. The same structural motif (i.e. η1-bonded aryl group) is also found for the [Zr(η-C5H4CH2C6H5)2]2+ dication.

Theoretical and experimental 1H, 13C, 15N, and 17O NMR chemical shifts for 5-halogenouracils

Theoretical and experimental 1H, 13C, 15N, and 17O NMR chemical shifts for uracil and 5-halogenouracils were determined and analyzed. The theoretical NMR chemical shifts were calculated based on the HF and MP2 GIAO approach using the 3-21G∗∗, MIDI!, polarized double zeta Hansen–Bouman, and the polarized triple zeta Sadlej basis sets. The solvent effect was checked at the HF/3-21G∗∗ level by surrounding the uracil molecule by seven water molecules. For all the nuclei studied in the series of halogenouracils, linear correlation was tested between the chemical shift (both experimental and theoretical) and halogen electronegativity. All the carbon–proton and one-bond carbon–carbon spin–spin coupling constants were measured and used to assign the carbon signals.

An NMR and computational study of a lithium amide and ethereal ligand exchange

The activation parameters for the exchange of an ethereal ligand in a chiral lithium amide was determined from full bandshape analysis of the dynamic NMR spectra. For diethyl ether, the activation parameters were deltaH++=11.0 kcalmol(-1) and deltaS++=12.0 cal K(-1)mol(-1). The exchange of the tetrahydrofuran ligand proceeds with a similar activation enthalpy deltaH++=11.2 kcalmol(-1); however, the entropy is close to zero. deltaS++=1.6 cal K(-1)mol(-1). The dissociation of the ethereal ligand was also modeled by means of semiempirical (PM3) and density functional (DFT) methods. The experimental 13C NMR chemical shifts for the carbons of uncoordinated and coordinated ethers were calculated with the GIAO-DFT (B3PW91/6-31G(d)) computational method.

Semi-synthesis of triterpene A-ring derivatives from oleanolic and maslinic acids. Part II. Theoretical and experimental 13C chemical shifts†

Maslinic acid was obtained from olive-pressing residues, and several derivatives were formed. Rearrangements of 2-tosyloxy-derivatives of methyl maslinate made out by acetolysis. The main product of these rearrangements contained a cyclopentanic A-ring as a result of a concerted 2(3) → 4-abeo rearrangement process. Experimental and theoretical (GIAO, B3LYP/6-31G*//MM+) 13C NMR chemical shifts for 20 compounds are given.