13C Nuclear Magnetic Resonance Shifts Research Articles

On simple, accurate calculations of atomic charges, bond properties and molecular energies. V. Conformational features of alkylcyclohexanes and related polycyclic hydrocarbons

Atomic charges of selected alkylcyclohexanes and related polycyclic molecules are calculated with the help of a simple approximation due to Del Re. The carbon charges duplicate those inferred from 13C nuclear magnetic resonance shifts within 0.30×10 −3 electron (root-mean-square deviation). The bond-energy formulas which depend explicitly on the charges of the bond-forming atoms, and proper consideration of butane– gauche interactions enable the calculation of the energy of atomization, Δ E a * , of molecules at their potential minimum. A convenient approximation for zero-point plus heat-content energies, supported by extended basis set Hartree–Fock and density functional calculations, simplifies the evaluation of standard enthalpies of formation, well within the uncertainties of the experimental data used for comparison.

Solid-State NMR, Crystallographic and Density Functional Theory Investigation of Fe−CO and Fe−CO Analogue Metalloporphyrins and Metalloproteins

We have synthesized and characterized the following four metalloporphyrins: Fe(OEP)(CO)(1-MeIm), Ru(OEP)(CO)(1-MeIm), Os(OEP)(CO)(1-MeIm), and Fe(TPP)(iPrNC)(1-MeIm), where OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinate, TPP = 5,10,15,20-tetraphenylporphyrinate, and 1-MeIm = 1-methylimidazole, using single-crystal X-ray diffraction, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT) methods. Unlike the situation found with the Fe-, Ru-, Os(TPP)(CO)(1-MeIm) analogues, which have ruffled porphyrins, all four systems here have essentially planar porphyrin rings, and a rule is developed that successfully predicts the presence or absence of ring distortion in a broad range of metalloporphyrins. In each of the three CO complexes, the M−C−O bond is close to linear and untilted, but with the iPrNC adduct, there are noticeable ligand distortions supporting the idea that RNC groups (but not CO) may be distorted in metalloproteins. Solid-state 13C, 15N, and 17O NMR shifts and shift tensors determined experimentally are in generally good agreement with those computed via DFT. For isocyanide binding to proteins, the experimental shifts are more deshielded than in the model system, and the effects which might contribute to this difference are explored theoretically. Unlike CO, electrostatic field effects are unlikely to make a major contribution to protein shielding. Neither are Fe−C−N tilt−bend distortions, although a bend at nitrogen is energetically feasible and also gives a large deshielding, as seen with proteins.

Calculation of DFT-GIAO NMR shifts with the inclusion of spin-orbit coupling

A formulation for the calculation of nuclear magnetic resonance (NMR) shielding tensors, based on density functional theory (DFT), is presented. Scalar-relativistic and spin-orbit coupling effects are taken into account, and a Fermi-contact term is included in the NMR shielding tensor expression. Gauge-including atomic orbitals (GIAO) and a frozen-core approximation are used. This formulation has been implemented, and H1 and C13 NMR shifts of hydrogen and methyl halides have been calculated and show good agreement with experiment. C13 NMR shifts of 5d transition metal carbonyls have been calculated and show improved agreement with experiment over previous scalar-relativistic calculations. For the metal carbonyls it is shown explicitly that the combination of spin-orbit coupling and magnetic field mixes spin triplet states into the ground state, inducing a spin density that then interacts with the nuclei of the metal carbonyl via the Fermi-contact term. Results indicate that the Fermi-contact contribution to the C13 NMR of the metal carbonyl ions increases with increasing oxidation state of the ion. It is reasoned that as the oxidation state increases, π back bonding decreases and σ bonding increases, within the metal–carbon bond, thus facilitating a greater transfer of spin density from the metal to the carbon nucleus, and thus increasing the Fermi-contact contribution to the NMR shielding of the carbon.

Theory of nuclear magnetic resonance in graphite and graphite intercalation compounds

The calculated results of 13C nuclear magnetic resonance shifts in graphite are presented. First the formulation of calculating the chemical shifts and Knight shifts for metallic energy bands systems developed by our group is reviewed. Then the shielding tensor is calculated based on this formalism with use of energy bands calculated by a numerical basis set LCAO Xα method. The calculated results are compared with a previous calculation of NMR shifts for graphite intercalation compounds. We also suggest a new kind of contribution to the NMR shift from the large Landau diamgnetism in graphite.

ChemInform Abstract: Shielding Contributions to 1H and 13C Nuclear Magnetic Resonance Shifts in Cyclohexane, Methylcyclohexane, and n‐Butane.

Chemischer InformationsdienstVolume 17, Issue 17 Physical Organic Chemistry ChemInform Abstract: Shielding Contributions to 1H and 13C Nuclear Magnetic Resonance Shifts in Cyclohexane, Methylcyclohexane, and n-Butane. H.-J. SCHNEIDER, H.-J. SCHNEIDERSearch for more papers by this authorG. SCHMIDT, G. SCHMIDTSearch for more papers by this author H.-J. SCHNEIDER, H.-J. SCHNEIDERSearch for more papers by this authorG. SCHMIDT, G. SCHMIDTSearch for more papers by this author First published: April 29, 1986 https://doi.org/10.1002/chin.198617053AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume17, Issue17April 29, 1986 RelatedInformation

An extension of the Beierbeck and Saunders parameters for the semiempirical calculation of the 13C nuclear magnetic resonance chemical shifts: the gauche-γ(X) effect in epoxides

An additional substituent parameter, the gauche–γ(X), was derived and a correction for the HC parameter of Beierbeck and Saunders is proposed; this new parameter would allow the semiempirical calculation of 13C nuclear magnetic resonance shifts of six-membered ring epoxides and, consequently, their stereochemistry.

Charge distributions and chemical effects. XL. Chemical bonds in benzenoid hydrocarbons

Benzenoid hydrocarbons were examined using a bond energy scheme featuring the role of atomic charges. The latter were conveniently deduced from appropriate correlations between theoretical results and 13C nuclear magnetic resonance shifts. Atomization energies calculated in this manner agree with their experimental counterparts to within 0.36 kcal mol-1 (average deviation). It appears that benzenoid hydrocarbons can be efficiently described in terms of local charge density properties. In the absence of any distinctive specific feature characterizing benzenoids, this particular description of chemical bonds ultimately results in a unifying genealogy smoothly relating to one another the various possible types of CC and CH bonds which are formed by sp2 and sp3 carbons.

Shielding contributions to 1H and 13C nuclear magnetic resonance shifts in cyclohexane, methylcyclohexane, and n-butane

Anisotropy effects (σan), linear and square electric field contributions (σE, σE2), and sterically induced polarizations (σst) are evaluated on the basis of MM2-generated molecular structures using a computer program (SHIFT) which allows the point of action of inducing and polarized bonds to be varied, and the number of interacting atoms or bonds to be selected. The linear electric field effect (σE) can be excluded as the source of the equatorial–axial1H shift difference, and ofthe methyl substituent effects, similarly σan for the methyl effect, and very likely for the equatorial–axial difference. Steric forces F(σst) describe distant shielding effects for 13C with a sensitivity of kc≅ 1 p.p.m. µdyn–1, and for 1H with kH; ≅ 0.05 p.p.m. µdyn–1 with the correct order of magnitude, including the expected 13C shielding in n-butane rotamers. In conclusion, the application of all now available computational options without arbitrary limitations shows that only a few limiting shielding mechanisms can be excluded, and that a parametrization of screening factors on the basis of hydrocarbon n.m.r. data alone is not feasible.

Van der Waals' induced 13C NMR shifts in crystalline amino acids and peptides.

Recent reports in the literature of solid-state 13C nuclear magnetic resonance (NMR) spectra of crystalline L-alanine [Naito, A., Ganapathy, S., Akasaka, K., and McDonnell, C. A. (1981) J. Chem. Phys. 74, 3190-3197] and L-leucine [Frey, M. H. and Opella, S. J. (1980) J. Chem. Soc. Chem. Commun., 474-475], recorded with cross-polarization and magic-angle spinning (CP-MAS), show downfield resonance shifts of several parts per million in their side-chain methyl groups, relative to their resonance positions in aqueous solution. Similar findings are reported here for crystalline aliphatic amino acids and L-alanine peptides, including tetra(L-alanine), which show similar, specific downfield shifts in their side-chain methyl resonances. Coupled with X-ray crystallographic data of these compounds, and previous gas and solution-phase 13C NMR studies, the CP-MAS 13C NMR data indicate that these downfield shifts are a result of van der Waals' interactions. This group have reported similar van der Waals' induced shifts of the same magnitude for 13C resonances of the side-chain methyl groups of 13C-enriched tetra(L-alanine) upon binding to high-affinity Fab' fragments of heterogeneous sheep anti-[poly(L-alanine)] antibodies in aqueous solution [Geller, S., Wei, S. C., Shkuda, G. K., Marcus, D. M., and Brewer, C. F. (1980) Biochemistry 19, 3614-3623]. The above findings show that van der Waals' induced 13C NMR shifts of similar magnitudes can be detected in specific antibody-hapten complexes and the side chains of crystalline aliphatic amino acids and peptides. The results also indicate that water possesses relatively little attractive van der Waals' interactions with aliphatic molecules.

Solvent effects on 13C nuclear magnetic resonance shifts of polar aliphatic compounds. A charge separation model

A model is proposed which explains the large solvent effects on the shielding of carbon atoms bearing polar bonds (ΔCα) with an increased charge separation in solvents of a higher dielectric constant. Using cis-1-iodo-1-methyl-4-t-butylcyclohexane and t-butyl chloride as conformationally homogeneous solutes, one obtains for a series of solvents a linear correlation (r > 0.95) for ΔCα in the expected range of 415 p.p.m./electron charge unit and 320 p.p.m./e, respectively. In a second set of experiments, the sensitivity of ΔCα against solvent changes for differing Cα–X bonds is obtained, again yielding a linear correlation (r > 0.99) with 380 p.p.m./e. In addition, a ΔCα enhancement with increasing carbon substitution at Cα is found for several alkyl iodides and rationalized in terms of an increasing contribution of the charged form to the resonance hydride.

13 C nuclear magnetic resonance study of five- and six-coordinated carbon in nonclassical organometallic compounds: Dimeric trialkyl-, tricyclopropyl-, and triarylaluminums and some nido and closo carboranes

A (13)C nuclear magnetic resonance spectroscopic study of dimeric trimethyl-, triethyl-, tricyclopropyl-, and triarylaluminums is reported. The five-coordinated bridging carbons are found consistently more shielded than the terminal carbons, in accordance with the increased p-character of the former. The nature of bridging two-electron three-centered Al-C-Al bonds is discussed. (13)C nuclear magnetic resonance shifts of several nido and closo carboranes containing five and six coordinated carbons and their (13)C-(1)H spin-spin coupling constants were also obtained. The relationship between the carbon chemical shifts and coordination number of the carbon atom is discussed. There is approximately a 20- to 40-ppm shielding of the (13)C chemical shifts of five- and six-coordinated carbons, compared with those of four valent carbons, with a simultaneous general increase of J(C-H) coupling constants.

Contact term contribution to lanthanide-induced13C nuclear magnetic resonance shifts in pyridine and β-picoline

Download options Please wait... Article information DOI https://doi.org/10.1039/C39720001343 Article type Paper Download Citation J. Chem. Soc., Chem. Commun., 1972, 1343-1344 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Contact term contribution to lanthanide-induced 13C nuclear magnetic resonance shifts in pyridine and β-picoline M. Hirayama, E. Edagawa and Y. Hanyu, J. Chem. Soc., Chem. Commun., 1972, 1343 DOI: 10.1039/C39720001343 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Masatoshi Hirayama Etsuo Edagawa Yoshiaki Hanyu