Carbon Shieldings Research Articles

Natural chemical shielding analysis of nuclear magnetic resonance shielding tensors from gauge-including atomic orbital calculations

Nuclear magnetic shielding tensors computed by the gauge including atomic orbital (GIAO) method in the Hartree–Fock self-consistent-field (HF-SCF) framework are partitioned into magnetic contributions from chemical bonds and lone pairs by means of natural chemical shielding (NCS) analysis, an extension of natural bond orbital (NBO) analysis. NCS analysis complements the description provided by alternative localized orbital methods by directly calculating chemical shieldings due to delocalized features in the electronic structure, such as bond conjugation and hyperconjugation. Examples of NCS tensor decomposition are reported for CH4, CO, and H2CO, for which a graphical mnemonic due to Cornwell is used to illustrate the effect of hyperconjugative delocalization on the carbon shielding.

Order parameters and carbon shielding tensors of bis-MSB from 13C NMR measurements in a nematic liquid crystal

Abstract The order and biaxiality of the fluorescent molecule p -bis(0-methylstyryl)benzene (bis-MSB) in the nematic phase of the liquid crystal ZLI-1167 have been determined as a function of temperature by means of a recently developed 13 C NMR approach. The method can also provide information on the carbon chemical shift tensors and we demonstrate that it can be readily applied to molecules containing a fairly large number of chemically different carbons, even if the assignment of the shifts in the isotropic phase is not available.

Ab Initio Study of 13C Shieldings for Linear π-Conjugated Systems. Theoretical Determination of the C12−C13 Conformation in the Chromophore of Rhodopsin

The ab initio shielding calculations are carried out to investigate the conformation dependence of 13C chemical shifts for conjugated compounds like the chromophore of a visual pigment rhodopsin (Rh). First, the calculations are applied systematically to 10 diene derivatives to obtain basic and universal relationships between their conformation and the shieldings of unsaturated carbons. It is indicated that the conjugated carbons are classified into two types according to the profiles of conformation dependence of the shieldings. The shieldings of the carbons forming the rotating bond exhibit complicated angular dependence. It is rigorously evidenced that the behavior of such carbon shieldings can be understood by considering the effect of π-orbital modification, a new concept introduced here. On the other hand, the shieldings of the other carbons essentially follow well-known mechanisms including the steric and charge density effects. One of the most important findings is that the steric effects are refl...

13C−13C Spin−Spin Coupling Tensors in Benzene As Determined Experimentally by Liquid Crystal NMR and Theoretically by ab Initio Calculations

This study reports experimentally and theoretically (ab initio) determined indirect CC spin−spin coupling tensors nJCC in benzene. The CC spin−spin coupling constants nJCC between the ortho, meta, and para (n = 1, 2, and 3) positioned carbons were experimentally determined in two ways: firstly by utilizing the 2H/1H isotope effect on the carbon shieldings in neat monodeuteriobenzene and recording the 13C satellite spectrum in a 1H-decoupled 13C NMR spectrum, and secondly by recording the 13C NMR spectrum of fully 13C-enriched benzene (13C6H6) and carrying out its complete analysis. The anisotropies of the corresponding coupling tensors, ΔnJCC, were resolved experimentally by liquid crystal 1H and 13C NMR using dipolar couplings corrected for both harmonic vibrations and deformations. The results obtained in three thermotropic liquid crystal solvents are in good mutual agreement, indicating the reliability of the determinations. The anisotropy of the ortho, meta, and para CC indirect couplings are ca. +17...

What Do Ab Initio Calculations Predict for the Structure of Lithiated Azaenolates of Peptides?

AbstractStructures and conformations of the azaenolate lithium salts of amides (formamide, acetamide, and N‐methylacetamide) and of the dipeptide model N‐formylalaninamide were investigated by means of ab initio MO theory. Four possible structures of the lithiated C‐enolates of acetamide were also included in the study. All structures were calculated at the HF/6‐31+G(d) and MP2(fc)/6‐31 + G(d)/HF/6‐31 + G(d) levels; the lithiated azaenolates of formamide were also investigated at higher theoretical levels (up to MP4(fc)/6‐311 + G(d,p)/MP2(fc)/6‐311 + G(d,p)). For the lithiated azaenolates of all amides investigated, the most stable structure contains a four‐membered ring in which the lithium ion is complexed by the oxygen and nitrogen atoms; the substituents attached to the carbon and nitrogen atoms of the azaenolate are in a cis arrangement. The lithiated azaenolates of acetamide are predicted to be more stable than the corresponding C‐enolates. To simulate solvation, calculations on complexes of the lithiated azaenolates of formamide with up to three molecules dimethyl ether were also performed, and all azaenolates of amides were also reoptimized by ab initio reaction‐field calculations. Both solvation models reduce the preference for lithium‐chelated cis structures. The Ramachandran maps of the dilithiated bis(azaenolate) of N‐formylalaninamide (having cis or trans arrangements of the azaenolate substituents) were scanned by MNDO calculations for conformational accessible regions. Thirteen stable structures were subsequently optimized at the HF/6‐31 + G(d) ab initio level. The global minimum resembles a peptide in C7 conformation, but other conformations, not known for peptides, are close in energy. The structures of dimers of the lithiated azaenolates of N‐methylacetamide and of glycinaldehyde were also calculated. The NMR chemical shielding of carbon, nitrogen, and oxygen atoms in all structures were predicted ab initio by using the gauge‐including atomic orbital (GIAO) method.

Order Parameters and Carbon Shielding Tensors of Some Anthracene Derivatives from 13C NMR Experiments

The orientational order parameters and approximate 13C shielding tensors of anthracene and of six of its 9,10-derivatives, 9,10-dihydroanthracene, 9,10-dimethylanthracene, 9,10-dibromoanthracene, anthraquinone, 9,10-diphenylanthracene, and 9,10-bis(phenylethynyl)anthracene, dissolved in the nematic liquid crystal ZLI-1167 have been determined using a recently proposed 13C NMR method. The biaxial order parameters obtained are compared with some current theoretical models of ordering. We find that none of these mean field approaches have fully satisfactory predictive capabilities.

Configurational Assignments of 5-Norbornene-2,3-dicarboximide and N-Phenyl-5-norbornene-2,3-dicarboximide by One- and Two- Dimensional NMR Spectroscopy

Abstract The H, 13C, and two-dimensional both homonuclear (H, H-Cosy) and heteronuclear (H, C-Cosy) correlated NMR spectra of endo- and exo-isomers of 5-norbornene-2,3- dicarboximide and N-phenyl-5-norbornene-2,3- dicarboximide have been examined. Two-dimensional spectra gave evidence for configurations based on long-range couplings. 13C resonances of carbons 1,4 and carbons 2,3 were ultimately assigned by means of gated decoupled spectra from which the coupling constants (JC, H) were determined. The literature assignments of the H NMR spectrum of N-phenyl-5-norbornene- endo-2,3-dicarboximide have been revised. The spectra of the imides were compared with the spectra of endo- and exo-isomers of 5-norbornene-2,3-dicarboxylic anhydride in order to examine the stereochemical effects of imide ring on the proton and carbon shieldings in 5-norbornene skeleton. When the imide ring is at endo-position of norbornene skeleton the protons of carbons 2 and 3 resonate at higher field than the protons of carbons 1 and ...

1H and 13C NMR studies on 5,11-dimethyl-5 H-indolo[2,3-b]quinoline and some of its derivatives

The 1H and 13C NMR spectra of the parent 5-methyl-5 H-indolo[2,3 b-quinoline] and 5,11-dimethyl-5 H-indolo[2,3 b]quinoline and nine various methyl, methoxy and fluoro derivatives of the latter have been measured and analysed by the use of COSY, HETCOR, COLOC and selective decoupling experiments. This gave unambiguous assignment of all signals in the spectra studied; proton-proton coupling constants across three, four and five bonds have also been determined. The most interesting result found concerns a very large increase in the shielding of carbon 1 upon introduction of a methyl group at C11. This effect is dramatically enhanced by subsequent substitution with a methoxy group at C2.

GIAO-CHF calculations of NMR chemical shifts for some N, N-dimethylacetamide derivatives using optimized geometries

NMR chemical shifts and shift anisotropies for hydrogen, carbon, nitrogen, oxygen, fluorine and chlorine atoms in four C-substituted N, N-dimethylamides ( XCON( CH 3) 2 where X = H, CH 3, F or Cl) have been calculated using ab initio GIAO-CHF method and optimized molecular structures. The molecular geometries for the studied molecules were obtained in the Berny optimization process implemented in the gaussian 92 code. In all the amides studied, two non-equivalent methyl groups were present due to hindered rotation about the CN bond. An activation energy of 18.2 kcal mor −1 was theoretically predicted for the internal rotation about the CN bond in N, N-dimethylformamide (DMF, HCON(CH 3) 2). It was found that the difference in hydrogen and carbon shielding constants of the methyl groups can be calculated with satisfactory precision. For dimethylcarbamyl fluoride (FCON(CH 3) 2) the ab initio calculations suggest that the 13C shielding of the cis-methyl group is lower than for the trans-methyl group, in contrast to the other derivatives ( X = H, CH 3 and Cl).

37Cl/ 35Cl isotope effects on the carbon and proton shielding in CH 3Cl: measurement and theory

A model is proposed to explain the small isotope shifts in the 1H resonance (≈ 0.2 ppb) and in the 13C resonance (≈ 6 ppb) of CH 3Cl when 35Cl is replaced by 37Cl. These shifts are related to the slight changes in the CCl and CH bond lengths and the HCCl interbond angles brought about by the isotopic substitution and to the derivatives of the shielding with respect to these parameters. The changes in geometry calculated from a quadratic and cubic force field are Δ r(CCl) = −0.000041 Å, Δ r(CH) = 0.000001 Å and Δθ(HCCl) = 0.00063° at 300 K when 37Cl replaces 35Cl. The derivatives are obtained from a large basis set calculation at the SCF level. Excellent agreement between calculated and experimental results is obtained. In the case of the 13C isotope effect, changes in the CCl bond length and the mean square of the CCl bond lengths are sufficient to account for the shift. For the proton shift the contributions from the slight changes in the CH bond length and HCCl angles, though they tend to cancel, are individually larger than experimental error and cannot be neglected even though the changes in the CCl bond length are again dominant.

Electric field effects on the carbon-13 nuclear magnetic shielding in several organic molecules

In a uniform electric field changes take place in the tensor components of nuclear shielding. For fields of small magnitude, a Taylor series expansion of each component in terms of the field can be defined. In the present work the coefficients of such expansions—sometimes called shielding polarizabilities and hyperpolarizabilities—have been calculated for the carbon nuclei in the molecules CH4, C2H6, C2H4, C2H2, HCN, CH3F, CH3CN and H2CO. The calculations have been carried out at the SCF level with large basis set wavefunctions in which derivative orbitals are added to basis sets suggested by Sadlej. The results show that the paramagnetic part of the carbon shielding is usually far more sensitive to an electric field than the diamagnetic part. The coefficients depend very strongly on the chemical bonding of the carbon atom containing the nucleus of interest.

Intermolecular and intramolecular effects on the 1H and 13C shielding in some gaseous hydrocarbons at various temperatures—experimental results

AbstractThe proton and 13C shielding constants in CH4, C2H6, C3H8, and some other gaseous hydrocarbons have been studied as functions of density at temperatures in the range 180–370 K. The linear coefficients describing the density dependence of the shielding, after correcting for bulk susceptibility, increase substantially as the temperature is reduced, indicating stronger intermolecular interactions. (Some of the required magnetic susceptibilities were determined in this work by an NMR method). The 13C measurements for CH4 are close to those of an earlier study; the results for the other gases are new. The linear coefficient is substantially greater for the carbon shielding of the methyl group in propane than for the methylene group at any temperature, but there is virtually no distinction between the linear coefficients for the proton shielding in this gas. Values for do0/dT, the temperature dependence of the shielding extrapolated to zero density, are also presented for both proton and 13C shielding in the hydrocarbons. They are positive and negative in different instances. It is shown from this and earlier gas‐phase studies that standard literature values of the methane 13C shielding relative to the 13 shielding in the other hydrocarbons are in error.

Magnetic properties of C 60 and C 70

The published 13C NMR spectra of C 60 give an absolute carbon shielding of σ = 43 ppm, i.e. 14 ppm less than for carbon in benzene. Extrapolation from basis ab initio coupled Hartree—Fock calculations with the origin of the vector potential at the nucleus gives σ=46 ppm as the basis-set limit. Minimal ab initio calculations on C 70 predict greatest diamagnetic shielding at the equator and least near the poles of this spheroidal cluster, in good agreement with the experimental assignment of the 13C NMR spectrum.

Substituent effects in various alkyl derivatives of 9aH-quinolizine-1,2,3,4-tetra-carboxylate studied by13C NMR spectroscopy and X-ray analysis

The 13C NMR spectra of 22 alkyl-substituted 9aH-quinolizine-1,2,3,4-tetracarboxylates have been obtained and X-ray analyses have been performed for three of them. The chemical shift differences between the parent 9aH-quinolizine and the methyl-substituted compounds can only be interpreted in terms of the usual α and β effects for 8-methyl-9aH-quinolizine. 6-, 7-, 9-, and 9a-methyl substituents cause not only a very large deshielding of the carbon at the position of substitution together with shielding changes at adjacent atoms, but also influence the shieldings of the other carbons in both rings of the compounds under study. The observed changes are interpreted in terms of steric hindrance between the methyl groups of ring B and the ester groups of ring A, and hyperconjugative effects introduced by the methyl groups.

High-resolution 1H, 2H, and 13C solid-state NMR of dimethylmalonic acid. Detection of a new mode of hydrogen motion

The proton and carbon shielding and the carboxyl deuteron quadrupole coupling tensors were measured in single crystals of dimethylmalonic acid. For the first time sufficient resolution has been achieved in solid-state multiple-pulse proton NMR to allow tracing out a methyl group proton shielding tensor. Its anisotropy is only 1.6 ppm. At room temperature only half as many carboxyl proton and deuteron resonances are observed as crystal symmetry predicts. This is the consequence of a novel hydrogen exchange process within dimeric units, -COOH-HOOC-. Lineshape analysis of deuteron spectra yields the temperature dependence of the rate k of the exchange process. Its activation energy is 66 kJ/mol; k( T = 281 K) = 5 × 10 4, s −1. A flip of the whole dimeric unit followed by a rapid concerted jump of the hydrogens along strongly asymmetric hydrogen bonds is thought to be the most plausible sequence of events. This hypothesis might be subjected to a test by 17O NMR.

13C NMR chemical shift substituent effects. 4–α‐monosubstituted acetones

Abstract13C NMR chemical shifts for some α‐heterosubstituted acetones were recorded. The carbonyl carbon shieldings are discussed in the light of the substituent electronic effects and were also empirically estimated by Tanaka et al.'s equation. An approximate linear relationship was observed between the 13C and 17O chemical shifts. A nonadditivity of substituent chemical shifts was observed for the α‐methylene carbon, and attributed to intramolecular interactions between the α‐heteroatom and the carbonyl group.

A13C nuclear magnetic resonance study of some linear alkynyl gold(I) and silver(I) complexes

Phenylacetylide gold(I) and silver(I) compounds of the type [n-Bu4N][M(C=CPh)2], (1) and [n-Bu4N][XMC=CPh], (2) have been studied by13C n.m.r. spectroscopy and their chemical shifts are reported for the first time. The shielding of the alkynyl carbon linked directly to the metal in (2) isca. 10–15 ppm less than the analogous carbon in (1) (M=Au), andca. 6–11 ppm less in the silver complexes (M=Ag). The variation in chemical shift depends on the nature of X(X=Cl, Br, I, C2Ph and Ph3P) and indicates greatly different degrees of polarization of the Au-C= (or Ag-C=) bonds in (1) and (2). I.r. spectra of the title compounds are reported and confirm the weakness of ML π back-bonding. Comparison is also made with13C and i.r. data for platinum, and mercury complexes.

Some CNDO/S parameterised calculations of carbon and nitrogen shieldings including two-centre integrals

Some Sum-Over-States shielding calculations for a number of carbon and nitrogen environments are reported. The CNDO/S parameterised calculations in which only one-centre integrals are included yield rather poor agreement with the experimental results. The inclusion of paramagnetic shielding contributions arising from two-centre integrals produces a significant improvement in the relationship between the calculated and observed nuclear shielding data.

Conformational and Spectral Investigations of Thiazolidinone Derivatives by1H and13C NMR Spectroscopy

Abstract The magnetic non-equivalence of isomeric proton and 13C nuclei has been investigated on rotational isomers of 3-aryl-2,4-thiazolidinediones and 3-aryl-2-arylimino-4-thiazolidinones. The detection of magnetic non-equivalence for proton nuclei was better in quinoline solutions. Free energies of activation have been estimated. The varying hetero atom in the hetero ring, in comparision to earlier data1–3, is seen to effect the additive shift parameters and the shielding of distant aryl carbon bonded to hetero ring.

13C NMR shifts and conformations of substituted indans

Abstract13C NMR spectra of indan derivatives bearing substituents in the 1, 2, 5 and 6 positions are reported and assigned by LIS measurements and other techniques. Epimeric indanes bearing vicinal oxygen and phenyl or benzyl substituents show ring carbon shielding in the cis relative to the trans isomers, which is compared with corresponding cyclopentane shifts, and indicates the predominance of envelope conformations with pseudoaxial oxygen substituents for the cis isomers. Acetylation shifts show consistently larger shielding at C‐β for the trans compounds. Introduction of oxygen at C‐5 leads to asymmetric shielding effects at the ortho carbon atoms as soon as there is a substituent in the para position which can participate in mesomeric forms.