C-13 magnetic relaxation rates and H-1 and C-13 paramagnetic shifts of Ni(II) complex of dopamine

The ¹H and ¹³C isotropic contact shifts and the ¹³C relaxation times of dopamine in aqueous solution have been measured in the presence of the Ni(II) ion. The pD dependence of the ¹H and ¹³C paramagnetic shifts was also investigated. From the analysis of the shifts at pD = 6.5 and from the INDO MO calculations on selected models of dopamine radicals, a dominant σ delocalization mechanism of the spin density is proposed. From the spin distribution on the ligand carbon atoms, the metal centered as well as the ligand centered dipolar contributions of the modified Solomon—Bloembergen equation were calculated and an estimate of the correlation time τ_c was given.     

Carbon-13 NMR studies of some paramagnetic transition-metal acetylacetonates some linewidth considerations

Carbon 13 isotropic shifts and linewidths have been measured for a series of paramagnetic transition-metal acetylacetonates. The results have been compared with the corresponding NMR parameters of the contiguous protons. The relaxation pathway of the ¹³C spins is controlled by hyperfine interactions with the unpaired electron spins and, in cases where the proton linewidth is also dominated by the hyperfine mechanism, good agreement is obtained between the experimental ¹³C/¹H linewidth ratio and that calculated on the basis of the contact shifts. Pseudocontact interactions can be important in determining the total isotropic shift for both the ¹³C and ¹H nuclei and contrary to previous studies, it is suggested that such effects are also important for V(III) and Mn(III) acetylacetonate.     

Anomalous 13C Isotropic NMR Shifts in Tetraalkylammonium Tetrahalometallate(II) Ion Pairs. I. Organic Substituent Effects

¹³C isotropic NMR shifts have been recorded for a number of tetrahalocobaltate(II) salts containing various quaternary ammonium cations in dichloromethane and nitrobenzene solution. The isotropic shifts are a consequence of ion- aggregate formation, and show great sensitivity to changes in the organic substituents about the -onium centre. The ¹³C shifts appear to be due in large measure to a predominantly Fermi contact (through-bond) mechanism, unlike the more usual shifts encountered in lanthanide-containing systems. The observed shift patterns correlate well with steric effects induced by the presence or absence of bulky substituents near the central cation nitrogen atom.     

Scaling factors for carbon NMR chemical shifts obtained from DFT B3LYP calculations

A linear scaling of the calculated chemical shifts is used in order to improve the accuracy of the DFT predicted ¹³C NMR chemical shifts. The widely applied method of GIAO B3LYP/6-311+G(2d,p) using the B3LYP/6-31G(d) optimized geometries is chosen, which allows cost-effective calculations of the ¹³C chemical shifts in the molecular systems with 100 and more atoms. A set of 27 ¹³C NMR chemical shifts determined experimentally for 22 simple molecules with various functional groups is used in order to determine scaling factors for reproducing experimentally measured values of ¹³C chemical shifts. The results show that the use of a simple relationship (δ_scalc = 0.95 δ_calc + 0.30, where δ_calc and δ_scalc are the calculated and the linearly scaled values of the ¹³C chemical shifts, respectively) allows to achieve a three-fold improvement in mean absolute deviations for 27 chemical shifts considered. To test the universal applicability of the scaling factors derived, we have used complex organic molecules such as taxol and a steroid to demonstrate the significantly improved accuracy of the DFT predicted chemical shifts. This approach also outperforms the recently recommended usage of the Hartree-Fock optimized geometries for the GIAO B3LYP/6-311+G(2d,p) calculations of the ¹³C chemical shifts.     

The use of 13C satellites in proton spectra for a study of the rα structure and of the internal rotation of p-chlorotoluene partially oriented in nematic phases

The proton magnetic resonance spectra with natural abundance ¹³C satellites of p-chlorotoluene in isotropic as well as in oriented solvents are analyzed. The indirect C? H and H? H couplings are determined from the isotropic sample and one bond ¹³C isotope effects on the proton chemical shifts are measured. The direct couplings derived from the oriented samples are corrected for harmonic vibration and used to determine the molecular r_α structure with a 6-fold hindering potential for the methl group. The resulting C? H and C? C bond lengths have errors which are smaller than 0.002Å. They deviate from the unsubstituted benzene molecule by between ?1.1 and ?0.5% for the C? H and between +0.3 and +0.5% for the C? C distances. It is not possible to obtain conclusive information about the hindering potential.     

29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

Nuclear magnetic resonance studies of boron compounds : XVI. Carbon-13 studies of organoboranes: Phenylboranes and boron-substituted aromatic heterocycles

¹³C and ¹¹B NMR data of 29 phenylboranes and 9 boron-substituted aromatic heterocycles (thiophene, N-methylpyrrole and furan) are discussed. The observed ¹³C chemical shifts of the para-carbon atoms in phenylboranes and the corresponding carbon atoms in the aromatic heterocycles are consistent with mesomeric interactions of the boryl group with the aromatic system. The trend of δ(¹³C(para)) in phenylboranes corresponds to that observed for isoelectronic phenylcarbocations. Low temperature ¹³C NMR and/or ¹³C {¹¹B, ¹H} heterocuclear triple resonance experiments were employed to obtain the ¹³C chemical shifts of the boron-bonded carbon atoms.     

Organylselenoacetylene und selenocyanate; 77Se- und 13C-NMR-chemische verschiebungen und 77Se- 13C-spin-kopplungskonstanten

⁷⁷Se and ¹³C NMR chemical shifts and ⁷⁷Se-¹³C spinspin coupling constants of mono- and bis(organylseleno)acetylenes and organyl selenocyanates are shown. The changes of ⁷⁷Se chemical shifts caused by variation of the organyl groups are well reflected by known increments. The δ(⁷⁷Se) and ¹J(⁷⁷Se¹³C) values of the investigated compounds are discussed in relation to the corresponding alkyl- and vinyl-selenides. The ¹J(⁷⁷Se¹³C) values of the selenoacetylenes and selenocyanates as well as alkyl- and vinyl-selenides are linearly dependednt (i) on ¹J(¹³C¹H) values of the corresponding hydrocarbons, hydrogen cyanide respectively and (ii) on the product of the s-characters of the coupled Se and C atoms. These linear correlations prove the predominance of the Fermi contact term for changes of the one-bond ⁷⁷Se¹³C coupling in dependence on hybridization.     

13C Pulse fourier transform NMR of menthol stereoisomers and related compounds

¹³C NMR spectra of menthol stereoisomers have been determined. The correlations of chemical shifts of these ring carbons with those of stereoisomeric 2-isopropylcyclohexanols are examined. Observed chemical shifts of 1-Me carbons are compared with those predicted from the chemical shifts of stereoisomeric 1-methyl-4-t-butylcyclohexanes. ¹³C NMR spectra of menthyl acetates, and cis and trans p -menthanes have also been examined.     

Configurational assignment of the geometric isomers of α,β-disubstituted vinyl methyl ethers from 13C nmr shift data

¹³C chemical shifts of the C atoms in the geometric isomers of some α,β-disubstituted vinyl methyl ethers have been measured. Configurational assignment is readily accomplished from the relative ¹³C chemical shift values of the β-C atoms, the signal of the Z isomer (with the β substituent in a cis position with respect of the MeO group) lying 11–15 ppm downfield from that of the E isomer. The higher chemical shifts of the β-C of the Z form are ascribed to reduced conjugation in the vinyloxy system, due to the nonplanar gauche configuration of the MeO group about the O-C(sp²) bond. Structural effects on the other ¹³C shift values are also discussed.     

Solid state NMR spectroscopy of metal carbonyls. The influence of site symmetry on the solid state spectrum of cis-(η5-C5H5)2Fe2(CO)4

Solid state carbon-13 NMR spectra of metal carbonyls are readily obtained using commercial instrumentation. The observed isotropic chemical shifts are in good agreement with solution values. Furthermore there is a one-to-one correspondence between crystallographically unique carbonyls and magnetically distinguishable carbonyls in the absence of accidental degeneracies. For cis-(η⁵-C₅H₅)₂Fe₂(CO)₄ the site symmetry is C₁ while the molecular symmetry is C_2ν. The lower solid state symmetry gives rise to more resonances in the solid spectrum than in solution. Magic angle tuning and chemical shifts were obtained using hexamethylbenzene as a standard.     

Aromatic lead(IV) compounds : VIII. 13C FT-NMR spectroscopy of compounds Arn Pb(CH3)4−n (n = 0–4)

¹³C chemical shifts and ²⁰⁷Pb¹³C coupling constants are reported for some arylmethyllead compounds. The value of J(²⁰⁷Pb¹³C) is shown to depend on the number and the nature of the aryl groups.     

Application of chemical shifts in 1H and 13C NMR spectra to configuration assignment of Schiff bases in the liquid phase

In ¹H and ¹³C NMR spectra of N-substituted dimethylketimines chemical shifts of protons and carbon atoms of the methyl groups in the cis-position with respect to the unshared electron pair of the nitrogen are larger than those of the CH₃ groups in the trans-position by 0.2–0.4 and 8–11 ppm respectively. This effect is accompanied by the reduction of the corresponding direct spin-spin coupling constant ¹³C-¹³C by 10 Hz. The experimental trends in the variation of the spectral parameters are well reproduced by ab initio quantum-chemical calculations. The discovered stereochemical dependence of the chemical shifts of ¹H and ¹³C may underlie a simple and efficient method of the configuration assignment in various compounds with a C=N bond.     

Structure of sulfur-stabilized carbanions. A 13C NMR study of some α-lithio-sulfoxides and sulfones

¹³C chemical shifts and ¹J(¹³CH) coupling constants provide information on the structures of some α-lithio-sulfoxides and sulfones.     

Quantum mechanical calculations on cellulose–water interactions: structures,energetics, vibrational frequencies and NMR chemical shifts for surfaces of Iα and Iβ cellulose

Periodic and molecular cluster density functional theory calculations were performed on the Iα (001), Iα (021), Iβ (100), and Iβ (110) surfaces of cellulose with and without explicit H₂O molecules of hydration. The energy-minimized H-bonding structures, water adsorption energies, vibrational spectra, and ¹³C NMR chemical shifts are discussed. The H-bonded structures and water adsorption energies (ΔE_ads) are used to distinguish hydrophobic and hydrophilic cellulose–water interactions. O–H stretching vibrational modes are assigned for hydrated and dry cellulose surfaces. Calculations of the ¹³C NMR chemical shifts for the C4 and C6 surface atoms demonstrate that these δ¹³C4 and δ¹³C6 values can be upfield shifted from the bulk values as observed without rotation of the hydroxymethyl groups from the bulk tg conformation to the gt conformation as previously assumed.     

Solid‐state 13C, 15N and 29Si NMR characterization of block copolymers with CO2 capture properties

Natural abundance solid‐state multinuclear (¹³C, ¹⁵N and ²⁹Si) cross‐polarization magic‐angle‐spinning NMR was used to study structures of three block copolymers based on polyamide and dimethylsiloxane and two polyamides, one of which including ferrocene in its structure. Assignment of most of the resonance lines in ¹³C, ¹⁵N and ²⁹Si cross‐polarization magic‐angle‐spinning NMR spectra were suggested. A comparative analysis of ¹³C isotropic chemical shifts of polyamides with and without ferrocene has revealed a systematic shift towards higher δ ‐values (de‐shielding) explained as the incorporation of paramagnetic ferrocene into the polyamide backbone. In addition, the ¹³C NMR resonance lines for ferrocene‐based polyamide were significantly broadened, because of paramagnetic effects from ferrocene incorporated in the structure of this polyamide polymer. Single resonance lines with chemical shifts ranging from 88.1 to 91.5 ppm were observed for ¹⁵N sites in all of studied polyamide samples. ²⁹Si chemical shifts were found to be around ?22.4 ppm in polydimethylsiloxane samples that falls in the range of chemical shifts for alkylsiloxane compounds. The CO₂ capture performance of polyamide‐dimethylsiloxane‐based block copolymers was measured as a function of temperature and pressure. The data revealed that these polymeric materials have potential to uptake CO₂ (up to 9.6 cm³ g^?1) at ambient pressures and in the temperature interval 30–40 °C. Copyright © 2016 John Wiley & Sons, Ltd.     

Electronic effects in the cyclopentadienyl ring. The 13C NMR spectraof monosubstituted ferrocenes

The chemical shifts of C(2,5) and C(3,4) carbons in the ¹³C NMR spectra of monosubstituted ferrocenes have been assigned using deuterium labelling. An analogy is observed between the shielding of C(2,5) and C(3,4) carbons of ferrocene derivatives and ortho- and para-carbons of benzene derivatives withthe same substituents. Electron-density distribution in the cyclopentadienyl ring is discussed on the basis of ¹³C NMR data.     

Transmission of substituent effects through the unsaturated system in ring-substituted α-methoxystyrenes and related compounds

A ¹³C NMR study of the transmission of substituent effects in m- and p-substituted derivatives of α-MeO-styrene and the E and Z forms of α-MeO-β-Me-styrene has been performed. The CC bond of vinyl ethers is a considerably weaker transmitter of substituent effects than that of ordinary alkenes, especially if p-π conjugation in the vinyloxy system is unhindered. In each series of compounds studied, a fairly good linear correlation exists between the ¹³C chemical shift of the β carbon and the Hammett σ parameter. The worst linear correlation was obtained for the C-β shifts of α-MeO-styrenes. The shifts concerned, however, were nicely correlated with the Hammett σ⁺ parameter.     

ATMP(氨基三甲叉膦酸)及其顺磁性Co(Ⅱ)配合物的核磁共振研究

本文用¹H、³¹P和¹³C核磁共振谱研究了ATMP(氨基三甲叉膦酸,以简式H₆L表示)及其顺磁性Co(Ⅱ)配合物。测定了不同C_co/C_ATMP摩尔比在不同pH值下的各向同性位移。定性地讨论顺磁性Co(Ⅱ)配合物在不同pH条件下的组成、电荷和空间构型变化对化学位移的影响。运用快速交换反应中化学位移与配合物浓度的关系,确定不同pH下的条件稳定常数。