The development of nuclear microprobe techniques for the spatial determination of13C and13C/12C ratios

Sensitive and selective nuclear reaction methods have been sought for the nuclear microprobe measurement of the spatial distributions of¹³C and¹³C/¹²C ratios. The¹³C(α, n)¹⁶O reaction, with neutron detection, is the most selective for¹³C, and has a sensitivity of ca. 100 ppm. The reactions¹³C(d, p)¹⁴C and¹²C(d, p)¹³C, with proton detection, are the most sensitive for the simultaneous measurement of¹³C and¹²C, with detection limits of 30 and 2 ppm respectively. Less sensitive alternative reaction pairs are;¹³C(³He, p)¹⁵N and¹²C(³He, p)¹⁴N;¹³C(d, nγ)¹⁴N and¹²C(d, pγ)¹³C;¹³C(³He, pγ)¹⁵N and¹²C(³He, pγ)¹⁴N. The conditions governing their use, particularly light element interferences, are detailed.     

13C?13C spin coupling constants in tetrahydronaphthylene,hexahydrobenzanthracene and benzo[a]pyrene derivatives

The ¹³C? ¹³C spin coupling constants have been determined in substituted [1-¹³C]tetrahydronaphthylenes, [5-¹³C]hexahydrobenzanthracenes and [5-¹³C]benzanthracene. In addition, the ¹³C? ¹³C spin coupling constants for 7-hydroxy[7-¹³C]benzopyrene, trans-7,8-dihydro[7-¹³C]benzopyrene-7,8-diol and trans-7,8-dihydro[10-¹³C]benzopyrene-7,8-diol are reported, together with the one-bond carbon-carbon coupling constants between C-4 and C-5 in selected 4,5-disubstituted benzopyrenes. Values for the directly bonded coupling constants and long-range coupling constants are similar to those reported previously for other aromatic and aliphatic systems. Substituent effects on carbon-carbon coupling are compared for similarly substituted cyclic and acyclic systems.     

13C?13C spin–spin coupling constants and 13C isotope effects on 13C chemical shifts in some 4-membered rings

The ¹³C? ¹³C spin–spin coupling constants in natural abundance oxetane, thietane, cyclobutanone, bromo-and chlorocyclobutane have been measured. Furthermore, the ¹³C isotope-induced changes in the chemical shifts of the different ¹³C nuclei in the molecules mentioned above are reported. These shifts are normally to higher magnetic field; in cyclobutanone, however, the resonance of the carbonyl carbon has shifted to lower field because of the substitution of ¹³C?3 for ¹²C?3.     

Substituenteneffekte auf die NMR-Spektren von Pentafulvenen. (13C, 13C)-NMR-Kopplungskonstanten (1J(C,C))

Substituent Effects on NMR Spectra of Pentafulvenes. ¹³C, ¹³C-NMR Coupling Constants (¹J(C, C)) ¹H- and ¹³C-NMR spectra of 6-monosubstituted pentafulvenes 1 – 8 have been analysed, and ¹J(C, C) coupling constants have been determined from ID-inadequate spectra of ¹³C satellites. It turns out that ¹³C,¹³C coupling constants of the ring C-atoms, and especially J(1,2)/J(3,4) and J(2,3), reflect the extent of π delocalisation in the fulvene ring. With increasing electron-donating capacity of the substituent R, J(1,2)/J(3,4) values are decreasing, while J(2,3) (and J(1,5)/J(4,5) as well) are increasing, and linear correlations of Hammett substituent constants σ⁺ and ¹J(C,C) values are obtained.     

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.     

An equilibrium and kinetic study of the complexation of lithium and sodium ions by the cryptand 4, 7, 13-trioxa-1, 10-diazabicyclo-[8. 5. 5]-eicosane (C21C5)

In the solid and solution state Li⁺ and Na⁺ form inclusive and exclusive cryptates respectively with C21C₅, in which Li⁺ resides inside and Na⁺ resides outside the C21C₅ cavity. Similar inclusive and exclusive structures are observed for [Li.C211]⁺ and [Na. C211]⁺. The logarithms of the stability constants in dimethylformamide for [Li.C21C₅]⁺, [Li.C211]⁺, [Na.C21C₅]⁺ and [Na.C211]⁺ are: 2.80, 6.99, 2.87 and 5.20; and^o the corresponding decomplexation rate constants are: 107, 0.013, 28800 and 12 s^–1 at 298.2 K. The relationships between cryptate structure, stability and lability are considered, as are solvent influences.     

Unterscheidung symmetrisch substituierter Kohlenstoffringe durch 13C,13C-Spin-Spin-Kopplung. Vergleich von herkömmlicher und INADEQUATE-13C-NMR-Spektroskopie

Distinction of Symmetrically Substituted Carbocyclic Compounds by ¹³C, ¹³C Spin-Spin Coupling. Comparison of Conventional and INADEQUATE¹³C-NMR Spectroscopy ¹³C,¹³C spin-spin couplings allow an easy distinction of symmetrically substituted carbocyclic compounds. On the basis of the number and ratio of ¹³C,¹³C satellite pairs, for instance head to head and head to tail products of photodimerisations can be distinguished. The informations derived from a conventional and an INADEQUATE ¹³C-NMR spectrum are compared.     

One-Bond 13C, 13C-Spin Coupling in η4-, η3- and η2-Olefin Transition Metal Complexes

One-bond ¹³C, ¹³C-spin-coupling constants have been measured, with natural isotope abundance, in η⁴-diene, η³-allyl and η²-ene transition-metal carbonyl complexes. Typical values of ¹J(C,C) are given for Fe-, Ru- and Os-complexes with the three types of olefinic ligands. The effects of substituents and the structural significance of the C,C-coupling constants are discussed.     

Strukturen sterisch überfüllter oder ladungsgestörter Moleküle. 9. Tetracyanethen-Kalium-Dimethoxyethan

Structures of Sterically Overcrowded or Charge Perturbed Molecules. 9. Tetracyanoethylene Potassium Dimethoxyethane The single crystal structure of [(NC)₂C?C(CN)₂ ^?K^⊕(H₃CO? CH₂CH₂? OCH₃)] reveals 4 formula units per monoclinic (P2₁/n) unit cell. The tetracyanoethylene radical anions are stapled within slightly undulated layers exhibiting alternating average intermolecular distances of 315 pm and 360 pm between their central C?C bonds. In- between the closer layers both are interspersed the eight-fold coordinated K^⊕ counter cations and the solvating dimethoxyethane molecules with contact distances K^⊕…?N of 282 to 306 pm and K^⊕…?O of 279 to 294 pm. The molecular halves of the radical anions (NC)₂C?C(CN)₂^? are twisted by 12° and the C?C, C? C and C?N bond length amount to 142, 142, and 114 pm, respectively. These structural features are discussed by comparison with those of analogous cyanohydrocarbon salts, with those of the neutral molecule, its largely interaction-free anion and its dianion as well as with results of geometry-optimized MNDO calculations.     

Long range 13C?13C coupling constants. A review

A large number of ¹³C? ¹³C coupling constants from a diversity of compounds are now available. Attempts have been made to classify this information in order to illustrate how these data are obtained, how signs are determined and how both two- and three-bond couplings can lead to structural information. Coupling constants in aromatic compounds are now well documented, substituent effect trends have been established and correlations with bond orders are noticed. Comparison of ¹³C? ¹³C couplings with ¹³C? ¹H couplings is mentioned. Theoretical calculations are dealt with, and these can now be properly evaluated as both magnitudes and signs of the experimental couplings are known. Finally, the occurrence of long range couplings in biosynthetic material is briefly surveyed.     

C and N stable isotope variation in urine and milk of cattle depending on the diet

The stable carbon and nitrogen isotopic composition of urine and milk samples from cattle under different feeding regimes were analysed over a period of six months. The isotope ratios were measured with isotope ratio mass spectrometry (IRMS). The δ ¹³C values of milk and urine were dependent on different feeding regimes based on C₃ or C₄ plants. The δ ¹³C values are more negative under grass feeding than under maize feeding. The δ ¹³C values of milk are more negative compared to urine and independent of the feeding regime. Under grass feeding the analysed milk and urine samples are enriched in ¹³C relative to the feed, whereas under maize feeding the ¹³C/¹²C ratio of urine is in the same range and milk is depleted in ¹³C relative to the diet. The difference between the ¹⁵N/¹⁴N ratios for the two feeding regimes is less pronounced than the ¹³C/¹²C ratios. The δ ¹⁵N values in urine require more time to reach the new equilibrium, whereas the milk samples show no significant differences between the two feeding regimes.     

Application of 1H and 13C NMR to the structural elucidation of tetrahydroquinoxalines condensed with five-membered heterocycles

¹H and ¹³C NMR spectral data for 21 N-methyltetrahydroquinoxalines annelated with furan, pyrrole, imidazole or thiazole rings are reported. Unambiguous assignments of the ring junction ¹³C resonances were made on the basis of selective decoupling experiments and with the aid of one-bond and long-range ¹³C–¹H coupling constants. The effects of five-membered heterocycles on the ¹H and ¹³C chemical shifts of the ring junction hydrogen and carbon atoms are considered. Values of one-bond ¹J(CH) and vicinal ³J(HH) coupling constants between the ring junction protons are also discussed as a diagnostic means for structural elucidation of tetrahydroquinoxalines condensed with five-membered heterocycles.     

Determination of relative signs of 13C?13C coupling constants in doubly labelled compounds

Analysis of the noise decoupled ¹³C spectra of doubly ¹³C labelled compounds where the two labelled carbons are identical, makes the determination of reltive signs of ¹³C? ¹³C coupling constants possible in a very simple way. The involved carbon form AA'X or AA'B spin systems.     

Pronounced stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the selenophenyl oximes as shown by NMR spectroscopy and GIAO calculations

In the ¹H and ¹³C NMR spectra of 1‐(2‐selenophenyl)‐1‐alkanone oximes, the ¹H, the ¹³C‐3 and ¹³C‐5 signals of the selenophene ring are shifted by 0.1–0.4, 2.5–3.0 and 5.5–6.0 ppm, respectively, to higher frequencies, whereas those of the ¹³C‐1, ¹³C‐2 and ¹³C‐4 carbons are shifted by 4–5, ～11 and ～1.7 ppm to lower frequencies on going from the E to Z isomer. The ¹⁵N chemical shift of the oximic nitrogen is larger by 13–16 ppm in the E isomer relative to the Z isomer. An extraordinarily large difference (above 90 ppm) between the ⁷⁷Se resonance positions is revealed in the studied oxime isomers, the ⁷⁷Se peak being shifted to higher frequencies in the Z isomer. The trends in the changes of the measured chemical shifts are well reproduced by the GIAO calculations of the ¹H, ¹³C, ¹⁵N and ⁷⁷Se shielding constants in the energy‐favorable conformation with the syn orientation of the? C?N? O? H group relative to the selenophene ring. Copyright © 2009 John Wiley & Sons, Ltd.     

13C chemical shielding tensor orientations in a phosphoenolpyruvate moiety from 13C rotational-resonance MAS NMR lineshapes

The absolute orientations of the three ¹³C chemical shielding tensors in the phosphoenolpyruvate (PEP) moiety in a PEP-model compound with known crystal structure are reported. The study uses a fully ¹³C-enriched polycrystalline sample of triammonium phosphoenolpyruvate monohydrate, (NH₄)₃(PEP)⋅H₂O, and ¹³C MAS NMR experiments fulfilling various different ¹³C rotational-resonance conditions. The absolute ¹³C chemical shielding tensor orientations are derived by iterative fitting, employing numerically exact simulations, of various rotational-resonance ¹³C MAS NMR lineshapes of the three-¹³C-spin system in fully ¹³C-enriched (NH₄)₃(PEP)⋅H₂O. The implications of the results of this study for future, biochemically oriented solid-state NMR studies on the PEP moiety are outlined.     

1H- and 13C-NMR Investigation of Nonafulvenes

¹H- and ¹³C-NMR spectra of a series of nonafulvenes 1 have been investigated. Most nonafulvenes are olefinic molecules with alternating bond lengths, their nine-membered ring deviating strongly from planarity. The 10-monosubstituted nonafulvenes contain 2 sterically different ring segments with a nearly planar (E)-diene system consisting of C(7), C(8), C(9), C(10), and R. Substituents R are influencing C(9) > C(7) > C(5). In symmetrically substituted nonafulvenes a fast process equilibrating olefinic conformers is operating so that pairs of ring protons and ring C-atoms are equivalent and only average substituent effects are observed for C(9) > C(7,2). ¹H- and ¹³C-NMR chemical shifts are not significantly influenced by changes of solvent or temperature. On the other hand, new ¹³C- and ¹H-NMR experiments completing previous investigations by Hafner and Tappe confirm that NMR spectra of 10,10-bis (dialkylamino)nonafulvenes are strongly dependent on solvent polarity and temperature. At ambient temperature and in unpolar solvents, nonplanar conformers are predominant, their spectral data fitting into the series of other nonafulvenes. At low temperature and/or in polar solvents, dipolar conformers are favoured which are characterised by charge separation and a planarised (but not necessarily completely planar) nine-membered ring with negative excess charge. The spectroscopic behaviour of nonafulvenes is reasonably explained by a qualitative scheme (Fig. 7) which is based on a model proposed by Boche for nonafulvenolates.     

Complete assignments of the 1H and 13C NMR spectra of 15 limonoids

Unambiguous and complete assignments of ¹H and ¹³C NMR chemical shifts for 15 limonoids, eight of them found in natural sources and seven other synthetic derivatives, are presented. The assignments are based on 2D shift‐correlated [¹H,¹H‐COSY, ¹H,¹³C‐gHSQC‐¹J(C,H), ¹H,¹³C‐gHMBC‐ⁿJ(C,H) (n = 2 and 3)] and NOE experiments. Copyright © 2003 John Wiley & Sons, Ltd.     

Simulation of 2D H homo- and H-C heteronuclear NMR spectra of organic molecules by DFT calculations of spin-spin coupling constants and H and C-chemical shifts

Simulation of 2D ¹H homo- and ¹H-¹³C heteronuclear NMR spectra of organic molecules are here suggested as a tool in the structure elucidation of organic compounds. DFT calculations of ¹H and ¹³C chemical shifts are performed on a sample compound, the ethyl ester of the exo-2-norbornanecarbamic acid, with the mPW1PW91 method using the 6-31G(d) basis set, following a full optimization of the geometry. Homo and heteronuclear spin-spin coupling constants are also calculated, providing full prediction of the common 2D ¹H-¹H COSY, 2D ¹H-¹³C HSQC, and 2D ¹H-¹³C HMBC.     

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.     

13C-NMR studies of polyfluorinated hydrocarbons,carboxylic acid derivatives,alcohols and ethers

δ¹³C values and coupling constants (¹J(¹³C¹H), ¹J(¹³C¹³C), ⁿJ(¹⁹F¹³C) are reported for 19 polyfluorinated organic compounds. It is shown that the shielding of carbon depends upon the number of fluorine atoms in α -position. If the R_F-group is linked to a π -system hyperconjugative $\text{and}$ η-π interaction accounts for the δ¹³C data. The values ¹J(¹³C¹H) and ¹J(¹³C¹³C) are in qualitative agreement with changes of the s-character of the respective bonds.