Proton n.m.r. spin-tickling studies of 13C-labeled compounds. The correlation of J(C,H) and J(H,H)

A tabulation has been compiled for twenty ¹³C? H coupling constants of various carboxylic acids and includes ²J(C,H), ³J(C,H) and ⁴J(C,H) values of olefins (both cis and trans); ³J(C,H), ⁴J(C,H) and ⁵J(C,H) values of aromatics; ³J(C,H) and ⁴J(C,H) values of acetylenes; and ²J(C,H) and ³J(C,H) values of rigid aliphatics. This tabulation has been completed in the present study by the spin-tickling proton n.m.r. study of ¹³C-carboxyl-endo-1,2,3,4,7,7-hexachloronorbornene-5-carboxylic acid, which has established that the ²J(C,H) value is negative and the ³J(C,H) values (both cis and trans) are positive in this system. A plot of these twenty J(C,H) values vs the corresponding J(H,H) values of geometrically equivalent model systems (where there is a proton in place of a carboxyl group) gives a correlation coefficient of 0·975 (with a slope of 0·62), indicating that carbon–proton and proton–proton couplings operate by similar mechanisms throughout this broad series of structural types.     

Linear dependence of 1J13C, 199Hg) and 2J(1H,C,199Hg) for dibenzylmercury in various solvents

The ¹J(¹³C, ¹⁹⁹Hg) spin–spin coupling constants have been studied for dibenzylmercury in various solvents. A linear correlation was found between ¹J(¹³C, ¹⁹⁹Hg) and the ²J(¹H, C, ¹⁹⁹Hg) constants obtained earlier for the same solutions.     

Order parameters based on (13)C(1)H, (13)C(1)H(2) and (13)C(1)H(3) heteronuclear dipolar powder patterns: a comparison of MAS-based solid-state NMR sequences

Order parameters describing conformational exchange processes on the nanosecond to microsecond timescale can be obtained from powder patterns in solid-state NMR (SSNMR) experiments. Extensions of these experiments to magic-angle spinning (MAS) based high-resolution experiments have been demonstrated, which show a great promise for site-specific probes of biopolymers. In this study, we present a detailed comparison of two pulse sequences, transverse Manfield-Rhim-Elleman-Vaughn (T-MREV) and Lee-Goldburg cross-polarization (LGCP), using experimental and simulation tools to explore their utility in the study of order parameters. We discuss systematic errors due to passively coupled (13)C or (1)H nuclei, as well as due to B(1) inhomogeneity. Both pulse sequences can provide quantitative measurements of the order parameter, but the LGCP experiment is capable of greater accuracy provided that the B(1) field is highly homogeneous. The T-MREV experiment is far better compensated for B(1) inhomogeneity, and it also performs better in situations with limited signal.     

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.     

A new technique for differentiating between 2J(C,H) and 3/4J(C,H) connectivities

We present a new pulse sequence that yields two simultaneously detected types of long‐range correlation spectra. The one spectrum is to show all ⁿJ(C,H) connectivities and the other is to show exclusively ²J(C,H) connectivities. The method is demonstrated by using strychnine as a test sample. A comparison with HMBC shows that the ²J(C,H)/ⁿJ(C,H) experiment supplies a ⁿJ(C,H) spectrum that is of equal standard with regard to sensitivity and spectral information. The additional ²J(C,H) spectrum allows the disentanglement of ²J(C,H) and ⁿJ(C,H) signals (n > 2) in HMBC type spectra, which greatly simplifies signal assignment and structure elucidation in general. Copyright © 2003 John Wiley & Sons, Ltd.     

J(77Se,1H) and J(77Se,13C) couplings of seleno‐carbohydrates obtained by 77Se satellite 1D 13C spectroscopy and 77Se selective HR‐HMBC spectroscopy

Seleno‐carbohydrates are those in which the oxygen of the glycosidic bond or the hydroxyl group is artificially replaced with selenium. This substitution changes ¹H and ¹³C chemical shifts and produces spin coupling constants involving ⁷⁷Se. Coupling constants, such as ^2‐3J(⁷⁷Se, ¹H), are likely to be useful for conformational analyses of glycans because such couplings are never observed in natural glycans. Several papers have discussed the relationship between ^2‐3J(⁷⁷Se, ¹H) and conformation; however, only few reports describe ^1‐3J(⁷⁷Se, ¹³C), which could also be useful. Here, we obtain ⁷⁷Se coupling constants of seleno‐carbohydrates from ⁷⁷Se‐selective HR‐HMBC and ⁷⁷Se satellites in 1D ¹³C spectra and examine their conformations using the Newman projection scheme.     

DFT calculation of 1J(119Sn,13C) and 2J(119Sn,1H) coupling constants in di- and trimethyltin(IV) compounds

We have tested several computational protocols, at the nonrelativistic DFT level of theory, for the calculation of 1J(119Sn, 13C) and 2J(119Sn, 1H) spin-spin coupling constants in di- and trimethyltin(IV) derivatives with various ligands. Quite a good agreement with experimental data has been found with several hybrid functionals and a double-zeta basis set for a set of molecules comprising tetra-, penta-, and hexa-coordinated tin(IV). Then, some of the protocols have been applied to the calculation of the 2J(119Sn, 1H) of the aquodimethyltin(IV) ion and dimethyltin(IV) complex with D-ribonic acid and to the calculation of 1J(119Sn, 13C) and 2J(119Sn, 1H) of the dimethyltin(IV)-glycylglycine and glycylhistidine complexes in water solutions. Solvent effects have been considered in these cases by including explicit water molecules and/or the solvent reaction field, resulting in a good agreement with experimental data. The proposed protocols constitute a helpful tool for the structural determination of di- and triorganotin(IV) derivatives.     

Temperature dependences of J(C,H) and J(C,D) in 13CH4 and some of its deuterated isotopomers

The nuclear spin—spin coupling constants J(C,H) and J(C,D) have been measured over the temperature range 200–370 K for the methane isotopomers ¹³CH₄, ¹³CH₃D, ¹³CHD₃ and ¹³CD₄. The coupling constants increase with increasing temperature for any one isotopomer and decrease with increasing secondary deuterium substitution at any one temperature. The results are entirely attributable to intramolecular effects and the data have been fitted by a weighted least-squares regression analysis to a spin—spin coupling surface thereby yielding a value for ¹J_e(C,H), the coupling constant at equilibrium geometry, and values for the bond length derivatives of the coupling. We find that ¹J_e(C,H) = 120.78 (±0.05) Hz which is about 4.5 Hz smaller than the observed value in ¹³CH₄ gas at room temperature. Results are also reported for J(H,D) in ¹³CH₃D and ¹³CHD₃ for which no temperature dependence was detected.     

Measurement of 1J(Ni,Calpha(i)), 1J(Ni,C'i-1), 2J(Ni,Calpha(i-1)), 2J(H(N)i,C'i-1) and 2J(H(N)i,Calpha(i)) values in 13C/15N-labeled proteins

Use of partial or selective (13)C/(15)N labeling of specific amino acid residues in a given protein to measure the values of (1)J((15)N(i),(13)C(alpha) (i)), (2)J((1)H(N),(13)C(alpha) (i)), (2)J((15)N(i),(13)C(alpha) (i-1)), (1)J((15)N(i),(13)C'(i-1)) and (2)J((1)H(N),(13)C'(i-1)) is described. This was achieved by recording a sensitivity-enhanced 2D [(15)N-(1)H] HSQC experiment, without mixing the spin states of C(alpha) and C' during the course of entire experiment.     

Direct measurement of 1J(13C13C) in coordinated double bonds

¹J(¹³C¹³C) has been determined in naturally abundant samples of η¹,η² -alkenyl-η⁵ -cyclopentadienylnickel complexes; for the coordinated double bonds ¹J(CC) lies between the value for free ethylene and cyclopropane, and therefore gives direct insight into the relevant bonding situation.     

13C-labeled platinum(IV)-carbohydrate complexes: structure determination based on (1)H-(1)H, (13)C-(1)H, and (13)C-(13)C spin-spin coupling constants

The reaction of D-mannose and D-allose with [PtMe(3)(Me(2)CO)(3)]BF(4) 1 in acetone affords complexes [PtMe(3)L]BF(4) 5 and 6 (5, L = alpha-D-mannofuranose; 6, L = beta-D-allofuranose). The coordination mode and conformation of the carbohydrate ligands in 5 and 6 in acetone-d(6) have been determined from an analysis of J(HH), J(CH), and J(CC) in complexes formed using site-specific (13)C-labeled D-mannose and D-allose. These coupling data are compared to those measured in (13)C-labeled complex [PtMe(3)L]BF(4) 2 (L = 1, 2-O-isopropylidene-alpha-D-glucofuranose) and 1, 2-O-isopropylidene-alpha-D-glucofuranose 3, whose solid-state structures are known, and in (13)C-labeled 1,2;5, 6-di-O-isopropylidene-alpha-D-glucofuranose 4. The preferred furanose ring conformations in 2 and 5 are very similar ((3)E/E(4) and E(4)/(o)E/E(1), respectively; eastern hemisphere of the pseudorotational itinerary), with platinum coordination involving O3, O5, and O6 of the saccharide. In contrast, the furanose ring of 6 prefers an (4)E/E(o)/(1)E geometry (western hemisphere of the pseudorotational itinerary) resulting from altered complexation involving O1, O5, and O6. Couplings within the exocyclic fragments of 2, 5, and 6 also support the existence of two different platinum coordination modes. In addition to establishing the structures and conformations of 2, 5, and 6 in solution, one-, two-, and three-bond J(CH) and J(CC) observed in these complexes provide new insights into the effect of structure and conformation on the magnitudes of these couplings in saccharides. Weak platinum(IV) complexation with the carbohydrate conformationally restricts the furanose and exocyclic fragment without introducing undesirable structural strain, thereby allowing more reliable correlations between structure and coupling magnitude.     

NMR study (1H, 13C, 31P) of the (C2H5)3-n. PXn compounds (X = Cl,Br, I; n = 0, 1, 2, 3)

The ¹H, ¹³C and ³¹P NMR data are reported for the (C₂H₅)_3-nPX_n compounds (X = Cl, Br, I; n = 0, 1, 2, 3). The chemical shifts are interpreted in terms of inductive and through-space effects of the X substituents together with a rehybridization of the phosphorous atom. The results of the coupling constants are in good agreement with the chemical shifts.