Carbon-13, 1H spin–spin coupling VI—biphenylene

¹³C, ¹H coupling constants for biphenylene have been obtained from the analysis of the ¹³C NMR spectrum of the natural abundance α-¹³C- and β-¹³C-isotopomers. The various mechanisms responsible for the observed results are discussed.     

Carbon-13, proton spin–spin coupling constants in some 2-substituted propanes

Carbon-13, proton coupling constants have been measured in eighteen different 2-substituted propanes. ¹J(C-2,H) shows variations similar to those observed previously for monosubstituted methanes. ²J(C-2,H) is essentially independent of the substituent at C-2, while ²J(C-1,H) varies over a range of at least 5 Hz. The latter coupling constant becomes more positive as the electronegativity of the substituent increases while ³J(CH) decreases as the electronegativity of the substituent increases. The observed trends in ⁿJ(CH) are compared with those calculated using semi-empirical molecular orbital theory at the INDO level of approximation.     

Carbon-13 fourier transform NMR studies of organotin compounds : III. Carbon-13 chemical shifts and tin-119—carbon-13 spin—spin coupling constants in acyclic,monocyclic and bicyclic organostannanes

Carbon-13 NMR parameters for 33 organotin compounds with a variety of structurul features were investigated in order to obtain information about the relationship between their structure and ¹³C NMR parameters. It was found that the substitution of a proton by a trialkyltin group generally produces an upfield shift for the directly bonded carbon. The γ-nuclei usually resonate at lower fields except where there is appreciable steric strain while the β-carbons undergo relatively constant shifts of approximately 3.5 to 4.5 ppm to lower fields. The magnitude of direct bond coupling Jz-sfnc;¹J(¹¹⁹Sn¹³C)z-sfnc; is influenced by the hybridization of the tin and the directly attached carbon atoms. In rigid organitins, the vicinal coupling constants show a Karplus type variation. In aliphatic organotins, the values of the vicinal¹¹⁹Sn¹³C coupling indicates a flexible molecular framework with a clear cut preference for certain conformations.     

Carbon-13 and aluminium-27 nuclear spin relaxation in triethylaluminum

Carbon-13 spin–lattice relaxation times and nuclear Overhauser enhancement factors are reported for triethylaluminium in toluene-d₈ solution in the temperature interval 208–318 K. Aluminium-27 linewidths are reported for the same solution in the temperature range 212–334 K. The effective correlation times at different positions in the molecule are determined and used for qualitative discussion of internal motions.     

Signs of heteronuclear spin–spin coupling constants in 15N-acetamide

By using digital deconvolution to improve spectral resolution, earlier NMR studies on ¹⁵N-enriched acetamide have been revised and extended to determine the signs of the heteronuclear spin-spin coupling constants. ¹J(¹³CO¹⁵N), ²J(¹³CH₃¹⁵N) and ³J(C¹H₃¹⁵N) are negative while ³J(¹H¹³CH₃)>0. The results, interpreted on the basis of the ‘selective decoupling’ formalism, were confirmed by computer simulation of the double resonance spectra. It is shown that ²J(¹H-α¹³CO) is significantly larger than ²J(¹H^{N 13}CO). Thus, jointly with {¹H-β}-¹³C′ double resonance experiments, {¹H-α}-¹³C′ experiments ought to be most helpful when assigning peptide group carbonyl resonances. The study provides valuable information for the interpretation of heteronuclear coupling constants in polypeptides.     

13C13C spin coupling constants in phenanthrene derivatives

The synthesis of six 1-X- (X = OH, OCH₃, OCOCH₃, CH₃, CHO and CN) phenanthrene derivatives with a ¹³C label at C-1 is described. An analysis of the ¹³C¹³C spin coupling constants shows the importance of π-interaction for the coupling constant transmission. Small ¹³C¹³C spin coupling constants over 6 bonds are reported.     

Carbon-13–fluorine-19 coupling constants in benzotrifluorides

The carbon–fluorine coupling constants in 33 different substituted benzotrifluorides (trifluoromethyl-benzenes) have been determined. The ³J(CF) to the ortho aromatic ring carbons varied between 1.7 and 5.6 Hz and, in a given molecule, were always larger than the ⁵J(CF) to the para carbon, which ranged between 0.7 and 1.7 Hz. Coupling to the meta carbons, ⁴J(CF), was not observed and is under 0.3 Hz.     

13C13C spin spin coupling constants in azulenes

[4-¹³C]- Azulene and [4-¹³C]-4-methylazulene have been synthesized. The ¹³C¹³C spin coupling constants have been measured and interpreted in terms of πMO theory.     

BIRD-J-resolved HMBC and BIRD-high-resolution HMBC pulse sequences for measuring heteronuclear long-range coupling constants and proton-proton spin coupling constants in complicated spin systems

Efficient pulse sequences for measuring long-range C-H coupling constants (J(C-H)) and proton-proton spin coupling constants (J(H-H)), named BIRD-J-resolved HMBC and BIRD-high-resolution HMBC, respectively, have been developed. In spin systems possessing a secondary methyl group positioned between protonated carbons (e.g. -CH(2)-CH(CH(3))-CH(2)-), the methine proton splits in a complicated fashion, resulting in difficulty in the determination of its spin coupling constants. For easy and accurate measurements of the long-range J(C-H) and J(H-H) in such a spin system, the BIRD pulse [90°x(H)-180°x (H/C)- 90° (-x)(H)] or [90°x(H)-180°x(H/C)-90° (-x)(H)180°x(C)] is incorporated into the J-resolved portion of the pulse sequence. As a result, the above secondary methyl group can be selectively decoupled, providing simplified cross-peak patterns, which are suitable for the accurate measurements of the long-range J(C-H) and J(H-H).     

Quenching echo modulations in NMR spectroscopy.

In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal pi rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor (15)N nuclei I=N (D) and S=N(A) in RNA Watson-Crick base pairs with homonuclear scalar couplings J(IS)=(2h)J(N(D), N(A)) across the hydrogen bonds.     

Carbon-13 chemical shifts and coupling constants in cis- and trans-1,2-di-tert-butylethylene

The spectral parameters of the title compounds were redetermined to resolve discrepancies between data reported in the literature.     

Carbon-13, 1H coupling constants for benzene and the tropylium ion

The ¹³C, ¹H spin–spin coupling constants for benzene and tropylium fluoroborate have been measured from the ¹³C NMR spectra of [D₅]benzene and the [D₆]tropylium ion using a new experimental technique which employs highly deuterated compounds and ²D-decoupling. For benzene the new data are in good agreement with earlier results. For the tropylium ion we find ¹J = 166.79, ²J ? 0, ³J = 9.99 and ⁴J = (?)0.64 Hz. Secondary isotope effects for ¹³C chemical shifts, including one over four bonds, are reported.     

Carbon-13?carbon-13 coupling constants of 13C-methyl labeled o- and p-substituted toluenes

From ¹³C-7 labeled toluene the following ¹³C-methyl o- and p-substituted toluenes were synthesized: o-NO₂, -NH₂, -I and ? CN; p-NO₂ and -NH₂. Each of these labeled compounds was studied by carbon magnetic resonance to determine all carbon-13? carbon-13 splittings involving the methyl carbon.     

Carbon-13 nuclear magnetic resonance studies on high spin iron(III) porphyrins

Carbon-13 NMR studies on a series of high spin iron(III) porphyrins, namely tetraphenylporphyrin iron(III) halides [Fe(TPP) X, X=Cl, Br, I] in CDCl₃ solution are reported. As expected the¹³C shifts are found to be an order of magnitude larger than the corresponding proton shifts. The dipolar contribution, which is quite important for the proton NMR, becomes much less significant for the¹³C shifts. No systematic variation in the¹³C shift across the series is observed, except for the meso-carbon which shows a small but gradual decrease in going from the chloro to the iodo complex. The¹³C shift for the various carbon atoms of the porphyrin ligand shows interesting pattern which is discussed in terms of spin delocalisation mechanisms.     

Carbon-13 nuclear magnetic resonance chemical shifts and 13C-199Hg coupling constants for symmetrical dialkylmercurials

The carbon-13 chemical shifts and coupling constants (J[¹³C? ¹⁹⁹Hg]) have been determined for a series of eleven symmetrically substituted organomercurials. Empirical substituent parameters can be calculated which correlate observed and predicted chemical shifts for dialkylmercurials.     

Carbon-13 n.m.r. study of 31P?13C spin–spin coupling constants in dichloro- and monochloro-vinyl phosphate derivatives

Carbon-13 chemical shifts and J(PC) coupling constants of 29 vinyl phosphate derivatives are presented. In the series of compounds (R₁O)₂P(O)OC¹(R)?C²X₂ (where 3 in R indicates the first carbon of the R₂ substituent) large differences were found between the ³J(P, O, C-1, C-3) and ³J(P, O, C-1, C-2) coupling constants of the chlorinated (X?CI) and the unsubstituted (X?H) derivatives. A possible explanation of this phenomenon is given on the basis of Jameson's s bond character theory. Strong stereospecificity of ³J(P, O, C-1, C-3) coupling constants was observed in the series of compounds (R₁O)₂ P(O)OC¹(R)?C²HR₃. Coupling constants varied between 3.2–4.9 Hz in the E isomers, while peaks could not be resolved in the Z isomers. The ³J(P, O, C-1, C-2) coupling constants were regularly 20–30% greater in the Z than in the E isomers.     

13C, 1H spin coupling constants of dimethylacetylene

¹³C, ¹H spin coupling constants of dimethylacetylene have been determined by the complete analysis of the proton coupled ¹³C NMR spectrum. For the methyl carbon ¹J(CH) = + 130.6₄ Hz and ⁴J(CH) = + 1.5₈ Hz, and for the acetylenic carbon ²J(CH) = ? 10.34 Hz and ³J(CH) = +4.30 Hz. The ⁵J(HH) long-range coupling constant (+2.79 Hz) between the methyl protons was also determined.     

Quenching and recoupling of echo modulations in NMR spectroscopy.

Trains of spin echoes are normally modulated by homonuclear scalar couplings. It has long been known that echo modulations are quenched when the pulse-repetition rates are much larger than the offsets of the coupling partners, because the spin systems behave as if they consisted of magnetically equivalent spins when the offsets are suppressed. This type of quenching of the echo modulations can occur when the radio-frequency (RF) pulses are ideal, that is, when they are perfectly homogeneous, properly calibrated to induce rotations through an angle, pi, and have an RF amplitude, omega(1)=-gammaB(1), that is strong compared to the largest offset, Omega(S)=omega(0S)-omega(RF), with respect to the carrier frequency. Recently, it was discovered that echo modulations can also be quenched when the RF pulses are nonideal, that is, when they are too weak to bring about an ideal rotation of the magnetization of the coupling partners, so that the effective fields associated with the RF pulses are tilted in the rotating frame. This phenomenon typically occurs when the pulse-repetition rates are much slower than the offset of the coupling partner. Under such conditions, it turns out, however, that for certain offsets, when the phase, Phi(S) (which arises from a free precession of the magnetization of the coupling partner, S, in the pulse interval, 2tau, and the pulse length, tau(pi)), approaches a multiple of 2pi, the echo modulations are restored. However, the frequencies of these echo modulations are not simply determined by the homonuclear scalar coupling, J(IS). The Fourier transforms of the echo trains (the so-called "J spectra") reveal surprising multiplet patterns, and the amplitudes of the echo modulations depend on the offsets of the coupling partners. Herein, we present a unified theory, based on an average-Hamiltonian approach, to describe these effects for two-spin systems. Experimental evidence of echo modulations in a system of two spins is presented. Experiments with three and more spins, backed up by extensive numerical simulations, will be presented elsewhere.