13C Spin-spin Coupling Constants Research Articles

Synthesis and NMR spectra of tetrahydrofuran-2-13C

An original method was developed for the synthesis of THF labeled selectively with the 13C isotope at the α-carbon atom. The effective reduction of symmetry brought about by the insertion of the isotopic label removes the excess degeneracy of the spin systems describing the multiplet structure of the 1H and 13C NMR spectra, making it possible to undertake a detailed analysis. Exact values for the 1H–1H spin-spin coupling constants through four bonds and also the 13C–1H and 13C–13C spin-spin coupling constants involving the α-carbon atom were determined for the first time. The isotopic chemical shifts of the protons and 13C nuclei caused by replacement of the 12C nucleus by 13C were determined. These data can be used to construct a quantitative model of the conformational behavior of THF as a molecular system in which pseudorotation in terms of vibrations with large amplitude is realized.

Conformational analysis of N-vinyl-2-phenylpyrrole

A conformational analysis of N-vinyl-2-phenylpyrrole was undertaken according to the results from experimental measurements and nonempirical quantum-chemical calculation of the high-level 13C–1H and 13C–13C spin-spin coupling constants. Angular relationships were established in the direct and vicinal J C,C and J C,H constants in the N-vinylpyrrole fragment, making it possible to use them in stereochemical investigations of the vinyl derivatives of pyrrole.

Methylation of aromatics with Me 3S +, Me 3Se +, and Me 3Te + in superacid media

Electrophilic reactivity of Me 3S + O Tf, Me 3Se + O Tf, and Me 3Te + O Tf ( O Tf= trifluoromethanesulfonate) in superacid solvents towards aromatics has been dem-onstrated, indicative of protosolvation of the remaining lone pair on the onium ions. When dissolved in superacids, Me 3Se + O Tf and Me 3Te + O Tf show measurable changes in their NMR ( 1H, 13C, 77Se, 125Te) chemical shifts and one bond 77Se- 13C, 125Te- 13C spin-spin coupling constants. The NMR data are compared with those of related selenonium cations and donor-acceptor complexes.

Carbon-to-carbon coupling constants - a new guide in the stereochemistry of heteroatomic compounds

A substantial positive contribution to the direct 13C 13C spin-spin coupling constants has been demonstrated to be induced by the proximity of the nitrogen and oxygen lone pairs in cis-orientation to the neighbouring coupling carbon nuclei.

Investigations to charge dependence of 13C 13C spin-spin coupling constants - aminobenzoic acids and their ionic species

The 13C 13C spin-spin coupling constants of ortho-, meta- and para-aminobenzoic acids, benzoic acid and aniline have been measured for the neutral compounds and both for the corresponding anions and cations. The results are compared with the behaviour of the 13C chemical shifts and discussed with respect to MO calculations.

Substituent effects in organometallic compounds RnM and changes in direct1J*M- 13C spin-spin coupling constants

Substituent effects in organometallic compounds RnM and changes in direct1J*M- 13C spin-spin coupling constants

Carboncarbon spin-spin coupling constants in monosubstituted benzenes

One-bond and several long-range 13C- 13C spin-spin coupling constants have been determined in natural abundance for about 30 monosubstituted benzenes. Of the one-bond couplings, only 1 J C1C2 shows a substantial substituent effect, which suggests that the use of 1 JCC is an attractive method for determining substituent patterns in aromatic systems. 1 J C1C2 values correlate with the limited number of 1 JCC values determined in the literature for other systems. Although a correl ation with the electronegativity of the first atom of the substituent, E X, is found for 1 J C1C2 (and 1 J C2C5), all other couplings being insensitive to substituent change, the difference of 6.2 Hz between XNO 2 and NH 2 indicates the presence of further interactions. Consequently, a new substituent parameter, group electron-withdrawing power, is defined and calculated by INDO MO theory; it gives a better correlation with 1 J C1C2 and resolves the problems encountered in the simple correlation of 1 J C1–C2 with E X. Previous theoretical calculations of J CC are reassessed in the light of the present experimental data and INDO MO calculations. Although the relative magnitudes of 1 J CC and 3 J CC as well as the general effects of substituents are correctly predicted, the magnitudes of 2 J CC are overestimated and specific substituent effects upon 1 J C1C2 and 1 J C2C5 are poorly reproduced.

A13C and31P magnetic double resonance study of organo-phosphorus compounds

Twenty-two organo-phosphorus compounds of a variety of structural types have been examined by1H—{13C} and1H—{31P} magnetic double resonance spectroscopy. The signs and magnitudes of the31P—H and31P—13C spin-spin coupling constants are sensitive to the valency of the phosphorus atom, and the nature of the groups attached to it. Parallel behaviour is noted between two types of coupling constant. The31P chemical shifts agree with results obtained by conventional31P single resonance spectroscopy, and the13C chemical shifts depend on the polarizability of the phosphorus atom and its associated groups.