NMR spin-spin couplings involving nuclei in the neighborhood of a carbonyl group. 3JCH couplings in alpha-substituted acetamides

Long-range proton-carbon coupling constants: NMR methods and applications

A general review of novel NMR methods to measure heteronuclear long-range proton-carbon coupling constants ((n)JCH; n>1) in small molecules is made. NMR experiments are classified in terms of NMR pulse scheme and cross-peak nature. A discussion about simplicity, general applicability and accuracy for each particular NMR experiment is presented and exemplified. Important aspects such as the sign determination and measurement of very small coupling values involving protonated and non-protonated carbons as well as the complementarity between different experiments are also discussed. Finally, a compilation of applications in structural and conformational analysis of different types of molecules since 2000 is surveyed.

Trivinylphosphine and trivinylphosphine chalcogenides: stereochemical trends of ³¹P-¹H spin-spin coupling constants

A combined theoretical and experimental study of the stereochemical behavior of (31)P-(1)H spin-spin coupling constants has been performed in the series of trivinylphosphine and related trivinylphosphine oxide, sulfide and selenide. Theoretical energy-based conformational analysis of the title compounds performed at the MP2/6-311G** level reveals that each of the four compounds of this series exists in the equilibrium mixture of five true-minimum conformers, namely s-cis-s-cis-s-cis, s-cis-s-cis-gauche, syn-s-cis-gauche-gauche, anti-s-cis-gauche-gauche and gauche-gauche-gauche, which were taken into account in the conformational averaging of (31)P-(1)H spin-spin couplings calculated at the second-order polarization propagator approach/aug-cc-pVTZ-J level of theory. All (31)P-(1)H spin-spin coupling constants involving phosphorus and either of the vinyl protons are found to demonstrate a marked stereochemical dependences with respect to the geometry of the coupling pathway and internal rotation of the vinyl group around the P-C bond which is of major importance in the stereochemical studies of the unsaturated phosphines and phosphine chalcogenides.

Conformational analysis and stereochemical dependences of (31)P-(1)H spin-spin coupling constants of bis(2-phenethyl)vinylphosphine and related phosphine chalcogenides

Theoretical energy-based conformational analysis of bis(2-phenethyl)vinylphosphine and related phosphine oxide, sulfide and selenide synthesized from available secondary phosphine chalcogenides and vinyl sulfoxides is performed at the MP2/6-311G** level to study stereochemical behavior of their (31)P-(1)H spin-spin coupling constants measured experimentally and calculated at different levels of theory. All four title compounds are shown to exist in the equilibrium mixture of two conformers: major planar s-cis and minor orthogonal ones, while (31)P-(1) H spin-spin coupling constants under study are found to demonstrate marked stereochemical dependences with respect to the geometry of the coupling pathways, and to the internal rotation of the vinyl group around the P(X)-C bonds (X = LP, O, S and Se), opening a new guide in the conformational studies of unsaturated phosphines and phosphine chalcogenides.

Stereochemical dependence of NMR geminal spin-spin coupling constants

In this work it was sought to explore the versatility of geminal spin-spin coupling constants, (2)J(XY) SSCCs, as probes for stereochemical studies. A set of compounds, where their experimental (2)J(XY) SSCCs through the X-C-Y molecular fragment are predicted to be sensitive to hyperconjugative interactions involving either bonding or antibonding orbitals containing the C carbon atom ('coupling pathway'), were analyzed. SSCC calculations were performed for some selected examples using the second order polarization propagator approximation (SOPPA) method or within the DFT-B3LYP framework. Hyperconjugative interactions were calculated within the Natural Bond Orbital (NBO) approach. Results are condensed in two qualitative rules: Rule I(M)-hyperconjugative interactions transferring charge into the coupling pathway yield a positive increase to the Fermi contact (FC), contribution to (2)K(XY) reduced spin-spin coupling constants (RSSCC), and Rule II(M)-hyperconjugative interactions transferring charge from the coupling pathway yield a negative increase to the FC contribution to (2)K(XY) RSSCC.

On the unusual 2J(C2-H(f)) coupling dependence on syn/anti CHO conformation in 5-X-furan-2-carboxaldehydes

A remarkable difference for (2)J(C(2)-H(f)) coupling constant in syn and anti conformers of 5-X-furan-2-carboxaldehydes (X = CH(3), Ph, NO(2), Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi-contact term transmission, a rather unusual dual-coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the J(H-H) in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the pi(C=C) electronic system. The homoallylic coupling pathway can be labeled as sigma*(C-H) <-- pi(C=C) --> sigma*(C-H). In the present case, this additional coupling pathway, using an analogous notation, can be labeled as sigma*(C(2)-C(C)) <-- LP(1)(O(1))...LP(2)(O(C)) --> sigma*(C(C)-H(f)) (sigma*(C(2)-C(C))) where O(1) and O(C) stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap.