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

Theoretical calculations of carbon-hydrogen spin-spin coupling constants

Structural applications of theoretical calculations of carbon-hydrogen spin-spin coupling constants are reviewed covering papers published mainly during the last 10-15 years with a special emphasis on the most notable studies of hybridization, substitution and stereoelectronic effects together with the investigation of hydrogen bonding and intermolecular interactions. The wide scope of different applications of calculated carbon-hydrogen couplings in the structural elucidation of particular classes of organic and bioorganic molecules is reviewed, concentrating mainly on saturated, unsaturated, aromatic and heteroaromatic compounds and their functional derivatives, as well as on natural compounds and carbohydrates. The review is dedicated to Professor Emeritus Michael Barfield in view of his invaluable pioneering contribution to this field.

Conformational and NMR study of some furan derivatives by DFT methods

4'-substituted neutral/protonated furfurylidenanilines and trans-styrylfurans are able to exist in two different conformations related to the rotation around the furan ring-bridge double bond. In this work, the equilibrium geometry and the corresponding rotational barrier of the benzene ring for each furan derivative conformation were calculated by DFT methods. The trend and shape of the rotational barrier are rationalized within natural bond orbitals as well as atoms-in-molecules approach. For the corresponding equilibrium geometries, (1)H and (13)C substituent induced shifts (SIS) were calculated and compared with experimental values. Calculated shielding constants are shown to be sensitive to the substituent effect through a linear fit with substituent's Hammett constants. An alternative approach was followed for assessing the effect of substituents over SIS through comparing the differences in isotropic shielding constants with NBO charges as well as with (1)H and (13)C experimental chemical shifts.

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.

Stereochemical dependence of 3JCH coupling constants in 2-substituted 4-t-butyl-cyclohexanone and their alcohol derivatives

Theoretical and experimental studies on (3)J(C2H6eq) NMR spin-spin coupling constants in both the 2-X-4-t-butyl-cyclohexanone (X = H, CH(3), F, Cl, and Br) and in their alcohol derivatives series are reported. Results thus found are rationalized in terms of the transmission of the Fermi contact contribution to such couplings. To this end, dependencies of (3)J(C2H6eq) couplings versus the C(2)-C(1)-C(6) angle are compared in both series for equatorial and axial X orientations. The main trend is described in terms of the rear lobes interaction. Besides, for X = halogen atom in equatorial orientation a rather strong interaction between oxygen and halogen lone pairs is observed, and its influence on (3)J(C2H6eq) couplings is discussed and rationalized in terms of different Fermi contact transmission pathways.

19F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

In the process of studying the chemistry of perfluoro[2.2]paracyclophanes (PFPCs), a novel class of compounds, it became necessary to identify some disubstituted products. To achieve this goal, we characterize in this work some monosubstituted PFPCs, identifying their (19)F-(19) F coupling patterns, and establishing a methodology for the assignment of their (19)F chemical shifts. The pattern of coupling constants indicates a skewed geometry in which the upper deck moves towards or away from the substituent, depending on the substituent electron-donor character and size. Quantum chemical calculations, performed at the HF/6-311 + G(d,p)//B3LYP/EPR-III level of theory, confirmed the conformations inferred from coupling constants and reproduced well the values of the couplings. Transmission mechanisms for the FC term of four- and five-bond (19)F-(19) F couplings are discussed in detail. Understanding the conformational preferences of PFPCs and how they are reflected by the coupling constants facilitates the assignment of (19)F chemical shifts in monosubstituted PFPCs and the identification of the disubstituted products.

Difference between ²JC2H3 and ²JC3H2 spin-spin couplings in heterocyclic five- and six-membered rings as a probe for studying σ-ring currents: a quantum chemical analysis

Adequate analyses of canonical molecular orbitals (CMOs) can provide rather detailed information on the importance of different σ-Fermi contact (FC) coupling pathways (FC term transmitted through the σ-skeleton). Knowledge of the spatial distribution of CMOs is obtained by expanding them in terms of natural bond orbitals (NBOs). Their relative importance for transmitting the σ-FC contribution to a given spin-spin coupling constants (SSCCs) is estimated by resorting to the expression of the FC term given by the polarisation propagator formalism. In this way, it is possible to classify the effects affecting such couplings in two different ways: delocalisation interactions taking place in the neighbourhood of the coupling nuclei and 'round the ring' effects. The latter, associated with σ-ring currents, are observed to yield significant differences between the FC terms of (2)J(C2H3) and (2)J(C3H2) SSCCs which, consequently, are taken as probes to gauge the differences in σ-ring currents for the five-membered rings (furan, thiophene, selenophene and pyrrol) and also for the six-membered rings (benzene, pyridine, protonated pyridine and N-oxide pyridine) used in the present study.

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.