Yury Yu. Rusakov

Structural trends of 77Se1H spin–spin coupling constants and conformational behavior of 2‐substituted selenophenes

Experimental measurements and second‐order polarization propagator approach (SOPPA) calculations of 77Se1H spin–spin coupling constants together with theoretical energy‐based conformational analysis in the series of 2‐substituted selenophenes have been carried out. A new basis set optimized for the calculation of 77Se1H spin–spin coupling constants has been introduced by extending the aug‐cc‐pVTZ‐J basis for selenium. Most of the spin–spin coupling constants under study, especially vicinal 77Se1H couplings, demonstrated a remarkable stereochemical behavior with respect to the internal rotation of the substituent in the 2‐position of the selenophene ring, which is of major importance in the stereochemical studies of the related organoselenium compounds. Copyright

Divinyl selenide: conformational study and stereochemical behavior of its 77Se1H spin–spin coupling constants

Theoretical energy‐based conformational analysis of divinyl selenide performed at the MP2/6‐311G** level is substantiated by the second‐order polarization propagator approach (SOPPA) calculations and experimental measurements of its 77Se1H spin–spin coupling constants, demonstrating marked stereochemical behavior in respect of the internal rotation of both vinyl groups around the SeC bonds. Based on these data, divinyl selenide is shown to exist in an equilibrium mixture of three nonplanar conformers: one the preferred syn‐s‐cis‐s‐trans and two minor anti‐s‐trans‐s‐trans and syn‐s‐trans‐s‐trans forms. Copyright

Experimental and computational studies of nJ(77Se,1H) selenium–proton couplings in selenoglycosides

Selenoglycosides are important starting materials in synthetic carbohydrate chemistry and play a role in biological interactions as well. Both aspects are influenced by the conformation around the glycosidic bond. Here, we present a combined experimental and computational approach to measure and evaluate nJ(77Se,1H) coupling constants for their use in conformational analysis. The measurements were carried out using a modified CPMG‐HSQMBC pulse scheme which yields pure absorption antiphase multiplets to allow accurate determination of the nJXH values regardless of the size of the proton‐proton couplings. Theoretical calculations were performed at the Second‐Order Polarization Propagator Approach (SOPPA) level. Population‐averaged values calculated for geminal and vicinal couplings are in a good agreement with experiment indicating an adequate theoretical level of the calculations. Experimental observations and computations alike have indicated that two‐bond 77Se‐1H couplings, 2J(77Se,1H), in a H1‐C1‐Se‐X moiety are very sensitive to the torsion angle around the C1‐Se‐bond and will, therefore, be useful for conformational studies. Copyright

Conformational analysis and diastereotopic assignments in the series of selenium-containing heterocycles by means of 77Se-1H spin-spin coupling constants: a combined theoretical and experimental study.

A combined theoretical and experimental study on the stereochemical behavior of 77Se‐1H spin‐spin coupling constants has been performed at the second‐order polarization propagator approach level together with heteronuclear multiple‐bond correlation technique in the series of selenium‐containing four‐, five‐ and six‐membered heterocycles including the derivatives of thiaselenetane, selenasilole, thiaselenole, thiaselenolane and dihydrothiaselenine. Geminal and vicinal 77Se‐1H spin‐spin couplings were shown to have the pronounced stereochemical dependences in respect with the topology of the coupling pathway, internal rotation of the side‐chain substituents and ring inversion providing a straightforward tool for the conformational analysis and diastereotopic assignments in the chiral organoselenium compounds. Copyright

Conformational analysis and stereochemical dependences of 31P–1H 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 31P–1H 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 31P–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. Copyright

Towards the versatile DFT and MP2 computational schemes for 31P NMR chemical shifts taking into account relativistic corrections

The main factors affecting the accuracy and computational cost of the calculation of 31P NMR chemical shifts in the representative series of organophosphorous compounds are examined at the density functional theory (DFT) and second‐order Møller–Plesset perturbation theory (MP2) levels. At the DFT level, the best functionals for the calculation of 31P NMR chemical shifts are those of Keal and Tozer, KT2 and KT3. Both at the DFT and MP2 levels, the most reliable basis sets are those of Jensen, pcS‐2 or larger, and those of Pople, 6‐311G(d,p) or larger. The reliable basis sets of Dunnings family are those of at least penta‐zeta quality that precludes their practical consideration. An encouraging finding is that basically, the locally dense basis set approach resulting in a dramatic decrease in computational cost is justified in the calculation of 31P NMR chemical shifts within the 1–2‐ppm error. Relativistic corrections to 31P NMR absolute shielding constants are of major importance reaching about 20–30 ppm (ca 7%) improving (not worsening!) the agreement of calculation with experiment. Further better agreement with the experiment by 1–2 ppm can be obtained by taking into account solvent effects within the integral equation formalism polarizable continuum model solvation scheme. We recommend the GIAO‐DFT‐KT2/pcS‐3//pcS‐2 scheme with relativistic corrections and solvent effects taken into account as the most versatile computational scheme for the calculation of 31P NMR chemical shifts characterized by a mean absolute error of ca 9 ppm in the range of 550 ppm. Copyright

Stereochemical study of the sterically crowded phenylselanylalkenes by means of 77Se1H spin–spin coupling constants

Stereochemical study of five sterically crowded phenylselanylalkenes obtained via the hydroselenation of either terminal or internal alkynes with benzeneselenol catalyzed by the nanosized Ni complexes has been carried out based on the experimental HMBC measurements and theoretical second order palarization propagator approach (SOPPA) calculations of their 77Se1H spin–spin coupling constants across double bond in combination with the energy‐based theoretical conformational analysis performed at the MP2/6‐311G** level. It has been found that studied phenylselanylalkenes adopt mainly skewed s‐cis conformation with the noticeable out‐of‐plane deviations of the phenylselanyl and phenyl groups. Copyright

Stereochemical Study of 2-Substituted N-Vinylpyrroles

Stereochemical study of five 2-substituted N-vinylpyrroles obtained via the Trofimov reaction was carried out based on the experimental measurements of their 13C–1H and 13C–13C spin–spin coupling constants substantiated by the high-level ab initio calculations of the parent 2-methyl-N-vinylpyrrole. The title compounds were shown to adopt a predominantly skewed s-trans conformer with a noticeable population (approximately 10%) of the higher-energy skewed s-cis conformation, however, with the exception of 2-tert-butyl-N-vinylpyrrole adopting almost entirely a skewed s-trans conformation.

Conformational Analysis of 2-Formylselenophene by Means of 13C–1H, 13C–13C, and 77Se–1H Spin–Spin Coupling Constants

Theoretical energy-based conformational analysis of 2-formylselenophene performed at the MP2/6–311G** level together with experimental measurements and SOPPA/aug-cc-pVTZ-J calculations of its 13C–1H, 13C–13C, and 77Se–1H spin–spin coupling constants showed that this compound predominantly adopts the s-cis conformation. Most of the spin–spin coupling constants under study, especially vicinal 77Se–1H couplings, demonstrate remarkable stereochemical behaviour with respect to the internal rotation of the formyl group, which is of major importance in stereochemical studies of the related selenium-containing compounds.

Resonance assignments of diastereotopic CH2 protons in the anomeric side chain of selenoglycosides by means of 2J(Se,H) spin‐spin coupling constants

Unambiguous resonance assignments of diastereotopic CH2 protons in the anomeric side chain of nine alkyl‐ and aralkylselenoglycosides have been carried out on the basis of experimental CPMG‐HSQMBC measurements and theoretical second order polarization propagator approach (SOPPA) calculations of geminal 77Se‐1H spin‐spin coupling constants involving diastereotopic pro‐R and pro‐S protons. Theoretical conformational analyses have been performed at the MP2/6‐311G** level. The conformational space of each of the selenoglycosides under study could be adequately described as a mixture of six interconverting conformers with the molar fractions depending on the nature of the side chain substituent at the selenium atom. The good agreement observed between measured and the weighted conformational averaged values of the calculated coupling constants provides a basis for reliable diastereotopic assignments in this type of carbohydrate structures. Copyright