Leonid B. Krivdin

Recent Advances in Theoretical Calculations of Indirect Spin–Spin Coupling Constants

Recent trends in theoretical calculations of indirect spin–spin coupling constants covering the period of May 2003–July 2006 are outlined with the emphasis on the currently used wavefunction- and DFT-based methods. Computational results dealing with the different spin–spin coupling constants in several representative inorganic, organic, and biomolecular species obtained within the ab initio and DFT frameworks are compiled and critically discussed to elucidate some illustrative structural trends and their practical applications.

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

On the accuracy of the GIAO-DFT calculation of 15N NMR chemical shifts of the nitrogen-containing heterocycles – a gateway to better agreement with experiment at lower computational cost

The main factors affecting the accuracy and computational cost of the gauge‐independent atomic orbital density functional theory (GIAO‐DFT) calculation of 15N NMR chemical shifts in the representative series of key nitrogen‐containing heterocycles – azoles and azines – have been systematically analyzed. In the calculation of 15N NMR chemical shifts, the best result has been achieved with the KT3 functional used in combination with Jensens pcS‐3 basis set (GIAO‐DFT‐KT3/pcS‐3) resulting in the value of mean absolute error as small as 5 ppm for a range exceeding 270 ppm in a benchmark series of 23 compounds with an overall number of 41 different 15N NMR chemical shifts. Another essential finding is that basically, the application of the locally dense basis set approach is justified in the calculation of 15N NMR chemical shifts within the 3–4 ppm error that results in a dramatic decrease in computational cost. Based on the present data, we recommend GIAO‐DFT‐KT3/pcS‐3//pc‐2 as one of the most effective locally dense basis set schemes for the calculation of 15N NMR chemical shifts. Copyright

Non‐empirical calculations of NMR indirect carbon‐carbon coupling constants. Part 5: Bridged bicycloalkanes

All possible J(C,C) of the bicarbocyclic frameworks together with J(C,H) and J(H,H) at bridgeheads in the series of six bridged bicycloalkanes, bicyclo[1.1.0]butane, bicyclo[2.1.0]pentane, bicyclo[3.1.0]hexane, bicyclo[2.2.0]hexane, bicyclo[3.2.0]heptane and bicyclo[3.3.0]octane, were calculated at the SOPPA level with correlation consistent Dunning sets cc‐pVTZ‐Cs augmented with inner core s‐functions and locally dense Sauer sets aug‐cc‐pVTZ‐J augmented with tight s‐functions and rationalized in terms of the multipath coupling mechanism and hybridization effects explaining many interesting structural trends. Copyright

Solvent effects in the GIAO-DFT calculations of the 15N NMR chemical shifts of azoles and azines

The calculation of 15N NMR chemical shifts of 27 azoles and azines in 10 different solvents each has been carried out at the gauge including atomic orbitals density functional theory level in gas phase and applying the integral equation formalism polarizable continuum model (IEF‐PCM) and supermolecule solvation models to account for solvent effects. In the calculation of 15N NMR, chemical shifts of the nitrogen‐containing heterocycles dissolved in nonpolar and polar aprotic solvents, taking into account solvent effect is sufficient within the IEF‐PCM scheme, whereas for polar protic solvents with large dielectric constants, the use of supermolecule solvation model is recommended. A good agreement between calculated 460 values of 15N NMR chemical shifts and experiment is found with the IEF‐PCM scheme characterized by MAE of 7.1 ppm in the range of more than 300 ppm (about 2%). The best result is achieved with the supermolecule solvation model performing slightly better (MAE 6.5 ppm). 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