A new method for determining positions of phenolic hydroxyl groups through silylation and application of H(Si)C triple-resonance NMR experiments

Triple Resonance Experiments for the Rapid Detection of 103Rh NMR Shifts: A Combined Experimental and Theoretical Study into Dirhodium and Bismuth-Rhodium Paddlewheel Complexes

A H(C)Rh triple resonance NMR experiment makes the rapid detection of ¹⁰³Rh chemical shifts possible, which were previously beyond reach. It served to analyze a series of dirhodium and bismuth–rhodium paddlewheel complexes of the utmost importance for metal–carbene chemistry. The excellent match between the experimental and computed ¹⁰³Rh shifts in combination with a detailed analysis of the pertinent shielding tensors forms a sound basis for a qualitative and quantitative interpretation of these otherwise (basically) inaccessible data. The observed trends clearly reflect the influence exerted by the equatorial ligands (carboxylate versus carboxamidate), the axial ligands (solvents), and the internal “metalloligand” (Rh versus Bi) on the electronic estate of the reactive Rh(II) center.

H(C)Ag: a triple resonance NMR experiment for (109) Ag detection in labile silver-carbene complexes

In silver complexes, indirect detection of (109) Ag resonances via (1) H,(109) Ag-HMQC frequently suffers from small or absent JHAg couplings or rapid ligand dissociation. In these cases, it would be favourable to employ H(X)Ag triple resonance spectroscopy that uses the large one-bond JXAg coupling (where the donor atom of the ligand X is the relay nucleus). We have applied an HMQC-based version of the H(C)Ag experiment to a labile silver-NHC complex (NHC=N-heterocyclic carbene) at natural (13) C isotopic abundance and variable temperature. In agreement with simulations, H(C)Ag detection became superior to (1) H,(109) Ag-HMQC detection above -20 °C.

The effect of solvent accessible surface on Hammett-type dependencies of infinite dilution (29)Si and (13)C NMR shifts in ring substituted silylated phenols dissolved in chloroform and acetone

Infinite dilution (29)Si and (13)C NMR chemical shifts were determined from concentration dependencies of the shifts in dilute chloroform and acetone solutions of para substituted O-silylated phenols, 4-R-C6 H4 -O-SiR'2 R″ (R = Me, MeO, H, F, Cl, NMe2, NH2, and CF3), where the silyl part included groups of different sizes: dimethylsilyl (R' = Me, R″ = H), trimethylsilyl (R' = R″ = Me), tert-butyldimethylsilyl (R' = Me, R″ = CMe3), and tert-butyldiphenylsilyl (R' = C6 H5, R″ = CMe3). Dependencies of silicon and C-1 carbon chemical shifts on Hammett substituent constants are discussed. It is shown that the substituent sensitivity of these chemical shifts is reduced by association with chloroform, the reduction being proportional to the solvent accessible surface of the oxygen atom in the Si-O-C link.

29Si-13C spin-spin couplings over an Si-O-Csp3 link

(29)Si-(13)C spin-spin couplings over one, two, and three bonds as well as other NMR parameters [delta((29)Si), delta((13)C), delta((1)H), (1)J((13)C-(1)H), and (2)J((29)Si-C-(1)H)] were calculated and measured for a series of trimethylsilylated alcohols of the types Me(3)Si-O-(CH(2))(n)CH(3) and Me(3)Si-O-CH(3-n)R(n)(n = 0-3; R = Me, Ph, or Vi). The signs of the coupling constants determined for selected compounds can likely be extended to all such compounds, as supported by theoretical calculations. Similar to couplings between other pairs of nuclei, the 2-bond and 3-bond (29)Si-O-(13)C couplings are of opposite signs ((2)J > 0 and (3)J < 0), and their relative magnitudes depend on the extent of branching at the alpha-carbon.

1H, 13C and 77Se NMR study of tetraselenafulvalene derivatives supported by novel H(Se)C and H(C)Se triple-resonance experiments at natural abundance

A series of substituted tetraselenafulvalenes (TSeFs) was studied in solution using 1H, 13C and 77Se NMR spectroscopy. Chemical shifts and heteronuclear coupling constant values were determined and assigned. Novel two-dimensional H(Se)C and H(C)Se triple-resonance correlation experiments were applied at natural abundance in order to accomplish 13C and 77Se signal assignments. Using this approach, all the signals were unambiguously assigned and atom connectivity was established in the studied TSeF derivatives. These experiments, allowing signal assignments of quaternary carbons, may find wide application in the study of substituted TSeF and other organoselenium compounds. To the best of our knowledge, triple-resonance experiments with proton detection have been applied to organoselenium compounds for the first time.