Chemical shifts isotope-induced

Isotopic labels (and especially enriched materials) have proven crucial in the investigation of the mechanisms of homogeneously catalyzed reactions [130]. Further, isotope effects on the rate or the equilibrium constant of a reaction can be diagnostic, and structural information can be provided by isotope-induced changes in the chemical shifts of neighbouring nuclei, and/or alterations in the coupling pattern of the detected spectra. The isotope- and position-specific information inherent to NMR techniques are ideally suited for the analysis of isotope effects in catalysis [131]. 

The introduction of isotopes into a compound alters the coupling pattern and the chemical shifts of the observed spectrum. As shovm in Eigure 1.18, deuterium-induced chemical shift variations have allo ved the estimation of the ratio of isomers 90a-d formed in Eq. (2) vhen R = Ph, R = CHjCOOH, and DCOOD/ Et3N is used for the hydrogen transfer [136]. The three sp -carbons Cl, C2 and C3 each afford a distinct singlet for the four possible isotopomers 90a-d (replacement of by shifts the resonances of the adjacent carbon nuclei to lo ver frequency) [137]. 

Also known are the deuterium-induced isotope effect on the Sn shielding of —0.05 ppm/D and the primary isotope effect of —11.6 7 Hz for Sn—H spin-spin coupling for the SnDnH3 + cations generated in fluorosulfuric acid at low temperature from SnD H4 . The chemical shift of 208 ppm found in the Sn NMR of the reaction mixture of Me4Sn and SbCls was attributed to the Me3Sn+ cation. 

The five-coordinate complexes of type 421 possess a rigid structure, which is retained in solution as indicated by comparing the Sn NMR in solution and in the solid state . For 421a-c, isotope-induced chemical shifts A ( C( Sn) were determined for the first time for five-coordinate tin nuclei at natural abundance of A positive sign of 1 Ai2/i3c(ii Sn) was found, in contrast with most tetracoordinate organotin compounds. 

Recently, deuterium-induced isotope effects on Li chemical shifts in organolithium compounds have been observed [51] and based on this observation, the isotopic fingerprint method was developed as a tool for structural investigations in the field of organolithium compounds. With this method, typical Li multiplets, which are characteristic of the aggregation state. 

B. Coxon, Deuterium isotope effects in carbohydrates revisited. Cryoprobe studies of the anomeriza-tion and NH to ND deuterium isotope induced 13C NMR chemical shifts of acetamidodeoxy and aminodeoxy sugars, Carbohydr. Res., 340 (2005) 1714-1721. 

Expectedly, the isotope-induced effect of the different tin isotopes " Sn and " Sn on chemical shifts of other nuclei is very small, considering the small difference in their masses. However, precise measurements of 30iO4d,e i5j NMR spectra revealed these tiny effects on C and N nuclear shielding, respectively [ A" " Sn( C), A" " Sn( N), A" /" Sn( N)]. 

The most important NMR parameters obtained for the hydroxyl protons are chemical shifts (6), vicinal proton-proton coupling constants (3J7hc,Oh), temperature coefficients (AS/7), deuterium-induced differential isotope shifts, and exchange rates ( ex)-119-123 These parameters may provide information on hydrogen bond interactions and hydration as well. Moreover NOEs and chemical exchanges involving hydroxyl groups observed by NOESY and ROESY experiments also add to the number of distance restraints used in conformational analysis. 

CP/MAS), gated H-decoupling techniques, deuterium-induced isotope effects chemical shifts, and molecular modeling has shown that this... 

F. 1. 186.5 MHz Sn NMR spectra (recorded by the refocused INEPT pulse sequence with CPD H decoupling) of tris(trunethyl)stannylamine in, (MejSn)3N, toluene, labelled partially ( 13%) with N (sample by courtesy of W. Storch, M. Vosteen, University of Munich). The broad lines originate from the Sn- N and Sn- N- Sn isotopomers. All sharp lines arise from the Sn- N and " Sn- N- Sn isotopomers. Note the isotope induced chemical shift i i4/isN(ii9sn) = -38.4ppb and the resolved C satellites of the sharp lines due to V( Sn,N,Sn, C) in the isotopomers containing N. 

---