Deuterium-induced isotope shift

A similar situation occurs in acetyl fluoride, where the nonlinearity of deuterium-induced isotope shifts on the 19F NMR signal implies a fractional population >1/3 for the conformer of CH2DCOF with C-D anti to F (38) and a fractional population <1/3 for the conformer of CHD2COF with C H anti to F (39).109 These populations arise because of the greater zero-point energy of a C H bond anti to F. They can be translated into deuterium IEs on the acidities of the conformers of acetyl or acetyl-d fluoride. 

The replacement of hydrogen atoms with deuterium caused large deuterium-induced isotope shifts in the 77Se NMR. For both conformers 45 and 46, the resonance of the deuterated compound shifted upfield with respect to the parent compound. The isotope shift (AS, an upfield shift) was 4.22 ppm for 45 and 4.03 ppm for 46. These values were definitely larger than that observed for 46 (AS = 3.78 ppm). These differences, AAS = +0.44 ppm for 45 and +0.25 ppm for 46, can be considered as the isotope shifts due to the C-H- -Se nonbonded interaction <1996BCJ1825>. 

Very recently an isotopic finger print method which rests on deuterium-induced isotope shifts for Li resonances has been proposed by Gunther and exemplified with applications from the aggregation behavior of cyclopropyllithium systems and mixed aggregates formation between methyllithium and lithium salts. By using one- and two-dimensional NMR experiments based on scalar spin-spin coupling and nuclear overhauser effects the structural aspects associated with benzyllithium and the formation of polylithium systems by lithium reduction of biphenylenes have been discussed. 

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. 

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. 

Figure 5 Deuterium-induced isotopic fingerprints in Li NMR spectra of partially deuteriated organolithium aggregates (a) phenyllithium monomer (THF/penta-methyldiethylenetriamine — 122°Q (b) phenyllithium dimer (Et20/tetramethylethyl-enediamine -111°C) (c) methylhthium tetramer (R=CHj EtjO, — 92 Q (d) fluxional phenyllithium tetramer (Et20, — 102 C). The measu H/ H isotope shifts for 5( Li) are 19.2, 10.4, 15.6, and 7.0 ppb, respectively. All systems were Li labelled and 50% of the organic ligands were perdeuterated v C Li) = 58.88 MHz...

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

According to deuterium-induced upfield H NMR isotope shifts in partially deuterated rigid cyclohexane-1,3-diols dissolved in CDC13 or benzene-c/6, the OH is preferentially solvent-exposed, while deuterium prefers to reside in the intramolecular hydrogen bond (49).121 In acetone-c/6 and DMSO-c/6 downfield isotope shifts indicate that the OH preferentially resides in the intramolecular hydrogen bond, while OD forms an external hydrogen bond to the acceptor solvent, S (50). 

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. 

Deuterium and even 0 can induce a small isotope shift in the position of a C signal and thus C can be used as a reporter nucleus for the presence of these isotopes. The NMR spectrum and the presence of couplings... 

The experimental arrangement is shown in Fig. 2.48. The output of a tunable dye laser at X = 486 nm is frequency-doubled in a nonlinear crystal. While the fundamental wave at 486 nm is used for Doppler-free saturation spectroscopy [261] or polarization spectroscopy [278] of the Balmer transition 2Si/2- P /2 the second harmonics of the laser at X = 243 nm induce the Doppler-free two-photon transition 15 i/2 25 i/2. In the simple Bohr model [279], both transitions should be induced at the same frequency since in this model v(lS-2S) = 4v(2S-4P). The measured frequency difference Av = v(lS-2S) — 4v(2S-4P) yields the Lamb shift vlCI ) = Av — 8v] 2S) — Avfs(45 i/2 4Pi/2) <5vl(45 ). The Lamb shift (5vl(2/S) is known and Avfs(45i/2-4Pi/2) can be calculated within the Dirac theory. The frequency markers of the FPI allow the accurate determination of the hfs splitting of the 15 state and the isotope shift Avis( H- H) between the 1S-2S transitions of hydrogen and deuterium (Fig. 2.38). 

Depending on the distance of the deuterium substitution from the other exchange site appropriate corrections for the deuterium-induced intrinsic chemical shift (6j ,r) must be taken into account to evaluate the splitting 5 (8 = 8 bs — 8i ) caused by the chemical isotope effect. 

ID and 2D) of long-chain N-alkyl lactosylamines has been reported. All H- and C-resonances of potassium sucrose octasulfate have been assigned with the help of 2D and deuterium-induced differential-isotope-shift (DIDIS) n.m.r. methods. A H-n.m.r. study in D2O showed that 3 -ketolactose (11) crystallized from methanol as the hemiacetal 12 with an equatorial methoxy group. A 500 MHz H-n.m.r. investigation of methyl fl-cellobioside confirmed the presence of an intramolecular hydrogen bond between HO-3 and 0-5 in DMSO, but not in DjO or CDjOD. ... 

Fig. 2.37. a Differential deuterium isotope shifts for C-NMR assignments for the tautomers of psicose (33 a-33 d) (384). Figure shows the carbon chemical shifts recorded in D2O numbers in parentheses represent the differential induced shift caused by deuterium exchange, b Number of carbon signals predicted for cellobiose (34) in OMSO-d under partial exchange numbers in parentheses represent number of observed signals (62)... 

Observe that all the mechanisms—that is, the classical indirect mechanism and the two quantum ones—predict a satisfactory isotope effect when the proton of the H bond is substituted by deuterium All the damping mechanisms induce approximately a l/y/2 low-frequency shift of the first moment and a 1 / y/2 narrowing of the breadth, which is roughly in agreement with experiment. As a consequence, the isotope effect does not allow us to distinguish between the two damping mechanisms. 

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