34S isotope shifts

T. M. Sanders-Loehr, J. Cusanovlch, M., unpublished results.) Assignments of bridging (-S-) and terminal (Cys) Fe-S vibrations based on S-34 Isotope shifts for oxidized ferredoxln from Clostridium pasteurlanum (2A,A3). 

A variant that combines this NMR titration method with NMR pH titration was developed much earlier by Forsyth and Yang.34 The 19F isotope shift between A-methyl or A-, A -dinictliyl-4-fluoroaniline and its CD3 or (CD3)2 isotopologue shows a maximum at a pH near the pAa of the anilinium ion. Rearranging Equation (19) and ignoring intrinsic isotope shifts leads to Equation (20), where SA and <5ah+ are chemical shifts of the aniline and the anilinium ion, respectively, assumed the same for both isotopologues. From this equation the IE on acidity could be evaluated at any pH, or preferably by averaging over all the pH values near the pKa. 

The four equatorial fluorines of p-tolyl-SF4Cl appear as a singlet at +137.7 ppm. Since sulfur has a significant (4.3%) 34-isotope, there appears a small signal just upfield that is due to the isotope effect on the fluorine chemical shift of the S-34 component. 

A recent study of the isotope shift in 34S-substituted k phases [92] has ruled out either a dominant role for the C—S stretching motions or a conventional BCS isotope effect involving the ET molecules as the relevant mass entity. There is still the possibility for the pairing in k phases to be related to a non-phonon-mediated mechanism such as the interplane exchange of spin fluctuations as proposed by Bourbonnais and Caron [82] in the context of one-dimensional superconductors. 

Subsequent RR studies, with better spectral quality and with isotope shift data, have shown the situation to be more complex, however. Figure 34 shows an interesting variation... 

Fig. 26. Heavy bars give approximate 6 " 0 of the oceans, based on the assumptions that the isotopic shift between cherts and sea water has remained constant at 34%o, and that the highest value for 6 0 from a suite of rocks is most likely to be primary. Solid lines are three calculated models, all with present ocean mass 3.89 10 moles O2, and S = 0 (Chase and Perry, 1972).

In a natSi sample doped with 34S, a negative sulphur IS of — 76 jeV (—0.61cm 1) with respect to 32S is observed for the Is (T2) line of S°, and this IS becomes —87peV (—0.70 cm-1) for the 2p (S2°) line in a sample doped with natS [233], in agreement with the isotopic shift model of [132]. Inversely,... 

Chemical shifts are shifted to higher field when deuterium replaces hydrogen either geminally or vicinally the stereochemical dependence of the vicinal-shift isotope effect has been investigated by means of cis- and trans-[2- H]fluorocyclohexane with spectra obtained under conditions of slow ring-inversion at — 85°C. The observed F isotope shifts (/p.p.m.) for (34) and (35) are shown. Values for (36) and (37) and the norbornyl systems (38) and (39) are also shown these were determined in order to ascertain the stereochemical dependence of the isotopic shift. The smallest... 

The experimental arrangement is shown in Fig. 7.43. The output of a tunable dye laser at A = 486 nm is frequency-doubled in a nonlinear crystal. While the fundamental wave at 486 nm is used for Doppler-free saturation spectroscopy [7.59] or polarization spectroscopy [7.76] of the Balmer transition 2S /2 — 4Fi/2, the second harmonics of the laser at A = 243 nm induce the Doppler-free two-photon transition IS1/22Si/2. In the simple Bohr model [7.77], both transitions should be induced at the same frequency since in this model y(lS —2S) = 4v(2S —4P). The measured frequency difference Ay = y(lS — 2S) —4y(2S —4P) yields the Lamb shift 5yL(lS) = Ay — 5yL(2S) — Ayfs(4Si/2 — 4P1/2) —Svl(4S), The Lamb shift 5yL(2S) is known and Ayfs (4Si/2 — 4P1/2) can be calculated within the Dirac theory. The frequency markers of the FPI allow the accurate determination of the hfs splitting of the IS state and the isotope shift Ayis( H — H) between the IS —2S transitions of hydrogen and deuterium (Fig. 7.34). 

Figure 35. Isotope shifts Ba of 6s So - 6sns So transitions. Dots refer to experimental data open circles were obtained using semiempirical (MQDT) wave functions. (Taken from Ref. 34.)...

Analogous C-shift patterns, although with different intrinsic isotope shifts superimposed, were observed by Roberts et al. (1984) in the C4H4D3 -cations. The isotopic perturbation for the CHD- and CHj-reson-ances at —95°C are 5.34 ppm for the CHD-endo-cation and 3.17 ppm in opposite direction for the CHD-exo-cation. 

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