Isotope effects, on chemical shifts

Isotope effects on chemical shifts can be divided into primary and secondary isotope effects. The primary ones are defined as  

The secondary isotope effect, AY(X), is the difference between the chemical shift of nucleus Y for the two types of molecules substituted with either the light or the heavy isotope, n is the number of bonds between the isotope and the nucleus in question (Y). Y could typically be The light and heavy pairs are 

Isotope effects are furthermore divided into intrinsic and equilibrium isotope 

From these two equations the ratio can be easily obtained, ffowever, this is complex the equilibrium part is much larger than the intrinsic part. If this is the case we obtain the very simple ratio AC-1(XD)/AC-3(XD) = (5CSFI-5C=S)/ (5C=0-5C0H). For an illustration of this principle see later in this section. 

One way of analyzing equilibrium systems is described by the equation given by Bordnereta/. [21] 

---

MI1 P. E. Hansen, Isotope Effects on Chemical Shifts as a Tool in Struc-... 

The measurement of deuterium isotope effects on chemical shifts is a useful tool for studies of tautomeric equilibrium (for details see the reviews 42 44). The deuterium isotope effect AX(D) is defined as the difference between chemical shifts in deuterated and non-deuterated sample [26]. 

The deuterium isotope effects on chemical shift consists of intrinsic isotope effect (direct perturbation of the shielding of X atom) and equilibrium isotope effect (perturbation of the equilibrium caused by the isotopic substitution). The values of deuterium isotope effects are to some extent independent of chemical shifts and allow determination of the mole fraction of the forms in equilibrium. 

Abstract Isotope effects on chemical shifts are versatile tools in structure elucidation,... 

ISOTOPE EFFECTS ON CHEMICAL SHIFTS AND SPIN-SPIN COUPLING CONSTANTS... 

Isotope effect on chemical shift is observed after the substitution of neighbouring mX isotope in the molecule with the heavier m X isotope. According to notation proposed by Gombler6 isotope shift is given by formula ... 

Theoretical calculations of isotope effects on chemical shifts and coupling constants and comparison with experimental values were used as a method of the calculation methods verification. Several papers were dedicated to this topic, mostly the review articles.32-35... 

Fenn, M. D., and Spinner, E., Proton and deuteron magnetic resonances of the strongly hydrogen-bonded complexes (HC02)H and(HC02)D " in aqueous solution. The primary isotope effect on chemical shift, and isotopic fractionation of labile hydrogen in H D mixtures, J. Phys. Chem. 88, 3993-3997 (1984). 

From these results it appears that the triple bond transmits isotope effects on chemical shifts very well. This result has to be compared with the observed effect on the solvolysis rate of 4-chloro-4-methyl-2-pentyne(Do) and (Da-l, , )- The rate... 

The presence of an aromatic moiety clearly has very important consequences for the NMR and IR spectra and the structural element mentioned above also illustrates the vibrational coupling between the hydroxy group and the aromatic ring vibrational modes. The NMR part will cover analysis of chemical shifts both in solution and in the solid. The nuclei immediately coming to mind are H, and O as these are vital parts of the phenol moiety, but others such as N and can also be present. Furthermore, as isotope effects on chemical shifts depend on vibrations they combine NMR and IR theory. Ab initio calculations of NMR properties such as chemical shifts and isotope effects can be very useful in studying some of these systems. These types of calculations are likewise invaluable in interpreting vibrational spectra. 

Deuterium isotope effects on chemical shifts of phenols of which the OH proton has been exchanged by deuterium can be measured in two different ways. If the OH(D) proton is exchanging slowly (see Section II.B) two different resonances are observed, one due to the protio and one due to the deuterio species (see Figure 1). The relative intensities will depend on the H D ratio, perhaps not in a quantitative way due to fractionation (see Section II.O). If exchange is fast on the NMR time scale only one resonance for the X-nuclei (e.g. C) is observed, the position of which depends on the H D ratio. In order to determine the isotope effects properly, a series of experiments must be conducted varying the H D ratios of the exchanging species, typically 1 5, 1 2, 1 1, 2 1 and pure solvent . The exchanging species is typically H2O D2O but could equally well be deuteriated alcohols, ROD. 

This problem has been addressed using isotope effects. Two different cases are found due to the interaction present. Two typical examples are seen in 42 and 43. In 43 steric twist is observed and in 42 steric compression is found. Spectroscopically these effects are characterized by the pairs of H chemical shifts and deuterium isotope effects on chemical shifts 50H, AX(OD), AX=0(0D), ACH3(OD) and AOH(CD3). The latter is especially useful, but requires that deuterium is incorporated into methyl groups of, e.g., acetyl groups . In 43 the number of intervening bonds are six (H—C8—C8a— Cl—C(0)—C—H) whereas for the steric compression cases (42) the number of bonds is five (0-C2-C3-C(0)-C-H). 

TABLE 2. Deuterium isotope effects on chemical shifts of mbazoic acids... 

R700 J. Abildgaard and P. E. Hansen, Model Quantum-Mechanical Calculations in Chemistry. Structure, Vibrational Spectra, Chemical Shifts, and Isotope Effects on Chemical Shifts , Wiad. Chem., 2000, 54, 845 R701 J. Goralski, J. Grams, I. Ludomirska, T. Paryjczak and I. Rzeznicka, Carbonaceous Deposits on Catalysts and Methods for their Study , Wiad. Chem., 2000, 54, 591... 

A review with 116 references was given. Isotope effects on chemical shifts, nA C(D), nA H(D), 1A N(D) and 1A C( 0), and solvent isotope effects in proteins are reviewed and references are provided to related cases. 

X (D)], where n is the number of intervening bonds between the deuterium and the observed nucleus, X . The most studied of these effects is the two-bond isotope effect on chemical shifts, AC(XD), produced upon H/D exchange at XH. This isotope effect has been correlated with d(XH) and, when XH takes part in an intramolecular hydrogen bond, to the hydrogen-bond enthalpy The magnitude of... 

We did not observe an isotope-dependent chemical shift under our experimental conditions, if a shift is measured for one of the bromine isotopes, it will be the same, within experimental error, for the other isotope. This is consistent with expected isotope effects on chemical shifts. Based upon our preliminary findings, we suggest that the lUPAC chemical shift scale for both bromine isotopes may be converted to an equivalent scale for solids and that the solid-state bromine standard may be either NaBr(s) or KBr(s). If KBr(s) is used,... 

---