Spin saturation transfer

It sometimes happens that a fluxional exchange process is suspected on chemical grounds but the low-temperature spectrum is seen at all accessible temperatures, and so the exchange is slow on the NMR timescale. An example is shown in Eq. 10.8, where we have to postulate exchange to account for the chemistry of the system, but it is too slow to affect the NMR lineshapes. In such circumstances, we can sometimes use spin saturation transfer. The principle of the method is to irradiate one of the resonances in the spectrum of one of the two species and watch for the effects on the spectrum of the other species. If we irradiate the MeA protons in 10.9a, we see a diminution in the intensity of the resonance for Meg in 10.9b. This shows that Mca in 10.9a becomes Mcb in 10.9b in the course of the exchange likewise, irradiation at the frequency of He affects the intensity of the Hd- In this way we can obtain mechanistic information about the fluxional process. 

The most useful feature of the method is not so much the rate data, but that it tells us which protons are exchanging with which, and so allows us to solve some difficult mechanistic problems. In certain circumstances the nuclear Overhauser effect (NOE) (Section 10.7) can affect the experiment, and must also be taken into account.  

FIGURE 10.7 The inversion recovery method for determining Ti. (a) A 180° pulse inverts the spins. They partially recover during the wait time and are sampled by a 90° pulse, (b) Varying the wait time allows us to follow the time course of the recovery process, as seen in a stacked plot of the resulting spectra (c). 

The rate of relaxation is given by Eq. 10.10, in which h is Planck s constant, y is the gyromagnetic ratio of the nuclei involved, is the rotational correlation time (a measure of the rate of molecular tumbling in solution), z is the Larmor frequency, I is the nuclear spin, and r is the distance between the two nuclei  

Unfortunately, we do not know Tj in Eq. 10.10. If we did, we could calculate the H—H distance. It turns out that on cooling the sample, T passes through a minimum value. Equation 10.10 predicts that this should happen when = 0.62/(1). Since we know o), we can calculate Tc at the minimum and so estimate the H—H distance directly. A number of precautions need to be taken because rotation of the H2 about the M—(H2) bond reduces the relaxation rate, and certain metals, notably Re, Nb, V, Mn, Co, and Ta, cause a substantial, but easily calculable, dipole-dipole relaxation of attached protons because both 7 and I are high. We also assume isotropic (random) motion of the molecule, which is not the case for such systems as IrH5L2 and Cp ReH5, where the MH unit has a low moment of inertia and so spins rapidly. 

It sometimes happens that a tluxional exchange process is suspected on chemical grounds, but a low-temperature limiting (static) spectrum is seen at all accessible 

The most useful feature of the method is not so much the rate data but that it tells us which protons are exchanging with which, and so allows us to solve some 

We now need to look at how we can determine the Ti for any signal, something that we need to do in the spin saturation transfer experiment If we imagine 

FIGURE 10.6 Fluxionality of CpFe(CO)2(V-Cp), showing the faster collapse of the Hb resonance, indicating a 1,2 rather than a 1,3 fluxional shift. Only the resonances for the Cp group are shown. 

For Cp, it is impossible to distinguish between a Woodward-Hoffmann orbital symmetry-allowed 1,5 shift and a least-motion 1,2 shift because they are equivalent. In an ri -CyHy system, the two cases are distinguishable, Woodward-Hoffman giving a 1,4 and least motion a 1,2 shift. A similar analysis shows that (r -C7H7)Re(CO)5 follows a least motion and T] -C7H7SnMe3 a Woodward-Hoffmann path. 

Another important case of fluxionality is bridge-terminal exchange in carbonyl complexes. The classic example is [CpFe(CO)2]2, which shows separate Cp resonances for cis and trans CO-bridged isomers in the NMR below -50°C, but one resonance at room temperature owing to fast exchange. 

When fluxional exchange is too slow to detect by the methods of Section 10.5, we may still be able to use spin saturation transfer. To 

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This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

These experiments while moving beyond simple structure determination, still depend primarily on structure determination as their primary tool. A number of experiments have moved beyond this to exploit other characteristics of NMR. For example, spin saturation transfer can be used to study internal rotation of molecules (59, 60), and line shape analyses can be used to study intramolecular exchange processes (61), conformational interchange (62) and... 

Kinetics of Internal Rotation of A ,A -Dimethylacetamide A Spin-Saturation Transfer Experiment 60... 

Alkylaryl ethers and diaryl ethers undergo protonation on either oxygen or carbon, depending upon the acidity of the medium87 (Scheme 4.3). Both the C-protonated species 28 and the (9-protonated species 29 have been observed. The evidence mainly comes from NMR and UV data. Sommer et al.88 have used para-anisaldehyde as an indicator in acidity measurements in the superacidity range. The barrier of rotation around the Cipso—0 bond upon (9-protonation has been used as a criterion in such studies. The torsional barrier around the phenyl-alkoxy bond in the C-protonated forms of aromatic ethers has also been measured by spin-saturation transfer measurements.89... 

Saturation transfer difference. The origins of the STD experiment1721 can be traced to the spin-saturation transfer experiment or Forsen-Hoffman experiment from the 1960s.1911 In the STD experiment, a subset of the protein II resonances are saturated by means of a train of frequency-selective radiofrequency pulses applied to a narrow spectral region devoid of ligand resonances. The saturation is transferred by spin diffusion ( H- H crossrelaxation pathways) to the rest of the protein, a process that becomes more efficient with... 

Fluxionality studies have been carried out on 343 (Scheme 44).137 No coalescence of the two allene hydrogens (HI and H6) was observed in temperature ranges where the complex is stable (ca. 100°C). However, spin saturation transfer was observed between 60 and 100°C, which yielded activation parameters (AH = 26.8 1.3 kcal/mol A5J = 15.1 1.3 eu) for migration of the metal between the two double bonds. At 80°C, 343... 

In addition to evaluation of line widths and peak separations, analysis of T values and spin saturation transfer can be performed to access a wide range of timescales. Under conditions of slow exchange between forms A and B and upon irradiation at vb, the time constant associated with state A (ta) can be determined from the measured T of nucleus A in the presence of exchange and the T of nucleus A in the absence of exchange ... 

The activation parameters for the exchange reactions of 17 and 18 were determined by a combination of variable-temperature ll NMR lineshape analysis16 and spin saturation transfer experiments.17 Rate data for 17 were measured over a temperature range of 100 "C. Rates for compound 18 were measured over a 65 °C range. The enthalpy of activation was found to be considerably smaller in the case of 17 (12.2(2) kcal/mol) relative to 18 (17.6(3) kcal/mol). Ion pair dissociation is therefore facilitated by the presence of a lone pair of electrons on the boron substituent. The entropy of activation for 17 is -2.3(6) eu, while that of 18 is 8(1) eu. The more positive entropy of activation measured for 18 may be interpreted as the creation of two independent particles from a closely associated ion pair. 

It is also possible to make these compounds by the reaction of LiR with CpsUTHF (421). Spin saturation transfer experiments suggest the equilibrium ... 

NMR spin saturation transfer experiments indicate that this process is facile at 75°C and a r 2-CF3 group is postulated as the transition state structure. 

Agostic interactions shown in structures 99-101 were found to be involved in selective H-D exchange processes with R-OD (R = D, CHj, CDs) catalyzed by complexes 98. process induced by complex 98a was shown by spin saturation transfer experiments to be highly selective, with only one of the methylene protons participating in the exchange (H in bis-agostic intermediate 99 is not involved). 

Re(Tp)(CO)(L)(ri2-ethylene)] (L = BuNC, PMe3, py, Meim, NH3) (Fig. 2.50) have been prepared by the reaction of [Re(Tp)(CO)(L)(rj2-cyclohexene)] with ethylene. These complexes have been used to determine the rates of the propeller-like rotation of ethylene about the ethylene-rhenium bond using spin-saturation transfer experiments at low temperatures.223... 

C to give distinct Me and H resonances. This suggests that an alkane complex Cp Os(dmpm)(CH4)+ is formed reversibly from the Me/H complex over 100 times per second at - 100°C. Spin saturation transfer experiments provide definitive evidence for exchange, wherein AH is 7.1 0.9 kcal/mol (AG = 8.1 kcal/mol at... 

The methods available to determine reaction rates by NMR spectroscopy include line broadening, spin saturation transfer and exchange spectroscopy (EXSY). The latter approach can be completed by both 1 and 2 D procedures. Several studies in the literature have served to illustrate that the dynamic behaviom of complexes detected through the parahydrogen effect can be examined. A general feature of this approach has involved the refocusing of the initial antiphase magnetisation. 

The conformation of [5]metacyclophane (4a) (Structures 1) and its derivatives 4b-4d (s. Scheme 9) was investigated by Bickelhaupt [31d, 56]. While the parent hydrocarbon 4a did not show temperature dependence in the HNMR, 4b-4d exhibited dynamic behavior due to the conformational change between the two conformers A and B (Scheme 18). The conformer A predominates over B by 6-8 times the ratio is dependent on the substituent. The barrier for the flipping was estimated to be 13.2-13.4 kcal/mol (296 K) from the line-shape analysis and the spin saturation transfer experiments. 

Spin saturation transfer studies of the 31p NMR of the iso-propyl complex yield data regarding the rate of interconversion of the dihydride and molecular hydrogen complexes. Thus ki is calculated to be 63 s-1 (299 K) and k-i is calculated to be 12.4 s-l(300 K). These values are in reasonable agreement with the corresponding values determine by stopped-flow kinetics for the cyclohexyl complex as described above ki = 37 S l(298 K) and k-i = 18 S l (298 K). Activation parameters for conversion of the dihydrogen to dihydride reaction (kq step) were determined based on analysis of the spectral changes in the temperature range 288-310 K. The values Zij = 16.0 0.2 kcal/mole, AI = 10.1 1.8 kcal/mole and -19.9 6.0 cal/mole deg were determined. 

PARAMETERS DETERMINED BY IH NMR STUDIES OF DIHYDROGEN-DIHYDRIDE EQUILIBRIUM (Kequil= 0.2-OJ) AND SPIN-SATURATION TRANSFER... 

In typical examples for R = CHj and for other migrants the mechanism of 1,2-shift (allowed sigmatropic 1.2, 1.6,1.10 or 1.14 shifts according to the Woodward-Hoffmann classification was specially established by such methods as that of spin saturation transfer 32,150.272.273) method of labelled atoms and others. 

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