Temperature Dependence of the Chemical Shift

Fig. 11 63CuMAS-NMR at 9.0 kHz spinning speed, partial spectra of y-Cul (ZB structure) diluted in an inert matrix, showing broadening of first three STs as temperature is increased. The spectra shift to the right due to the temperature dependence of the chemical shift. Quantitative analysis of the broadening yields an activation energy for Cu+ hopping of 0.64 eV. Reprinted from [122]...

Figure 8. A plot of temperature dependence of the chemical shift for C4 of 2 for dioxane (curve labeled with stars), methanol (squares), dimethyl sulfoxide (rhombuses) and water (triangles).

Figure 4.18 Temperature dependence of the chemical shift of the hydride resonance of (triphos)Ru(CO)H2 in CD2CI2 (squares) and in the presence of a twofold excess of (CF3)2CH0H (circles). (Reproduced with permission from ref. 28.)...

Fig. 13 Temperature dependence of the chemical shift on going through the phase transition [16]...

Figures 15 and 16 show the temperature dependence of the chemical shift for 80% deuterated KD2PO4 and RbH2P04. Obviously, iso varies significantly with temperature in the paraelectric phase and shows a clear break at Tc of 202 K for DKDP and 147 K for RbH2P04, respectively. The shift exhibits a distinct discontinuity at Tc while the line width shows an abrupt increase below Tc, in agreement with the (close to) first-order nature of the phase transition, and an anticipated pronounced distortion in the PO4 moiety. The cause of the line width increase below Tc has yet to be explained, but is at least partly due to a lack of the increased spinning speed to average out the enhanced chemical shift anisotropy below Tc.

In carbocation 139, the )a-H appears at 5 H -0.78 at 203 K and -1.34 at 159 K. The relatively low shielding and temperature dependence of the chemical shift indicates the involvement of the equilibrium between the bridged and the unbridged structures. The more shielded chemical shift at lower temperatures indicates that the equilibrium favors the bridged structure as the temperature is lowered, that is, the bridged structure is favored enthalpically while the unbridged structure is favored entropically. 

Hyperthermia is an important therapeutic tool in the treatment of tumors. Its application does however require a tight control of the temperature, hence the need for contrast agents that would allow its constant monitoring during the entire therapy. Magnetic resonance spectroscopy has already been proposed for measuring the temperature of a sample. Indeed, Aime et al. [101] measured the temperature dependence of the chemical shift of a methyl group of an Yb(III)... 

The occurrence of the singlet at room temperature is caused by the different temperature dependence of the chemical shifts of the two phosphorus nuclei contained in the molecule. In the case of R = Et... 

The temperature dependence of the chemical shifts of the base and sugar resonances of poly(dA-dT) in 0.1 M phosphate buffer is plotted in Figure 3. There are upfield and downfield shifts associated with the noncooperative premelting transition between 5 and 55°C while only downfield shifts are observed for most of the base and sugar protons on raising the temperature above 65°C in the noncooperative postmelting transition temperature range. 

The temperature dependence of the chemical shifts of the base resonances in poly(dA-dT) and poly(dA-5brdU) are plotted in Figure 5. These data demonstrate that the adenosine H-8 and H-2 protons exhibit very similar behavior over the entire temperature range and are not perturbed by the substitution on the pyrimidine 5 position. 

Figure 39. The temperature dependence of the chemical shifts and linewidths of the base resonances of poIy(dA-dT) (O) and the Nuc/D = 50 netropsin poly-(dA-dT) complex ( J in 0.1 M cacodylate, 4.4mWl EDTA, 2HiO, pH 7.25...

The temperature dependence of the chemical shift for both lines, as shown in Figure 12.12, is quite similar until at approximately -0 °C both chemical shifts are constant (neglecting the wiggle in the iPP chemical shift curve) and above -0 °C the chemical shifts decrease linearly with temperature with practically the same slope. At first sight it seems that the Xe chemical shifts for both components are mainly determined by the free volume, which increases linearly with temperature above Tg [16], while the line widths of the Xe resonances are mainly determined by the mobility of the polymer chains. This would imply that the increase of the free volume with temperature of (amorphous) iPP and EP must be approximately the same in the temperature range investigated here. 

Figure 12.12 The temperature dependence of the chemical shifts which result from fitting of the spectra in Figure 12.10 by two lines...

Figure 12.18 shows the temperature dependence of the 129Xe chemical shifts of Xe in EPDM/carbon black N110, in the bound rubber fraction of EPDM/N110 and in the carbon black N110 itself. It shows that the temperature dependence of the chemical shift of the broad resonance in the bound rubber sample shows a similar temperature dependence as the chemical shift of the pure EPDM and that of Xe adsorbed on the carbon black. 

High-frequency, p.m.r. spectroscopy has proved to be an extremely important tool in studies of the structures, conformations, and inter-and intra-molecular base-stacking interactions of nucleosides and nucleotides. The temperature dependence of the chemical shift of the base proton at position 6 (H-6) has been studied at 220 MHz for uridine (36), cytidine (38), and a number of their mono- (37, 39) and... 

The NMR spectrum of the C4H7 cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species 13). A small temperature dependence of the chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species 13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl 14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

Fig. 26. The temperature dependence of the chemical shift of the peptide NH signals of Cyclo-(Pro-Sar-Gly)2...

Mauri s group have published a number of papers on calculated O NMR parameters. Ref. 167 is particularly interesting as it considers vibrational effects on the NMR shielding by averaging the chemical shift over fluctuations of the nuclear positions. They show that vibrational corrections are crucial to reproducing the temperature dependence of the chemical shift. Truflandier et al have published the first periodic calculations of NMR shielding for a transition metal nucleus, albeit on a d° system (V04 ). 

The conformation of cycloheptene oxide has been examined via the C nmr spectra of deuterated compounds on the basis of the temperature-dependence of the chemical shifts of the individual signals. In phenyl-substituted oxiranes, the C shifts have revealed the inductive and hyperconjugative effects of the oxirane ring, and thus the ring behaves as an electron-acceptor. " ... 

The a value for an isolated molecule does not depend on pressure and temperature [37], while in the case of hydrogen-bonded complexes such a dependence does exist it is due to the fact that with increasing temperature some parts of hydrogen bonds become broken and the proton signal is shifted towards stronger magnetic fields. In order to explain the temperature dependence of the chemical. shift, the model of two states is typically employed [37-39], according to which the measured chemical shift can be written as... 

Next, one should consider the line width of the one-dimensional spectra of both nuclei. The detected nucleus should be the one with the smaller linewidth, since a short T2 leads to signal losses during the pulse sequence. Compared with metal nuclei usually has the smaller linewidth. Because of the chemical shift anisotropy and the temperature dependence of the chemical shift of heavy nuclei, the measurement of is therefore favoured in these cases. For detection, in addition, the large NOE enhancement caused by proton irradiation has to be taken into account. Heteronuclei usually have no directly bound proton their NOE factors are therefore lower. To facilitate further discussion, the y values and the natural abundances of the nuclei used for C-X correlation are given in Table 1. 

However, measurement of the temperature dependence of the Si NMR chemical shift of 4 in solution provided evidence that neither of the two options prevailed (Fig. 3) in fact, the evidence suggested that a third, unexpected response took place, namely the neutral dissociation of the dative N->Si bond (Eq. 3). The Si NMR spectra of 4 in toluene-d solution as a function of temperature are shown in Fig. 3. There is a distinct temperature dependence of the chemical shift, such that at higher temperatures the Si resonance moves to lower field, in accord with a decrease in coordination number. This temperature dependence of 4 is in sharp contrast with that observed for 1 (where R = f>Bu, X = i-Bu) because of ionization (Eq. 1). This is shown in Fig. 3. 

Figure 8.22. A. MAS NMR spectra of KO2 at temperatures in the vicinity of the phase transition between the a (cubic) and p (tetragonal) forms. The a-form is solely present below 388K, but between 353 and 423K the two polymorphs coexist. Above 423 K only the P-form is present. B. Temperature dependence of the chemical shifts of the a and p-forms of KO2. From Krawietz et al. (1998), by permission of the American Chemical Society.

The temperature dependences of the chemical shifts in a series of cuprous halides have been determined by Becker (1978). These results suggest that CuBr is the most ionic of these compounds and Cul is the most covalent. The Cu shifts of the low temperature (cubic) phases increase with temperature due to increasing vibrational overlap. At higher temperatures the Cu shifts become increasingly diamagnetic with temperature, reflecting the highly disordered state of the Cu under these conditions (Becker 1978). 

It was suggested that the temperature dependence of the chemical shifts in the C NMR spectrum of ion 101 was due to an equilibrium between two or more energetically similar structural isomers of C4H7+ that interconvert rapidly, even at -155°C. Using chemical shift arguments, the major contributing isomer was assigned by Roberts and coworkers to the nonclassical bicyclobutonium structure 104. 2°... 

Fig. 8. (left) Schematic of the tautomerism of semibullvalenes and (right) the measured temperature dependence of the chemical shift difference between carbons 6 and 2. (Fig. from ref. 71, Copyright 1998 by Academic Press, reproduced by permission of the publisher.)... 

Fluorine has a high NMR sensitivity and, having similar chemical properties to protons, fluorine-substituted compounds are used widely in the pharmaceutical and medical industries. In addition to many studies carried out in the gas phase, the temperature dependence of the chemical shifts of liquid-state hexafluorobenzene, 1,4-ditrifluoromethylbenzene (DTFMB), 1,4-dibromo-tetrafluorobenzene (DBTFB) and CFCI3 have been measured.Plots of chemical shift vs. temperature were highly linear for hexafluorobenzene (0.0022 ppm K from 280 to 340 K) and CFCI3 (0.0048 ppm K from 220 to... 

Although this theory predicts the temperature dependence of the metal chemical shifts, it also predicts, for example, that an isotope shift should be independent of the remoteness of substitution, since only the vibrational frequencies of the whole molecule are considered. In practice a large dependence of the isotope on the position of substitution is observed experimentally. A theory which successfully explains both the intrinsic temperature dependence of the chemical shift and the observed isotope shifts is based on the expansion of the nuclear shielding as a function of powers of displacement coordinates. The intrinsic temperature-dependent nuclear shielding can be expressed as ... 

A proposed explanation of the discrepancy of results between the liquid crystal and other methods is the existence of a temperature dependence of the chemical shift in the isotropic and nematic phase (P. K. Bhattacharyya, private communication). Thus, Eq. (7) is modified so that the difference (0Nem — o ) is explicitly a function of the... 

Below 130°K the methylene protons give two non-averaged resonances separated at 123°K by T27 p.p.m. but still with a temperature dependence of the chemical shift, which shows that there is still interconversion of forms I and II. By studying line widths of the methylene resonance in the fast-exchange limit, the activation energy for the inversion was found to be 6-3 + 0-5 kcal mole-1. Earlier published spectra for the resolution of distinct 7,7-methyl groups in 2-t-butyl-3,7,7-trimethyltropilidene at 173°K (Conrow et al., 1963) have been used by Anet (1964) to compute AO+ = 9-2 kcal mole-1 at — 87° for a similar inversion process in this molecule. 

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