Scaling of Chemical Shifts

The chemical shift ranges reveal broad patterns of periodicity despite wide differences in structure. A more precise comparison of chemical shifts is that for compounds with corresponding structure. These also appear to be scaled by ratios of For example, a plot of the Te vs. the Se chemical shifts for the analogous compounds gives a straight line with a slope of 1.8 which can be compared with 1.56 for the ratio of for the free atoms. Corresponding figures for Pb and Sn 

Excitations which contribute to the paramagnetic term must be magnetic dipole allowed. Thus the AE used in the approximate theory usually corresponds to the lowest energy transition of this type (in terms of one-electron excitations, n- n, n-KT, n a, 7 r, etc.) or else some effective average over a few of them, the most important involving orbitals centered on the nucleus of interest. Such transitions have, e.g., Px Py, d y or corresponding/ /components which involve a 

Deshielding with decrease in atomic electron density is a well-known pattern for all nuclei. It may result (e.g.) from a more negative ionic charge, or from increase in oxidation number or in the electronegativity of substituents. The effect can be related to increase in the magnitude of the paramagnetic term [equations (10)-(13)], via an increase in at least in part. For example, the radial terms for C , C, and C  

Effects of atomic charge on the radial term may be reinforced by parallel changes in the energy term. Thus, in aromatic compounds decrease in charge density also reduces the shielding by reducing AE o Alternatively, effects of changes in 

The general increase in shielding accompanying an increase in negative charge has led to many correlations between chemical shifts and calculated charge densities. For example, there are many S/q relationships in carbon shielding, where 5 is a 

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Also because of the Larmor equation (1.8), the frequency or field differences /1vs or ABS are proportional to the swept radio frequency Vj (in MHz) or the field strength of B0 (in T). Therefore, chemical shifts dvs (or ABS) obtained at different radio frequencies v, (or field strengths B0) have to be adjusted to the same radio frequency (or field) before comparison. In order to get chemical shift values which are independent of the frequency or field strength used, the d scale of chemical shifts is introduced. <5 values are obtained by dividing the frequency differences Avs (in Hz) by the frequency iq used (in MHz = 106 Hz). 

A different method of referencing is used in the E-scale of chemical shifts, sometimes referred to as universal referencing . (We shall reserve the term absolute scale , which is also used in this context24, for a different concept see below.) In this scale the chemical shift is given as the absolute frequency of the signal that would be observed in a magnetic field in which the XH NMR signal of TMS appears exactly at 100 MHz. Thus... 

The most common scale of chemical shifts is the 8 (delta) scale, which we will use (Figure 13-8). The signal from tetramethylsilane (TMS) is defined as 0.00 ppm on the 8 scale. Most protons are more deshielded than TMS, so the 8 scale increases toward the left of the spectrum. The spectrum is calibrated in both frequency and ppm 8. 

An NMR spectrum plots the intensity of a peak against its chemical shift measured in parts per million (ppm). The common scale of chemical shifts is called the 5 (delta) scale. The proton NMR spectrum of tert-butyl methyl ether [CH30C(CH3)3] illustrates several important features ... 

Figure 15.4 High-resolution NMR spectrum of ethanol showing 5-scale of chemical shifts. The line at 5 = 0 corresponds to the TMS trace added as a reference.

Because the Larmor frequency is proportional to the strength of the magnetic field, there is no absolute scale of chemical shift. Thus, a frequency difference (Hz) is measured from the resonance of a standard substance [tetramethylsilane (TMS) in and C NMR] and divided by the absolute value of the Larmor fi equency of the standard (several MHz), which itself is proportional to the strength of the magnetic field. The chemical shift is therefore given in parts per million (ppm, S scale), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of l 10 . 

Note An old scale of chemical shift, no longer used, was called the tau (t) scale. On this scale, the resonance position of TMS was defined to be 10.00. To convert lvalues to rvalues, merely subtract them from 10. You will find rvalues in the older literature. 

A) Schematic diagram of a simple nuclear magnetic resonance (NMR) spectrometer. The sample is placed in solution in a long, thin tube and spins in a probe sitting in a magnetic and surrounded by radio-frequency (RF) coils B) proton NMR spectrum of ethanol (QH O) with tetramethylsilane (TMS) added as internal standard. On the 8-scale of chemical shifts,... 

Delta (8) scale (Section 14.1B) A common scale of chemical shifts used in NMR spectroscopy in which the absorption due to tetramethylsUane (TMS) occurs at zero parts per million. 

There exist other, less often used, scales of chemical shifts. The generalization of NMR spectra allows experimental results on chemical shifts to be exhibited in the form depicted in Figure 8.10. Since the protons in similar atomic groups have somewhat different d, the chemical shift values are plotted on the abscissa axis in the form of segments (not points) of (5i (in m.u.) the corresponding functional atomic groups are plotted on different levels on the ordinate axis. The black strips correspond to those cases, which are met with more often. 

Frequently the chemical shifts (Table 2.3) of molecular fragments and functional groups containing nitrogen complement their H and C shifts. The ammonia scale of N shifts used in... 

The rate of formation of hydrogen ions can be determined by observing the chemical shift for the water molecules in the solution. Since the rate of protonation is very rapid compared to a typical NMR time scale, the chemical shift may be said to be a linear combination of the contributions of the protonated water molecules and the unprotonated water molecules. That is,... 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

Figure 3.18 Chemical shift scales and chemical shifts of some compounds. (Adapted with permission of Nelson Thornes Ltd. from Figure 2.12 of Akitt, J. W. NMR and Chemistry, 3rd ed 1992.)...

Chemical shifts are measured in ppm from the appropriate standard. The early literature on C shifts is given relative to certain compounds, but nowadays the same standard as hydrogen is used (tetramethylsUane TMS). With other nuclei the situation is more complex and there has been some discussion on the use of a certain standard. To avoid this problem, all data are given relative to the standard used in the original hteramre whereas its conversion to other scales for chemical shifts is given for each nucleus of interest. No such problem exists, of course, with coupling constants. 

Another important point to consider in the interpretation of chemical shifts is the fact that the diffusion path over which xenon travels must be known, as the chemical shifts are averaged if various pore sizes (including extra-particle pore space) are sampled on a time scale rapid compared to the inverse chemical shift differences characteristic of the different pores [12]. Thus the chemical shifts observed may be dependent on the morphology of the sample, depending on the amount of inside versus outside xenon. 

Changes in energy needed to flip protons are called chemical shifts. The location of chemical shifts (peaks) on a NMR spectrum are measured from a reference point that the hydrogens in a standard reference compound—(CH).Si or tetramethylsilane (TMS)—produce. The amount of energy necessary to flip protons in TMS is assigned the arbitrary value of zero 8. Chemical shifts are measured in parts per million magnetic field strength difference (8-scale), relative to TMS. 

The combined strategy of calculating the 19F chemical shifts has been studied for fluorobenzenes [62] in several solvents. Here crw has been found to be the dominant contribution to the total solvent-induced change of chemical shift the authors have neglected the solvent magnetic anisotropy contribution cra which is related to the short-range interactions. To obtain the agreement with the experimental data, the term [Pg.137]

A typical demonstration of the utility of theoretical calculations in the assignment of uncertain signals is reported by Quin et al.30 31 As already found by Bagno,116 the experimental values reported in the literature for S-methyltetrahydrothiophenium salt 1, 750 ppm (referred to CS2), did not fit well with the scaled values, calculated by the B3LYP DFT approach and the EMPI method, which were 87.4 and 121 ppm respectively. With the aid of AIM calculations, it was verified that this discrepancy cannot be ascribed to intermolecular interactions in solution, neither salt formation nor interaction with counterions. The experimental redetermination of chemical shift values has given a value of 95 ppm (ext. ref. CS2), in agreement with calculated values. For S-methylthianium ion 16, a value of 68 ppm has been calculated, compared to an experimental value reported in the literature of 670 ppm.29... 

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