Chemical Shift

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Chapter: Organic Chemistry : Structure Determination of Organic Compounds

The range of frequencies over which protons absorb in most organic molecules depends on the applied field.


CHEMICAL SHIFT

The range of frequencies over which protons absorb in most organic molecules depends on the applied field. For example, for an applied field of 14,000 G, most protons will absorb over a range of 600 Hz beginning at the value of 60 × 106 Hz (60 MHz), or from 59,999,400 to 60,000,000 Hz. At 23,486 G this range is 1000 Hz near the value of 100 MHz or from 99,999,000 Hz to 100,000,000 Hz. Thus the actual range of frequency of absorption depends on the magnetic field of the instrument. (This is exactly as expected since the energy gap between the spin states and hence the frequency of absorption are dependent on the applied field.) To compare absorption values from different instruments, a dimensionless scale must be devised that is independent of the magnetic field of the instrument. This is accomplished by using the absorption of tetramethylsilane (TMS) as a spectral anchor. The frequency of absorption of a given set of protons is measured relative to the frequency of absorption of TMS. This absorption frequency difference Δv in hertz (cps) is expressed as δ, the chemical shift of the protons in ppm, where


The chemical shift δ is dimensionless and independent of the spectrometer. Since normal absorption ranges Δv are about 0 – 600 Hz for an operating frequency of 60 × 10 6 Hz, or 0 – 1000 Hz at 100 × 106 Hz, and so on, chemical shifts range from 0 to 10 ppm for most protons.

In practice a small amount of TMS (<1%) is added to the NMR sample, the TMS signal is set at 0 ppm, and the protons of the sample are then measured in parts per million relative to TMS. The choice of TMS as a standard is useful because nearly all other protons absorb at frequencies lower than TMS. It is rou-tine to present NMR spectra with low frequency on the left and high frequency on the right (Figure 11.5). Thus the TMS signal defines δ = 0 ppm on the right side of the spectrum and other proton signals are found to the left or downfield from TMS from 0 to about 10 ppm. 


It is also normal to describe signals having larger chemical shifts as being downfield from protons with smaller chemical shifts. The left side of the spectrum is termed low field and the right side high field.

With a method available to measure differences in chemical shifts between protons, it is appropriate to ask why different protons experience different Heff’s even though a single H0 is applied to the sample. The explanation lies in the fact that nuclei are surrounded by electron clouds (Figure 11.6). In the applied field H0, electron pairs in bonds surrounding the hydrogens act to counter the applied field by induced fields (Hind). The result is that the nucleus is shielded from the applied field by its electron cloud. (Nuclei which are more shielded come at higher fields and have lower chemical shifts.)

Thus it is the electron density around the nucleus which shields the nucleus from the applied field. It follows that the greater the electron density around a proton, the larger will be the induced field Hind and that proton will be more shielded. It will appear more upfield and will have a smaller chemical shift (δ value). Conversely the lower the electron density around a proton, the less shielded it will be, the more downfield it will be, and it will have a larger δ value (Figure 11.7).


Structural features which withdraw electrons from protons cause downfield shifts and larger δ values, while structural features which increase electron density around protons cause upfield shifts and lower δ values. For example, chemical shifts for methyl chloride, dichloromethane, and chloroform are δ = 3.0, δ = 5.5, and δ = 7.1, respectively. The inductive effects of increasing numbers of chlorine atoms decrease the electron density about the hydrogens and result in increasing chemical shifts.


Likewise 1,2,2-trichloropropane discussed previously has the two-proton sig-nal downfield from the three-proton signal. This is because the methylene protons are influenced by the inductive effects of three chlorine atoms, two vicinal and one geminal, while the methyl group is influenced by only two vicinal chlorine atoms. The electron density is higher at the methyl hydrogens, which are more shielded and occur at higher fields than the two protons of the methylene group.


Consideration of a series of compounds containing methyl groups illustrates clearly the influence of the electron density on chemical shift. As the electron-withdrawing ability of groups attached to the methyl group increase, progressive downfield shifts are evident and δ values increase. Conversely TMS comes very far upfield because silicon – carbon bonds are polarized toward carbon and result in very high electron density about the methyl hydrogens of TMS.


Although the influence of electron density on chemical shift is clear, it is not the only factor which determines the chemical shift, as seen from the following series of compounds:


Comparing the methyl groups, we find that typical saturated aliphatic methyl groups come at 0.9 – 1.1 ppm. However, attaching a methyl group to a double bond gives a change to 1.8δ. Attaching the methyl group to an aromatic ring moves it further downfield to 2.4δ. Attachment to a triple bond moves it back upfield to 1.3δ. Analogous but even larger changes in chemical shift are seen for protons directly attached to double bonds, aromatic rings, and triple bonds. Simple electron density shielding arguments cannot satisfactorily account for these large changes in chemical shift.

For example, the greater s character of sp2 orbitals and hence greater effective electronegativity of sp2-hybridized carbon might account for the downfield shift of the protons of a methyl group when it is attached to an olefinic carbon rather than a saturated sp3 carbon; however, the sp2 carbons of aromatic rings should induce the same downfield shift. In fact, aromatic methyl groups are shifted significantly further downfield. By the same argument, attachment of a methyl group to the sp-hybridized carbon of an acetylene, which has even greater s character, should cause the chemical shift to move even further downfield. In fact, propargylic methyl groups are found at higher field than allylic methyl groups.

It is clear that there are other factors at work which influence the chemical shifts of different types of protons.

Simple shielding of the hydrogen nucleus by its surrounding cloud of electrons is isotropic in that the induced magnetic field is the same for any orientation of the hydrogen relative to the magnetic field. This is due to the fact that the electron cloud around the hydrogen nucleus behaves as though it is spherical (or nearly so). Other types of electron clouds (double bonds, aromatic clouds, triple bonds) are not spherically symmetric. As a consequence, the induced fields for these types of bonds are not the same at different orientations of the functional group in the magnetic field. This anisotropic shielding, or anisotropy, leads to regions of shielding and deshielding around the functional group that are averages of the orientations possible.


Aromatic rings have among the strongest anisotropy of any group. Above and below the ring there is a strong shielding region (Hind is in opposition to the applied field) while in the plane of the ring there is a strong deshielding region (Hind is in the same direction as the applied field). This phenomenon is termed ring current and has been used as a criterion to establish whether a compound is aromatic (Figure 11.8). Consequently protons and groups attached to the ring are in the plane of the ring and thus are strongly deshielded and come at low fields relative to a comparable proton in a nonaromatic compound. Aromatic protons normally come at δ > 7 ppm and benzylic methyl groups come at δ 2.4, which are both significantly shifted downfield due to the anisotropy of the aromatic ring. (The shift of benzylic protons is less than the shift of aromatic protons because they are further from the aromatic ring than the protons directly attached to the ring.)


If protons could be positioned in the center of or above the aromatic ring, they would fall in the shielding region and should come at high field. For example, 18-annulene is an aromatic compound (4n + 2, n = 4). The protons on the outside of the ring lie in the deshielding region and have δ = 9.3 ppm while those on the inside of the ring fall in the shielding region and have δ = −3.0. They come at higher field than TMS due the anisotropic shielding from the ring current. For the same reason, the central protons in p-cyclophanes come at higher fields because they are placed over the aromatic ring in the shielding region.

Double bonds contain one σ bond and one π bond, which results in anisotropic shielding, as shown in Figure 11.9. There is a conical shielding region normal to the molecular plane and a deshielding region in the molecular plane. This is true for all double-bonded functional groups such as olefins, carbonyl groups, and imines, and it explains why olefinic protons (δ 5) and aldehyde protons (δ = 9 – 10) absorb at such low fields.


Acetylene (and nitriles), because of their cylindrical symmetry, have shielding regions along the triple-bond axis (Figure 11.10). Thus groups attached to the triple bond are constrained to the shielding region and are shifted upfield relative to similar vinyl protons. Thus acetylenic protons come at δ = 2 – 3 and propargylic methyl groups are upfield from allylic methyl groups.

The chemical shift of a given proton is thus determined by a combina-tion of isotropic shielding by the electron cloud surrounding the proton and by anisotropic shielding due to the presence of nearby functional groups which are strongly anisotropic. These factors are usually sufficient to give unique chemical shifts for most protons in a molecule, and they can normally be distinguished using modern high-field NMR spectrometers (200 – 300 MHz). Furthermore the integration of these signals gives the numbers of the different types of protons.

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