A CH group in the middle of a carbon skeleton resonates at about 1.7 ppm—another 0.4 ppm downfi eld from a CH2 group. It can have up to three substituents and these will cause further downfi eld shifts of about the same amount as we have already seen for CH3 and CH2 groups. Three examples from nature are nicotine, the methyl ester of lactic acid, and vitamin C. Nicotine, the compound in tobacco that causes the craving (although not the death, which is doled out instead by the carbon monoxide and tars in the smoke), has one hydrogen atom trapped between a simple tertiary amine and an aromatic ring at 3.24 ppm. The ester of lactic acid has a CH proton at 4.3 ppm. You could estimate this with reasonable accuracy using the guidelines in the two summary boxes on pp. 273 and 274. Take 1.7 (for the CH) and add 1.0 (for C=O) plus 2.0 (for OH) = 4.7 ppm—not far out. Vitamin C (ascorbic acid) has two CHs. One at 4.05 ppm is next to an OH group (estimate 1.7 + 2.0 for OH = 3.7 ppm) and one is next to a double bond and an oxy gen atom at 4.52 ppm (estimate 1.7 + 1 for double bond + 2 for OH = 4.7 ppm). Again, not too bad for a rough estimate.

An interesting case is the amino acid phenylalanine whose CH₂ group we looked at a moment ago. It also has a CH group between the amino and the carboxylic acid groups. If we record the ¹H NMR spectrum in D₂O, in either basic (NaOD) or acidic (DCl) solutions, we see a large shift of that CH group. In basic solution the CH resonates at 3.60 ppm and in acidic solution it resonates at 4.35 ppm. There is a double effect here: CO₂H and NH₃⁺ are both more electron withdrawing than CO₂⁻ and NH₂ so both move the CH group downfield.

● A simple guide to estimating chemical shifts

 We suggest you start with a very simple (and therefore necessarily oversimplified) picture, which should be the basis for any further refinements. Start methyl groups at 0.9, methylenes (CH2) at 1.3, and methines (CH) at 1.7 ppm. Any functional group is worth a 1 ppm downfi eld shift except oxygen and halogen which are worth 2 ppm. This diagram summarizes this approach.

The guide above is very rough and ready, but is easily remembered and you should aim to learn it. However, if you want to, you can make it slightly more accurate by adding further subdivisions and separating out the very electron-withdrawing groups (nitro, ester OCOR, fluoride), which shift by 3 ppm. This gives us the summary chart on this page, which we suggest you use as a reference. If you want even more detailed information, you can refer to the tables in Chapter 18 or better still the more comprehensive tables in any specialized text (see the Further reading section).

مواضيع ذات صلة

How electron donation and withdrawal change chemical shifts

Uneven electron distribution in aromatic rings

The benzene ring current causes large shifts for aromatic protons

Summary chart of proton NMR shifts

CH and CH2 groups have higher chemical shift than CH3 groups

Methyl groups give us information about the structure of molecules

Proton chemical shifts tell us about chemistry

Protons on saturated carbon atoms Chemical shifts are related to the electronegativity of substituents

Regions of the proton NMR spectrum

Integration tells us the number of hydrogen atoms in each peak

The differences between carbon and proton NMR

Kinetic versus thermodynamic products