1.8. What Can We Learn? Chemical Environments and Functional Groups

There are four practical pieces of information that can be extracted from a typical ¹H NMR spectrum. The third of these is an educated guess at the functional groups in the molecule (what kind of functional group(s) each signal’s hydrogens could be part of). For now, the differences in shape for each signal (see Section 1.9) should be ignored.

1.8.1. Functional Groups and Standard Chemical Shift Ranges

The chemical shift for an NMR signal is determined by the ΔE value for the nuclei that generated it (see Section 1.4). This value is dependent on several things, primarily the strength of the external magnetic field as experienced by those nuclei. This is principally influenced by the amount of electron density near the nucleus (see Section 1.6.1).

Recall that a functional group is a collection of specific atoms connected in a specific way. Because the connectivity of atoms in a functional group is the same in every molecule the atoms in any given functional group will have approximately the same amount of electron density around them in every molecule. This means that, barring outside factors, hydrogen atoms from a given functional group will have approximately the same chemical shift in any molecule. For example, the amount of electron density around the hydrogen atom of the aldehydes in propanal and cinnamaldehyde is roughly the same despite the rest of the structure being significantly different (Figure 1.23) and the chemical shift for those hydrogens is roughly the same in each compound’s ¹H NMR spectrum.

Figure 1.23 – Examples of Similar Chemical Shifts for Hydrogens of the Same Functional Group.

This can be generalized to almost all functional groups. Most commonly these ranges are summarized as a table or chart. Excellent examples of both are available from a large number of sources online, or in textbooks dedicated to NMR spectroscopy. For consistency, a simple table is provided with typical ranges for chemical shifts of hydrogens in common functional groups (Figure 1.24). Hydrogen atoms attached to oxygen or nitrogen tend to have very high variability in their chemical shifts (see Section 1.10.2) and are included separately from those attached to carbon atoms. These are only the “normal” ranges. Outside factors (see Section 1.8.2) can cause signals to fall outside of these ranges.

Figure 1.24 – Typical ¹H NMR Chemical Shifts for Common Functional Groups.

1.8.2. Chemical Shift and Electron Density Complications

Standard chemical shifts are compiled by measuring the chemical shifts of hydrogen atoms from a large number of “normal” molecules. However, it is possible even for simple molecules to produce values outside of these ranges. Because chemical shift is affected by the amount of electron density nearby, having electronegative atoms close to a group will affect the chemical shift of that group.

The induction effect removes electron density from nearby atoms (including hydrogens) which affects chemical shift (Figure 1.25). All standard rules for assessing strength of induction apply: electronegativity, multiple electronegative atoms, proximity, etc. In the vast majority of cases having electronegative atoms makes signals for nearby hydrogen atoms deshielded (moves them upfield, increases the ppm value).

Figure 1.25 – Examples of Effects from Nearby Electronegative Atoms on ¹H NMR Chemical Shifts.

Note that the effect does not have to be pronounced (Figure 1.26). Seeing an increase in ppm probably means there are nearby electronegative atom(s) but NOT seeing an increase does not necessarily mean there are none nearby. At the same time, other complex factors may result in an increase in chemical shift. This is particularly common when groups add or possess conjugation. This is already highlighted in most standard chemical shift tables. Both of these are mentioned only to reinforce that the effects of nearby functional groups can affect chemical shift and may be complex.

Figure 1.26 – Examples of Small Effects from Nearby Electronegative Atoms and Complex Additional Effects on ¹H NMR Chemical Shifts.

1.8.3. How To Determine Possible Functional Groups from Chemical Shifts

It is possible to analyze a ¹H NMR spectrum and determine possible functional groups in the corresponding molecule. Doing this with only a spectrum is often challenging. This is much easier when additional information is given, such as the molecular formula. Once signal shapes are understood (see Section 1.9) this task again becomes simpler.

Determine the approximate chemical shifts of all signals. Because signal shapes have not yet been discussed individual signals are highlighted for simplicity. If you are given chemical shifts for signals skip this step.

Check chemical shifts against table ranges. Remember to use any additional information you may be given or can infer. For example, after the blue signal is identified as a carboxylic acid we know there are electronegative atoms that might increase chemical shifts so ranges slightly above standard values need to be considered.

Apply logic to look for likely functional groups. Sometimes multiple signals may indicate a similar functional group “seen” from both sides. If there is ambiguity be cautious in assessment. In general, avoid considering OH or NH options unless the peak shape indicates one of these is likely (see Section 1.10.2) or it is unambiguous (e.g. a carboxylic acid)

Based on chemical shifts, multiple signals may be from hydrogens adjacent to alkenes.

There may be an alkene.

Based on chemical shifts, multiple signals may be from hydrogens adjacent to carbonyls.

There is a carbonyl in the carboxylic acid.

There may be another carbonyl.

There are no likely Ar-H signals.

There are probably no aromatic rings (rule out options with Ar).

Etc.

A common task involves students performing the assessment and then choosing from potential options. For example, if asked to choose which molecule best corresponds to the spectrum from the options below it is obvious without any extra work or analysis that only Molecule B is possible (no other options have carboxylic acids). Checking the chemical shifts of the remaining signals would support this choice.

This procedure is often done with extra data provided. For example, if the molecular formula for this molecule were given as C₇H₁₂O₄, much better conclusions could be drawn: rule out halides, rule out NH options, two oxygens used in carboxylic acid means there are two left to be carbonyl(s), etc.

Most commonly this procedure is done as part of the larger process of confirming whether a spectrum matches a candidate structure (see Sections 1.11 and 1.12).