H NMR Spectroscopy Basics

Introduction

Proton Nuclear Magnetic Resonance (also known as Proton NMR or 1H NMR) is a technique that applies nuclear magnetic resonance to the H-1 nuclei. This method is used to ascertain the molecular structure of the substance. When natural hydrogen (H) is used in samples, the hydrogen predominantly comprises the isotope 1H (hydrogen-1), which has a proton as its nucleus. H NMR allows chemists to elucidate the structure of a molecule by determining the connectivity of the carbons and protons. NMR spectra are typically recorded in solution, and it’s crucial to prevent solvent protons from interfering. For NMR usage, deuterated solvents are favored. Examples include deuterated water (D2O), deuterated acetone ((CD3)2CO), deuterated methanol (CD3OD), deuterated dimethyl sulfoxide ((CD3)2SO), and deuterated chloroform (CDCl3). The Proton NMR spectra of most organic compounds are distinguished by chemical shifts ranging from +14 to -4 ppm and by spin-spin coupling among protons. The integration corresponding to each proton mirrors the abundance of individual protons.

Chemical Shifts

Chemical shift values, denoted by δ, are not exact but are generally considered as a point of reference. These values can deviate within a range of ±0.2 ppm, and sometimes even more. The precise value of the chemical shift is influenced by factors such as the molecular structure, the solvent used, temperature, the magnetic field in which the spectrum is recorded, and the presence of other functional groups nearby. Hydrogen nuclei are particularly sensitive to the hybridization state of the atom to which they are attached and to electronic effects. Groups that withdraw electron density tend to deshield nuclei (move downfield), causing these nuclei to resonate at higher δ values, while shielded nuclei resonate at lower δ values.

Substituents that withdraw electron density, such as -OH, -OCOR, -OR, -NO2, and halogens, induce a downfield shift of about 2–4 ppm for H atoms on al[ha-carbons and less than 1–2 ppm for H atoms on beta-carbons. "Alpha carbon" refers to an aliphatic C atom directly bonded to the substituent in question, and "beta carbon" refers to an aliphatic C atom bonded to the alpha-carbon. The presence of carbonyl groups, olefinic fragments, and aromatic rings introduces sp2 hybridized carbon atoms into an aliphatic chain, resulting in a downfield shift of 1–2 ppm at the alpha carbon. It’s important to note that labile protons (-OH, -NH2, -SH) do not have a characteristic chemical shift. However, their resonances can be identified by the disappearance of a peak when the sample is treated with D2O, as the deuterium replaces the proton. This technique is known as a D2O shake.

Spin-Spin Coupling (Peak Splitting):

Apart from the chemical shift, NMR spectra also facilitate structural determinations through spin-spin coupling and integrated intensities. Nuclei, possessing their own minute magnetic fields, exert influence on each other, altering the energy and consequently the frequency of nearby nuclei as they resonate. This phenomenon is referred to as spin-spin coupling. Scalar coupling is an interaction between two nuclei via chemical bonds, typically observable up to three bonds away (3-J coupling). The impact of scalar coupling can be comprehended by examining a proton with a signal at 1 ppm. In a hypothetical molecule, another proton exists three bonds away (for example, in a CH-CH group). The neighboring group’s magnetic field causes the 1 ppm signal to bifurcate into two peaks, one a few hertz above 1 ppm and the other the same number of hertz below 1 ppm. Each of these peaks possesses half the area of the original singlet peak. The magnitude of this splitting, or the difference in frequency between peaks, is termed the coupling constant. For aliphatic protons, a typical coupling constant value would be 6-7 Hz.

The coupling constant is not dependent on the strength of the magnetic field as it is induced by the magnetic field of another nucleus, not the spectrometer magnet. Hence, it is expressed in hertz (frequency) rather than ppm (chemical shift).

A peak is divided by n identical protons into components whose sizes correspond to the ratio of the nth row of Pascal’s triangle. As the nth row comprises n+1 components, this type of splitting adheres to the “n+1 rule”: a proton with n neighbors manifests as a cluster of n+1 peaks.

Peak Area (Integration):

Area under each peak = peak integration
Peak areas may be given as non-normalized values or as normalized values
- If values are non-standardized, each value can be divided by the lowest peak area so that the lowest value becomes 1
- If the number of protons for a specific signal (peak) is known, it can be manually set on the spectrum to that number of protons. For example, if you know that the aldehyde signal, around 10 ppm, corresponds to 1 proton, then setting that integration value to 1 will reveal the number of protons for every other signal on the spectrum as well
Usually, H NMR spectra given in courses will provide normalized peak areas. In this case, the peak area simply corresponds to the number of protons in the given signal

Types of Protons

Homotopic Protons

All protons are the same/equivalent
Appear as one signal on the spectrum

Enantiotopic Protons

The protons are equivalent
If one is replaced by a deuterium, the two resulting molecules would be enantiomers, so the protons are said to be enantiotopic
Appear as one signal on the spectrum

Diastereotopic Protons

The protons are adjacent to a stereocenter and see different electronic environments
If one is replaced by a deuterium, the two resulting molecules would be diastereomers, so the protons are said to be diastereotopic
Appear as different signals on the spectrum

Putting it All Together

Step 1:

Look at the chemical shift. Try to approximately compare the chemical shift of the signal on your spectrum to the diagram on page 18 (or any similar diagram) to see what type of proton it may be. Note that the ranges on the diagram are approximate and the actual chemical shift depends on the specific electronic environment of the proton in question.

Step 2:

Look at the peak integration. Often, normalized integrations will be given and will directly tell you the number of protons corresponding to the peak. If the integration is not normalized, follow the procedure described above.

Step 3:

Look at the coupling pattern for each peak, it tells you how many neighboring protons are present. This is the final clue that an H NMR spectrum gives you, and the clues given for each peak can be put together to deduce the connectivity of the molecule.

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