Chemical Shift in Proton NMR Spectroscopy

 Nuclear Magnetic Spectroscopy(NMR):

The nuclei of certain elements, including 1H nuclei (protons) and 13C (carbon-13) nuclei, behave as though they were magnets spinning about an axis. When a compound containing protons or carbon-13 nuclei is placed in a very strong magnetic field and simultaneously irradiated with electromagnetic energy of the appropriate frequency, nuclei of the compound absorb energy through a process called magnetic resonance. The absorption of energy is quantized.

A graph that shows the characteristic energy absorption frequencies and intensities for a sample in a magnetic field is called a nuclear magnetic resonance (NMR) spectrum. As a typical example, the proton (1H) NMR spectrum of 1-bromoethane is shown in Fig.9.1. We can use NMR spectra to provide valuable information about the structure of any molecule we might be studying. In the following sections we shall explain how four features of a molecule’s proton NMR spectrum can help us arrive at its structure.

Chemical Shift in Proton NMR Spectroscopy
 Proton NMR Spectroscopy

1. The number of signals in the spectrum tells us how many different sets of protons there are in the molecule. In the spectrum for 1-bromoethane (Fig. 9.1) there are two signals arising from two different sets of protons. One signal (consisting of four peaks) is shown in blue and labeled (a). The other signal (consisting of three peaks) is in red and is labeled (b). These signals are shown twice in the spectrum, at a smaller scale on the baseline spectrum, and expanded and moved to the left above the base spectrum. [Don’t worry now about the signal at the far right of the spectrum (labeled TMS); it comes from a compound (tetramethylsilane) that was added to the 1-bromoethane so as to calibrate the positions of the other signals.]

2. The position of the signals in the spectrum along the x-axis tells us about the magnetic environment of each set of protons arising largely from the electron density in their environment. We’ll learn more about this in Section Chemical Shift.

3. The area under the signal tells us about how many protons there are in the set being measured. We’ll learn how this is done in Section 9.2B. 4. The multiplicity (or splitting pattern) of each signal tells us about the number of protons on atoms adjacent to the one whose signal is being measured. In 1-bromoethane, signal (a) is split into a quartet of peaks by the three protons of set (b), and signal (b) is split into a triplet of peaks by the two protons of set (a).

Chemical Shift:

 The position of a signal along the x-axis of an NMR spectrum is called its chemical shift.

 The chemical shift of each signal gives information about the structural environment of the nuclei producing that signal.

Counting the number of signals in a 1H NMR spectrum indicates, at a first approximation, the number of distinct proton environments in a molecule. Tables and charts have been developed that allow us to correlate chemical shifts of NMR signals with likely structural environments for the nuclei producing the signals. Table 9.1 and Fig. 9.2, for example, are useful for this purpose. 1H NMR chemical shifts generally fall in the range of 13–0 ppm (d).

Chemical Shift in Proton NMR Spectroscopy
Chemical Shift in Proton NMR Spectroscopy

Chemical Shift in Proton NMR Spectroscopy
Chemical Shift in Proton NMR Spectroscopy

The chemical shift of a signal in an NMR spectrum depends on the local magnetic environment of the nucleus producing the signal. The local magnetic environment of a nucleus is influenced by electron density and other factors we shall discuss shortly. The physical meaning of chemical shift values relates to the actual frequency of the NMR signals produced by the nuclei. The practical importance of chemical shift information is that it gives important clues about molecular structure. Each NMR signal indicates the presence of nuclei in a different magnetic environment. Chemical shifts are measured along the spectrum axis using a delta (d) scale, in units of parts per million (ppm). When comparing one signal with another:

A signal that occurs further to the left in the spectrum than another (i.e., at a higher d or ppm value) is said to occur downfield.

 A signal to the right is said to occur upfield. The terms upfield and downfield relate to the strength of the magnetic field (higher versus lower, respectively) that is required to bring the nuclei into resonance.

The 1HNMR spectrum of 1,4-dimethylbenzene (p-xylene), shown in Fig. 9.3, is a simple example that we can use to learn how to interpret chemical shifts. First, note that there is a signal at d 0. The signal at d 0 is not from 1,4-dimethylbenzene, but from tetramethylsilane (TMS), a compound that is sometimes added to samples as an internal standard to calibrate the chemical shift scale. If the signal from TMS appears at zero ppm, the chemical shift axis is calibrated correctly. 

Next we observe that there are only two other signals in the 1H NMR spectrum of 1,4dimethylbenzene, at approximately d 7.0 and d 2.3. The existence of just two signals implies that there are only two distinct proton environments in 1,4-dimethylbenzene, a fact we can easily verify for ourselves by examining its structure. We say, then, that there are “two types” of hydrogen atoms in 1,4-dimethylbenzene, and these are the hydrogen atoms of the methyl groups and the hydrogen atoms of the benzene ring. The two methyl groups produce only one signal because they are equivalent by virtue of the plane of symmetry between them. Furthermore, the three hydrogen atoms of each methyl group are equivalent due to free rotation about the bond between the methyl carbon and the ring. 

Chemical Shift in Proton NMR Spectroscopy
Chemical Shift in Proton NMR Spectroscopy

The benzene ring hydrogen atoms also produce only one signal because they are equivalent to each other by symmetry. Referring to Table 9.1 or Fig. 9.2, we can see that 1H NMR signals for hydrogen atoms bonded to a benzene ring typically occur between d 6 and 8.5, and that signals for hydrogen atoms on an sp3 carbon bonded to a benzene ring (benzylic hydrogens) typically occur between d 2 and 3. Thus, chemical shifts for the signals from 1,4-dimethylbenzene occur where we would expect them to according to NMR spectral correlation charts. In the case of this example, the structure of the compound under consideration was known from the outset. Had we not known its structure in advance, however, we would have used chemical shift correlation tables to infer likely structural environments for the hydrogen atoms.

Post a Comment