Nmr  (Page 2/6)

Because many solvents also have protons present, their use in obtaining NMR spectra is problematic. The signal due to the protons in a typical organic solvent would be so large that it would swamp any signal due to the sample you want to measure - sort of like trying to see a tiny flashlight in broad daylight outdoors. In order to remedy this problem, one could choose solvents which do not have protons such as ${\text{CS}}_2$ or ${\text{CCl}}_4$ ; however, these are not suitable solvents for modern FT spectrometers. A better solution is to use solvents in which the protons have been replaced by deuterium. Such solvents, known as deuterated solvents, have very similar properties to their proton-analogues. Thus deuterated benzene is very similar to normal benzene. While deuterium does have a spin (spin = 1), the frequency at which the deuterium nucleus resonates in a magnetic field is sufficiently different from that of protons so that its presence does not interfere with the detection of proton signals. In reality, not all protons of a solvent are replaced in deuterated solvents such that a residual peak due to the presence of a small quantity of protons can usually be observed. This peak usually serves as a good reference point for determining the chemical shifts of peaks in the sample since the peak locations of common deuterated solvents are well known. One can also add a small amount of TMS [tetramethylsilene, $\text{Si}({\text{CH}}_3{)}_4$ ] to the sample and use its peak to serve as a reference peak as well.

Table 2. Some commonly used deuterated solvents.

Because the field strengths are so high, it is potentially dangerous for persons with pacemakers to enter into the fringe field region of these magnets. The magnets will also erase the magnetic information stored on IDs and credit cards. The stronger magnets have been known to pull heavy tools up into them if someone with tools walks too close to the magnet. This often causes severe damage to the magnet.

In this set of exercises, we are going to concentrate on ${}^{1}H$ NMR spectroscopy since it is the most widely used and simplest of the NMR-active nuclei to discuss.

Chemical shift

Since the effect being measured involves the measurement of spin states of a nucleus, the values of $\Delta$ E will be affected by the local magnetic field of a nucleus being examined.

The local magnetic field is, in turn, affected by the chemical environment of the nucleus. $\Delta$ E thus becomes a measure of the chemical environment of the nucleus. Hydrogen atoms bonded to ${\text{sp}}^3$ carbon atoms are found in different regions of the NMR spectrum from hydrogen atoms attached to alkene ${\text{sp}}^2$ carbon atoms, alkyne sp carbon atoms, and aromatic ${\text{sp}}^2$ carbon atoms, oxygen, nitrogen, metals, etc.