An Introduction to NMR Spectroscopy Shelby Feinberg and Steven Zumdahl Nuclear magnetic resonance spectroscopy NMR has risen to the same level of importance as electronic and vibrational spectroscopy as a tool for studying molecular properties particularly structural properties Although the following discussion of nmr specifically deals with hydrogen atom nuclei in organic molecules the principles described here apply to other types of molecules as well Many other types of nuclei 13C 19F 31P etc have nuclear spins and thus can be studied using NMR techniques Certain nuclei such as the hydrogen nucleus but not carbon 12 or oxygen 16 have a nuclear spin The spinning nucleus generates a small magnetic field called When placed in a strong external magnetic field called Ho the nucleus can exist in two distinct spin states a low energy state A in which is aligned with the external magnetic field Ho and a high energy state B in which is opposed to the external magnetic field Ho figure 1 Alignment of with Ho is the more stable and lower energy state Ho Ho Figure 1 Spin 1 2 Parallel A Low Energy Spin 1 2 Anti parallel B High Energy In NMR transitions from the more stable alignment A with the field to the less stable alignment B against the field occur when the nucleus absorbs electromagnetic energy that is exactly equal to the energy separation between the states E This amount of energy is usually found in the radiofrequency range The condition for absorption of energy is called the condition of resonance It can be calculated as the following E h H h 2 h Planck s constant H the strength of the applied magnetic field Ho at the nucleus the gyromagnetic ratio a constant that is characteristic of a particular nucleus the frequency of the electromagnetic energy absorbed that causes the change in spin states There are three features of NMR spectra that we will focus on the number and size of signals the chemical shift and spin spin coupling Number and Size of Signals Let s consider how the NMR spectrometer can distinguish between hydrogen nuclei and produce multiple signals Magnetically equivalent hydrogen nuclei produce one signal These hydrogen nuclei experience the same local environment For example in a molecule such as diethyl ether Figure 2 there are two sets of magnetically equivalent hydrogens The hydrogens labeled a are six magnetically equivalent methyl hydrogens while the hydrogens labeled b are four magnetically equivalent methylene hydrogens Notice that the methyl a hydrogens are all located adjacent to a carbon containing two hydrogen atoms Additionally the methylene b hydrogens are all located adjacent to an oxygen atom and a carbon atom containing three hydrogen atoms Figure 2 CH3 a CH2 b O CH2 CH3 b a Because of rapid rotations about sigma bonds and molecular symmetry the six methyl hydrogens a and the four methylene hydrogens b comprise two individual magnetically equivalent groups of hydrogens The methyl hydrogens a experience a different total magnetic field than the methylene hydrogens b because of different local magnetic fields As a result the resonance energy E corresponding to the frequency of absorption will be different for these two groups of hydrogen nuclei Thus the NMR spectrometer can distinguish between groups of hydrogen nuclei which experience different local magnetic fields Different frequencies are required for the two different groups of hydrogens thereby producing two distinct signals The areas of the signals are directly proportional to the number of hydrogens So in the case of diethyl ether the two peaks have an area ratio of 3 2 or 6 4 because there are six methyl hydrogens a and four methylene hydrogens b We will look at an actual spectrum presently The Chemical Shift When an organic molecule is placed in an external magnetic field H o each hydrogen nucleus experiences a total field that is the sum of H o and two other local magnetic fields one produced by bonding and non bonding electrons He and the other produced by neighboring protons which possess a nuclear spin Hh Thus the applied magnetic field H o is constant while the magnetic fields produced as H e and Hh are not constant The magnetic field produced by the neighboring electrons He determines the position relative frequency of an nmr peak called the chemical shift The magnetic fields produced by neighboring nuclei H h are smaller than He and cause splitting of an NMR peak called spin spin coupling This will be discussed later For the moment let s assume that Hh is zero There is no spin spin coupling Under these conditions the total magnetic field H experienced by a particular nucleus is given by H o He That is we are assuming that the resonance energy is only dependent on the sum of the external magnetic field and the magnetic field produced by neighboring electrons A mathematical representation of the value of this resonance energy is the following E h Ho He h 2 If we vary the frequency of the electromagnetic energy until h E absorption will cause a transition in spin states and a signal will be recorded by the NMR spectrometer The position of an NMR signal is recorded relative to the position of the signal of an internal standard This standard is commonly tetramethyl silane TMS CH 3 4Si If TMS absorbs at frequency s and the hydrogen nucleus of interest absorbs at the spectrometer will record a signal at s 10 6 where o is the spectrometer frequency commonly 60 o megacycles per second is called the position of absorption or the chemical shift expressed as parts per million ppm on a scale of 0 10 ppm TMS absorbs at 0 0 ppm while most nuclei absorb downfield towards 10 ppm The value of is independent of Ho but it DOES depend on He Figure 3 gives a few examples of how neighboring atoms affect the chemical shifts of hydrogen atoms Figure 3 H O H H C C C C C C C O O RCOH RCH H H X N H H C satd C C or O 10 9 8 Downfield Deshielding 7 6 Ho 5 4 3 Upfield Shielding 2 TMS 1 0 ppm Hydrogen nuclei which absorb at large are said to be deshielded from the external magnetic field These signals appear downfield towards 10 ppm from TMS because the frequency at which they absorb differs greatly from the frequency of the TMS hydrogens As a result the value of s is large Deshielding is caused by adjacent atoms which are strongly electronegative e g oxygen nitrogen halogen or groups of atoms which possess electron clouds e g C O C C aromatics Let s now look at the nmr spectrum of benzyl acetate Figure 4 Figure 4 O