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Nuclear Magnetic Resonance Spectroscopy Topics 1. Theory behind NMR 2. Chemical shifts 3. Integration of signals 4. Splitting of signals One of the most powerful tools available to chemists for determining molecular structure. The test sample can be recovered since the measurement does not destroy the molecule. Theory behind NMR The proton in the nucleus has a positive charge. The protons act as if they were spinning and they generate a magnetic field. Nuclei which have an odd number of protons can behave as if they were tiny bar magnets. Of importance to organic chemists is that the hydrogen nucleus, one proton, and the carbon-13 nucleus, 13 protons, both have this property. We will discuss proton magnetic resonance spectroscopy, i.e. that arising from the hydrogen nucleus. When a molecule containing hydrogen atoms is placed in a strong magnetic field, the spinning proton of the nucleus of each hydrogen atom aligns either with the magnetic field or against the magnetic field. The energy difference between the two states is very very small, but those protons aligned with the field are of slightly lower energy than those aligned against the field. Therefore, there are a few more protons aligned with the field. When a beam of electromagnetic energy of the proper energy is applied to the sample, some of the protons which are aligned with the field absorb energy and flip to become aligned against the field. This is the state calle ‘resonance’. As the magnetic field strength is increased, the energy needed to flip the proton becomes higher so that the frequency of the electromagnetic energy which causes resonance is also higher. Chemical Shift If all of the protons in a molecule resonated at exactly the same energy, then there would be only one signal and the spectrum would not be useful. However, protons in a molecule are surrounded by electrons and these electrons also set up tiny electric fields around the proton. These fields make small changes in the energy needed to cause the proton to resonate. Therefore the environment of the proton in the molecule affects the energy at which is resonates. The energy at which the proton in an organic molecule resonates is measured relative to the energy at which the protons in tetramethylsilane, (CH3)4Si resonate. All of the protons in tetramethylsilane (TMS) are identical and so only one signal results. This molecule was chosen as the reference since the protons in it resonate at a slightly higher frequency than the protons in organic molecules so there is no interference. The position of the TMS signal is set to zero and the signals of the other protons are expressed relative to this signal on the so-called delta(δ)-scale where δ = peak distance in Hertz (Hz) from the TMS peak spectrometric frequency in MHz Thus 1δ = 1 part per million of the spectrometer operating frequency and 1δ = 60 Hz at a spectrometer frequency of 60MHz, 100 Hz at a frequency of 100 MHz and 600 Hz at a frequency of 600 MHz. The reason for measuring the chemical shift in this way is that the δ value of the signal is constant regardless of the operating frequency of the spectrometer. A proton that gives a signal at 2.5 δ on a 60 MHz instrument will also give a signal at 2.5 δ on a 600 MHz instrument. Protons with a high electron density around them resonate at high field – closer to TMS. These are called ‘shielded protons’. They require a higher energy to resonate. Protons that are less shielded resonate at lower energy and so their signals are further downfield from TMS. Each chemically non-equivalent set of protons gives a signal at a different value of δ . Integration The area under each peak is proportional to the number of protons giving rise to the peak. Spin-Spin Splitting This is a very useful feature of the signals in an NMR spectrum. The number of peaks in a signal from a proton or set of equivalent protons on a given carbon atom is equal to n + 1, where n is the number of protons on adjacent carbons.