Resonance frequencies and chemical shifts

A particular nucleus (e.g. H, C, P) absorbs characteristic radiofrequencies, i.e. it resonates at a characteristic frequency. If an NMR spectrometer is tuned to a particular resonance frequency, only a selected NMR active nucleus is observed. For example, only H nuclei are observed if a 400 MHz spectrometer is tuned to 400 MHz, but if the same spectrometer is retuned to 162MHz, only nuclei are observed. This is analogous to tuning a radio and receiving only one station at a time. 

In a H NMR experiment, protons in different chemical environments resonate at different frequencies. The same is true of, for example, non-equivalent nuclei in a NMR experiment, or non-equivalent nuclei in a F NMR spectroscopic experiment, and so on. Each signal in an NMR spectrum is denoted by a chemical shift value, 6, a value that is given relative to the signal observed for a specified reference compound (see below). 

The parameter 8 is independent of the applied magnetic field strength and is defined as follows. The frequency difference (A /), in Hz, between the signal of interest and some defined reference frequency (yf) is divided by the absolute frequency of the reference signal (eq. 4.12)  

Typically, this leads to a very small number. In order to obtain a more convenient number for 6, it is usual to multiply the ratio in eq. 4.12 by 10 . This gives 6 in units of parts per million, ppm. The lUPAC defines 6 according to eq. 4.12, but eq. 4.13 gives a method of calculating S in ppm. 

It follows that you use eq. 4.14 to work out the frequency difference between two spectroscopic peaks in Hz when 

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NMR spectroscopists are also interested in nuclear spin relaxation times. The relaxation time measures the time required for an excited nucleus to return to the ground state. Two types of relaxation times are involved spin-lattice relaxation time Ti, the time constant for thermal equilibrium between the nuclei and crystal lattice, and spin-spin relaxation time Tz, the time constant for thermal equilibrium between nuclei themselves. Information on molecular dynamics can be obtained from these relaxation times. Generally, T Tz for low-viscosity liquids and T Tz for solids. A combination of information in molecular dynamics (from relaxation times), molecular structure (from spin-spin interaction), molecular identification (from resonance frequency and chemical shift), and spin density (from signal intensity) make the NMR an extremely versatile tool. 

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