NMR is built on the principle that almost every nucleus has at least one

Nmr is built on the principle that almost every

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NMR is built on the principle that almost every nucleus has at least one (sometimes several) isotope with a non-zero quantum spin number. Nuclei with a non-zero quantum spin number will, when placed in an external strong magnetic field B 0 , align themselves either parallel or opposite to the magnetic field. There is a small difference in energy between these two states, ΔE, where the difference depends on the interaction between the nuclei and the static magnetic field. By supplying an r.f. pulse of the correct frequency ν the nuclei can be forced to change their orientation if the resonance condition is roughly fulfilled: Δ E = h ν (1.2) where h is Plank’s constant. The r.f. pulse supplied during an NMR experiment is typically broad enough to excite all nuclei of one type, regardless chemical shift, in a sample and its center frequency is generally set in the middle of the resulting spectrum. The difference in resonance frequency between different types of nuclei or indeed different isotopes is generally too large to allow for more than one type of nuclei to be detected during a single experiment. 6 4 2 0 -2 Chemical shift (ppm) Figure 1.3: 1 H spectrum of 10 mM TMA Br in D 2 O. The peak at 1.1 ppm is TMA + and the peak at 2.2 ppm is residual 1 H signal from D 2 O. Once a sample has been placed within the NMR magnet, the individual spins align themselves along the orientation of the magnetic field. This takes a non-zero time which is referred to as longitudinal relaxation time or T 1 . A second type of relaxation is T 2 , the spin - spin relaxation time of a sample which reflects relaxation due to interactions with closely connected nucleus. Once an r.f. pulse is supplied, the spins will be forced out of alignment and (if nothing more is done to the sample) they will start to oscillate and an induced signal can be detected. This is know as a free induction decay , FID. The FID is then Fourier transformed from the time domain to the frequency domain and analyzed. An example of a resulting NMR spectra is shown in figure 1.3 . As the r.f. pulse is operating at a set frequency dependent on the field strength, the power supplied to the sample will depend on the length and amplitude of the r.f. pulse. The typical length of an r.f. pulse is in the order of μ s and during an experiment one typically uses what is known as 90 and 180 pulses. The angular notation refers to the flip angle the pulse exacts on the spins.
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1.2. NMR 9 rf acq 90 o Figure 1.4: A standard 1D experiment, used to obtain the spectra in figure 1.3 . Depending on what is sought, the sequence of r.f. pulses required can vary greatly and normally the experiment is repeated several times. Should the sample be diluted several scans will be required, as the signal to noise ratio increases as N where N is the number of scans. A single run of pulses is referred to as a pulse sequence and is typically displayed graphically (see Figure 1.4 ) with boxes along a time axis with a FID displayed to represent data acquisition at the end of the sequence. There are several ways
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