Shift in H-NMR Spectroscopy

Atomic number Mass number Spin quantum number 

The above relationships emerge from the fact that the protons and neutrons in the nucleus possess spins. Protons will thus form pairs with other protons in the nucleus with opposite spins. Similarly, neutrons will pair with other neutrons in the same nucleus but with opposite spins. In nuclei which have even numbers of protons and neutrons, all the spins will be paired and the spin number / will be zero. However if there is an odd number of either protons or neutrons, the spin quantum number I will have a quantized value of i, 1, etc. If the sum of protons and neutrons is even, I will be zero or a multiple of 1. If the sum is odd, I will be an integral multiple of j. The nuclei with which the organic chemist is most frequently concerned are and both of which have a spin quantum number of Other elements which may be of interest are and P which also have / = i while deuterium, and have a spin quantum number of 1. 

In nuclei such as or which have / = 1, there will be (2 x 1) -h 1 orientations, i.e., three different orientations, each having its characteristic 

As mentioned earlier, electromagnetic radiation of the correct frequency can cause transitions between adjacent energy levels. The relationship between the electromagnetic frequency v and the magnetic field strength Bq is governed by the Larmor equation 

In order to cause transitions between the two spin states, a second radio frequency field is applied perpendicular to the original field. When the value of this electromagnetic radiation reaches the precessional frequency of the nucleus, absorption of energy will occur. It is important to note that the frequency of the electromagnetic radiation required to induce transitions from one nuclear spin state to the other is exactly equal to the precessional frequency cdq of the nucleus. The precessional frequency coq is directly proportional to the applied magnetic field Bq and also to the gyromagnetic ratio 

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The discussions in this section briefly described the use of local descriptors. Another application of local descriptors is the characterization of atoms in nuclear magnetic resonance (NMR) spectroscopy. This is described later with an application for the prediction of chemical shifts in H-NMR spectroscopy, where protons were represented by their local RDF descriptors. 

Another approach has been developed for the prediction chemical shifts in H-NMR spectroscopy. In this case, special proton descriptors were applied to characterize the chemical environment of protons. It can be shown that 3D proton descriptors in combination with geometric descriptors can successfully be used for the fast and accurate prediction of H-NMR chemical shifts of organic compounds. The results indicate that a neural network can make predictions of at least the same quality as those of commercial packages, especially with rigid structures where 3D effects are strong. The performance of the method is remarkable considering a relatively small data set that is required for training. A particularly useful feature of the neural network approach is that the system can be easily dynamically trained for specific types of compounds. 

The prediction of chemical shifts in H-NMR spectroscopy is usually more problematic than in C-NMR. Experimental conditions can have an influence on the chemical shifts in H-NMR spectroscopy and structural effects are difficult to estimate. In particular, stereochemistry and 3D effects have been addressed in the context of empirical H-NMR chemical shift prediction only in a few specific situations [81,82]. Most of the available databases lack stereochemical labeling, assignments for diastereo-topic protons, and suitable representations for the 3D environment of hydrogen nuclei [83]. This is the point where local RDF descriptors seemed to be a promising tool. 

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