Hydrogen nucleus, properties

The NMR signal arises from a quantum mechanical property of nuclei called spin . In the text here, we will use the example of the hydrogen nucleus (proton) as this is the nucleus that we will be dealing with mostly. Protons have a spin quantum number of V . In this case, when they are placed in a magnetic field, there are two possible spin states that the nucleus can adopt and there is an energy difference between them (Figure 1.1). 

Various phenomena related to the quantum effects of the nuclei This area includes the properties of hydrogen bonds and proton transfer at low temperatures caused by quantum effects of the hydrogen nucleus. 

Today Bohr s concept of an electronic orbit is no longer tenable, but the modern quantum theory substitutes for the orbit a probability distribution, in which, in the case of the hydrogen atom, the probability is concentrated in the region where the Bohr orbit was. For a free electron (i.e., one not bound to the hydrogen nucleus), the probability distribution looks like that of a wave—a confined region of oscillations, called a wave packet. These wavelike properties are extremely difficult to observe under normal conditions because typical wavelengths of electrons are extremely short—around 10 or m. Wavelike... 

In the opening paragraph a proton is said to have the property of spin. General chemistry usually discusses the spin quantum number, 1, which is the orientation of the intrinsic angular momentum of an electron, with values of +V2 or -V2. The hydrogen nucleus may assume additional values for I. An atom with an even number of protons or an even number of neutrons will not exhibit spin. That is, the spin quantum number will be zero. If an element has an odd number of protons or an odd number of neutrons, then 1 will have a positive value. If there is one odd proton or neutron, I = 1/2 with two odd particles, 1 = 1 with three odd particles, 1 = 3/2, and so on. Table 14.4 shows several common nuclei, the number of protons and neutrons, and the spin quantum number. 

The treatment of the latter appears important for the following reason. As shown in II, there exist molecules with two nuclei, where both term systems, the one symmetric and the one anti-symmetric in the nuclei appear (for instance N2 and Nj ) If we can trust the empirically measured specific heat of hydrogen, this is also the case for H2. For a closer investigation of the properties of the hydrogen nucleus that are hidden behind this fact, one can consider the molecules H2O, NH3 and CH4. As will be shown, only very subtle properties of the spectra or specific heats of these substances can provide the information and the present measurements are not sufficient for a decision. 

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