The resonance condition depends on having exactly the right combination of external magnetic field and radio frequency.
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If you replaced the isolated proton with one that was attached to something, it wouldn't be feeling the full effect of the external field any more and so would stop resonating (flipping from one magnetic alignment to the other). Suppose you were using a radio frequency of 90 MHz, and you adjusted the size of the magnetic field so that an isolated proton was in the resonance condition. The effect of the electrons is to cut down the size of the external magnetic field felt by the hydrogen nucleus. What we've said so far would apply to an isolated proton, but real protons have other things around them - especially electrons. The importance of the hydrogen atom's environment This flipping of the proton from one magnetic alignment to the other by the radio waves is known as the resonance condition. It's possible to detect this interaction between the radio waves of just the right frequency and the proton as it flips from one orientation to the other as a peak on a graph. (BBC Radio 4 is found between 92 - 95 MHz!) The energy needed to make this flip depends on the strength of the external magnetic field used, but is usually in the range of energies found in radio waves - at frequencies of about 60 - 100 MHz. It is possible to make it flip from the more stable alignment to the less stable one by supplying exactly the right amount of energy. Again, the alignment where it is opposed to the field is less stable (at a higher energy). Hydrogen nuclei also behave as little magnets and a hydrogen nucleus can also be aligned with an external magnetic field or opposed to it. It is very unstable opposed to the Earth's field, and as soon as you let it go again, it will flip back to its more stable state. Provided it isn't sealed in some sort of container, you could twist the needle around with your fingers so that it pointed south - lining it up opposed to the Earth's magnetic field. If you have a compass needle, it normally lines up with the Earth's magnetic field with the north-seeking end pointing north.
On this page we are focussing on the magnetic behaviour of hydrogen nuclei - hence the term proton NMR or 1H-NMR. Nuclear magnetic resonance is concerned with the magnetic properties of certain nuclei. This page describes what a proton NMR spectrum is and how it tells you useful things about the hydrogen atoms in organic molecules. WHAT IS NUCLEAR MAGNETIC RESONANCE (NMR)? The background to nuclear magnetic resonance (nmr) spectroscopy
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