In position a), there will be a repulsive interaction between the two, in position c)it will be attractive, and somewhere in between(position b))the interaction will be zero. Now our molecule is not static, but pushed around all the time in our sample solution. A rotation of the molecule will change its orientation in the static magnetic field and cause a modulation of the field strenght a spin actually through the effects of the neighbouring spins. Thus, the rotation of the other spins generates an electromagnetic field -just like in an electric generator, but not with a well-defined frequency, but with a wide range of frequency components, due to the random nature of the molecular rotation The easiest approximation to describe the rotation of a molecule in solution is the assumption of the molecule as a rigid sphere. If we calculate how"much"of a certain frequency is generated in the random rotation, we find the following expression for the"spectral density"at a frequency o J()= It means that we have a maximum at @=0 and the spectral density then drops off for higher frequencies. How fast is controlled by the parameter tc, the molecular rotaional correlation time(or just" correlation time"). A long correlation time means a rather sluggish rotation, and a fast drop off of J(very small high-frequency contributions). A short t, corresponds to a fast random rotation causing a much wider frequency distribution/more high-frequency contributions95 In position a), there will be a repulsive interaction between the two, in position c) it will be attractive, and somewhere in between (position b)) the interaction will be zero. Now our molecule is not static, but pushed around all the time in our sample solution. A rotation of the molecule will change its orientation in the static magnetic field and cause a modulation of the field strenght a spin actually "experiences" – through the effects of the neighbouring spins. Thus, the rotation of the other spins generates an electromagnetic field – just like in an electric generator, but not with a well-defined frequency, but with a wide range of frequency components, due to the random nature of the molecular rotation. The easiest approximation to describe the rotation of a molecule in solution is the assumption of the molecule as a rigid sphere. If we calculate how "much" of a certain frequency is generated in the random rotation, we find the following expression for the "spectral density" at a frequency w: J c c (w) t w t = + 2 1 2 2 It means that we have a maximum at w=0 and the spectral density then drops off for higher frequencies. How fast is controlled by the parameter t c , the molecular rotaional correlation time (or just "correlation time"). A long correlation time means a rather sluggish rotation, and a fast drop off of J (=very small high-frequency contributions). A short t c corresponds to a fast random rotation, causing a much wider frequency distribution / more high-frequency contributions