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<keywords content="diffusion rates, dimensionless units, hidden units, radian, relaxation rates, rotational correlation times, SI supplementary units, spherical harmonics" />
A major point of confusion in the NMR field is that of the dimensionless and hidden radian unit. This often results in people mistakenly writing Hertz units when inverse seconds (or rad/s) should have been used. This issue can be traced back to the [http://en.wikipedia.org/wiki/International_System_of_Units SI] organisation itself. Specifically the SI supplementary units definitions whereby the assumption is made that if a process is rotational, a physicist should know that radian units are implicit.
== Introduction == '''Warning: ''' If hard core NMR or physics theory is not to your taste, please do not read any further!
This concept is quite important for understanding relaxation in NMR or, in fact, any rotational process in physics. It is important for understanding the model-free equations, for reduced spectral density mapping, for SRLS, and for relaxation dispersion. The reason is because R1 and R2 are measured in rad/s. As I describe in section 1, the radian unit can be dropped because it is plainly obvious that NMR and relaxation is an angular process and hence radian units are implied (that was sarcastic). Hence R2 can be said to be in units of 1/s, but never, ever Hz. Also note that because of the SI conventions described below, describing the correlation time in s units does not prove that there are no radian units. But reporting rates as Hz implies no radian units whereas reporting as 1/s instead often means radian units are present.