 However, the relation "η"IS(max)= obscures how the NOE is related to internuclear distances because it applies only for the idealized case where the relaxation is 100% dominated by dipole-dipole interactions between two nuclei I and S. In practice, the value of ρI contains contributions from other competing mechanisms, which serve only to reduce the influence of "W"0 and "W"2 by increasing "W"1.Sometimes, for example, relaxation due to electron-nuclear interactions with dissolved oxygen or paramagnetic metal ion impurities in the solvent can prohibit the observation of weak NOE enhancements.The observed NOE in the presence of other relaxation mechanisms is given by

where ρ⋇ is the additional contribution to the total relaxation rate from relaxation mechanisms not involving cross relaxation.