characterized in terms of two separate processes, each with their own time constants. One process, called T1, is responsible for the loss of resonance intensity following pulse excitation. The other process, called T2, characterizes the width or broadness of resonances. Stated more formally, T1 is the time constant for the physical processes responsible for the relaxation of the components of the nuclear spin magnetization vector M parallel to the external magnetic field, B0 (which is conventionally designated as the z-axis). T2 relaxation affects the coherent components of M perpendicular to B0. In conventional NMR spectroscopy, T1 limits the pulse repetition rate and affects the overall time an NMR spectrum can be acquired. Values of T1 range from milliseconds to several seconds, depending on the size of the molecule, the viscosity of the solution, the temperature of the sample, and the possible presence of paramagnetic species (e.g., O2 or metal ions). T1 The longitudinal (or spin-lattice) relaxation time T1 is the decay constant for the recovery of the z component of the nuclear spin magnetization, Mz, towards its thermal equilibrium value, . In general, In specific cases: If M has been tilted into the xy plane, then and the recovery is simply i.e. the magnetization recovers to 63% of its equilibrium value after one time constant T1. In the inversion recovery experiment, commonly used to measure T1 values, the initial magnetization is inverted, , and so the recovery follows T1 relaxation involves redistributing the populations of the nuclear spin states in order to reach the thermal equilibrium distribution. By definition, this is not energy conserving. Moreover, spontaneous emission is negligibly slow at NMR frequencies. Hence truly isolated nuclear spins would show negligible rates of T1 relaxation. However, a variety of relaxation mechanisms allow nuclear spins to exchange energy with their surroundings, the lattice, allowing the spin populations to equilibrate. The fact that T1 relaxation involves an interaction with the surroundings is the origin of the alternative description, spin-lattice relaxation. Note that the rates of T1 relaxation (i.e., 1/T1) are generally strongly dependent on the NMR frequency and so vary considerably with magnetic field strength B. Small amounts of paramagnetic substances in a sample speed up relaxation very much. By degassing, and thereby removing dissolved oxygen, the T1/T2 of liquid samples easily go up to an order of ten seconds. Spin saturation transfer Especially for molecules exhibiting slowly relaxing (T1) signals, the