initial rate at which the NOE grows is proportional to rIS−6, which provides other more sophisticated alternatives for obtaining structural information via transient experiments such as 2D-NOESY. Two-dimensional NMR The motivations for using two-dimensional NMR for measuring NOE's are similar as for other 2-D methods. The maximum resolution is improved by spreading the affected resonances over two dimensions, therefore more peaks are resolved, larger molecules can be observed and more NOE's can be observed in a single measurement. More importantly, when the molecular motion is in the intermediate or slow motional regimes when the NOE is either zero or negative, the steady-state NOE experiment fails to give results that can be related to internuclear distances. Nuclear Overhauser Effect Spectroscopy (NOESY) is a 2D NMR spectroscopic method used to identify nuclear spins undergoing cross-relaxation and to measure their cross-relaxation rates. Since 1H dipole-dipole couplings provide the primary means of cross-relaxation for organic molecules in solution, spins undergoing cross-relaxation are those close to one another in space. Therefore, the cross peaks of a NOESY spectrum indicate which protons are close to each other in space. In this respect, the NOESY experiment differs from the COSY experiment that relies on J-coupling to provide spin-spin correlation, and whose cross peaks indicate which 1H's are close to which other 1H's through the chemical bonds of the molecule. The basic NOESY sequence consists of three 90° pulses. The first pulse creates transverse spin magnetization. The spins precess during the evolution time t1, which is incremented during the course of the 2D experiment. The second pulse produces longitudinal magnetization equal to the transverse magnetization component orthogonal to the pulse direction. Thus, the idea is to produce an initial condition for the mixing period τm. During the NOE mixing time, magnetization transfer via cross-relaxation can take place. For the basic NOESY experiment, τm is kept constant throughout the 2D experiment, but chosen for the optimum cross-relaxation rate and build-up of the NOE. The third pulse creates transverse magnetization from the remaining longitudinal magnetization. Data acquisition begins immediately following the third pulse and the transverse magnetization is observed as a function of the pulse delay time t2. The NOESY spectrum is generated by a 2D Fourier transform with respect to t1 and t2. A series of experiments are carried out with increasing mixing times, and the increase in NOE enhancement is followed. The closest protons show the most rapid