Wednesday, July 23, 2008
The 2D EXchange SpectroscopY (EXSY) technique is exactly the same as the same as the 2D NOESY technique. The pulse sequences are identical. The method provides off-diagonal responses for spins which exchange slowly with one another (either conformationally or chemically) and also between spins with NOE's. The EXSY method is useful for showing exchange when the rate of the exchange is greater than or of the same order as the T1 relaxation rate (1/T1) but less than the frequency difference between the two spins (in the absence of exchange). Depending on the experimental conditions, the responses due to exchange are often much more intense than those due to NOE's. The figure below shows the 300 MHz 2D 1H EXSY spectrum of N,N-dimethylacetamide at room temperature. At this temperature, the molecule exhibits slow rotation about the (CH3)2N - C bond such that both methyl groups exchange with one another rotationally yet are distinct in the spectrum. This is evident by the cross peaks in the spectrum between the two methyl groups on the nitrogen. Note that the off diagonal peaks are of the same phase as the diagonal peaks.