Two-Dimensional WMR Spectrometry Thomas C. Farrar Department of Chemistry University of Wisconsin Madison, wls. 53706 This article is the second in a two-part series. In part one (ANALYTICAL CHEMISTRY, May 15) we discussed one-dimensional nuclear magnetic resonance (NMR) spectra and some relatively advanced nuclear spin gymnastics experiments that provide a capability for selective sensitivity enhancements. In this article we present an overview and some applications of two-dimensional NMR experiments. These powerful experiments are important complements to the one-dimensional experiments. As in the more sophisticated one-dimensional experiments, the two-dimensional experiments involve three distinct time periods: a preparation period, to; an evolution period, tl; and a detection period, tp.
For example, in an ERNST experiment (enhancement via refocused nuclear spin-polarization transfer, discussed in Reference 1)the preparation period involves a proton relaxation period and a uI2 pulse on the proton spins. Sometimes the preparation period also contains a numher of 13C pulses that are used to ensure that the carhon magnetization, M . ( W ) , is zero,at the
obtain the frequency domain signal. The ERNST experiment and all of the others discussed in Reference 1are one-dimensional experiments, because only a single time response is recorded and only a single Fourier transform is required to obtain the desired frequency domain data. The FID signal, as we have seen, is the result of phase modulation arising from precessing nuclear
end of the preparation period. The evolution period may contain and proton pulses. The evolution times are often multiples of 1/(44, where J is the JCHspin-coupling constant. During the detection period for the ERNST experiment the decoupler is turned on and the carhon free induction decay (FID) signal is detected (recorded). After the experiment is completed, the FID signal is Fourier-transformed to
magnetizations with different chemical shifts and spin-couplinzconstants and, consequently, different resonance frequencies. In all of the experiments discussed so far, the phase modulation effects of chemical shifts and spin couplings are usually present during either the tl or the t p periods, or both. By carefully controlling the time periods during which the chemical shift phase modul4tion and the spin-coupling
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A carbon (?., -,x p-.-w .otates the magnetization into the xy plane where it begins to dephase because of chemical shift end spin-coupling interactions. ~ n ean r mlutlon time T = (t,)/2 = 1/(4J]. me carbon a and 6 magnetizationsare $0' out of phase with one m m r . A C a r m rIpulse lnltiates lhe retocusing 01 both me Chemical shin and me Spin-Wupling dephaslng. After B fvther evOlutiOn time 7 = 1/(4J]. me mgnetlZallonSare COmpletely
