TRIPLE-QUANTUM NMR BY SELECTIVE TRIPLE-RESONANCE

In high-resolution nuclear magnetic resonance spectra of scalar coupled spins in isotropic phase, it is possible to convert single-into triple-quantum coherences and vice versa by irradiating simultaneously at the frequencies of three chemical shifts. A triple resonance condition can be achieved by modulating a radio-frequency carrier by a product of two cosinusoidal audiowaves, chosen so that three of the four sidebands coincide with three selected chemical shifts. When used in conjunction with two-dimensional Fourier transform spectroscopy, it is possible to obtain triple-quantum multiplets with very high resolution within a limited time span. The linewidths can give an accurate measure of the relaxation rates of triple-quantum coherences.