Collective long-lived zero-quantum coherences in aliphatic chains

In nuclear magnetic resonance, long-lived coherences constitute a class of zero-quantum (ZQ) coherences that have lifetimes that can be longer than the relaxation lifetimes T-2 of transverse magnetization. So far, such coherences have been observed in systems with two coupled spins with spin quantum numbers I = 1/2, where a term |S-0 >< T-0| + |T-0 >< S-0| in the density operator corresponds to a coherent superposition between the singlet |S-0 > and the central triplet |T-0 > state. Here, we report on the excitation and detection of collective long-lived coherences in AA ' MM ' XX ' spin systems in molecules containing a chain of at least three methylene (-CH2-) groups. Several variants of excitation by polychromatic spin-lock induced crossing (poly-SLIC) are introduced that can excite a non-uniform distribution of the amplitudes of terms such as |S0S0T0 >< S0S0T0|, |S0T0S0 >< S0T0S0|, and |T0S0S0 >< T0S0S0|. Once the radio frequency fields are switched off, these are not eigenstates, leading to ZQ precession involving all six protons, a process that can be understood as a propagation of spin order along the chain of CH2 groups before the reconversion into observable magnetization by a second poly-SLIC pulse that can be applied to any one or several of the CH2 groups. In the resulting 2D spectra, the omega(2) domain shows SQ spectra with the chemical shifts of the CH2 groups irradiated during the reconversion, while the omega(1) dimension shows ZQ signals in absorption mode with linewidths on the order of 0.1 Hz that are not affected by the inhomogeneity of the static magnetic field but can be broadened by chemical exchange as occurs in drug screening. The ZQ frequencies are primarily determined by differences Delta J between vicinal J-couplings.