Intermolecular double-quantum coherence NMR spectroscopy in moderate inhomogeneous fields.

Intermolecular multiple-quantum coherences (iMQCs) can be utilized to retrieve high-resolution NMR spectra in inhomogeneous magnetic fields. The application of selective pulses in pulse sequences can greatly simplify 2D iMQC spectra. However, so far high-resolution iMQC methods are mainly used in relatively small field inhomogeneities. In this paper, we took the IDEAL-II sequence as an example to study their applicability in moderate inhomogeneous magnetic fields. The experimental and simulation results show that high-resolution NMR spectra can be obtained in moderate inhomogeneous fields if the excitation range of selective pulse is properly set. Once the field inhomogeneity reaches a certain degree, the appearance of undesirable intermolecular cross-peaks due to the distant dipolar field produced by solute spins is inevitable. The spectral quality may vary with sample even in the same moderate inhomogeneous fields, depending on the chemical shift distributions and the J coupling networks of the components under study. The conclusions drawn in this paper are generally applicable to all high-resolution iMQC methods utilizing selective RF pulses.