NMR Techniques Based On Heteronuclear Intermolecular Multiple Quantum Coherences And Their Applications

Intermolecular multiple-quantum coherences (iMQCs) or multiple spin echo (MSE) in highly polarized spin systems have become a hot topic in the NMR community since 1990s. Several research groups have explored the intrinsic and extrinsic properties of iMQCs for their applications in NMR, ranging from high-resolution NMR spectroscopy in inhomogeneous fields to MR imaging. In this work, several pulse sequences were designed and analyzed to identify the characteristics of signals from heteronuclear iMQCs. The results can be regarded as an extension and supplement of iMQC theory and its applications, and can besummarized as follows:1. General theoretical expressions for the heteronuclear CRAZED (COSY Revamped with Asymmetric Z-gradient Echo Detection) pulse sequence with arbitrary flip angles were derived using dipolar field treatment and signals originating from heteronuclear intermolecular single-quantum coherences (iSQCs) in highly-polarized two spin-1/2 systems were discussed in order to find the optimal flip angles for maximal signal intensity. The results show that signals from heteronuclear iSQCs decay slower than those from intermolecular double-quantum coherences (iDQCs) or intermolecular zero-quantum coherences (iZQCs). Magical angle experiments validate that heteronuclear iSQC signals are insensitive to the imperfection of radio-frequency pulse flip angles.2. The CRAZED pulse sequence was used to study the signal features from intermolecular multiple-quantum coherences between spin 1/2 and quadrupolar nuclei in solution NMR. General theoretical expressions were derived using the raising and lowering operator formalism. Theoretical predictions for the relationships between relative signal intensities and the optimal pulse flip angles are in excellent agreement with experimental results. Both the magic angle gradient and two-dimensional (2D) experiments validate that the signals are indeed from heteronuclear iMQCs.3. A modified heteronuclear CRAZED pulse sequence was proposed to detect high-resolution NMR spectra via heteronuclear iZQCs or iDQCs in inhomogeneous fields. It overcomes the difficulty of selective excitation in the homonuclear CRAZED experiments when the chemical shifts of solvent and solute are close to each other. In the one-dimensional (1D) spectra extracted from the 2D data, the chemical shifts, coupling constants, multiplet patterns, and relative peak areas are almost independent of the magnetic field inhomogeneity.4. A pulse sequence based on heteronuclear iSQCs was proposed to achieve fast acquisition of 2D high-resolution spectra in inhomogeneous fields. Because the width of the F1 dimension is determined by the line-broadening instead of chemical-shift of the modulating spins, the total acquisition time is greatly reduced. This new pulse sequence overcomes the problem of spectral peaks overlapping in highly inhomogeneous fields using heteronuclear iZQC or iDQC pulse sequence. In addition, the data of heteronuclear iSQCs can be post processed using the system built-in command, which makes the heteronuclear iSQC method more convenient and universal than iZQC and iDQC methods.