Selective multiple quantum ¹H nuclear magnetic resonance in chemically exchanging systems: Applications to liquid crystalline solutions

Multiple quantum (MQ) proton NMR spectroscopy is applied to chemically rearranging systems. These systems include liquid crystalline solutions of s-trioxane, which undergoes ring inversion and cyclooctatetraene (COT), which undergoes a bond shift rearrangement. The MQ spectra are recorded by a modified version of the selective detection method of Wokaun and Ernst. In this modification the phases of both the rf pulses and of the detector are cycled in concert, yielding a more efficient and potentially more accurate method for recording the MQ transitions. Experimental fourth and sixth order MQ spectra are presented for respectively the s-trioxane and COT solutions over wide temperature ranges. The results are compared with simulated spectra calculated using exact as well as approximate expressions for the dynamic line shapes. Potential uses and advantages as well as limitations of MQ spectroscopy for studying dynamic systems are discussed.