Pure absorption multiple quantum spectra of molecules dissolved in liquid crystalline solvents

Abstract

The use of time reversal pulse sequences to obtain multiple quantum spectra of molecules in thermotropic liquid crystalline phases is described. Several studies have already demonstrated that in order to obtain multiple quantum coherence of high order in polycrystalline solids, it is necessary to utilize pulse trains that produce a preparation propagator that is the adjoint of the mixing propagator. Such pulse sequences produce multiple quantum powder spectra that are pure absortive, thus avoiding destructive phase interference that would occur if standard multiple quantum pulse sequences were used. However even in cases where all single quantum transitions are well-resolved, standard multiple quantum pulse sequences yield multiple quantum spectra of low signal-to-noise because single quantum coherent states are projected out of phase. Sensitivity may be improved by projecting the full two dimensional transform, but this may not be practical in cases involving moderately large numbers of strongly coupled spin one-half nuclei. If the time reversal sequences are used however single quantum coherent states are projected in phase and the full two dimensional transform need not be calculated. The pure absorption double quantum spectrum of oriented benzene has been obtained using time reversal pulse trains and demonstrates a considerable increase in sensitivity over standard methods. Practical aspects of applying multiple pulse sequences to thermotropic systems are considered.