State-correlated two-dimensional NMR spectroscopy: Separation of local dipolar fields of protons in nematic phase of 4'-methoxybenzylidene-4-acetoxyaniline

A novel two-dimensional NMR experiment, state-correlated two-dimensional NMR (SC-2D NMR) spectroscopy, was successfully performed between the nematic phase and the isotropic phase of 4'-methoxybenzylidene-4-acetoxyaniline (APAPA) by using pulsed microwave heating. In this experiment, local dipolar fields of the individual protons in the nematic phase were well separated as cross sections in the F1 dimension of the SC-2D NMR spectrum. The cross sections for the aromatic protons showed main doubler patterns with a common splitting of 9.7 kHz, giving an order parameter of 0.41 for the aromatic rings of APAPA. The cross section for the methyl protons of the methoxy group clearly showed a triplet pattern with a splitting of 10.2 kHz, while the cross section for the methyl protons of the acetoxy group showed an unresolved singlet with small shoulders. Computer simulation, based on the evolution of the density matrix operator, taking the order of the ring and the local dipolar couplings of the methyl protons into account, reproduced well the triplet pattern of the methoxy group. It also showed that the difference in the splitting between the methyl and methoxy groups originates from the difference in the orientation of the two methyl axes with respect to the molecular axis. The time dependence of spectral mixing between cross sectional patterns for individual protons of APAPA was studied as a function of the mixing time introduced before the microwave pulse, and was reproduced well for the methoxy protons by a Solomon type equation that takes into account exchange of magnetizations due to dipolar interactions in the liquid crystalline phase. (C) 1996 American Institute of Physics.