QUANTUM TREATMENT OF THE EFFECTS OF DIPOLE-DIPOLE INTERACTIONS IN LIQUID NUCLEAR-MAGNETIC-RESONANCE

Experimental observation of anomalous intermolecular cross-peaks in tw o-dimensional solution NMR spectra have attracted significant recent a ttention. Extremely simple guise sequences on extremely simple samples with large equilibrium magnetization give resonances in the indirectl y detected dimension which are simply impossible in the conventional d ensity matrix framework of NMR. Here we extend a recently proposed den sity matrix treatment [Science 262, 2005 (1993)] to calculate the exac t time evolution for a variety of pulse sequences, This density matrix treatment explicitly removes two fundamental assumptions of the stand ard theory-it includes the dipolar interaction between spins in soluti on (which is only partially averaged away by diffusion) and completely removes the high temperature approximation to the equilibrium density matrix [exp(-beta H)approximate to 1-beta H]. We compare this quantum mechanical treatment to a corrected classical model, which modifies t he dipolar demagnetizing field formulation to account for the effects of residual magnetization, and show that the quantum picture can be re duced to this corrected classical model when certain assumptions about the retained dipolar couplings are valid. The combination of quantum and classical pictures provides enormously better predictive power and computational convenience than either technique alone. (C) 1996 Ameri can Institute of Physics.