Comparison of spin dynamics, characterized by Multiple Quantum evolution under different Hamiltonians

CLAUDIA M. SÁNCHEZ; ANA K. CHATTAH; PATRICIA R. LEVSTEIN; R.H. ACOSTA

La Falda, Córdoba, Argentina

Workshop; Mini-School on Disordered Systems 2008 and 6th International Workshop on Disordered Systems (6IWDS-2008); 2008

ICTP (Trieste)

In this work we present multiple quantum solid state NMR experiments to study the many-body dynamics of spin systems. A pulse sequence is designed to characterize the dipolar evolution and to encode it in different coherence numbers. This pulse sequence, inspired in that reported in ref. [Cho et al. Phys. Rev. B 72 054427 (2005)], allows the study of the dipolar Hamiltonian rotated with respects to the y axis (H0xx). Initially the system is in equilibrium with the Zeeman field, successive rotations of the sequence of 2 pulses around z-axis encode the coherence orders in the Zeeman base after the dipolar evolution during a time t. A spin-lock r.f. field and a final lecture pulse are used to refocus the multiple-spin terms back to observable single quantum coherence terms. Two structurally different systems are studied. The first one is the polycrystalline Adamantane which conforms an infinite 1H system with an uniform set of intermolecular interactions. The second compound is the liquid crystal 5CB in the nematic mesophase, were only intramolecular interactions survive conforming a finite set of protons. The difference in the arrangement in the 1H-1H interactions of both systems is correlated with the behavior of the coherence order buildups. Spin counting measurements give information on the geometrical arrangement of spins and cluster formation. The experiments also give insight on the sources of decoherence during the evolution. A comparison between the evolution under dipolar and double quantum Hamiltonian is discussed.