Spin dynamics in Adamantane and Liquid Crystal 5CB Charaterized by Multiple Quantum Dipolar Evolution.

A.K. CHATTAH; C.M. SANCHEZ; P.R. LEVSTEIN; R.H. ACOSTA

Pacific Grove, EEUU

Congreso; 49th ENC - Experimental Nuclear Magnetic Resonance Conference; 2008

In this work we present multiple quantum solid state NMR experiments to study the many-body dynamics of spin systems.A new 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 reference1, allows the study of the dipolar Hamiltonian rotated withrespects to the y axis (H0xx). Initially the system is in equilibrium with the Zeeman field and a rotation around the z axisencodes the coherence orders in the Zeeman basis after the dipolar evolution during a time 􀁗. A spin-lock r.f. field and afinal 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 infinite1H system with an uniform set of intermolecular interactions. The second compound is the liquid crystal 5CB in thenematic mesophase, were only intramolecular interactions survive conforming a finite set of protons. The difference in thearrangement 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.2 Theexperiments also give insight on the sources of decoherence during the evolution. A comparison between the evolutionunder dipolar and double quantum Hamiltonian is discussed.