The Direct DIVAM Experiment: A Spin Dynamics Analysis

Abstract

Domain selection in polymer NMR is limited to experiments specifically suited to each structural domain owing to its particular spin dynamics and relaxation properties. The DIVAM experiment can be tuned to select for signal from the domain of interest, making it possible to obtain signals specific to different domains using only one experiment. An early description of this sequence explains this tunability using a simple one-spin-relaxation model, thereby limiting the selection mechanism to incoherent processes and thus ignoring the coherent terms such as chemical shift anisotropy (CSA), dipolar coupling and offset terms. Experiments have shown that when the DIVAM sequence is applied directly to the nucleus of interest, referred to as direct DIVAM (DD), transient behavior is observed in the signal intensity on the sample spinning time scale. This indicates that the coherent terms are involved in the selection process; the exact role of these terms is explored in this work. SIMPSON simulations illustrate that the CSA and offset terms can play a dominant role in domain selection; however, the dipole term was relatively ineffective and required large values before substantial selection was predicted. Using a one-spin-relaxation model, which now includes a chemical shift evolution term, an analytical expression for the signal intensity was provided as a function of interpulse delay (tau), excitation angle (theta), relaxation time (T2), and offset frequency (Deltanu). These indicate that the selection behavior varies substantially with differing time scales and excitation angles. For small angles and long delay times DD behaves primarily as a relaxation filter, whereas for larger angles and short delay times the coherent terms take over dominated by the CSA interactions. The DD sequence can therefore be set to select on the basis of the transverse relaxation rate or the strength of the CSA interaction, depending on the excitation angle used.