Is the manifestation of the local dynamics in the spin–lattice NMR relaxation in dendrimers sensitive to excluded volume interactions?

Abstract

The effect of excluded volume (EV) interactions on the manifestation of the local dynamics in the spin–lattice NMR relaxation in dendrimers has been studied by using Brownian dynamics simulations. The study was motivated by the theory developed by Markelov et al., [J. Chem. Phys., 2014, 140, 244904] for a Gaussian dendrimer model without EV interactions. The theory connects the experimentally observed dependence of the spin–lattice relaxation rate 1/T_¹H on the location of NMR active groups with the restricted flexibility (semiflexibility) of dendrimers. Semiflexibility was introduced through the correlations between the orientations of different segments. However, these correlations exist even in flexible dendrimer models with EV interactions. We have simulated coarse-grained flexible and semiflexible dendrimer models with and without EV interactions. Every dendrimer segment consisted of two rigid bonds. Semiflexibility was introduced through a potential which restricts the fluctuations of angles between neighboring bonds but does not change orientational correlations in the EV model as compared to the flexible case. The frequency dependence of the reduced 1/T_¹H(ω_H) for segments and bonds belonging to different dendrimer shells was calculated. It was shown that the main effect of EV interactions consists of a much stronger contribution of the overall dendrimer rotation to the dynamics of dendrimer segments as compared to phantom models. After the exclusion of this contribution the manifestation of internal dynamics in spin–lattice NMR relaxation appears to be practically insensitive to EV interactions. For the flexible models, the position ω_max of the peak of the modified 1/T_¹H(ω_H) does not depend on the shell number. For semiflexible models, the maximum of 1/T_¹H(ω_H) for internal segments or bonds shifts to lower frequencies as compared to outer ones. The dependence of ω_max on the number of dendrimer shells appears to be universal for segments and bonds in dendrimer models with and without EV interactions.