Multinuclear and Solid State NMR of Gels

Abstract

Over the past six decades, nuclear magnetic resonance spectroscopy has been an integral part of synthetic organic and organometallic chemistry, as well as biochemistry. Beyond solution state experiments, increasing developments have opened new avenues to study materials in their solid state. Between two extremes (i.e., solution and solid), there exist several other forms of materials, especially soft materials such as gels and liquid crystals. Traditionally gels have been studied using solution state NMR spectroscopic methods. However, the viscosity of complex viscoelastic fluids such as gels affects the molecular tumbling, which in turn affects the chemical shift anisotropy and dipolar and quadrupolar interactions, resulting in broad spectral lines. Therefore, the importance of solid state (SS) NMR in understanding the structural details of self-assembled soft materials has remained unexplored for several decades. Nevertheless, promising results in understanding weak interactions in polymer gels have been explored using solid state cross polarization (CP) and high resolution (HR) magic angle spinning (MAS) NMR spectroscopy. However, similar studies and the possibility to utilize SS NMR spectroscopy to study hydro- and organogels derived from low molecular weight gelators have been limited until recently. In this chapter, we will focus on the application of SS NMR to study xerogels, aerogels and native gels with selected examples.