NMR and isothermal calorimetric titrations, as well as X-ray structures, show the binding of a nitrate anion inside guanidinium-based macrocycles. All six lone pairs of nitrate are complemented by oriented hydrogen bond donors from a guanidinium and two urea subunits.
![Enthalpy driven nitrate complexation by guanidinium-based macrocycles](https://www.iciq.org/wp-content/uploads/2014/03/Enthalpy-driven-nitrate_eng_noticies_articles_2007_19.gif)