Canonical nucleobases are attached to the sugar-phosphate backbone in nucleic acids and act as molecular recognition units. Their complementary base-pairing is central to the transfer of genetic information. Moreover, base-pairing of free canonical nucleobases (without being attached to the sugar-phosphate backbone) might have been a starting point for the origin of nucleic acids. However, it has been known for almost 60 years that there is no molecular recognition and complementary base-pairing of free canonical nucleobases in water. Instead, they form energetically favored stacking structures and hydrogen bonds with water molecules. We recently showed that base-pairing of free canonical nucleobases does not occur even in the solid-state reaction environment devoid of water. This inability of free canonical nucleobases to spontaneously form Watson-Crick base pairs is known as a "base-pairing paradox", making it unclear how the chemical selection of nucleobases occurred from a larger pool of similar molecules likely available on the prebiotic Earth. On one hand, it has been proposed that RNA is a product of evolution. Under this assumption, proto-RNA forms could have been constituted by different components, including nucleobases, whose chemical bonding was easier under prebiotic conditions. An alternative scenario is that RNA formed de novo on the prebiotic Earth and bypassed chemical selection on the level of nucleobases. For example, prebiotic syntheses of canonical nucleosides and nucleotides have been reported and include the formation of the glycosidic bond between more reactive substances and subsequent building of nucleobases on the sugars. Nevertheless, the base-pairing of free canonical or non-canonical nucleobases may affect their chemical properties and remains largely unexplored in the literature. Hence, it serves as a motivation for this work.